ALBERT

Description: This study presents a three-phase tri-state buck–boost integrated inverter suitable for stand-alone and/or grid-connected photovoltaic (PV) energy applications. The usage of the special features of the tri-state operation coupled with a modified space vector modulation allows the inverter to present a remarkable degree of freedom for the controllers’ design, that is, input and output can be independently controlled. As the input can be autonomously controlled, this converter is very attractive for interfacing renewable energy sources as PV panels, once the main duty cycle D 1 controls the maximum power point tracking and the second duty cycle D 2 is responsible to feed the output. It should be pointed out that there are no power electrolytic capacitors in this integrated inverter, which results in a major advantage for the structure working life and it is the first time in the literature that this inverter is used to feed power into the grid. Throughout this study the system is described, the modulation and control schemes are presented and interesting experimental results are available in order to confirm the outstanding features of this proposal.

Description: The purpose of this study is to analyse possible strategies for control and management of an existing low-voltage (LV) network, also with a supervisor control and data acquisition system with future internet potentialities, in order to flatten the network voltage profile, to improve the power factor at medium voltage supplying busbars, to minimise the overall power losses and then to reduce operating costs. In particular, operational records of the LV network have been acquired over a period of 8 months, with a time rate of 4 snap-shots per hour (23,232 measures), and different possible scenarios (each of which compatible with the pilot plant control system and power management) have been analysed. On the base of simulation results carried out by power flow (PF), and optimal PF, studies, the effectiveness, in terms of cost–benefits analysis, of the installation of a suitable sized battery energy storage system, as well as of the implementation of a demand response strategy, has been also assessed. The main results of simulations, measurements and experimental tests are reported and discussed in detail.

Description: In recent years, the concept of micro-grid has appeared as an appropriate way for the integration of distributed energy resources (DERs) in the distribution networks. However, micro-grids have encountered a number of challenges from control and protection aspects. One of the main issues relevant to the protection of micro-grids is to develop a suitable protection technique which is effective in both grid-connected and stand-alone operation modes. This study presents a micro-grid protection scheme based on positive-sequence component using phasor measurement units and designed microprocessor-based relays (MBRs) along with a digital communication system. The proposed scheme has the ability to protect radial and looped micro-grids against different types of faults with the capability of single-phase tripping. Furthermore, since the MBRs are capable of updating their pickup values (upstream and downstream equivalent positive-sequence impedances of each line) after the first change in the micro-grid configuration (such as transferring from grid-connected to islanded mode and or disconnection of a line, bus or DER either in grid-connected mode or in islanded mode), they can protect micro-grid lines and buses against subsequent faults. Finally, in order to verify the effectiveness of the suggested scheme and the designed MBR, several simulations have been undertaken using DIgSILENT PowerFactory and MATLAB software packages.

Description: The increasing share of variable renewable energy sources for electricity increases the operating reserve requirements of power systems. This contribution studies the techno-economic feasibility of procuring reserve capacity as an ancillary service from wind power producers. Research and demonstration have shown that wind power is able to meet the technical requirements of providing these balancing services. This study shows how acceptable availability levels can be achieved when using probabilistic forecast models in combination with day-ahead procurement. The participation of wind power is analysed with a unit commitment model in order to determine the impact on generation system scheduling, as well as the electricity generation costs. Simulations show how wind power becomes a potential provider of downward reserve capacity when facing high reserve capacity requirements during periods with low demand and high renewable generation. In contrast, its participation in the upward reserve capacity remains limited, explained by the high opportunity cost.

Description: In recent years, configuring battery energy storage system (BESS) in wind farm has become the most popular method to smooth wind power fluctuation. The effectiveness of wind power fluctuation smoothing relies on the charging/discharging power control and state of charge (SOC) regulation of BESS. This study proposed a novel optimising SOC control approach for BESS in wind farm which could regulate wind power fluctuation in a suitable level and maintain SOC in an optimal range by utilising the wind power prediction information. A wind power prediction-based optimal SOC calculation module is designed to obtain an optimal range of SOC which makes BESS have enough capacity to smooth wind power fluctuation in a finite future period of time. It takes minimising over-charging and over-discharging time as optimisation objectives and biggest variations of wind farm outputting power within 1 and 10 min as restraint. A fuzzy self-adjusting of filter-based real-time control module is designed to smooth the wind power fluctuation in real time and regulate SOC to the optimal range. Finally, the effectiveness of the proposed approach was demonstrated through implementing it in wind power smoothing control simulation experiments.

Description: In this study, a method for solving a probabilistic three-phase power flow in radial distribution networks and taking into account the technical constraints of the system is presented. Regulation of voltage is one of the main problems to be taken into account in networks with distributed generation. The present study introduces a probabilistic model to determine the performance of the distribution system. This study considers the random nature of lower heating value of biomass and loads. This study presents a new hybrid technique combining the shuffled frog leaping algorithm with probabilistic three-phase power flow that is solved by Monte Carlo method. Feasible results are achieved in a few iterations. The results show that the proposed technique can be applied to keep the voltages within the limits specified at each node of a distribution network with biomass power plants. The outcomes are attained using the unbalanced distribution system IEEE-13 node and connecting biomass power plants at some nodes. This study shows that the power losses and voltage unbalance are reduced as a result of the inclusion of distributed generation.

Description: The detection of islanding effect is one of the important issues for photovoltaic (PV) power system since islanding is dangerous to utility equipment and workers, and result in severe injuries and death. The novelty of this study is combining extension neural networks and Chaos synchronisation (CS) on islanding detection of grid connected PV systems based on non-autonomous Chua's circuit. Combining CS and extension neural network type-2 (ENN-2), this research is to propose a novel detection method at detecting and distinguishing the occurrence of islanding effect based on non-autonomous Chua's circuit. Simulation and experimental designs through powersim (PSIM) were applied to mimic PV power system to demonstrate the effectiveness of the proposed method. Results show that the accuracy of ENN-2 achieves 98.4% on detecting the islanding effect for PV power system.

Description: Fuel cells have a key role in the potential provision of combined generation of heat and power. Sophisticated power management algorithms have been developed to reduce hydrogen consumption. An accurate analysis of the interaction between the fuel cell system and the front-end power converter is fundamental to achieving high performances. Accurate modelling of the entire system is required, especially the primary energy source, including the auxiliary components. Fuel cell modelling techniques have been presented extensively in the literature. Models are usually oriented to give technical insight concerning the fuel cell systems. The importance of fuel cell modelling in the design and control of front-end power converters seldom has been addressed, and this has led to highly complex model structures and the need to assume the values of several parameters that cannot be measured easily in a power electronics laboratory. Such models are not well suited for designing renewable energy systems. Hence, semi-empirical models oriented to the design of power systems are presented and compared. Both white-box and black-box modelling approaches are presented, and both models were tested on a 5-kW commercial fuel cell system provided by Nuvera. Both modelling approaches were validated comparing the simulation and experimental results.

Description: It is anticipated that concentrated solar power (CSP) plant will become one of the significant resources for electric power generation in the near future. As the smaller scale CSP-based generations are primarily connected in distribution systems, there is a need to assess risk-based security for mid- to long-term planning. To quantify the risk of static and dynamic security with CSP, a method based on chronological simulation has been proposed. A clustering, algorithm-based scenario-reduction technique has been used to utilise the data efficiently for chronological simulation while preserving the temporal information. A number of composite risk indices that truly reflect the system risk have been used to address the static and dynamic security of a distribution system. The proposed method has been applied to a medium voltage distribution system, 43-bus system, to evaluate the static and dynamic risk with CSP and electric vehicles using the load profiles of North Queensland. Power profiles of a CSP system at Rockhampton, North Queensland, have been used for the simulation studies.

Description: The authors develop a stochastic model for assessing the life-cycle cost and availability of wind turbines resulting from different maintenance scenarios, with the objective to identify the most cost-effective maintenance strategy. Using field-data-based reliability models, the wind turbine – in terms of reliability – is modelled as a serial connection of the most critical components. Both direct cost for spare parts, labour and access to the turbine, as well as indirect cost from production losses are explicitly taken into account. The model is applied to the case of a Vestas V44–600 kW wind turbine. Results of a reliability-centred maintenance analysis of this wind turbine are used to select the most critical wind turbine components and to identify possible maintenance scenarios. This study reveals that corrective maintenance is the most cost-effective maintenance strategy for the gearbox and the generator of the V44 turbine, while the cost benefit of condition-based maintenance using online condition-monitoring systems increases with higher electricity price, turbine capacity and remoteness of sites.

Description: High voltage AC (HVAC) transmission system is preferred to be used in off-shore wind power plant which is located within 40km from the seashore. Capacitance of HVAC cables is naturally higher than that of overhead lines, and it should be properly compensated. P model of three-core XLPE cable which is widely used in off-shore wind power plant was applied in the paper. Based on this model, impacts of reactive power compensation on HVAC transmission system, including capacity and location of compensation equipment, were analyzed and simulated then. Research showed that inductive compensation at off-shore side of the cable is more effective to decrease power loss along the cable and increase its transmission capacity than that at onshore side, while the latter is more appropriate to improve the power factor at onshore access point of the wind power plant to grid. Compensation capacity was optimized to realize the best economy with designated certain parameters of HVAC system by Sequential Quadratic Programming (SQP) method. Finally by SQP costs of 35kV and 220kV HVAC transmission system were compared, critical distance and transmission capacity of these two systems adoption were found. These results can be a reference of HVAC transmission system design of off-shore wind power plant.

Description: Renewable energy sources generate stochastic electric power that depends on factors that vary widely. The average wind speed throughout Lithuania varies from 3.1–6.1 m/s at a height of 50 m. Low, stochastic wind energy dominates in the European Union and Lithuania. In Lithuania, the recorded hydro energy potential of 472 small rivers is approximately 585 MW of power. Range of runoff highly varies of Lithuanian small rivers. These are huge wind and hydro energy resources that low energy potential is poorly utilised because of the very complex structures of power converter systems and their low quality of converted power. The application of a dual electric system that can operate with either I = constant and U = variable or U = constant and I = variable appliance is one of the possibilities to solve renewable energy sources low-intensity stochastic energy utilisation problems. This research article presents the features of a dual electric system and the simulation schemes, research results, aspects of practical applications of the inverter – three-phase dual electric system converter with the simple control system that confirms highly effective power extraction in low and high-intensity stochastic energy.

Description: To achieve renewable energy targets, more wind turbine generators (WTGs) are being integrated into many power networks around the world. However, unlike traditional synchronous generators, modern WTGs are driven by power electronic devices which provide almost zero inertia and frequency response to frequency related events such as generator tripping. Wind manufacturers and researchers have been working on synthetic WTG inertia support, which can better utilise the rotational nature of WTGs. However, synthetic inertia alone may be insufficient under certain circumstances to prevent automatic under frequency load shedding after generation tripping, which causes security concerns for network operation. Consequently, WTG active power control (APC, similar to governor control) should be activated to improve network security. However, APC will costly reduce WTG output from the maximum power point and result in financial concerns. Therefore when and how much APC service should be activated becomes an extremely important question, which has not been addressed in the literature. This study develops a new method to quickly estimate frequency response caused by generator tripping hence system operators can use this proposed method to continuously evaluate inertia and headroom competency and accordingly activate the amount of WTG inertia and APC required for reliable system operation.

Description: In this study, a fuzzy-PI controlled grid interactive inverter has been designed and implemented. The proportional and integral gains of the PI controller are decided and tuned by the fuzzy logic controller (FLC) according to required operation point of the system. Thus, adaptive nature of the FLC and robust structure of the PI controller are synthesised. Eventually, an adaptive PI controller which can adopt changes because of different operation conditions, grid disturbances and natural effects with fast transient response is obtained. Simulation studies are validated with experimental results. Both simulation and experimental results show that proposed system has fast dynamic response and tracks reference current with a low overshot and short settling time. In addition, the waveform of the inverter output current is sinusoidal and also the current is in same phase and frequency with the line voltage. Furthermore, the total harmonic distortion level of the inverter current meets the international standards. In addition, the proposed inverter system is compared with conventional PI controlled grid interactive inverter with various proportional and integral gains.

Description: This study proposes a new method to improve low-voltage ride through of the doubly-fed induction generator (DFIG) wind turbines (WTs) using gate-controlled series capacitor (GCSC) in series with the generator rotor. Using GCSC, as soon as a voltage dip occurs at the generator's terminals, the GCSC inserts its capacitor in series with the rotor, increasing the voltage seen by the rotor winding. Consequently, the rotor over-current is limited and transmission of the inrush energy to the rotor side converter is reduced. Moreover, by this method, the chopper is not required for protecting the DC-link capacitor. Unlike the Crowbar method, which control of the generator is lost during the fault, the proposed method in this study avoids the loss of control during the grid faults. To validate the effectiveness of the proposed method, extensive time-domain simulation is performed using MATLAB/SIMULINK on a 1.5 MW DFIG-based WT.

Description: The power-voltage characteristic curve of photovoltaic systems under partially shaded conditions exhibits multiple peaks and renders conventional maximum power point tracking techniques ineffective. This study proposes a hybrid optimisation algorithm incorporating genetic algorithm (GA) in the initial stages of tracking followed by traditional perturb and observe (P&O) algorithm. Although GA and P&O methods do not guarantee convergence to global maximum power point when employed separately, the fusion of the two methods leads to confirmed global convergences with least time. The excellent performance of the combined method is illustrated through extensive simulation and experimental results.

Description: In this study, a microgrid with two voltage source inverters (VSIs), operating in islanded or grid-connected mode is analysed. The active power control by droop coefficients and reference frequencies is demonstrated. In addition, the inductive and resistive droop strategies are compared by considering the complex line impedance. By measuring the line impedance in the prototype, it was found that, even for the low-voltage microgrid, the line impedance was not predominantly resistive, but complex. For this line impedance condition, an investigation to determine the best droop control law considering the partial derivatives. Observing the partial derivatives and the experimental results obtained it is demonstrated that inductive droop control is better to control the active power for complex line impedance. For a scenario with resistive unbalanced loads and different power sharing between the inverters, the proposed control, implemented in the a–b–c reference frame, was validated. A novel absolute damping factor is developed to improve the transient response and reduce the reactive power flow. The VSIs are connected to the grid without transformers to reduce the connection costs. Simulation and experimental results are presented to confirm the improvements achieved using the implemented control method.

Description: With the rapid growth of the wind energy systems in the past years and their interconnection with the existing power system networks, it has become very significant to analyse and enhance the transient stability of the wind energy conversion systems connected to the grid. This study investigates the transient stability enhancement of a grid-connected wind farm using doubly-fed induction machine-based flywheel energy storage system. A cascaded adaptive neuro-fuzzy controller (ANFC) is introduced to control the insulated gate bipolar transistor switches-based frequency converter to enhance the transient stability of the grid-connected wind farm. The performance of the proposed control strategy is analysed under a severe symmetrical fault condition on both a single-machine infinite bus model and the IEEE-39 bus New England test system. The transient performance of the system is investigated by comparing the results of the system using the proposed ANFCs with that of the black-box optimisation technique-based proportional–integral controllers. The validity of the system is verified by the simulation results which are carried out using PSCAD/EMTDC environment.

Description: Power balancing algorithm and dc fault ride through strategy in a DC grid with a DC hub are proposed in this article. Inductor-capacitor-inductor (LCL) dc hub is used in the DC grid to match different voltage levels and to prevent DC fault spreading. An automatic power balancing control is developed for the hub controller, and for DC grid terminals including wind farms and passive AC loads. Collectively these controls enable power balancing for any DC fault without the need for hardware braking systems. The challenges of connecting/disconnecting of DC grid segments and a DC grid black start with non-isolated dc hubs are resolved with new controls and specific operating sequence. A 3-terminal test dc grid is developed using a 3-port LCL dc hub where one terminal is connected to a wind farm, another terminal is connected to an ac grid and a third terminal to a passive ac load. Simulations on this difficult test system are performed for a range of DC faults, power steps and a black start using PSCAD/EMTDC and results verify conclusions.

Description: This study investigates the feasibility of integrating a single-switch active rectifier layout in an offshore wind farm. The proposed active rectifier topology consists of cascade connection of a diode-rectifier and a DC–DC boost converter. A DC collection grid is considered inside the wind farm, which supplies power to a land grid. Each wind conversion units within the DC collection grid includes a turbine, a generator, and an active rectifier. The DC–DC stage of the active rectifier allows regulating the frequency of generator, controlling the DC voltage to follow the reference signal, mitigating the distortion or ripple in the current signal, and maintaining the fault-ride through capability of the wind farm. The AC output signal from the wind-turbine-generators is converted to DC signal through a non-controlled full-bridge diode rectifier and a controlled DC–DC converter. The dynamic model of the DC–DC boost converter cascaded with the diode-rectifier is derived; and the contribution of pulse width modulation (PWM)-controller to the mitigation of signal variations is evaluated. The results prove consistency of the closed loop PWM-controller, which reduces significantly the gain of disturbances. The results are presented in the form of small-signal transfer functions and simulated using MATLAB software.

Description: An improved control strategy for the three-phase grid-connected inverter with space vector pulse-width modulation (SVPWM) is proposed. When the grid current contains harmonics, the d - and q -axis grid currents will be interacted, and then the waveform quality of the grid current will be poorer. As the reference output voltage cannot directly reflect the change of the reference grid current, the dynamic response of the grid-connected inverter is slow. In order to solve the aforementioned problems, the d - and q -axis grid currents in the decoupled components of the grid current controller can be substituted by the d - and q -axis reference grid currents, respectively. The operating principles of the traditional and proposed control methods are illustrated. Experimental results for a 15-kVA three-phase grid-connected inverter with SVPWM verify the theoretical analysis. Compared with the traditional control strategy, the grid-connected inverter with the improved control strategy has a high waveform quality of the grid current, small ripple power and fast dynamic response.

Description: This study presents a direct power control strategy for a grid-connected doubly fed induction generator (DFIG) without phase-locked loop (PLL) in a synchronous reference frame. In the proposed control system, a virtual phase angle, produced by a fixed angular speed, is used to replace the actual one of the network voltage for synchronous coordinate transformations, thus, PLL can be eliminated. The steady-state and dynamic behaviours of DFIG active and reactive powers are analysed with the potential grid frequency variations. It is found that the natural components of the flux linkages are the root cause of power and torque pulsations during the transient frequency variations. Thus, an auxiliary demagnetising current control scheme is proposed and designed to damp the natural flux linkages caused by the transient frequency variations. Then, based on the mathematic model, the control system performance is theoretically analysed. Finally, the simulation and experimental results are provided to demonstrate that the developed control system is adaptive to frequency deviations and achieve a decoupling control of both active and reactive powers.

Description: This study investigates the probabilistic risk of gear tooth flank micro-pitting in wind turbine (WT) gearboxes and shows how relatively slow rate of supervisory control and data acquisition (SCADA) data, recorded during operation, can be used to analyse the onset of gear surface damage. Field measured time series of SCADA signals, including wind speed, generator power and rotational speed, were used to obtain the statistical variation of gear shaft torque and rotational speed. From the SCADA data obtained over a 2.2 year period random number datasets of smaller sizes were selected. Based on these random number datasets the effect of gear shaft torque and rotational speed variations on the probabilistic risk of gear micro-pitting was investigated. Determinations of the gear tooth flank contact stress and lubricant film thickness were based on the technical report of gear micro-pitting, ISO/TR 15144-1 (2010). The study has shown that the considered pinion gear is subjected to high load conditions resulting in high contact stresses. The variation of rotational speed causes greater sliding between the gear teeth. The results of specific lubricant film thicknesses have shown that there is considerable risk of gear micro-pitting under the operational conditions recorded from the SCADA field data.

Description: This study proposes a method for detecting possible faults in wind turbine systems in advance such that the operating state of the fan can be changed or appropriate maintenance steps taken. In the proposed method, a chaotic synchronisation detection method is used to transform the vibration signal into a chaos error distribution diagram. The centroid (chaotic eye) of this diagram is then taken as the characteristic for fault diagnosis purposes. Finally, a grey prediction model is used to predict the trajectory of the feature changes, and an extension theory pattern recognition technique is applied to diagnose the fault. Notably, the use of the chaotic eye as the fault diagnosis characteristic reduces the number of extracted features required, and therefore greatly reduces both the computation time and the hardware implementation cost. From the experimental results, it is shown that the fault diagnosis rate of the proposed method exceeds 98%. Moreover, it is shown that for oil leaks in the gear accelerator system, the proposed method achieves a detection accuracy of 90%, whereas the multilayer neural network method achieves a maximum accuracy of just 80%.

Description: Renewable energy sources are expected to have a considerable share in the global energy mix over the next years and consequently further research is required to enhance the planning strategies, modify prudently the existing standards and set quality levels on delivered power in order to accommodate the penetration of these technologies. Depending on the distribution grid dynamics and the load demand, the amplitude of voltage may vary significantly, and therefore proper simulation models are required for performing dedicated studies. In this study, a simplified and accurate model representing the distribution grid dynamics is introduced for power quality studies in the presence of distributed photovoltaic (PV) generators. The proposed simplified distribution grid model (SDGM) is validated using measurements and it is then used along with a verified PV system model to estimate the voltage variation and PV capacity of the distribution grid under investigation. Moreover, a sensitivity analysis is performed by varying the parameters of the SDGM to assess how the PV capacity is altered. As a last step, common voltage regulation schemes are analysed through the use of the proposed model, validating operational modes that can accommodate high distributed PV capacities.

Description: The operational characteristics of a stand-alone hybrid-powered system are investigated. The sources are conventionally powered synchronous generator and photovoltaic (PV) array. The voltage of the common coupling point is controlled by the automatic voltage regulator of the synchronous generator via which the maximum power point (MPP) of the PV generator is tracked using the open-circuit voltage method. Numerical simulations based on the non-linear dynamical model in dq stationary reference frame are conducted. Power control unit is incorporated to control the driving torque of the synchronous generator once the PV generator becomes unable to meet the requirements of the motor because of reduction of the solar irradiance and/or increase of the load. The analysis includes the response of the system after step changes in the load coupled to the motor and after step changes in the levels of the solar irradiance. The steady-state characteristics of the induction motor in case of hybridisation at various solar intensities are presented and compared with the case when the motor is powered by fixed terminal voltage. It is concluded that the proposed system can run at wide range of operating conditions and robust enough to withstand system parameters step changes.

Description: Owing to increasing the penetration level of the inverter-based distributed generations (IBDGs) in power systems, their fault ride-through (FRT) capability has become one of the essential issues of new grid codes. This study proposes a novel DC-link fault current limiter (DLFCL)-based FRT scheme to improve the FRT capability in IBDG units. The DLFCL module has almost no effect on normal operation of IBDG. When a short-circuit fault occurs in power system, the DLFCL module effectively limits the inverter output current and protects its switching devices. The employed DLFCL does not need any control, measurement and gate driving system. Also, it has simple configuration and it is not mandatory to use a superconductor inductor in its power circuit so, it has low initial cost. By using the proposed scheme, it is possible to provide continues operation of IBDG even at zero grid voltage. Analytical analysis is presented in details. The effectiveness of the proposed approach is approved through extensive simulation studies in PSCAD/EMTDC environment. An experimental setup is used to verify the main concept of the proposed approach.

Description: This paper analyses the performance of maximum power point tracking in a grid connected permanent magnet synchronous generator-based wind energy conversion system using a linear relationship between optimum speed and wind velocity to increase the speed of the controller response. Furthermore, the proposed large step forward and the small step reverse initial tracking improves the accuracy of the existing large step perturb and observe algorithm and is also capable of tracking maximum power for rapidly varying wind conditions. The system consists of back-to-back sinusoidal pulse-width modulation converters. The generator side converter is used to track the maximum wind power using the proposed method through field oriented control. The role of the grid side inverter is to transfer the generated wind power from DC-link to the grid and regulate the DC-link voltage. Moreover, this paper examines the performance of the grid side inverter in grid synchronisation with a non-linear inductive load. Moreover, it analyses the concept of the proposed method using the theoretical aspects. A model for the complete system is developed in MATLAB/SIMULINK and the performance of the proposed control scheme is validated through simulations.

Description: Using both modal analysis and time domain simulations, it is shown that sub-synchronous resonance (SSR) phenomena in a series-compensated DFIG wind farms may cause system instability. In order to damp the SSR, both the rotor-side converter (RSC) and grid-side converter (GSC) controllers of the doubly-fed induction generator (DFIG) are utilised. The objectives are to design an SSR damping controller (SSRDC) with an optimum input control signal (ICS), and to identify the optimum point within the RSC and GSC controllers where to introduce the SSRDC. The optimum ICS is identified using residue-based analysis and root-locus method. The criterion for choice of optimal ICS is that it should make the SSR mode stable without decreasing or destabilising the other system modes. Moreover, since the dynamics of the DFIG-based series compensated wind farm change with varying operating point conditions, the operation of the SSRDC is optimised using the adaptive gain-scheduling method.

Description: Power generation from renewable energy resources is on the increase in most countries, and this trend is expected to continue in the foreseeable future. In an effort to enhance the integration of renewable power generation from solar and wind into the traditional power network, there is need to address the vulnerabilities posed to the grid as a result of the intermittent nature of these resources. Variability and ramp events in power output are the key challenges to the system operators due to their impact on system balancing, reserves management, scheduling and commitment of generating units. This has drawn the interest of utilities and researchers towards developing state of the art forecasting techniques for forecasting wind speeds and solar irradiance over a wide range of temporal and spatial horizons. The main forecasting approaches employ physical, statistical, artificial intelligence and hybrid methodologies. This study provides the rationale for forecasting in power systems, a succinct review of forecasting techniques as well as an assessment of their performance as applied in the literature. Also, techniques for improving the accuracy of forecasts have been presented together with key forecasting issues and developing trends.

Description: This study proposes a probabilistic multi-objective optimisation method for the operation of three-phase distribution networks incorporating active network management (ANM) schemes including coordinated voltage control and adaptive power factor control. The proposed probabilistic method incorporates detailed modelling of three-phase distribution network components and considers different operational objectives. The method simultaneously minimises the total energy losses of the lines from the point of view of distribution network operators and maximises the energy generated by photovoltaic (PV) cells considering ANM schemes and network constraints. Uncertainties related to intermittent generation of PVs and load demands are modelled by probability density functions (PDFs). Monte Carlo simulation method is employed to use the generated PDFs. The problem is solved using ɛ -constraint approach and fuzzy satisfying method is used to select the best solution from the Pareto optimal set. The effectiveness of the proposed probabilistic method is demonstrated with IEEE 13- and 34-bus test feeders.

Description: Differential-mode inverter topologies are promising for renewable energy generation since they offer advantages such as passive elements sizes reduction, having decreased power losses and reduced total cost. Single phase buck–boost differential-mode current-source inverters (DMCSIs) can deliver wider range of output voltage above or below the input dc voltage, which is necessary for high-efficiency modern renewable energy applications. Since these inverters have continuous input currents, they are suitable for maximum power point tracking (MPPT) operation of photovoltaic systems. Yet, the investigation, operation and implementation of these types of inverters have not been extensively discussed. As a drawback, the total dc side input current of a single-phase inverter consists of a dc component and an undesirable ac component. This ac component frequency is double the output voltage frequency and thus it affects MPPT resulting in reduced total efficiency. In this study, five promising DMCSIs are proposed and compared in terms of their total efficiencies, high-frequency ripple current in the dc side, total harmonic distortion, devices sizes and ratings. Moreover, the sliding mode controller's design and possible methods of eliminating the input second harmonic current are discussed. 2.5 kW bidirectional inverters are implemented to confirm the design flexibility of the proposed DMCSIs.

Description: This study presents a wind–solar photovoltaic based standalone hybrid energy system (HES) for an un-electrified village for central region of India – Madhya Pradesh. The inputs for the designing of HES are wind speed, solar radiation, temperature and the load demand which are variable with respect to time. In this study, hourly values of meteorological data and hourly load demand are considered over a year. For sizing and performance analysis of this standalone HES, ant colony optimisation technique has been used. The performance analysis of the system is done for the various parameters such as total cost of the system, power generated by various sources, state of charge of battery, contribution of various sources, continuity of supply to the load demand and unmet load. The obtained optimal configuration of the proposed HES is found to provide minimal energy cost with excellent performance and reduced unmet load.

Description: The load and hydrogen pressure changes in a proton exchange membrane fuel cell (PEMFC) result in instability and reduce the efficiency of the output power. This study proposes an extension sliding mode controller (ESMC) for maximum power point tracking (MPPT) of the fuel cells to enhance system efficiency and to reach the steady state of the output power. The experimental platform is set at a 200-watt PEMFC in this study. Results show that the tracking speed of the ESMC is 0.95 s, and the booster conversion efficiency is 94.5%; both are superior to other conventional MPPT methods. This study demonstrates a number of key technologies for renewable energy systems.

Description: This study analyses the voltage dip behaviour of the doubly-fed induction generator (DFIG) connected to a common dc link on both the stator and rotor sides, via a diode bridge and a voltage source inverter (VSI), respectively. After a voltage dip in the dc grid, the rotor VSI can transiently lose control so that rotor currents circulate through the VSI free-wheeling diodes. However, during the uncontrolled period, both stator and rotor voltages are clamped to the same dc-link voltage and no rotor overvoltage occurs. In order to analyse the behaviour of the system during the first periods of the uncontrolled current transients, this study presents a simplified average model which is validated through simulations. Voltage dips are classified into three types depending on their severity. A simple analytical expression is derived for the maximum amplitude of the current during the voltage dip, and an overcurrent chart in the plane ‘rotor speed – dip amplitude’ is deduced. These results provide deep insight into the DFIG-DC system behaviour and are useful for design purpose.

Description: A control strategy is formulated for photovoltaic generators (PVG) operating in parallel in a Microgrid. A radial microgrid is considered and when it is isolated from the utility grid, the PVG's must operate in the voltage-frequency control mode in the absence of a storage or a diesel generator unit. It is observed that the conventional P-f and Q-v droop control method fails to deliver correct power references for inverters operating in parallel when network impedance is unequal, and therefore an active power-reactive power-frequency-derivative (PQFD) droop control is proposed to achieve an error free power sharing. Seamless operation from the grid connected mode to isolated mode and vice-versa is achieved with the help of the aforementioned control, which in addition assists in the alleviation of sluggish response in the presence of Induction motor (IM) loads by providing appropriate damping. For a transient free transfer from isolated mode to grid connected mode, a synchronisation control is formulated which helps in the proper reconnection. The controller designed is tested in both Microgrid and the grid connected mode and during transition. The controller gains used in the paper have been obtained with the help of a small signal analysis of the system considered.

Description: All the sources are required to supply the scheduled power to the system as directed by the system operator, within a time frame of operation. The fluctuating nature of wind makes supplying the scheduled power a challenging task. To address this, gas turbines are proposed to augment the wind generator to work as buffer owing to their high ramping capability. However, the cost of gas turbine increases with increased ramp rates, whereas, operating efficiency and the life time decreases. Hence, to relieve the buffering gas turbines from higher ramp rate requirements, emulation of inertia of the wind generator is proposed in this paper. The controller is designed to operate in inertia emulation mode whenever there is a retardation of rotor of the wind turbine and the proposed ramp controller will be in action only for first few seconds after the fall in wind speed. While emulating the inertia, the performance of the proposed control philosophy with different ramp rates is investigated in this paper. Guide lines for the selection of the ramp duration and ramp rate for seamless transition from inertia emulation mode to maximum power extraction mode are also suggested in this paper.

Description: Price-elastic load (PEL) is a typical demand response resource. PEL provides an effective option to address the challenge of wind power output fluctuation. This study proposes a stochastic scheduling model, considering uncertainties of variable wind power and PEL response. A PEL stochastic security-constrained economic dispatch model using chance-constrained programming is presented for scheduling. The authors propose that the chance-constrained transmission interface flow constraints should be considered to control the probability of transmission interface flow limit violation via the parameter of security probability. An approach is suggested to transform the stochastic model into a deterministic quadratic programming problem via linearising the branch flow constraints and transmission interface flow constraints with sensitivity analysis method. Then, the probabilistic transmission interface flow constraints are equivalently transformed into conventional constraints. Case simulations on a provincial power system with 151 buses demonstrate the effectiveness and feasibility of the proposed mechanism, models, and methods.

Description: Integrating cooling systems with photovoltaic (PV) modules represents a very important aspect of keeping modules within acceptable operating temperatures. The objective of this study is to analyse the performance of each series-connected PV cell in photovoltaic thermal (PVT) modules. The importance of this analysis is due to the consideration of the variable operating temperature values of the PV cells, resulting in a different performance of each PV cell. The PVT modules are cooled with water serving as both a heat sink and a solar heat collector. This enhances the electrical efficiency value of the PVT modules by preventing them from elevated temperature values, while the recovered thermal energy can be utilised to increase the overall PV effectiveness. The new consideration in this study over the existing ones is the temperature distribution of each PV cell is applied for the calculation of the current voltage characteristics of the PV modules, which makes the analysis more accurate. The variance in the temperature values is a result of the temperature gradient of the fluid flowing through the pipes where the heat exchange differs in the flow direction. The potential of this study becomes more important for countries of high ambient temperatures and high solar intensity.

Description: Doubly fed induction generators (DFIGs) are widely used in wind power systems; hence their reliability model is an important consideration for production assessment and economic analysis of wind energy conversion systems. However, to date mutual influences of reliability analysis, production estimations and economic assessments of wind farms have not been fully investigated. This study proposes a reliability model for DFIG wind turbines considering their subcomponent failure rates and downtimes. The proposed production estimation algorithm leads to an economic assessment for wind farms. A comprehensive spare parts management procedure is then presented in the study. As a case study, practical data from a prominent wind farm is used and the procedure of the study is verified.

Description: The three-phase quasi-Z-source AC–DC converter possesses many attractive attributes for applications with wide AC input variations, such as variable speed wind generator applications. This is due to its buck–boost capability, high input power factor, low output ripple and low rating of components. Detailed steady-state and small-signal models of the converter are derived using circuit averaging and the synchronous reference frame. From the steady-state model, it is shown that the three-phase quasi-Z-source rectifier (QZSR) has all the advantages of the three-phase Z-source rectifier (ZSR) as well as further advantages such as lower rated capacitors. The small-signal model reveals that the QZSR has non-minimum-phase zeroes in its control-to-output voltage and control-to-inductor L 1 current transfer-function similar to the ZSR. Right-half-plane zeroes tend to limit the maximum control bandwidth and destabilise the wide-bandwidth feedback loop. The key elements in the design of a closed-loop controller, namely, the transfer functions and the block diagram models are derived. Simulations and experimental results are presented to validate the models developed.

Description: This study analyses a surge capacitor failure on the neutral bus in a ±500 kV high voltage direct current (HVDC) converter station. Both overvoltage and insulation of the surge capacitor are investigated through simulation, practical record data analysis, and experimentation. A highly accurate HVDC model is established on the basis of the topological structure and parameters of the HVDC project. Using this model, the failure process is simulated by combining neutral bus overvoltage record analysis and voltage divider performance test. Results indicate that the 246 kV uncharacteristically high overvoltage on the neutral bus is a measurement error caused by voltage divider fault. From the field test, difference equation of the voltage divider is derived to reconstruct the actual overvoltage, the amplitude of which is 73 kV. The amplitude is significantly lower than that of the switching impulse withstand level of the surge capacitor, which should not cause capacitor failure. Therefore, the primary cause of capacitor failure is insulation damage rather than overvoltage. Finally, maintenance measures are proposed and applied in the station.

Description: The modular multilevel converter (MMC) will be used in power systems more and more widely. The main circuit parameter design of the MMC impacts the initial investment and the operating performance of the system. In this study the parameter design problem of several important elements in the main circuit of the MMC, such as the link transformer, the arm reactor, the sub-module (SM) capacitor and the SM power electronic devices are analysed. Based on the fundamental equivalent circuit of the MMC, the design method for the rated valve side no-load voltage of the link transformer is proposed; and the variation range of the rated valve side no-load voltage is determined as (1.00–1.05)/2 times of the DC side voltage. By introducing the concept of the equivalent capacity discharging time constant H , the nearly constant relationship between the fluctuation ratio of the SM capacitor voltage and the H -value for different projects is discovered, which means the H -value is independent of particular projects. In this study, the corresponding parameters of several practical projects are analysed and the recommended H -value is given as 40 ms. The voltage and current stress of the SM power electronic devices is investigated for the four extreme operating conditions by the analytical formulae of the MMC; and it is discovered that the current stress of the SM power electronic devices changes greatly with the operating condition. By introducing the concepts of the phase unit series resonance frequency and the circulating current resonance frequency, the principle for designing the parameter of the arm inductor is established. Through analysing the corresponding parameters of several real projects, the recommended value of the phase unit series resonance frequency is given as the rated frequency of the connected AC system.

Description: This study focuses on the possibility of modular multilevel converter (MMC) loss optimisation by modulation without any additional circuit, where a common-mode voltage is injected into a nearest level modulator. Owing to the common-mode voltage injection, the proposed modulation strategy increases the fundamental voltage. Therefore, for a given voltage, operation is possible with the submodule (SM) voltage of at least one SM inserted each arm. This reduces the number of SMs required, resulting in reduction of conduction losses and leading to significant cost savings and/or returns on investment for facilities. Furthermore, the injection of common-mode voltage also decreases the total SM transitions, resulting in reduction of switching losses. The proposed modulation strategy is valid under both normal and unbalanced ac fault conditions for MMCs. The common-mode voltage extraction method is provided and its harmonic components are extracted. Then, the SM saving capability of the injection-based nearest level modulation is discussed and the SM saving limit is presented in relation with modulation indexes. The optimisation of both conduction losses and switching losses is analysed. The effectiveness of the proposed modulation strategy under both normal and unbalanced ac fault condition is verified by simulations of a 101-level three-phase MMC in MATLAB/Simulink.

Description: In a multi-terminal high-voltage direct current (HVDC) system, DC circuit breakers (DCCBs) are conventionally connected in a star configuration to enable isolation of a DC fault from the healthy system parts. However, a star connection of DCCBs has disadvantages in terms of loss, capacity, reliability and so on. By rearranging the star connection, a novel delta configuration of DCCBs is proposed. The total loss of the proposed delta configuration is only 33.3% of that of star configuration, yielding a high efficiency. Moreover, any DC fault current is shared between two DCCBs instead of one DCCB in the faulty branch suffering the fault current. Thus, DCCB capacities in the proposed delta configuration are only half of those in a star arrangement. Additionally, in the case of one or two DCCBs out of order, the power can still be transferred among three or two terminals, thereby affording high supply security of all HVDC links. On the basis of the DCCB delta configuration, two novel DC fault protection structures with external and internal DC inductances are proposed. Their characteristics are discussed and it is shown a DC fault can be isolated using slow DCCBs. The results demonstrate DC fault tolerant operation is achieved by using the proposed DC fault protection structures with delta configuration.

Description: A novel analytical approach to reliability evaluation of voltage-source converter-based multi-terminal direct current (VSC-MTDC) integrated offshore wind plants is presented in this study. To treat the multi-terminal grid precisely and efficiently, the approach combines the state evaluation method and network flow method. A DC line equivalent component is also proposed to consider the operation modes, failure clean measures as well as interactions between different DC lines of VSC-MTDC. Employing this approach, the authors can obtain both frequency and probability indexes of reliability. The usefulness and effectiveness of the proposed method are verified by case studies. The comparison between three popular schemes is also carried out.

Description: Interconnected voltage-source converter based multi-terminal high-voltage DC (HVDC) systems provide better system redundancy, higher flexibility and capability of exchanging power between multiple areas. Recent developments in modular multi-level converter technology makes multi-terminal HVDC (MTDC) transmission more promising than before. This study provides an in-depth analysis of the power flow and transmission loss for MTDC systems with different grid topologies. A voltage–power droop control based algorithm is applied to solve the power flow problems. The main contribution of this study is the novel way of determining and calculating the transmission line losses for different MTDC network topology configurations. Three MTDC system topologies are investigated. Simulated case studies are used to observe the system power flow and transmission losses for all three topologies.

Description: The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.

Description: A high-power double frequency bidirectional symmetrical DC–DC resonant converter is proposed to reduce the sizes of the passive components and increase the cost-effectiveness. Inspired by the bipolar and unipolar modulations for the single-phase inverter, an asymmetrical duty cycle double frequency modulation is originated for high-power bidirectional resonant converters whose switching frequency and power density are significantly limited by the high-power devices. The equivalent operating frequency of the resonant tank becomes twice of the switching frequency, which effectively reduces the volumes and costs of the passive components, especially for high-power high-voltage applications. Moreover, the proposed converter has the same output characteristics for the two power flow directions, which not only symmetrises the bidirectional frequency control, but also simplifies the design of the resonant components. The converter operation principle is illustrated and the converter performances are analysed including the principle of the double frequency modulation, output characteristics and soft switching condition. Finally, a 5 kW prototype is built to verify the analysis, and the resonant components for the conventional bidirectional resonant converter are also designed and compared to illustrate the advantages of the proposed converter.

Description: Traditional low voltage high power wind energy conversion system (WECS) requires heavy cables and large step-up grid interfacing transformers. In recent years, power rating of offshore wind turbines already exceeds 10 MW, and medium voltage (MV) converters have become strongly demanded. Modular multilevel converter (MMC) is suitable for MV high power applications. However, there are two technical difficulties for MMC when applied in MV permanent magnet synchronous generator (PMSG) WECS. The first one is the large sub module (SM) voltage fluctuation caused by low frequency and high amplitude PMSG phase current. The other one is the low voltage ride through (LVRT) problem. Overall control strategy based on carrier-phase-shifted -sinusoidal pulse width modulation technique of the MMC WECS is proposed. For the first difficulty mentioned, a second-order circulating current injection method based on closed-loop control is proposed. For the LVRT problem, a distributed braking chopper solution is provided. The effectiveness of the injection method and the distributed braking chopper solution are validated by simulation of a 5 MW/3.3 kV MMC WECS. Influences of the current injection amplitude on SM voltages and arm currents are discussed and a trade-off method is given to optimise system design, which is also verified by simulation.

Description: The generation of offshore wind power is less predictable. This can cause the overload of offshore DC transmission system and thus requires the curtailment of wind power. To reduce the amount of wind power curtailment, a method of optimising DC power flow using DC power flow controller (DC-PFC) is proposed. The analytical expression of coordinating DC-PFCs and converters in controlling the power flow of the DC system has been created. Method has been developed to optimise the power flow of DC grids within which control setting changes automatically in different wind conditions to reduce both the power curtailment and power losses. The proposed method has been demonstrated and validated on a 9-port DC system. It is concluded that both the curtailment of wind power and power losses are effectively reduced by inserting DC-PFCs into DC grids.

Description: DC wind farm (DCWF) with series-connected DC wind turbines (DCWTs) is proved to be a potential solution of offshore wind power collection. The coupling behaviour of series-connected DCWTs is described in detail. Possible wind energy curtailment during the period of wind turbine voltage limitation and its key impact factors are firstly quantitatively derived. A novel variable speed control strategy is proposed under voltage limiting condition of the DCWT to improve its wind energy capture. This control algorithm can be implemented in the local DCWT controller without the communication need. The specific variable speed control strategy of the DCWT is realised in a generalised averaged model. Dynamic simulation cases under different operation conditions have been conducted to evaluate the effectiveness of the proposed control strategy. It is found that the proposed control strategy might be a solution for wind energy curtailment, which will significantly improve the performance of the series-connected DCWTs between the time scale above seconds and below minutes.

Description: This study presents a ‘seek’ and ‘check’ approach to determine the operating area of the modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. First, the equivalent circuit for analysing the operating area of MMC-HVDC systems is derived and the operating area is represented by discrete operating points. Then, three operating constraints are proposed according to the MMC operation principles, i.e. the voltage, current and voltage stability constraints. Therefore, the determination of the operating area is transformed into checking whether the operating point meets the proposed constraints. The case studies are performed on a one-terminal MMC-HVDC system under six typical short circuit ratios. The results show that the top boundary of the operating area is limited by the voltage constraint, and the bottom boundary of the operating area is limited by the current constraint. The influence of the third harmonic injection and the ac impedance characteristic on the operating area is also studied. Lastly, the validity of the ‘seek’ and ‘check’ approach is proved by time-domain simulation results.

Description: DC droop control strategy is usually used to improve load sharing in increasingly popular DC distribution networks. However, the conventional droop control strategy suffers from considerable terminal voltage drop and influences by line impedance. In this study, the authors propose a new robust droop control strategy to overcome these drawbacks. They first develop a mathematical model of the proposed robust controller and use the load terminal voltage as a feedback signal. They further treat the line impedance as part of the equivalent output impedance of individual power converters and minimise the inaccuracy of load sharing by regulating robust coefficients. They analyse the influence of the robust coefficients on the system stability. They have verified the robust control strategy with both power system computer-aided design/electromagnetic transients including DC simulation and control hardware in the loop semi-experimental method. It is shown that the robust concept improves the load sharing accuracy and suppressing the load voltage fluctuation.

Description: This study introduces an advanced decentralised control method for DC microgrids which can be adopted both in the grid-connected mode and the islanded mode with a small change. Compared with the conventional droop control methods, there are three improvements in this proposed control method. First, in the grid-connected mode, the proposed control method can output the specified current, which is useful for the economic dispatch of the microgrid. Second, in the islanded mode, the proposed control method can realise accurate current sharing among distributed generations (DGs). Third, in the islanded mode, the proposed control method can directly restore the average voltage of the bus without the extra secondary control, which can ensure the power quality and reduce the complexity of the control system. The high reliability of the proposed control method is illustrated by the analysis of the stability. Also, the time-delay effect caused by the finite bandwidth of the communication network is analysed and the limited bandwidth of the communication network can be calculated out. All the conclusions are verified by the corresponding simulation tests based on MATLAB/Simulink, also the real-time hardware-in-loop tests are conducted to evaluate the performance of the control method when applied practically.

Description: In this study, the control objectives such as maximum power point tracking (MPPT), synchronisation with grid and current control are realised for single-stage utility-scale photovoltaic (PV) system which is also capable of injecting the reactive power into the grid. For utility-scale PV system, the single-stage scheme is used for high efficiency and simple power converter topology. The proposed MPPT approach utilising a modified particle swarm optimisation method is interrelated with other control schemes. The MPP changes with variation in solar irradiation and temperature. Also, the PV power characteristic of large system is characterised by multiple peaks under partial shaded condition. The conventional methods such as hill climbing and incremental conductance methods do not work properly in these conditions. The proposed method is suitable to all conditions with reduced steady-state fluctuations. The overall system is simulated in PSCAD and the effectiveness of the proposed method is tested on various conditions.

Description: This study presents an inverse hyperbolic secant function based islanding detection technique for distribution network containing various types of distributed generations. The proposed scheme is based on rate of change of inverse hyperbolic secant function of negative sequence voltage signal. Different non-islanding events along with various islanding conditions have been simulated by modelling IEEE 34-bus system using PSCAD/EMTDC software package. The proposed scheme is able to detect islanding condition even with zero active and reactive power mismatches. Moreover, it eliminates non-detection zone completely and at the same time also remains stable during diversified non-islanding events including change in network topology. The simulation results indicate that the proposed scheme not only detects islanding condition rapidly, but also provides better stability in case of critical non-islanding events during which most of the existing schemes issue nuisance tripping. Furthermore, comparative evaluation of the proposed scheme with the recently proposed techniques in the literature clearly indicates superiority of the proposed scheme. At the end, issues related to practical implementation of the proposed scheme have also been discussed.

Description: Recent advances in renewable energy technologies and changes in the electric utility infrastructures have increased the interest of the power utilities in utilisation of distributed generation (DG) resources to generate electricity. The recent trends in the development and utilisation of DG resources for power generation application are subject to the deregulation of the electric power sector and technical constraints to extend distribution and transmission networks to some areas. The electric power system planners, regulators and the policy makers have derived many benefits from integration of DG units into the distribution networks. These benefits depend on the characteristics of DG units such as photovoltaic (PV), wind system and reciprocating engines, characteristics of the loads, local renewable resources and network configuration. This study comprehensively reviews various research works on the technical, environmental and economic benefits of renewable DG integration such as line-loss reduction, reliability improvement, economic benefits and environmental pollution optimisation. These benefits can be optimised if all the renewable DG units are optimally sized, located and configured. This study also reviews the current status of renewable DG technologies based on different characteristics and the operational issues of integration of renewable DG into the electric power systems.

Description: This study presents a control algorithm of a grid tied solar photovoltaic (PV) system using a dual reference phase shifted pulse width modulation technique for a single-phase cascaded N -level inverter. The grid tied inverter consists of several five-level hybrid inverter modules which are connected to the solar PV panels having equal DC voltage magnitudes. The proposed control technique for the multilevel inverter generate the near-sinusoidal current with near-unity power factor to the grid and balance the capacitor voltages across the DC links. The voltage and frequency of the multilevel inverter can be adjusted using closed loop control, such that, the maximum power flow to the grid can be achieved even under varying grid voltage. The performance of the complete system has been verified under different transient states. Experimental results verify the effectiveness of the proposed technique.

Description: An improved differential evolution (IDE) algorithm-based Elman neural network (ENN) controller is proposed to control a squirrel-cage induction generator (SCIG) system for grid-connected wind power applications. First, the control characteristics of a wind turbine emulator are introduced. Then, an AC/DC converter and a DC/AC inverter are developed to convert the electric power generated by a three-phase SCIG to the grid. Moreover, the dynamic model of the SCIG system is derived for the control of the square of DC-link voltage according to the principle of power balance. Furthermore, in order to improve the transient and steady-state responses of the square of DC-link voltage of the SCIG system, an IDE-based ENN controller is proposed for the control of the SCIG system. In addition, the network structure and the online learning algorithm of the ENN are described in detail. Additionally, according to the different wind speed variations, a lookup table built offline by the dynamic model of the SCIG system using the IDE is provided for the optimisation of the learning rates of ENN. Finally, to verify the control performance, some experimental results are provided to verify the feasibility and the effectiveness of the proposed SCIG system for grid-connected wind power applications.

Description: In recent years, multi-crystalline solar grade silicon (mc-SoG-Si), instead of expensive electronic-grade Si, is being considered in photovoltaic industry for production of solar modules. These materials usually contain a comparable amount of acceptors (e.g. boron) and donors (e.g. phosphorus) and are therefore called compensated mc-SoG-Si. The electrical parameters, e.g. majority carrier mobility ( μ ), majority carrier density ( p ) and resistivity ( ρ ), of compensated mc-SoG-Si which affect performance of the solar cells vary non-linearly with temperature due to several complex mechanisms. In this study, the authors propose artificial neural network (ANN)-based models to predict the three electrical parameters of mc-SoG-Si material. Using a limited amount of measurement data, the authors have shown that the ANN-based models can predict the three electrical parameters of a given sample over a wide temperature range of 70–400 K and a specific range of compensation ratio. The authors have shown with extensive simulated results that these models can predict the three parameters with a maximum error of ±10%.

Description: In this study the potential for further reduction in greenhouse gas emissions (GHG) of c -Si, multi-crystalline silicon (mc-Si) and CdTe photovoltaic (PV) systems are assessed to inform climate change mitigation in the UK. The amount of CO 2 equivalent released in grams for each kWh of electricity over the lifetime of the PV system (gCO 2 -e/kWh) has been assessed using a model to predict the change in GHG as a result of engineering improvements in the PV module and system operation. The scenarios modelled here are for two different locations in the UK (Midlands and South West England) to give a typical range. Similarly a range of carbon intensity in manufacture is considered by taking manufacture in China and the UK. This study considers the impact of; system and inverter lifetimes, continuing trends of increases in module conversion efficiencies and reduction in carbon intensity during manufacture. For most of the scenarios considered the extension of the PV system lifetime to 40 years makes the largest impact while extending inverter lifetime for deployment of thin film PV modules becomes more significant and cannot be ignored. All of the four interventions considered in this study can significantly contribute to a reduction in the carbon footprint of PV by the near term target of 2020. For all three module systems considered the model shows the potential for the carbon footprint to be reduced to one third of the 2012 values giving a range of 11 – 38 gCO 2 -e/kWh, which is within the range of current wind generation in the UK.

Description: Photovoltaic (PV) systems are frequently covered by performance guarantees, which are often based on attaining a certain performance ratio (PR). Climatic and electrical data are collected on site to verify that these guarantees are met or that the systems are working well. However, in-field data acquisition commonly suffers from data loss, sometimes for prolonged periods of time, making this assessment impossible or at the very best introducing significant uncertainties. This study presents a method to mitigate this issue based on back-filling missing data. Typical cases of data loss are considered and a method to infer this is presented and validated. Synthetic performance data is generated based on interpolated environmental data and a trained empirical electrical model. A case study is subsequently used to validate the method. Accuracy of the approach is examined by creating artificial data loss in two closely monitored PV modules. A missing month of energy readings has been replenished, reproducing PR with an average daily and monthly mean bias error of about −1 and −0.02%, respectively, for a crystalline silicon module. The PR is a key property which is required for the warranty verification, and the proposed method yields reliable results in order to achieve this.

Description: In this study, we present the conjugate refractive reflective homogeniser (CRRH) to be used in a 500× Cassegrain photovoltaic concentrator. The CRRH is a dielectric crossed v-trough lined with a reflective film whilst maintaining an air gap between them. This air gap between the two surfaces helps in trapping the scattered light from the refractive geometry and ensures both total internal reflection and standard reflection of the escaped rays. A 10–42% drop in optical efficiency has been shown to occur due to varying the surface roughness of the homogeniser in these ray trace simulations for the Cassegrain setup. The CRRH increased the overall optical efficiency by a maximum of 7.75% in comparison with that of a standard refractive homogeniser simulated within the same concentrator system. The acceptance angle and flux distribution of these homogenisers was also investigated. The simple shape of the CRRH ensures easy manufacturing and produces a relatively uniform irradiance distribution on the receiver. The theoretical benefit of the CRRH is also validated via practical measurements. Further research is required but a 6.7% power increase was measured under a 1000 W/m 2 solar simulator at normal incidence for the experimental test.

Description: To assess the systemic value and impacts of multiple photovoltaic (PV) systems in urban areas, detailed analysis of on-site electricity consumption and of solar PV yield at relatively high temporal resolution is required, together with an understanding of the impacts of stochastic variations in consumption and PV generation. In this study, measured and simulated time-series data for consumption and PV generation at 5 and 1 min resolution for a large number of domestic PV systems are analysed, and a statistical evaluation of self-consumption (SC) carried out. The results show a significant variability of annual PV SC across the sample population, with typical median annual SC of 31% and inter-quartile range of 22–44%. About 10% of the dwellings exceed an SC of 60% with 10% achieving 14% or less. The results have been used to construct a Bayesian network model capable of probabilistically analysing SC given consumption and PV generation. This model provides a basis for rapid detailed analysis of the techno-economic characteristics and socio-economic impacts of PV in a range of built environment contexts, from single building to district scales.

Description: Deployment of grid-connected photovoltaics (PV) in the UK has increased rapidly. By 2014 there were over 650,000 installations (over 5 GWp), spread over different market segments (on site of existing domestic and non-domestic electricity demand customers, or connected directly to the network, e.g. solar farms). This rapid deployment and diverse market segmentation raises questions about impacts upon the electricity network. Here the authors present a novel geographical information system framework which maps current PV deployment and electricity demand to sensitive spatial resolution and by market segment. This is used to understand how current PV deployment affects power flows between the high-voltage (HV) and low-voltage (LV) network. The analysis reveals that overall, current LV PV generation is significantly below summer daytime LV demand – with over half of the areas investigated showing electricity demand five times greater than peak PV generation. Interestingly a small number of areas exhibit peak PV generation greater than demand, where reverse power flow from LV to HV may occur. The framework is hence capable of identifying the areas where network impacts are likely to occur and will also be useful to consider how integration strategies, such as energy storage and demand response could facilitate further PV deployment.

Description: Large-scale adoption of solar photovoltaics (PV) in the built environment requires automation of roof suitability surveying over large geographical areas. Furthermore, as local PV installation density increases, electricity network operators require clearer information on the overall impact the large number of different rooftop PV systems will have on the stability of the local network. Knowledge of roof features (tilt angle, azimuth angle and area) and localised in-plane irradiance data is essential to meet both of these requirements. Such information is currently not available (except by individual roof surveying by PV consultants) and has to be generated. This study demonstrates the automated extraction of building roof plane characteristics from existing wide-area, aircraft-based light detection and ranging data. These characteristics are then aggregated statistically and scaled-up to produce a UK-wide map of average roof tilt variation. Validation of roof tilt with site measurements taken by four different methods demonstrates a mean absolute error of 3°. For major roof plane azimuth angles, banded into compass octants, accurate detection was achieved in 100% of cases, validated by inspection of aerial photography. This is sufficient for calculating in-plane irradiance for a more detailed automated assessment.

Description: This study deals with the use of a Landsman converter for maximum power point tracking in solar photovoltaic (SPV) array-based water pump driven by a permanent magnet brushless DC (BLDC) motor. The primary function of a DC–DC Landsman converter is to optimise the power output of SPV array and it also provides the safe and soft starting of the BLDC motor with an appropriate control. Amongst various DC–DC converters, Landsman converter meets the desired performance of proposed water pumping system. The starting, dynamic and steady-state behaviours of the SPV array fed BLDC motor driven water pump are presented to demonstrate the novelty of the proposed system. The SPV fed water pumping system under study is simulated using MATLAB/SIMULINK environment and validated on a developed prototype of the system in order to manifest its performance under practical operating conditions.

Description: This study presents a novel sensorless model predictive control (MPC) strategy of a wind-driven doubly fed induction generator (DFIG) connected to a dc microgrid. In this configuration, the stator is directly connected to the dc microgrid through a diode rectifier, and the rotor is fed by only a voltage source converter (VSC). This connection structure brings considerable benefits such as simple control scheme and reduced power converter cost. In order to obtain sensorless operation, a simple and effective sensorless position detection technique has been proposed. It is based on the detection of the stator frequency, and it is designed to operate without any machine parameters. Thus, the proposed sensorless method offers as a model-free solution. In addition, the MPC strategy has been used as a current controller to overcome the weaknesses of the inner control loop and to consider the discrete-time operation of the VSC. The proposed control system is robust against machine parameters, speed, and load variations. To verify the dynamic and steady-state performances of the proposed sensorless MPC scheme under various speed and load conditions, experimental studies are performed with 6.4 kW DFIG. Experimental results aim to show that the proposed sensorless MPC strategy works properly at steady state and in transient.

Description: The annual yield for solar photovoltaic (PV) electricity generation in the UK is calculated for the installed capacity at the end of 2014 and found to be close to 960 kWh/kWp. This value is derived by averaging expected PV yield in different regions of the UK, weighted according to the regionally installed generating capacity from UK government statistics and has an estimated uncertainty of ±5%. The mean generating yield can vary with time as newly deployed PV may change the regional distribution of installed power and the variation over the period 2010–2014 is charted. The installed generating capacity at September 2015 was 8.19 GWp and, based on the above yield, should generate around 7860 GWh of electricity in a typical year or 2.6% of UK consumption (2014). Based on current trends, Solar PV electricity should exceed 3% of UK consumption in 2016.

Description: Variable-step incremental conductance (Inc.Cond.) technique, for photovoltaic (PV) maximum power point tracking, has merits of good tracking accuracy and fast convergence speed. Yet, it lacks simplicity in its implementation due to the mathematical division computations involved in its algorithm structure. Furthermore, the conventional variable step-size, based on the division of the PV module power change by the PV voltage change, encounters steady-state power oscillations and dynamic problems especially under sudden environmental changes. In this study, an enhancement is introduced to Inc.Cond. algorithm in order to entirely eliminate the division calculations involved in its structure. Hence, algorithm implementation complexity is minimised enabling the utilisation of low-cost microcontrollers to cut down system cost. Moreover, the required real processing time is reduced, thus sampling rate can be improved to fasten system response during sudden changes. Regarding the applied step-size, a modified variable-step size, which depends solely on PV power, is proposed. The latter achieves enhanced transient performance with minimal steady-state power oscillations around the MPP even under partial shading. For proposed technique's validation, simulation work is carried out and an experimental set up is implemented in which ARDUINO Uno board, based on low-cost Atmega328 microcontroller, is employed.

Description: This study addresses the development and operation of the European continental electricity system with a high penetration of wind and photovoltaic (PV) generation. The main focus of the work is the assessment of the impact of inertia reduction, due to wind and PV power electronics interface, on frequency stability indicators, as the rate of change of frequency and the frequency nadir following a large generation loss. The analysis is based on dynamic frequency stability studies, performed for every hour of the year and over a large number of weather scenarios. The outputs of these simulations are used to perform statistical analysis of these indicators and to estimate the critical instantaneous penetration rate of wind and PV, which the European continental synchronous area can accommodate from a system dynamics point of view. The results show that a single critical instantaneous penetration rate cannot be defined, since the frequency dynamic behaviour depends on parameters that change from one period to the following. Instead, this critical penetration rate should be calculated for every dispatch period. This study also highlights the growing importance of load self-regulating effect's contribution to frequency stability in the future system.

Description: Wind turbine generators (WTGs) equipped with the inertial response feature have been in operation on the Hydro-Québec system since 2012. As part of the validation and performance test program carried out at each wind power plant, on-line monitoring was used to evaluate the inertial response during under frequency events. This study presents the status of the ongoing assessment using field measurements and simulation results for type-III and type-IV WTGs equipped with the inertial response feature required by Hydro-Québec TransÉnergie since 2006.

Description: Solar photovoltaic (PV) system installations are rapidly increasing in distribution networks. These PV systems include power electronic devices which have an influence on the power quality of the grid in the form of harmonic distortion. The aim of this study is the harmonic impact of PV systems on distribution networks. A comprehensive harmonic behaviour analysis has been performed on the IEEE-13 bus distribution network with high PV systems penetration. A certain level of harmonics is also injected into the IEEE network through non-linear loads to resemble a realistic scenario. The investigation has been carried out through simulations of three case studies, namely PV system integrations at a single node in particular with and without the presence of background distortions in the supply and finally PV penetration at multiple nodes with supply distortions. Furthermore, an evaluation study has been conducted at the University of Queensland PV site to validate simulation results. This study has highlighted the PV systems harmonic contributions on real distribution networks and the impact of harmonics propagation on transformer K -factor. Results show that the total harmonic distortions of current and voltage are exceeding the limits when the number of PV systems increases, leading to transformer overloading and heating.

Description: This paper presents an implementation of sliding mode controller (SMC) along with a proportional and integral (PI) controller for a DSTATCOM (Distribution STATic COMpensator) for improving current induced power quality issues and voltage regulation of three-phase self-excited induction generator (SEIG). The use of SMC for regulating the DC link voltage of DSTATCOM offers various advantages such as reduction in number of sensors for estimating reference currents and the stable DC link voltage during transient conditions. The use of PI controller for terminal voltage control gives the error free voltage regulation in steady state conditions. The voltage regulation feature of DSTATCOM offers the advantages of single point voltage operation at the generator terminals with the reactive power compensation which avoids the saturation in the generator. Other offered advantages are balanced generator currents under any loading condition, harmonic currents mitigation, stable DC link voltage and the reduced number of sensors. The SMC algorithm is successfully implemented on a DSTATCOM employed with a three-phase SEIG feeding single phase or three phase loads. The performance of the proposed control algorithm is found satisfactory for voltage regulation and mitigation of power quality problems like reactive power compensation, harmonics elimination, and load balancing under nonlinear/linear loads.

Description: This study deals with photovoltaic power plant modelling and its integration within the distribution network. It presents a simulation model of a whole photovoltaic power plant including the solar cells, boost converter with maximal power point tracking, voltage-oriented control and inductor-capacitor-inductor (LCL) filter. In such a sense, the applied inverter has many advantages such as a controllable power factor and sinusoidal input current, while the switching frequency of the power switch is relatively high. Therefore, it could cause high-frequency harmonics around the switching frequency. The traditional way of solving these problems is the usage of LCL filters, where the basic requirement is to achieve sufficient filtering with inductor and capacitor values as small as possible. In addition, emphasis is given to a comparison between two optimisation methods – particle swarm optimisation and differential evolution that are used for the parameters of proportional–integral (PI) controllers determination. These PI controllers represent the main part of the voltage-oriented control.

Description: The power generation of regular water waves from a horizontal-axis single Savonius rotor is experimentally investigated in an experimental wave flume in the context of the performance assessment of the rotor in intermediate-to-shallow water depths for different parametric conditions. A number of prototypes with multiple blades of 3–5 with the same diameter and blade curvature angle are fabricated in-house and tested to measure both the long-term average torque and power generated at different submergence levels, wave heights and wave periods. The experiments are performed in the wave-current flume equipped with a piston-type wave maker with an active wave absorption capability. The energy performance of the rotor is comparatively assessed for each experimental test case based on quantitative comparison of obtained wave-to-mechanical energy conversion efficiency (ECE) to suggest a possible optimum blade number, positioning and wave physical conditions for the investigated range of wave flow characteristics. It is found that higher ECE values can be achieved at higher wave heights and blade numbers for the lower wave periods when the rotor is placed at the water surface and/or just below the surface. This study provides a proper guideline for performance analysis of such device(s) for further studies.

Description: This study explores the integration issues of next-generation high-penetration photovoltaic (PV) systems, where the grid is becoming more decentralised and vulnerable. In that case, the PV systems are expected to be more controllable with higher efficiency and reliability. Provision of ancillary and intelligent services, such as fault ride-through and reactive power compensation, is the key to attain higher utilisation of solar PV energy. Such functionalities for the future PV inverters can contribute to reduced cost of energy, and thus enable more cost-effective PV installations. To implement the advanced features, a flexible power controller is developed in this study, which can be configured in the PV inverter and flexibly change from one to another mode during operation. Based on the single-phase PQ theory, the control strategy offers the possibilities to generate appropriate references for the inner current control loop. The references depend on system conditions and also specific demands from both system operators and prosumers. Besides, this power control strategy can be implemented in commercial PV inverters as a standardised function, and also the operation modes can be achieved online in predesigned PV inverters. Case studies have verified the effectiveness and flexibilities of the proposal to realise the advanced features.

Description: The conventional algorithm of perturb and observe (P&O) is widely applied due to its simplicity, low cost and easy implementation. However, it suffers from instabilities during rapid changes of weather and/or oscillation around maximum power point (MPP) at steady state. Instabilities occur due to the incorrect decision taken by the conventional P&O algorithm at the first step change in duty cycle during the rapid change in radiation. The reason for the steady-state oscillation is the continuous perturbation and tradeoff between step sizes and the convergence time. This study presents a modified P&O algorithm to overcome such drawbacks. It uses a constant load technique to help the conventional P&O algorithm for recognising the cause of power change and to enable it in taking the right decision at first step change in duty cycle during rapid change of weather. The proposed algorithm is simulated using a single solar photovoltaic module of 80 W and a DC/DC boost converter. It is validated experimentally and implemented within an embedded microcontroller. The experimental setup presents a proposed model-based design methodology that uses measurements’ data for MPP tracking systems’ design. It combines hardware-in-the-loop simulation and prototype testing using actual weather measurements. Simulation and experiments show excellent results.

Description: This study deals with the development of a control system for voltage and frequency regulation of a micro hydro power generation system, fed by self-excited induction generator (IG). The generation system considers a four-wire three-phase grid, composed by three phases plus neutral conductor, fed by three-phase three-wires IG. The neutral conductor is created from the neutral point of star connection of excitation capacitors bank. The four-wire configuration allows the connection of three-phase loads in star or delta configuration, as well as single-phase loads. The voltage regulation is performed through a four-legs distribution static synchronous compensator (DSTATCOM) shunt connected to the AC bus. It is considered the employment of adaptive control techniques and the compensation of load current unbalances, which offers suitable voltage regulation in conditions of system's parameters variations such as load connection and disconnection, and changes in IG's unmodelled parameters, and voltage unbalance compensation, respectively. Frequency regulation is obtained by electronic load control connected to the DC bus of DSTATCOM. Experimental results were obtained to demonstrate the good performance of the voltage and frequency regulation control system during loads transient, including three-phase and single-phase loads.

Description: The time synchronization protocol is indispensable in various applications of wireless sensor networks, such as scheduling, monitoring, and tracking. Numerous protocols and algorithms have been proposed in recent decades, and many of them provide micro-scale resolutions. However, designing and implementing a time synchronization protocol in a practical wireless network is very challenging compared to implementation in a wired network; this is because its performance can be deteriorated significantly by many factors, including hardware quality, message delay jitter, ambient environment, and network topology. In this study, we measure the performance of the Flooding Time Synchronization Protocol (FTSP) and Gradient Time Synchronization Protocol (GTSP) in terms of practical network conditions, such as message delay jitter, synchronization period, network topology, and packet loss. This study provides insights into the operation and optimization of time synchronization protocols. In addition, the performance evaluation identifies that FTSP is highly affected by message delay jitter due to error accumulation over multi-hops. We demonstrate that the proposed extended version of the FTSP (E-FTSP) alleviates the effect of message delay jitter and enhances the overall performance of FTSP in terms of error, time, and other factors.

Description: The Internet of Things (IoT) will feature pervasive sensing and control capabilities via the massive deployment of machine-type communication devices in order to greatly improve daily life. However, machine-type communications can be illegally used (e.g., by criminals or terrorists) which is difficult to monitor, and thus presents new security challenges. The information exchanged in machine-type communications is usually transmitted in short packets. Thus, this paper investigates a legitimate surveillance system via proactive eavesdropping at finite blocklength regime. Under the finite blocklength regime, we analyze the channel coding rate of the eavesdropping link and the suspicious link. We find that the legitimate monitor can still eavesdrop the information sent by the suspicious transmitter as the blocklength decreases, even when the eavesdropping is failed under the Shannon capacity regime. Moreover, we define a metric called the effective eavesdropping rate and study the monotonicity. From the analysis of monotonicity, the existence of a maximum effective eavesdropping rate for a moderate or even high signal-to-noise (SNR) is verified. Finally, numerical results are provided and discussed. In the simulation, we also find that the maximum effective eavesdropping rate slowly increases with the blocklength.

Description: GNSS receiver data crowdsourcing is of interest for multiple applications, e.g., weather monitoring. The bottleneck in this technology is the quality of the GNSS receivers. Therefore, we lay out in an introductory manner the steps to estimate the performance of an arbitrary GNSS receiver via the measurement errors related to its instrumentation. Specifically, we do not need to know the position of the receiver antenna, which allows also for the assessment of smartphone GNSS receivers having integrated antennas. Moreover, the method is independent of atmospheric errors so that no ionospheric or tropospheric correction services provided by base stations are needed. Error models for performance evaluation can be calculated from receiver RINEX (receiver independent exchange format)data using only ephemeris corrections. For the results, we present the quality of different receiver grades through parametrized error models that are likely to be helpful in stochastic modeling, e.g., for Kalman filters, and in assessing GNSS receiver qualities for crowdsourcing applications. Currently, the typical positioning precision for the latest smartphone receivers is around the decimeter level, while for a professional-grade receiver, it is within a few millimeters.

Description: The power transmission lines are the link between power plants and the points of consumption, through substations. Most importantly, the assessment of damaged aerial power lines and rusted conductors is of extreme importance for public safety; hence, power lines and associated components must be periodically inspected to ensure a continuous supply and to identify any fault and defect. To achieve these objectives, recently, Unmanned Aerial Vehicles (UAVs) have been widely used; in fact, they provide a safe way to bring sensors close to the power transmission lines and their associated components without halting the equipment during the inspection, and reducing operational cost and risk. In this work, a drone, equipped with multi-modal sensors, captures images in the visible and infrared domain and transmits them to the ground station. We used state-of-the-art computer vision methods to highlight expected faults (i.e., hot spots) or damaged components of the electrical infrastructure (i.e., damaged insulators). Infrared imaging, which is invariant to large scale and illumination changes in the real operating environment, supported the identification of faults in power transmission lines; while a neural network is adapted and trained to detect and classify insulators from an optical video stream. We demonstrate our approach on data captured by a drone in Parma, Italy.

Description: The number of connected sensors and devices is expected to increase to billions in the near future. However, centralised cloud-computing data centres present various challenges to meet the requirements inherent to Internet of Things (IoT) workloads, such as low latency, high throughput and bandwidth constraints. Edge computing is becoming the standard computing paradigm for latency-sensitive real-time IoT workloads, since it addresses the aforementioned limitations related to centralised cloud-computing models. Such a paradigm relies on bringing computation close to the source of data, which presents serious operational challenges for large-scale cloud-computing providers. In this work, we present an architecture composed of low-cost Single-Board-Computer clusters near to data sources, and centralised cloud-computing data centres. The proposed cost-efficient model may be employed as an alternative to fog computing to meet real-time IoT workload requirements while keeping scalability. We include an extensive empirical analysis to assess the suitability of single-board-computer clusters as cost-effective edge-computing micro data centres. Additionally, we compare the proposed architecture with traditional cloudlet and cloud architectures, and evaluate them through extensive simulation. We finally show that acquisition costs can be drastically reduced while keeping performance levels in data-intensive IoT use cases.

Description: One of the most significant challenges in Internet of Things (IoT) environments is the protection of privacy. Failing to guarantee the privacy of sensitive data collected and shared over IoT infrastructures is a critical barrier that delays the wide penetration of IoT technologies in several user-centric application domains. Location information is the most common dynamic information monitored and lies among the most sensitive ones from a privacy perspective. This article introduces a novel mechanism that aims to protect the privacy of location information across Data Centric Sensor Networks (DCSNs) that monitor the location of mobile objects in IoT systems. The respective data dissemination protocols proposed enhance the security of DCSNs rendering them less vulnerable to intruders interested in obtaining the location information monitored. In this respect, a dynamic clustering algorithm is that clusters the DCSN nodes not only based on the network topology, but also considering the current location of the objects monitored. The proposed techniques do not focus on the prevention of attacks, but on enhancing the privacy of sensitive location information once IoT nodes have been compromised. They have been extensively assessed via series of experiments conducted over the IoT infrastructure of FIT IoT-LAB and the respective evaluation results indicate that the dynamic clustering algorithm proposed significantly outperforms existing solutions focusing on enhancing the privacy of location information in IoT.

Description: There is strong demand for real-time suspicious tracking across multiple cameras in intelligent video surveillance for public areas, such as universities, airports and factories. Most criminal events show that the nature of suspicious behavior are carried out by un-known people who try to hide themselves as much as possible. Previous learning-based studies collected a large volume data set to train a learning model to detect humans across multiple cameras but failed to recognize newcomers. There are also several feature-based studies aimed to identify humans within-camera tracking. It would be very difficult for those methods to get necessary feature information in multi-camera scenarios and scenes. It is the purpose of this study to design and implement a suspicious tracking mechanism across multiple cameras based on correlation filters, called suspicious tracking across multiple cameras based on correlation filters (STAM-CCF). By leveraging the geographical information of cameras and YOLO object detection framework, STAM-CCF adjusts human identification and prevents errors caused by information loss in case of object occlusion and overlapping for within-camera tracking cases. STAM-CCF also introduces a camera correlation model and a two-stage gait recognition strategy to deal with problems of re-identification across multiple cameras. Experimental results show that the proposed method performs well with highly acceptable accuracy. The evidences also show that the proposed STAM-CCF method can continuously recognize suspicious behavior within-camera tracking and re-identify it successfully across multiple cameras.

Description: The paper proposes a sensors platform to control a barrier that is installed for vehicles entrance. This platform is automatized by image-based license plate recognition of the vehicle. However, in situations where standardized license plates are not used, such image-based recognition becomes non-trivial and challenging due to the variations in license plate background, fonts and deformations. The proposed method first detects the approaching vehicle via ultrasonic sensors and, at the same time, captures its image via a camera installed along with the barrier. From this image, the license plate is automatically extracted and further processed to segment the license plate characters. Finally, these characters are recognized with the help of a standard optical character recognition (OCR) pipeline. The evaluation of the proposed system shows an accuracy of 98% for license plates extraction, 96% for character segmentation and 93% for character recognition.

Description: Wireless sensor networks are widely used in many fields. Nodes in the network are typically powered by batteries. Because the energy consumption of wireless communication is related to the transmission distance, the energy consumption of nodes in different locations is different, resulting in uneven energy distribution of nodes. In some special applications, all nodes are required to work at the same time, and the uneven energy distribution makes the effective working time of the system subject to the node with the largest energy consumption. The commonly used clustering protocol can play a role in balancing energy consumption, but it does not achieve optimal energy consumption. This paper proposes to use the power supply line to connect the nodes to fully balance the energy. The connection scheme with the shortest power line length is also proposed. On the basis of energy balance, the method of transmitting data with the best hop count is proposed, which fully reduces the power consumption of the data transmission. The simulation results show that the proposed method can effectively reduce the energy consumption and prolong the lifetime of wireless sensor networks.

Description: High frequency guided-waves offer a trade-off between the high sensitivity of local bulk ultrasonic thickness measurements and the large area scanning of lower frequency guided-waves, so it has been a growing interest for corrosion inspection with the dispersive SH1 mode. However, according to the dispersive curve, it is hard to generate the pure SH1 mode since the non-dispersive SH0 mode will be excited simultaneously. Thus, this paper investigates a transducer design method to generate a pure SH1 guided-wave, where the dual periodic-permanent-magnet electromagnetic acoustic transducers (PPM EMATs) are placed on exactly opposite positions either side of the plate symmetrically. The suppression effect for SH0 and the enhancement effect for SH1 of the dual PPM EMATs are mainly discussed by theoretical analysis and simulation analysis, and the influence of positioning errors of PPM EMATs placed on opposite sides of the plate on its performances are analyzed. Employing the proposed dual PPM EMATs, some experiments are performed to verify the reliability of finite element simulation. The results indicate that the dual PPM EMATs can suppress the SH0 mode and generate the pure SH1 mode effectively. Moreover, the longitudinal and lateral positioning errors can affect the dual PPM EMATs performances significantly.

Description: In this paper, we report a wireless gas sensor based on surface acoustic waves (SAW). For room temperature detection of oxygen gas, a novel nanostructured ZnxFeyO gas-sensitive film was deposited on the surface of a SAW resonator by an oblique magnetron co-sputtering method. The measurements of X-ray diffraction (XRD) and a scanning electron microscope (SEM) showed that the crystal phase composition and the microstructures of ZnxFeyO films were significantly affected by the content of Fe. The experimental results showed that the sensors had a good response to O2 at room temperature. The max frequency shift of the sensors reached 258 kHz as the O2 partial pressure was 20%. Moreover, X-ray photoelectron spectroscopy (XPS) was performed to analyze the role of Fe in the sensitization process of the ZnxFeyO film. In addition, the internal relationship between the Fe content of the film and the sensitivity of the sensor was presented and discussed. The research indicates that the nanostructured ZnxFeyO film has a good potential for room temperature O2 gas detection applications.

Description: According to the survey on various health centres, smart log-based multi access physical monitoring system determines the health conditions of humans and their associated problems present in their lifestyle. At present, deficiency in significant nutrients leads to deterioration of organs, which creates various health problems, particularly for infants, children, and adults. Due to the importance of a multi access physical monitoring system, children and adolescents’ physical activities should be continuously monitored for eliminating difficulties in their life using a smart environment system. Nowadays, in real-time necessity on multi access physical monitoring systems, information requirements and the effective diagnosis of health condition is the challenging task in practice. In this research, wearable smart-log patch with Internet of Things (IoT) sensors has been designed and developed with multimedia technology. Further, the data computation in that smart-log patch has been analysed using edge computing on Bayesian deep learning network (EC-BDLN), which helps to infer and identify various physical data collected from the humans in an accurate manner to monitor their physical activities. Then, the efficiency of this wearable IoT system with multimedia technology is evaluated using experimental results and discussed in terms of accuracy, efficiency, mean residual error, delay, and less energy consumption. This state-of-the-art smart-log patch is considered as one of evolutionary research in health checking of multi access physical monitoring systems with multimedia technology.

Description: Smart grids incorporating internet-of-things are emerging solutions to provide a reliable, sustainable and efficient electricity supply, and electric vehicle drivers can access efficient charging services in the smart grid. However, traditional electric vehicle charging systems are vulnerable to distributed denial of service and privileged insider attacks when the central charging server is attacked. The blockchain-based charging systems have been proposed to resolve these problems. In 2018, Huang et al. proposed the electric vehicle charging system using lightning network and smart contract. However, their system has an inefficient charging mechanism and does not guarantee security of key. We propose a secure charging system for electric vehicles based on blockchain to resolve these security flaws. Our charging system ensures the security of key, secure mutual authentication, anonymity, and perfect forward secrecy, and also provides efficient charging. We demonstrate that our proposed system provides secure mutual authentication using Burrows–Abadi–Needham logic and prevents replay and man-in-the-middle attacks using automated validation of internet security protocols and applications simulation tool. Furthermore, we compare computation and communication costs with previous schemes. Therefore, the proposed charging system efficiently applies to practical charging systems for electric vehicles.

Description: This paper presents a mathematical model of measuring blood flow based on electromagnetic induction for predicting the rate of arterial stenosis. Firstly, an electrode sensor was used to collect the induced potential differences from human skin surface in a uniform magnetic field. Then, the inversion matrix was constructed by the weight function theory and finite element method. Next, the blood flow volume inversion model was constructed by combining the induction potential differences and inversion matrix. Finally, the rate of arterial stenosis was predicted based on mathematical relationship between blood flow and the area of arterial stenosis. To verify the accuracy of the model, a uniform magnetic field distribution of Helmholtz coil and a 3D geometric model of the ulnar artery of the forearm with different rates of stenosis were established in COMSOL, a finite element analysis software. Simulation results showed that the inversion model had high accuracy in the measurement of blood flow and the prediction of rate of stenosis, and is of great significance for the early diagnosis of arterial stenosis and other vessel diseases.

Description: Magnetic Induction Tomography (MIT) is a non-invasive imaging technique that has been widely applied for imaging materials with high electrical conductivity contrasts. Steel production is among an increasing number of applications that require a contactless method for monitoring the casting process due to the high temperature of hot steel. In this paper, an MIT technique is proposed for detecting defects and deformations in the external surfaces of metal, which has the potential to be used to monitor the external surface of hot steel during the continuous casting process. The Total Variation (TV) reconstruction algorithm was developed to image the conductivity distributions. Nonetheless, the reconstructed image of the deformed square metal obtained using the TV algorithm directly does not yield resonable images of the surface deformation. However, differential images obtained by subtracting the image of a perfect square metal with no deformations from the image obtained for a deformed square metal does provide accurate and repeatable deformation information. It is possible to obtain a more precise image of surface deformation by thresholding the differential image. This TV-based threshold-differencing method has been analysed and verified from both simulation and experimental tests. The simulation results reported that 0.92 % of the image region can be detected, and the experimental results indicated a 0.57 % detectability. Use of the proposed method was demonstareted in a MIT device which was used in continuous casting set up. The paper shows results from computer simulation, lab based cold tests, and real life data from continoeus cating demonstating the effectiveness of the proposed method.

Description: Gas mixture quantification is essential for the recording and reproducing odors, because an odor consists of multiple chemical compounds. Gas mixture quantification using field asymmetric ion mobility spectrometry (FAIMS) was studied. Acetone, ethanol, and diethyl ether were selected as components of a ternary gas mixture sample as representatives of the ketone, alcohol, and ether chemical classes, respectively. One hundred and twenty-five points with different concentrations were measured. The results were evaluated by error hypersurface, variance, and the coefficient of variation. The error hypersurface showed that it is possible to reach the target composition by following the error-hypersurface gradient. Successful convergence was achieved with the gradient descent method in a simulation based on the measurement data. This result verified the feasibility of the quantification of a gas mixture using FAIMS.

Description: In this study, a novel strain gauge arrangement and error reduction techniques were proposed to minimize crosstalk reading and simultaneously increase sensitivity on a decoupled six-axis force–moment (F/M) sensor. The calibration process that comprises the least squares method and error reduction techniques was implemented to obtain a robust decoupling matrix. A decoupling matrix is very crucial for minimizing error and crosstalk. A novel strain gauge arrangement that comprised double parallel strain gauges in the decoupled six-axis force–moment sensor was implemented to obtain high sensitivity. The experimental results revealed that the maximum calibration error, F/M sensor measurement error, and crosstalk readings were reduced to 3.91%, 1.78%, and 4.78%, respectively.

Description: Conventional eddy current testing (ECT) using a pickup coil probe is widely employed for the detection of structural cracks. However, the inspection of conventional ECT for steel structures is difficult because of the magnetic noise caused by the nonuniform permeability of steel. To combat this challenge, we have developed a small magnetic sensor probe with a dual-channel tunneling magnetoresistance sensor that is capable of reducing magnetic noise. Applying this probe to a complicated component of steel structures—such as the welds joining a U-shaped rib and deck plate together—requires the reduction of signal fluctuation caused by the distance (liftoff) variations between the sensor probe and the subject. In this study, the fundamental crack signal and the liftoff signal were investigated with the dual-channel sensor. The results showed that the liftoff signals could be reduced and differentiated from the crack signals by the differential parameters of the dual-channel sensor. In addition, we proposed an extraction technique for the crack signal using the Lissajous curve of the differential parameters. The extraction technique could be applied to the inspections not only for flat plates but also for welded angles to detect cracks without the influence of the liftoff signal.

Description: The objective of this paper is to study the ability of polymer optical fiber (POF) to be inserted in a knitted fabric and to measure both pressure and friction when walking. Firstly, POF, marketed and in development, have been compared in terms of the required mechanical properties for the insertion of the fiber directly into a knitted fabric on an industrial scale, i.e. elongation, bending rigidity, and minimum bending radius before plastic deformation. Secondly, the chosen optical fiber was inserted inside several types of knitted fabric and was shown to be sensitive to friction and compression. The knitted structure with the highest sensitivity has been chosen for sock prototype manufacturing. Finally, a feasibility study with an instrumented sock showed that it is possible to detect the different phases of walking in terms of compression and friction.