ALBERT

Description: An electronic load interface (ELI) for improving the operational margin of a photovoltaic (PV) dual-converter system under dynamic conditions is presented. The ELI ?? based on a modified buck??boost converter ?? interfaces the output of the converters and the load system. It improves the operational margin of the PV dual-converter system by extending the conditions under which the dual-converter system operates at the maximum power point. The ELI is activated as and when needed, so as minimise system losses. By employing the ELI, utilisation and efficiency of a PV dual-converter system increases. In general, the concept of the ELI can be applied to multi-converter PV systems ?? such as multi-converter inverters, and multi-converter DC??DC converter systems ?? for performance and efficiency improvement.

Description: Grid synchronisation is crucial for integration of renewable energy sources and distributed generation systems into the grid. Accurate and fast estimation of the fundamental positive sequence component is a necessity for synchronisation of threephase distributed generation systems with the grid. In this study, a simple synchronisation technique based on the realcomplex- coefficient filters is proposed. It has a very simple structure, and can directly estimate the fundamental positive and negative sequences under the distorted, unbalanced and offset conditions with no need of a phase-locked loop. Furthermore, the dc offset can also be extracted precisely. For optimising the steady-state and dynamic performance, a parameter tuning guidance is provided from the eigenvalue analysis point of view. In addition, a simple auxiliary unit is presented to remain its frequency-adaptive feature. Finally, experimental results based on a 32-bit fixed-point TMS320F2812 DSP verify the effectiveness of the proposed synchronisation method.

Description: For the grid-connected doubly-fed induction generator (DFIG)-based wind energy conversion system (WECS), many improved control algorithms have been developed for the rotor-side converter (RSC) to suppress the overcurrents in the rotor-side under voltage dips. However, such objective can hardly be achieved under severe grid fault conditions because of the limitation of RSCs output voltage. An analysis tool is proposed to estimate the the theoretical control limit of the RSC in suppressing the shortcircuit rotor currents during grid faults in this study. The tool is based on the optimisation theory and takes the practical constraints of the RSC into account. To execute the analysis, a simplified DFIG model with decoupled stator and rotor fluxes is presented, and the low-voltage ride through (LVRT) problem can be formulated as an optimisation problem, which intends to suppress the rotor winding currents with voltage constraints. The Pontryagin??s minimum principle is employed to solve the optimisation problem and the results can identify the control limit of the RSC. A case study based on a typical 1.5 MW DFIG-based WECS under various grid voltage dips is carried out to validate the analytical method. The proposed method is also further verified by experimental tests on a scaled 3 KW DFIG system. The results are expected to help the manufacturers to assess and improve their RSC controllers or LVRT measures.

Description: The increasing share of renewable energy sources for electricity, driven by variable output technologies such as wind and solar photovoltaics, is expected to have an impact on the operational reserve requirements of power systems. This study applies a probabilistic approach to estimate reserve requirements and establishes a methodology that makes it possible to distinguish between different categories of reserves based on the imbalance drivers of wind power. The methodology is based on sizing fast-response reserves based on the distribution of output fluctuations inside the settlement period, and sizing slowresponse reserves based on the distribution of the average prediction error over the settlement period. The main advantage of this methodology is a reduction of the fast-response reserves, which are generally assessed as expensive compared to slowresponse reserves. This approach is applied in a case study and compared with alternative strategies. The results for 500 MW of wind power installed in a North Sea country confirm these reductions and show that with the suggested approach the required fast-response and slow-response reserves, respectively, amount to 7 and 23??26% of the installed wind power capacity.

Description: This study presents an improved sliding mode model reference adaptive system (SM-MRAS) speed observer for the fuzzy control of direct-drive wind power generation system with a permanent magnet synchronous generator (PMSG). The SM-MRAS speed sensorless observer is described and the corresponding algorithm is derived. The designed fuzzy controller is compared with the conventional PI controller by simulations and experiments. A dc motor is controlled to simulate the wind turbine and an active machine-side converter with space vector pulse width modulation control is adopted to realise the maximum power extraction. A 250-W PMSG experimental platform is built and the experiment results verify the validity of the proposed SM-MRAS speed observer.

Description: This study proposes a power angle control strategy of grid-connected doubly fed induction generators (DFIGs) to improve the transient performance during grid fault ride-through (FRT). The proposed power angle control can efficiently restrict the rotor-side current under a limited converter rating, support the terminal voltage of DFIGs and hence improve system stability during the fault period. In this study, the power angle of DFIGs in a multi-machine power system is defined firstly and then the relation between the defined power angle and rotor-side current surge and terminal voltage dip of DFIGs are analysed, respectively. Based on the analysis, control of DFIG power angle is implemented expediently on the existing flux magnitude and angle control (FMAC) scheme of DFIGs as an improved FMAC scheme. In this study, two test examples are presented to demonstrate and validate the effectiveness of improved FMAC with power angle control and to compare it with the conventional PQdq control and FMAC. Simulation results show that it can significantly enhance the FRT capability of grid-connected DFIGs.

Description: Increasing penetration of intermittent energy sources in power systems has been requiring higher operational flexibility and cross-border transmission capacity. Intermittent generation, especially wind and solar power, has grown significantly in the Iberian Peninsula during the last decade and it will keep growing in the next year. Owing to limited interconnection capacity with the French and the Moroccan power systems, generation intermittency has been dealt with mostly within Spain and Portugal, helped by a flexible generation mix. Despite the relatively high flexibility of the Iberian system, in some situations, renewable generation output had to be curtailed. In general, renewable energy curtailment is an emergency action taken in case of network constraints or in case of generation surplus at single node. This emergency action is expected to be more common in the near future if no additional operating measures are taken. This study focuses on the expansion of interconnection capacity that would be required in order to integrate intermittent generation surplus in the Iberian Peninsula in the period 2020??2050. For this purpose, the trade-off between transmission investment costs and the costs of renewable energy curtailment is analysed. Results for different intermittent generation levels and price scenarios are presented and discussed.

Description: High-performance solar cells and photovoltaic modules exhibit high internal capacitance, limiting the speed of their transient responses including the current??voltage characteristics scans. This study proffers a model-based method to obtain optimal scan time during the current??voltage performance characterisation of a solar cell or module while preserving a pre-set accuracy. Static model parameters are extracted from the quasi-static current??voltage characteristic, whereas the capacitive character, modelled by two bias voltage dependent capacitances, is determined from the open-circuit voltage decay measurement. The obtained model is used to calculate the optimal current??voltage curve scan time. Efficacy of the proposed method is demonstrated through test results obtained on three wafer-based solar cells. I??V curve errors determined by the proposed method at different scan times are in good agreement with the measurements. Results show that in order to achieve 〈 0.5% error in curve fitting, determined scan times of tested crystalline silicon solar cells lie within the range of 3.6??45 ms for constant angle step semiconductor curve tracer. Use of a capacitive-based curve tracer, however, requires approximately twice that time to retain a comparable error.

Description: The authors present a four-season model representing the aggregate output of a possible British wind fleet circa 2030, suitable for providing synthetic wind time series or a statistical characterisation of the transitional behaviour at timescales of 1 h and above. The model is fitted to an aggregated power output time series derived from historic onshore anemometry data and shown to provide a good fit to both long-term and transitional statistics. The authors show that the use of a constant factor to extrapolate anemometer-height wind speeds to hub height leads to an excessive diurnal variation in the implied wind power output. They adjust the model parameters to compensate for this and to account for the offshore component that is not present in the raw data. The complete parameter set is presented.

Description: Single-sensor maximum power point tracking algorithms for photovoltaic systems are presented. The algorithms have the features, characteristics and advantages of the widely used incremental conductance (INC) algorithm. However, unlike the INC algorithm which requires two sensors (the voltage sensor and the current sensor), the single-sensor algorithms are more desirable because they require only one sensor: the voltage sensor. The algorithms operate by maximising power at the DC-DC converter output, instead of the input.

Description: Wind turbine (WT) technology is currently driven by offshore development, which requires more reliable, multimegawatt turbines. Models with different levels of detail have been continuously explored but tend to focus either on the electrical system or the mechanical system. This study presents a 4.5 MW doubly-fed induction generator (DFIG) WT model with pitch control. The model is developed in a simulation package, which has two control levels, the WT control and the DFIG control. Both a detailed and a simplified converter model are presented. Mathematical system block diagrams of the closed-loop control systems are derived and verified against the simulation model. This includes a detailed model of the DClink voltage control - a component which is usually only presented in abstract form. Simulation results show that the output responses from the two models have good agreement. The grid-side converter control with several disturbance inputs has been evaluated for three cases and its dynamic stiffness affected by operating points are presented. In addition, the relation of pitch controller bandwidth and torsional oscillation mode has been investigated using a two-mass shaft model. This model can be employed to evaluate the control scheme, mechanical and electrical dynamics and the fault ride-through capability for the turbine.

Description: This study presents a new single-phase topology for the power injection system of a grid-connected photovoltaic generation system that is based on the parallel association of two voltage-source inverters: one is a three-level inverter with clamping diodes (or neutral point clamped) with a low switching frequency strategy (equal to the grid frequency) and the other is a two-level inverter which operates with a current hysteresis band strategy and an usual switching frequency (5??20 kHz). The main advantage of the proposed topology is to optimise the system design, allowing high power injection with a significant reduction of system losses (mainly switching losses) and achieving an increase of the energy injected into the grid. The performance of the proposed topology and its control algorithms are tested and validated using a laboratory prototype.

Description: As wind turbines increase in power output, their size and mass grows as well. The development of offshore wind farms demands higher reliability to minimise the maintenance cost. Direct drive electrical generators offer a reliable alternative to conventional doubly fed induction generator machines since they omit the gearbox from the drive train. A fundamental issue for these generators is their large size which makes them difficult to manufacture, transfer and assemble. This study will investigate an analytical and a finite element analysis optimisation technique developed for minimising the structural mass of a direct drive generator. Both tools seek to minimise the mass of three different permanent magnet direct drive (PMDD) generators with 5 MW nominal power output while keeping a set of deflection criteria under limitations. The results indicate that the structural mass of a 5 MW PMDD generator can be effectively reduced with the help of these design tools. The research concludes in favour of a specific transversal flux PMDD topology, of which the electromagnetic topology benefits the structural design.

Description: In this paper, a low-power, direct-drive, double-sided linear switched reluctance generator (LSRG) is designed and constructed as one of the components of generator matrix for wave power utilizations by direct energy conversion. This machine has the characteristics of simple construction, mechanical robustness, and maintenance free. Following the theoretical background of linear power generation, characteristic investigation is performed by finite element analysis, which shows that each phase can be controlled individually and normal forces can be approximately counteracted from the double-sided structure. Both preliminary power simulation and experimental results show that there is an optimized generation region for turn-on and turn-off position control strategy. The closed loop test based on Pulse Width Modulation (PWM) of current waveforms also demonstrate that for a wide range of wave speed excitations, PWM scheme should be combined with other control strategies for uniform current level regulations.

Description: This study presents a backstepping power control (BPC) design for the grid-side voltage source converter (GSVSC) in a voltage source converter-based high-voltage dc (VSC-HVDC) wind power generation system. First, a dynamic model by taking parameter variations and external disturbances into account is derived on the basis of the space-vector theory to achieve the decoupling control characteristic of the GSVSC in the VSC-HVDC. Moreover, based on the backstepping design procedure, a BPC scheme is developed in the sense of Lyapunov stability theorem for the GSVSC to satisfy multiple objectives of a stable HVDC bus and the grid connection with a unity power factor. The salient feature of the proposed BPC is the introduction of additional error terms into the control laws to reduce the chattering phenomena in traditional backstepping control. In addition, the effectiveness of the proposed BPC scheme is demonstrated by numerical simulations on a doubly-fed induction generator wind farm with VSC-HVDC grid connection, and its advantage is indicated in comparison with a traditional proportional-integral control strategy under a wide range of operating conditions and the possible occurrence of uncertainties.

Description: The present study has as main goal to make a technical and financial feasibility study of a photovoltaic powered reverse osmosis (PV-RO) and pumping system developed at the Renewable Energies Laboratory, Federal University of Ceara, in Fortaleza, Brazil, with the aim to supply drinking water for human consumption at low cost from brackish water in semi-arid areas. The research intends to determine parameters like drinking water production, specific energy consumption and specific costs, as well as the optimal system size regarding financial viability. The used configurations allow comparing two strategies: with and without batteries. The research was conducted with different levels of brackish water salinity, to identify a viability limit. The technical and financial analysis show that the unit produces satisfactory and competitive results, comparing with different plants in the world, concluding that, assuming a 2748 mg/l brackish water well, the configuration that brings the best cost/benefit is the no battery plant using 3 PV panels, which provides a daily production of 175 l of drinking water at 324.60 mg/l, with a specific consumption of 3.12 kWh/m3 and presenting a competitive specific cost of 10.32 US$/m3.

Description: The interaction of the dynamics of doubly fed wind generators with the electromechanical mode of nearby synchronous generators (SGs) can affect the small signal stability of power systems with high penetration levels of wind power. In this study, a novel approach is developed to investigate these interactions and their impact on the damping of power system oscillations. In this approach it is not necessary to model the dynamic behaviour of system SGs and only the frequencies of system oscillations are important. This approach is based on the sensitivity of SGs electromechanical eigenvalue with respect to variations in the Jacobian matrix of power system. By applying this approach to a test system, two of the wind farm (WF) modes are identified to have interaction with the SG electromechanical mode, one has detrimental and the other has beneficial impact, which decrease the damping of system electromechanical oscillation altogether. By using the modal analysis technique, the WF controller gains are retuned to shift the detrimental and beneficial interacting modes away and towards the frequency of system oscillation, respectively. It is shown that this results in the improvement of system oscillation damping whereas the dynamic performance of WF remains satisfactory.

Description: This study presents two methods for designing power oscillation damping (POD) controllers for wind farms comprising doubly fed induction generators (DFIGs). The first is the residue method, which uses linear feedback. The second method uses a non-linear signal as feedback. Here linear matrix inequalities (LMIs) and regional pole placement are used to determine the feedback gains for multiple wind farms simultaneously so that the power system satisfies a minimum damping ratio. The impact of the designed POD controllers in wind farms is demonstrated in a test power system. Modal analysis is used to design controllers using both the residue and LMI methods, and dynamic simulations are used to demonstrate the contribution of the wind farms to power system damping. Numerical simulations show that DFIGs, such as those found in wind farms, are capable of damping oscillations, and also illustrate the effectiveness of using non-linear feedback controllers

Description: Distributed generation systems (DGS) with fixed-speed induction-generator-based wind turbines (FSWT) are sensitive and vulnerable to voltage disturbances and reactive power deficiency. Consequently, the control and protection strategies for such a DGS should be prompt and precise to avoid undesired wind generator cutting off. This study investigates and analyses the dynamic characteristics of an FSWT to reveal the behaviours of an FSWT under disturbances. Then a fast control strategy for stabilising an islanded DGS is proposed. The main contributions of the control strategy are: the total amount of reactive power compensation and load shedding may be determined precisely; all the operation execution points may be decided accurately to improve DGS stability. Furthermore, the implementation of the fast control strategy is also discussed. A case study is presented to demonstrate the effectiveness of the proposed method.

Description: Owing to geographic relationships of wind farms, wind speeds at different wind sites show certain dependence between them, which may create a considerable impact on the reliability of power systems containing wind energy. A new technique for establishing multivariate distribution of wind speeds using copulas is presented to model wind speed dependence in the study. The separation between marginal distributions of each single wind speed and dependence structure is a basic feature of the copula method, which leads to the effectiveness and simplicity in dealing with the dependence in a multivariate distribution. The case studies using two actual wind speed data indicate that the copula model can give a valid representation of multiple wind speed series. The impact of wind speed dependence between two wind regimes on system reliability is illustrated using a commonly recognised reliability test system. The results confirm that system reliability will decrease as the wind speed dependence increases.

Description: This study proposes a fault forecasting system for proton exchange membrane fuel cells (PEMFCs), which uses a set of wireless sensors to accomplish fuel cell (FC) condition monitoring. The software interface of the FC fault forecast system uses LabVIEW software. Owing to the time delay condition of FC reactions, this study first forecasts all the operation features of the FC using a Grey prediction model. Then, an extension diagnosis method uses the forecasted values of the features to forecast the future operational conditions. Thus, the complex condition monitoring and fault forecasting problem of the FC can be implemented effectively. To demonstrate the effectiveness of the proposed method, comparative studies using a multilayer neural network and k -means algorithm are conducted on 400 sets of field-test patterns of 200 W PEMFC with rather encouraging results.

Description: In this study, a novel design tool for energy harvesting integrated circuits (IC) and system synthesis is developed with visual basic software. The proposed tool can be used to shorten the design time-to-market. A temperature compensation technique is then adopted to ensure that the circuit characteristics are stable and have low thermal sensitivity. In addition, a smart meter system is developed to measure energy harvester information by using an online system. Thus, users can obtain the proposed system information at all times. The accurate and superior performance of the proposed IC and system synthesis were confirmed by computer simulations and experimental results.

Description: With the current increase in wind power penetration into the energy market, control and operation of wind turbine generators becomes a major research topic. Wind turbine based on doubly fed induction generator (DFIG), which is sensitive to grid disturbances, is widely used. Under an unbalanced grid voltage condition, oscillations of the DFIG's electromagnetic torque and instantaneous stator powers strongly affect the dynamic performance of the DFIG. In this study, a new configuration based on vector proportional'integral (VPI) controller is proposed to eliminate such oscillations. This new configuration is employed in the rotor side converter (RSC) of the DFIG. With the proposed VPI control strategy, decomposition of sequential components and mathematical complexity are reduced. Compared with the conventional field-oriented control based on the standard single PI controller, the VPI controller can successfully eliminate torque and stator power oscillations. The effectiveness of the proposed control strategy is validated through simulation results obtained on a 1.5'MW DFIG-based wind turbine system model built in MATLAB/Simulink.

Description: A common practice in the analysis of voltage source inverters is the assumption of a voltage-type input source even in the case of renewable energy sources (RES). When a converter is supplied by an RES, such as a photovoltaic (PV) generator, the dc-link voltage is usually controlled by the inverter to extract maximum power from the source. This study presents an improved dynamic model of grid-connected three-phase renewable-energy-related inverter. For the proposed model to correspond to reality, the true nature of the input source has to be taken into account: an input-voltage-controlled converter has to be analysed so that it is supplied by a current source. Since all real sources are non-ideal, an explicit method to include the source effect on the inverter dynamics is presented, which has not been previously published in the literature. This study shows that the dynamics of the converter change completely when the operating point of a PV generator shifts from the constant current to the constant voltage region. This study shows also that assuming a voltage-type input source for a PV inverter leads to a model that cannot predict the existence of higher-order dynamics and a right-half plane zero in the control-to-output transfer function.

Description: An electrical structure of a variable-speed wind turbine based on an externally excited synchronous generator; a passive diode rectifier; and a boost converter is discussed in this study. The clear advantage of such a system is its lower-semi-conductor devices count. A brief theoretical explanation of such a system is included. A boost converter normally utilies an inductor (energy storage) to boost the voltage level from its input to a higher output value. This study analyses the possibility of using the generator inductance as a boost inductor. It is discussed and verified in the study that for the given switching frequency of the boost converter ( f s =1= kHz), the generator sub-transient inductance (not the synchronous inductance) appears as an equivalent inductance seen by the boost converter. The parasitic capacitors present in the generator terminals are often neglected from design issues. It is presented in the study that such capacitors can be a major issue when high-frequency switching is applied to the voltage at the generator terminals. Some major results from the experimental work are included. The experimental setup used in this work is a scaled down 7.5=kVA system.

Description: A current controller suitable for three-phase photovoltaic systems is presented. The proposed controller ensures the injection of sinusoidal and balanced currents, with high immunity to both imbalances and harmonics present in the supply voltage, without using a phase-locked loop to synthesise the reference currents to be injected. A fully digital domain controller design is proposed, that takes into account the digital signal processing delay as part of the system to stabilise.

Description: The installation of a new wind farm requires the previous acquisition of a great amount of wind speed data in order to perform a correct wind energy assessment. These data are obtained from a meteorological mast installed at each site. Real wind measurements at the meteorological mast include many erroneous data, which must be detected and eliminated before working with them. To localise wrong data, a new method based on the Kalman filter is proposed. This filter is applied to a short-term prediction like the one used in wind power forecasting. The filter parameter tuning is based on general verification metric curves tested with real data from different sites. The filter equations are adapted to different sites depending on the intensity of turbulence and wind direction. In this study all the parameters are explained in detail and the filter is evaluated, showing nice results for wind energy assessment of candidate sites.

Description: This study presents and thoroughly compares different alternatives to create DC links in distribution networks by means of voltage source converters (VSC). Such links give rise to fully controllable loops in nowadays radially operated systems, allowing higher levels of distributed generation penetration. Suitable VSC-based devices are reviewed and compared from several points of view, including topologies, degrees of freedom, operating ranges and performance. Several case studies, involving actual urban and rural distribution networks, are worked out to show the potential benefits provided by the use of the proposed smart links.

Description: The connection of high penetrations of renewable generation such as wind to distribution networks requires new active management techniques. Curtailing distributed generation during periods of network congestion allows for a higher penetration of distributed wind to connect, however, it reduces the potential revenue from these wind turbines. Energy storage can be used to alleviate this and the store can also be used to carry out other tasks such as trading on an electricity spot market, a mode of operation known as arbitrage. The combination of available revenue streams is crucial in the financial viability of energy storage. This study presents a heuristic algorithm for the optimisation of revenue generated by an energy storage unit working with two revenue streams: generation-curtailment reduction and arbitrage. The algorithm is used to demonstrate the ability of storage to generate revenue and to reduce generation curtailment for two case study networks. Studies carried out include a single wind farm and multiple wind farms connected under a 'last-in-first-out' principle of access. The results clearly show that storage using both operating modes increases revenue over either mode individually. Moreover, energy storage is shown to be effective at reducing curtailment while increasing the utilisation of circuits linking the distribution and transmission networks. Finally, renewable subsidies are considered as a potential third revenue stream. It is interesting to note that under current market agreements such subsidies have the potential to perversely encourage the installation of inefficient storage technologies, because of increased losses facilitating greater "utilisation" of renewable generation.

Description: A standalone wind/solar/battery hybrid power system, making full use of the nature complementarity between wind and solar energy, has an extensive application prospect among various newly developed energy technologies. The capacity of the hybrid power system needs to be optimised in order to make a tradeoff between power reliability and cost. In this study, each part of the wind/solar/battery hybrid power system is analysed in detail and an objective function combining total owning cost and loss of power supply probability is built. To solve the problems with non-linearity, complexity and huge computation, an improved particle swarm optimisation (PSO) algorithm is developed, which integrates the taboo list to broaden the search range and introduces 'restart' and 'disturbance' operation to enhance the global searching capability. The simulation results indicate that the proposed algorithm is more stable and provides better results in solving the optimal allocation of the capacity of the standalone wind/solar/battery hybrid power system compared with the standard PSO algorithm.

Description: Wind power plants (WPP) are for power system stability studies often represented with aggregated models where several wind turbines (WT) are aggregated into a single up-scaled model. The advantage is a reduction in the model complexity and the computational time, and for a number of study types the accuracy of the results has been found acceptable. A large WPP is, however, both modular and distributed over a large geographical area, and feasibility of aggregating the WTs, thus, have to be reassessed when new applications are introduced for WPPs. Here, the power oscillation damping capabilities are investigated for a WPP, which includes the full layout of the collector grid and where the WTs are represented individually. With this approach, the influence of the WT control in terms of impact on oscillatory modes is assessed for the WTs individually. The initial results encourage that park level control is possible. Time domain simulations support that each WT contribute to a common WPP response. Park level active and reactive power-based power oscillation damping controllers (POD) are designed and the positive damping contribution is demonstrated. Keeping the POD designs unchanged, the impact of WPP aggregation is investigated and it is shown that the level of WPP aggregation only has limited impact on the resulting modal damping. The study is based on a non-linear, dynamic model of the 3.6 MW Siemens Wind Power WT.

Description: This study presents a second-order sliding-mode control (2-SMC) scheme for a wind turbine-driven doubly fed induction generator (DFIG). The tasks of grid synchronisation and power control are undertaken by two different algorithms, designed to command the rotor-side converter at a fixed switching frequency. Effective tuning equations for the parameters of both controllers are derived. A procedure is also provided that guarantees bumpless transfer between the two controllers at the instant of connecting the DFIG to the grid. The resulting 2-SMC scheme is experimentally validated on a laboratory-scale 7 kW DFIG test bench. Experimental results evidence both the high dynamic performance and the superior robustness achieved with the proposed control scheme.

Description: An intelligent controlled three-phase squirrel-cage induction generator (SCIG) system for grid-connected wind power application using wavelet fuzzy neural network (WFNN) is proposed in this study. First, the indirect field-oriented mechanism is implemented for the control of the SCIG system. Then, an AC/DC power converter and a DC/AC power inverter are developed to convert the electric power generated by a three-phase SCIG from variable-voltage and variable-frequency to constant-voltage and constant-frequency. Moreover, the intelligent WFNN controller is proposed for both the AC/DC power converter and DC/AC power inverter to improve the transient and steady-state responses of the SCIG system at different operating conditions. Three online trained WFNNs using backpropagation learning algorithm are implemented as the tracking controllers for the DC-link voltage of the AC/DC power converter and the active power and reactive power outputs of the DC/AC power inverter. Furthermore, the network structure and the online learning algorithm of the WFNN are introduced in detail. Finally, some experimental results are provided to demonstrate the effectiveness of the proposed SCIG system for wind power.

Description: The study presents a wind generator system based on a new converter configuration with a rectifier with near sinusoidal input currents (RNSICs-1 converter with DC capacitors connected in parallel with diodes). A detailed analysis of the system for different values of the load current is presented and the advantages of the solution are emphasised. The new converter configuration is characterised by smaller power losses, reduced electromagnetic interference (EMI) problems, low harmonic input currents, high reliability, as well as reduced costs. This original configuration could also be used for small hydro interconnection with squirrel cage induction generator (SCIG) and partial variable-speed wind turbine (typically 60??100% synchronous speed).

Description: As more renewable energy sources, especially more wind turbines (WTs) are installed in the power system; grid codes for wind power integration are being generated to sustain stable power system operation with non-synchronous generation. Common to most of the grid codes, wind power plants (WPPs) are requested to stay connected and inject positive-sequence reactive current in order to boost positive-sequence grid voltage during short-circuit grid faults, irrespective of the fault type; symmetrical or asymmetrical. However, as shown in this study, when WPPs inject pure positive-sequence reactive current in case of asymmetrical faults, as a conventional method (CM) in accordance with the grid code requirement, positive-sequence grid voltage is boosted, but also higher negative sequence voltage in the grid and higher overvoltages at the non-faulty phases occur. In this study, an alternative injection method, where WTs are injecting both positive and negative sequence currents during asymmetrical faults, providing improved grid support, is given and compared with the CM. In addition, effect of coupling between positive, negative and zero sequences when WPPs are injecting currents during asymmetrical faults, is investigated, which was not considered in the wind power impact studies before.

Description: Small wind turbine generator (WTG) below 300 W capacity is suitable to generate power in residential or city area, where no large space for installing kilowatt scale WTG. The modularised micro-inverter which can feed WTG power to the grid directly has the merit of increasing the installed flexibility and omitting the battery in this application. This study proposes a grid-tied micro-inverter combining the boost and flyback-based inverter topology and specially designed for the small WTG with permanent magnetic (PM) generator. The boost converter is used to step up the input voltage and track the maximum power point (MPP) of the WTG. The flyback-based inverter is used to feed power to the grid with unity power factor. A fast MPP tracking (MPPT) method is presented to track the wind power to fit for the wind speed with fast variation. The variable frequency peak current mode control method is adopted for the control of the flyback-based inverter. An interleaving technique is also applied to the inverter for increasing its power capacity. Quantitative controller design of the boost converter with the WTG input and the grid-tied flyback-based inverter is presented. A 240 W experimental system is built. A WTG emulator implemented with the M-G set is developed for testing the proposed system. Some simulation and measured results are provided for demonstrating the effectiveness of the proposed system.

Description: How much capacity credit should be given to wind power in generation system adequacy analysis is a question of great interest around the world. Both a theoretical analysis and an accurate evaluation on the wind power capacity credit are essential for understanding its contribution to power system reliability. Current evaluation techniques usually rely on numerical calculation procedures that do not provide an analytical analysis, or are based on assumptions that are valid only for small wind penetration. This study presents a rigorous model based on the definition of the reliability function. The derivation of the model is presented and a fast and accurate method for calculating the capacity credit is developed based on this model. The proposed method does not require strong hypotheses and is thus widely applicable, especially when current evaluation techniques might cause large errors, for example, when the wind power penetration is large and the wind power and load profile are not statistically independent. The model is used to explain how the statistical characteristics of the load and wind power affect the capacity credit, from both a statistical and chronological perspective. Numerical tests demonstrate the correctness of the proposed model and its potential applicability under different circumstances.

Description: Measurements of breaking wave forces on vertical circular columns have been re-analysed, where substantial magnification over forces because of non-breaking non-linear waves may occur, by a factor of up to 2.8. The analysis shows that this factor increases as the depth parameter kd decreases, being close to unity for kd ≫ 1.5 (k is the wave number and d is depth). Non-breaking wave forces are predicted reasonably by non-linear stream function wave theory using Morison's equation with empirical drag and inertia coefficients. A study was then made of the magnitude of wave overturning moment in relation to hub moment due to wind on standard 2 and 5 MW turbines with a 6 m diameter column. This showed that the wave moment in extreme conditions is greater than wind 'hub' moment for depths greater than about 7 and 13 m for the 2 and 5 MW turbines, respectively. Monopiles are normally used in depths below about 30 m and extreme moments due to waves occur when waves are depth-limited and defined by the Miche criterion, well below the limiting value of kd for the onset of depth-induced breaking. The maximum moment for these depths is expected to be predicted reasonably.

Description: The use of renewable energy sources has increased year-on-year. Thus, there is an increasing rate of small generating units connected directly to distribution networks and micro-grids close to consumers. At the same time, these micro-sources must provide stability and reliability of electrical energy to the power network to which they are connected. In the technical literature, several studies have been done to ensure power systems with traditional generating sources to operate in a stable and reliable way, but there is an issue regarding generation uncertainty when a distribution system has many micro-sources. This is because of the uncertainty of primary sources, for example, wind and radiation intensity, and could result in intermittent generation. In this study, stability and reliability of voltage in a power system with distributed generation is analysed using simulation techniques. In the proposed method in this study voltage security analysis is jointly considered with probability laws. Moreover reliability theory is also considered in the proposed voltage collapse analysis methodology. The responsibility of generator in the voltage collapse process, the probabilistic risk of voltage collapse of each operating point and the probability of enlarging the system load as a function of different operating points are the outcome of the methodology, and it is validated by using the IEEE34 test feeder.

Description: This study analyses the integration impact of battery energy storage systems (BESSs) on the short-term frequency control in autonomous microgrids (MGs). Short-term frequency stability relates with the primary or speed control level, as defined in the regulations of the classical grids. The focus is on autonomous MGs that dynamically behave similarly to the classical power systems. This is the systems case with classical distributed generators (DGs), but which can also contain renewable energy sources (RESs) in a certain penetration level. During MG islanded operation, the local generators take over most of the frequency control process, by means of their automatic generation control, which include inertia response and primary control. However, RES-based DGs are rarely able to provide grid frequency support, as they lack controllability and usually the power conversion chain does not have the possibility of storing and releasing energy when required by the system. Therefore the need of boosting the MG power reserves by adding energy storage systems is often a requirement. The study highlights the improvement in the MG short-term frequency stability brought by an original BESS control structure enhanced with both inertial response and an adaptive droop characteristic during battery state-of-charge limitations. The conducted analysis is accomplished by adopting aggregated models for the involved control mechanisms. The developed model is analysed in frequency domain, whereas an experimental test bench including a real-time digital simulator with BESS controller in a hardware-in-the-loop structure is used for assessing the system performances.

Description: Wind turbines are increasingly being expected to provide oscillation damping to the power system to which they are connected. In this study, power oscillation damping control of variable speed wind turbines is studied. An energy storage device with a bidirectional DC/DC converter connected to the DC link of a fully rated converter-based wind turbine is proposed. As system oscillation is often induced by an AC fault, it is desirable for wind turbines to ride through the fault first and then provide a damping effect. During the fault period, the energy storage system (ESS) is controlled to assist the fault ride through process, and the line side converter (LSC) is controlled to provide AC voltage support in accordance with the grid code. Methods based on regulating the active power output of the ESS and modulation of reactive power output of the LSC are proposed so as to damp the oscillations of the power system. Matlab/Simulink simulations based on a simplified Irish power system demonstrate the performance of the ESS and LSC during fault periods and validate the damping effect of the proposed system.

Description: This study presents the dynamic modelling of a linear permanent magnet generator for extracting energy from ocean waves. Translator position, calculated from measured generator voltage, is used as input for the simulation model. Instantaneous power from the simulation model has been compared with the measurements from the Lysekil research site. The power output from the model considering the air gap flux variation is precisely matching with the measured values before core saturation. The generator dynamic model is modified by including the saturation effect. Although a simple mathematical expression is considered for representing the saturation, the model gives accurate power spectrum close to the experimental results. The presented model is a first step towards the system model that can simulate the entire electric system including electric grid. As such, the generator model can be used for further analysis of the wave energy conversion system.

Description: The droop control method is usually selected when several distributed generators (DGs) are connected in parallel forming an islanded microgrid. This is because of the advantages it offers such as flexibility, absence of critical communications etc. Besides, several studies add a fictitious impedance to improve the performance of the original droop method. However, only a few studies deal with the design of this fictitious impedance, which is necessary to ensure an improvement in the dynamics and stability of the microgrid. In addition, these studies do not consider load variations for the design of the fictitious impedance, which is a habitual event in these systems. On the other hand, some studies propose a restoration control to bring the frequency and voltage amplitude of the microgrid to their nominal values. However, these do not deal with the design of the dynamics of this control to maintain a good transient and to ensure the stable performance of the microgrid. This study proposes the design of a fictitious impedance that ensures the stable operation of an experimental microgrid without power oscillations during load jumps and throughout its entire load range. This study also proposes a new restoration control that allows to take into account the possible inertias, delays etc. of the DGs and reduces the bandwidth of the required communications. Moreover, the proposed restoration control is properly designed to guarantee a good transient and the satisfactory performance of the microgrid. Experimental results confirm the validity of the proposed controls.

Description: A coupled electromechanical and hydrodynamic time-domain simulation of a direct-drive generator connected to a heaving buoy for wave energy conversion is presented. The system is based on a novel direct-drive power take-off unit referred to as snapper. The simulation is based primarily in MATLAB using its built-in ordinary differential equation solvers. These solvers act on the data derived from electromagnetic finite element analysis and from the WAMIT wave interaction simulation software. Test results of a generator prototype for comparison with the electromechanical simulation are presented. Results from wave tank tests of a full system incorporating the power take-off are also provided for comparison with the hydrodynamic model.

Description: It is a pleasure to write the Editorial for this third IET Renewable Power Generation Proceedings Special Issue from the Science & Research Track of the 2012 EWEA Annual Event, organised by the European Wind Energy Association (EWEA) in Copenhagen in April 2012. EWEA2012 hosted more than 10 000 delegates including more than 3000 scientific researchers and teachers.

Description: Standard blade element momentum (BEM) codes use Prandtl??s tip-loss correction which relies on simplified vortex theory under the assumption of optimal operating condition and no wake expansion. The various tip-loss functions found in the literature are listed. A simple comparison between them shows important differences in Annual Energy Production which reveal a large uncertainty in current BEM-based computations. A new tip-loss correction for implementation in BEM codes has been developed using a lifting-line code to account for the effect of wake expansion, roll-up and distortion under many operating conditions. A database of tip-loss corrections is established for further use in BEM codes. This model is closer to the physics of the flow and hence, a better assessment of the performance of wind turbines by this method is expected.

Description: Recent developments in LiDAR technology have led to much interest in the possibility of improving turbine control by using a turbine-mounted LiDAR, to provide advance information about the approaching wind field. This could significantly reduce turbine loads, bringing improved cost-effectiveness, especially for large turbines. There have also been claims of direct increases in energy capture as a result of using such preview information. This study reports on an independent study employing detailed analytical methods to evaluate the likely benefits of LiDAR-assisted control and advise LiDAR manufacturers about the characteristics of their systems, which are most likely to be useful for this application. Accurate simulation models are vital for assessing the performance of LiDARs and controllers which use them. Current models use Taylor??s frozen turbulence hypothesis, but this is not strictly valid when LiDAR is used to measure upstream wind speeds, as the measured wind cannot be assumed to convect unchanged to the turbine. A method for avoiding the frozen turbulence assumption is proposed, and simulation results are presented to illustrate the effect on fatigue load reductions which LiDARassisted control might achieve. A detailed assessment of possible LiDAR benefits is made using the UPWIND generic 5 MW turbine as an example.

Description: This study investigates wound rotor induction machine bearing fault detection by stator current analysis. The research first establishes an analytic machine model that enables high fidelity simulation of a range of machine bearing defects. The timestepped simulation results are then used to examine stator current spectral signatures of typical bearing faults. The calculations indicate that a number of low magnitude fault specific frequency components appear in the current signal as a result of air-gap variations produced by an incipient bearing fault. However, the considerably low magnitude levels at which these components are exhibited would make the detection of bearing fault using conventional current signature analysis techniques challenging. An alternative technique based on spectral analysis of complex current signals is therefore proposed in order to improve fault detection. The validity of the findings of this work is confirmed by analysis of measured data obtained on a 30 kW commercial machine test rig that can be configured to introduce a range of different bearing fault severities.

Description: Certain parts of a wind turbine, for example, the gearbox require significant time and heavy lifting equipment in the event of catastrophic failure necessitating replacement. Continuous condition monitoring has the potential to catch problems early, enable scheduled preventative maintenance and thereby reduce turbine downtime, reduce the number of site visits and prevent secondary damage. Accelerometers applied to mechanical components of the drive train are traditionally used for condition monitoring, but require their own data acquisition system and analysis software. In contrast, the electrical current and voltage are continuously measured and could also be used for condition monitoring more cheaply. An experimental data acquisition system has been installed on a small (25 kW) onshore turbine in Leicestershire, UK to compare three-phase currents and voltages on the stator windings with six accelerometer signals. Data have been recorded before and after a gearbox failure and replacement. Data were analysed using both Fourier transform and Morlet continuous wavelet transform methods. Results show that the stator voltages show the same radial and axial mode vibration frequencies as the accelerometers, and could therefore be used for condition monitoring. Furthermore, the stator currents show torsional modes of vibration not picked up by the accelerometers.

Description: Offshore wind energy is catching increasing worldwide interest. However, access and maintenance offshore can be difficult and will be more costly than onshore, and hence, availability is correspondingly lower. As a result, there is a growing interest in wind turbine condition monitoring with condition-based rather than responsive and scheduled maintenance. A non-linear state estimation technique (NSET) model is presented here to model a healthy wind turbine gearbox using stored historical data. These data capture the inter-relationship between the model input and output parameters. The state vectors comprising the data should cover as much as turbine operational range, including the extreme conditions in order to obtain an accurate model performance. A model so constructed can be applied to assess the operational data. Welch??s t-test is employed in the fault detection algorithm, together with suitable time series filtering, to identify incipient anomalies in the turbine gearbox before they develop into catastrophic faults. Two case studies based on 10-minute supervisory control and data acquisition data from a commercial wind farm are presented to demonstrate the model??s effectiveness. Comparison is made with neural network modelling, and the NSET approach is demonstrated to be superior.

Description: The Gearbox Reliability Collaborative has conducted extensive field and dynamometer test campaigns on two heavily instrumented wind turbine gearboxes. In this study, the load sharing behaviour between six bearings in the planetary stage is described using a combined approach of measurement and simulation. First, planet-bearing data are analysed to characterise planetary stage behaviour in different environments. Second, a method is described for integrating the measured responses of the planetary stage into an advanced model of the bearing life that significantly changes the life prediction. Third, a sensitivity study of the planet bearings is conducted using multibody gearbox models. Various levels of gearbox flexibility and different planet assembly fits are investigated and compared with experimental observations. Measurements in the dynamometer and field show that bearing loading differs significantly between the six planet bearings. The relative loading behaviour of the planetary stage bearings is directly influenced by boundary conditions of the planet carrier pins. Assembly differences between two identically designed gearboxes cause different load sharing behaviour. Simulations are used to quantify the effect of different component flexibilities. Reduced order models are developed to accurately predict bearing loading in a cost-efficient manner.

Description: Control algorithms for the rotor- and grid-side power converters of a double-fed induction generator (DFIG)-based wind turbine under non-ideal grid voltage conditions are proposed, and guidelines for tuning the controller parameters are presented. The control schemes are based on sliding-mode control (SMC) theory. Apart from directly controlling the DFIG??s average active and reactive powers, the proposed methods also fulfil two additional control targets during voltage unbalance and harmonic distortion, that is, the rotor-side converter (RSC) eliminating electromagnetic torque fluctuations and the gridside converter (GSC) compensating for the stator current harmonics to ensure a sinusoidal total current from the overall generating unit. The described control strategies are proved to be robust against parameter deviations and of fast dynamic response. In spite of the discontinuous nature of the standard SMC, constant converter switching frequency is achieved. Besides, the RSC control algorithm does not require a phase-locked loop and, furthermore, there is no need for decomposing the grid voltage and different currents into symmetrical sequences or harmonic components in any of the converters?? control systems. Finally, the excellent performance of the system, as well as its robustness, is verified by means of simulation results under different grid voltage conditions.

Description: This study presents wind turbine converter stability analysis of wind farms in frequency domain. The interaction between the wind turbine control system and the wind farm structure in wind farms is deeply investigated. Two wind farms (i.e. Horns Rev II and Karnice) are taken into consideration in this study. It is shown that wind farm components, such as long high-voltage alternating current cables and park transformers, can introduce significant low-frequency series resonances seen from the wind turbine terminals that can affect wind turbine control system operation and overall wind farm stability. The same wind turbine converter control strategy is evaluated in two different wind farms. It is emphasised that the grid-side converter controller should be characterised by sufficient harmonic/noise rejection and adjusted depending on wind farms to which it is connected. Various stability indices such as gain margin, vector gain margin and phase margin are used in order to emphasise the differences between the two wind farms.

Description: Applying a land-based designed pitch controller on a floating wind turbine may cause severe instability. A common strategy to overcome this problem is to reduce the closed-loop bandwidth of the pitch control system. In doing so, the generator speed variation increases possibly leading to shutdowns because of overspeed. This study uses a parallel path modification to avoid instability without increasing the generator speed variation. The results of comprehensive simulations and load calculations carried out on a benchmark wind turbine are presented. These demonstrate that by using the proposed method it is possible to apply the land-based designed pitch controller on its floater-based equivalent.

Description: The work presented in this study describes a comparative study between different techniques aimed at identifying the damping values of an offshore wind turbine on a monopile foundation. It will be shown that damping ratios can directly be obtained from vibrations of the tower under ambient excitation from wave and wind loading. The results will be compared with the damping values obtained from a commonly used overspeed stop. Ambient vibration tests have the strong advantage of being more practical and less demanding for the wind turbine in comparison with the overspeed stop. Several identification algorithms, the standard exponential decay method, alternative procedures in the time domain as well as more advanced operational modal analysis techniques in the frequency domain will be applied to the experimental data. These data have been obtained during a short measurement campaign on an offshore wind turbine in the Belgian North Sea. The results of the used methods for estimating the modal damping of a wind turbine excited by ambient excitation will be discussed and compared. This study also presents some aspects related to the practical implementation of the measurements.

Description: Costs and losses have been calculated for several different network topologies, which centralise the turbine power electronic converters, in order to improve access for maintenance. These are divided into star topologies, where each turbine is connected individually to its own converter on a platform housing many converters, and cluster topologies, where multiple turbines are connected through a single large converter. Both AC and DC topologies were considered, along with standard string topologies for comparison. Star and cluster topologies were both found to have higher costs and losses than the string topology. In the case of the star topology, this is due to the longer cable length and higher component count. In the case of the cluster topology, this is due to the reduced energy capture from controlling turbine speeds in clusters rather than individually. DC topologies were generally found to have a lower cost and loss than AC, but the fact that the converters are not commercially available makes this advantage less certain.

Description: Construction of a wind farm without a reliable plant margin forecast can jeopardise potential returns on investment from the outset. Meteorological and topological factors influence the wind characteristics across any site which in turn affects wind farm output, critical for localised generation, and also the dynamic loading of the turbine structure. The models developed in this study follow the generally advocated use of probability density estimation as a means of representing wind resource characteristics but, owing to differences, in characterisation that may be encountered, do not assume a single distribution form across all sites. A mixture modelling approach is adopted that removes the need for choosing distribution forms on a site by site basis. Advancing previous work constructing statistical distributions over congruent wind speed and direction observations of the wind resource characteristics at a given site, the proposed model, as a consequence of using a mixture distribution, captures both recurring regimes in the site behaviour along with their frequency of occurrence. Preliminary results using data sets from a diverse range of locations in Scotland demonstrate the variation in the forms of model learned; comparisons of the model with current and alternate practices are given through visualisation and resource assessment illustrations.

Description: In this study, norm bounded linear quadratic Gaussian controller synthesis method is utilised to design a wide-area measurement signal (WAMS)-based power oscillation damping controller at photovoltaics (PV) plants. The uncertainties associated with the system are confined by the system matrices which are the affine functions of parameters belong to a convex polytopic region. A hybrid method based on Hankel singular value (HSV) and right-half-plane zeros (RHP-zeros) is utilised to assess and select the optimal feedback signal for the PV wide-area damping controller. First, the HSVs of the delayed subsystems are employed to preselect the candidate signals for damping the target mode. Then, the RHP-zeros and the modal interaction measures of the candidate signals are evaluated. Finally, the signal with minimum variance in HSV and modal interaction over the wide range of operating conditions is selected as the optimal feedback signal for the controller design. The wide range of operating conditions is obtained by the probabilistic distribution of loads, synchronous generators and PVs. The approach has been tested on a large 16 machine, 68 bus test system as compared to geometric measures of controllability/observability method and has shown improved performance.

Description: Power fluctuations could have a remarkable impact on power system operation and costs, in accordance with the increasing production of wind power worldwide. In systems with weak interconnections, such as Iberian Peninsula Power System, these fluctuations could be a security issue. In order to characterise the worst effects on power systems of wind power fluctuations, most negative wind power ramp rates can be used. In this study, one-year real data production of 17 wind farms, scattered over Spain, is analysed regarding the geographical smoothing effect on negative wind power ramp rates. Several control strategies are suggested to reduce the resulting aggregated negative ramps. For each strategy, the controlled power is compared with uncontrolled power output, studying the achieved reduction in negative ramps. The results give a comparison of such strategies, regarding energy losses and improvement for power system operation.

Description: For wave energy converters, the input resource has a typical period of 5-20 s depending on the site location and dominant seastates. Wave energy converters generally produce mechanical power twice per wave cycle and therefore depending on the storage available, the electrical power output has half the period of the input resource. These regular power changes induce a voltage change at the point of connection (POC) which is proportional to the amplitude of the power change and at the same frequency. Therefore the coupling of the input resource to the output power of a wave energy converter will cause voltage flicker at the POC, which may exceed the permitted limits under specific conditions. This study establishes the nature of the flicker issue from wave energy converters. Some practical tools for the evaluation of flicker from a device are introduced. These tools are suitable for early stage flicker assessment to assist in the design process only. They are not meant of substitutes for existing codes and standards which are outlined in this study. This study concludes that wave energy converters may exceed flicker emission limits as a result of the coupling of the resource to the output power, and this may be particularly severe when connected to weak grids. Some potential strategies for overcoming this problem are presented also.

Description: This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show the system's steady-state and dynamic performance and grid low-voltage ride-through capability. The medium-speed brushless DFIG in combination with a simplified two-stage gearbox promises a low-cost low-maintenance and reliable drivetrain for wind turbine applications.

Description: In this study, a novel network reinforcement model incorporating optimal wind farms (WFs) integration to power systems is proposed. Based on the reinforcement model, new transmission lines are added to the network for eliminating the transmission network congestion. In addition, a methodology for determining the WFs optimal capacities, the economical WF lines transfer rates and the appropriate reinforcement plan, whereas maximising the total benefit using cost-reliability analysis is presented. In this way, the objective of the proposed optimisation problem has been divided into a master problem and two subproblems. Benders decomposition is employed to connect the master and subproblems of the model. In order to assess the validity of the proposed method, it is applied to the IEEE 24-bus test system under three different scenarios.

Description: Concentrating solar power technology (CSP) will probably become increasingly present in power systems and will have to be included in generation adequacy studies. CSP has two special features which makes it different from other technologies: its daily and seasonal cycle and its possibility of storage. This study proposes a sequential Monte Carlo method, which includes series of possible trajectories of CSP production to find the capacity value of this technology. From the generated CSP production trajectories, values of loss of load probability, equivalent load carrying capacity and equivalent conventional power plant are calculated. Different hypotheses are considered, in order to study the effect of location of CSP plants, their storage capacity and demand profile on capacity credit. The study has a two fold objective: it intends to propose a way of including CSP in generation adequacy studies, and gives representative values of the contribution of this technology to overall power system reliability.

Description: The primary photovoltaic (PV) system investment decision-making criteria are economics. These criteria are focused on system efficiency and cost, which is reasonable in the context of generous financial support schemes. However, when financial support is phased out, the PV market becomes technologically diversified. Environmental concerns and other qualitative issues become significant, whereas efficiency and costs may fail to describe PV systems properly. This study presents the first application of outranking techniques for PV technologies within an overall framework that includes qualitative, economic, technical and environmental criteria. The multi-criteria analysis method, ELECTRE III, identifies optimal investment decisions from a pre-determined set of investment alternatives. A case study based on a grid-connected household PV system in the United Kingdom illustrates the methodology. The results suggest emerging excitronic PV technologies such as organic PV to lead the ranking when compared with inorganic technologies. These results have to be taken in the context of a number of assumptions and estimates as well as free market, no financial support schemes and promising future technology developments. The overall decision-making framework provides comprehensive mathematical evaluations that assist PV owners, policy makers and the business community decide on technology, financial support schemes or business strategies.

Description: Wind power is increasingly integrated in modern power grids, which brings new challenges to the power system operation. Wind power is fluctuating because of the uncertain nature of wind, whereas wind shear and tower shadow effects also cause periodic fluctuations. These may lead to serious forced oscillation when the frequencies of the periodic fluctuations are close to the natural oscillation frequencies of the connected power system. By using modal analysis and time-domain simulations, this study studies the forced oscillation caused by the wind shear and tower shadow effects. Three forced oscillation mitigation controllers are proposed and compared. A model of direct-drive-full-convertor-based wind farm connected to the IEEE 10-machine 39-bus system is adopted as the test system. The calculations and simulations are conducted in DIgSILENT PowerFactory 14.0. Results are presented to show the effectiveness of the controllers in mitigating the forced oscillation.

Description: This study presents a multi-objective framework to evaluate the integration of distant wind farms with associated transmission network upgrades on optimal power system planning. The presented approach also extends the technique to include the consideration of energy limitations associated with the installed hydro generation facilities. This study attempts to emphasise on the reliability implications rather than the production cost evaluation aspects. The decision making is based on hierarchal level II (HL-II) Expected Energy Not Served as an entire power system reliability assurance, and capital cost plus annual operational cost as an economical index. Non-dominated Sorting Genetic Algorithm is adopted to achieve the Pareto front of the aforementioned multi-objective problem. A fuzzy satisfying method, designated as the distance metric, is used to represents a trade-off between different objectives. To numerically evaluate the efficiency of the proposed method, simulation results on three case studies are provided. In spite of huge computation burden at HL-II reliability assessment, the results indicate high efficiency of the proposed method.

Description: This document describes the assessment of a decision support tool for wind power producers in real wind farms. This tool allows them to update their positions taken in daily markets into subsequent adjustment markets. The tool intends to maximise the revenues of wind power producers that participate in electricity markets through an optimisation procedure, making use of the possibilities of non-continuous adjustment markets and the probabilistic tools developed in the EU project ANEMOS.plus. Results are presented for nine Spanish wind farms and they are compared with those obtained with a standard strategy of market participation for wind power producers. For the examined cases, the revenues were overall increased in a 1.3% by using the decision support tool. This could encourage even more the participation of wind energy in electricity markets. The overall energy imbalance also increases, showing a potential risk for the whole system if all wind power producers would use such a tool.

Description: Compressed-air energy storage (CAES) is considered a promising energy storage system for many grid applications, including managing renewable variability and grid capacity concerns. However, compared with conventional generation such as coal or hydro, the cost of storage power of CAES is still high, which impedes its deployment. Therefore a standing question is how to operate CAES in the most efficient and economical fashion, that is, to exploit the system functions for maximum-possible benefit. This study investigates the CAES dynamic reactive capability used to stabilise wind farms under grid fault conditions. Two considered operation modes are motor mode with leading power factor and synchronous condenser mode. Analysis with a 60-MW wind farm and two types of popular wind turbines, namely stall-regulated and doubly fed induction-generatorbased WTs, shows that the CAES performance is comparable or better than that of an static var compensator in most situations investigated. Therefore the reactive-power-supply function should be considered in CAES design and operation to increase the system efficiency and value.

Description: The ability to influence electricity demand from domestic and small business consumers, so that it can be matched to intermittent renewable generation and distribution network constraints is a key capability of a smart grid. This involves signalling to consumers to indicate when electricity use is desirable or undesirable. However, simply signalling a time-dependent price does not always achieve the required demand response and can result in unstable system behaviour. The authors propose a demand response scheme, in which an aggregator mediates between the consumer and the market and provides a signal to a 'smart home?? control unit that manages the consumer's appliances, using a novel method for reconciliation of the consumer's needs and preferences with the incentives supplied by the signal. This method involves random allocation of demand within timeslots acceptable to the consumer with a bias depending on the signal provided. By simulating a population of domestic consumers using heat pumps and electric vehicles with properties consistent with UK national statistics, the authors show the method allows total demand to be predicted and shaped in a way that can simultaneously match renewable generation and satisfy network constraints, leading to benefits from reduced use of peaking plant and avoided network reinforcement.

Description: In this study, the design and implementation of a new portable thermoelectric generator of 100 W for low geothermal temperatures has been carried out. For this system, a new SCADA-based testing and measuring system equipped with special software has been developed and employed for the first time. Thus, effects of the hot??cold water flow rates, the temperature differences between the surfaces and the load resistance affecting output power and efficiency of the thermoelectric generator have been investigated by a single testing and measuring system device. In the established SCADA-based testing and measuring system, the hot-cold water flows passed throughout the surfaces of the thermoelectric generator were increased by up to 3.7 and 12.8 l/min, respectively. Then, the temperature difference between the surfaces of the thermoelectric generator was measured as 67°C. When the load resistance of the thermoelectric generator was about 15 Ω, the maximum power of the thermoelectric generator was obtained as 41.6 W and the conversion efficiency was calculated as 3.9%. Also, the SCADAbased testing and measuring system will open up a new stage in examination of various thermoelectric generators.