ALBERT

Description: In this paper, the power grid with penetration of renewable energy sources is modeled as a multigenerator interconnected power network. The power grid includes distributed energy resources including conventional synchronous generators and renewable energy sources; here called renewable generators that are connected to the grid via grid-tie inverters (GTIs). With the proposed modeling, the GTI resembles a synchronous generator with excitation control. The modeling takes into account the dc-link capacitor stored energy as a dynamical state, in contrast with the available methods, and through an appropriate controller assures the stability of the dc link and the entire grid without needing an abundant-energy dc link. Next, the power grid comprising the synchronous and renewable generators is converted to decentralized control form with subsystems in Brunovsky canonical form whose interactions with the rest of the grid are unknown. A decentralized adaptive neural network (NN) feedback controller is proposed with quadratic update law to stabilize the rotor speed and dc-link voltage oscillations in asymptotic fashion in the presence of grid disturbances. The proposed controller is then simplified. Though the solar power interacting with conventional synchronous generators is considered in this paper, the proposed modeling and controller design can be applied to many other renewable energy systems. Simulation results on the IEEE 14-bus power system with penetration of solar power are provided to show the effectiveness of the approach in damping oscillations that occur after disturbances.

Description: Decentralized methods for computing optimal real and reactive power setpoints for residential photovoltaic (PV) inverters are developed in this paper. It is known that conventional PV inverter controllers, which are designed to extract maximum power at unity power factor, cannot address secondary performance objectives such as voltage regulation and network loss minimization. Optimal power flow techniques can be utilized to select which inverters will provide ancillary services and to compute their optimal real and reactive power setpoints according to well-defined performance criteria and economic objectives. Leveraging advances in sparsity-promoting regularization techniques and semidefinite relaxation, this paper shows how such problems can be solved with reduced computational burden and optimality guarantees. To enable large-scale implementation, a novel algorithmic framework is introduced—based on the so-called alternating direction method of multipliers—by which optimal power flow-type problems in this setting can be systematically decomposed into subproblems that can be solved in a decentralized fashion by the utility and customer-owned PV systems with limited exchanges of information. Since the computational burden is shared among multiple devices and the requirement of all-to-all communication can be circumvented, the proposed optimization approach scales favorably to large distribution networks.

Description: The emergence of cloud computing has established a trend toward building massive, energy-hungry, and geographically distributed data centers. Due to their enormous energy consumption, data centers are expected to have a major impact on the electric power grid by significantly increasing the load at locations where they are built. Dynamic energy pricing policies in the recently proposed smart power grid technology can incentivize the cloud controller to shift the computation load toward data centers in regions with cheaper electricity or with excessive electricity generated by renewable energy sources, e.g., photovoltaic (PV) and wind power. On the other hand, distributed data centers in the cloud also provide opportunities to help the power grid with distributed renewable energy sources to improve robustness and load balancing. To shed some light into these opportunities, this paper considers an interaction system of the smart power grid with distributed PV power generation and the cloud computing system, jointly accounting for the service request dispatch and routing problem in the cloud with the power flow analysis in power grid. The Stackelberg (sequential) game formulation is provided for the interaction system under two different dynamic pricing scenarios: 1) real-time power-dependent pricing; and 2) time-ahead pricing. The two players in the Stackelberg games are the power grid controller that sets the pricing signal and the cloud controller that performs resource allocation among data centers. The objective of the power grid controller is to maximize its own profit and perform load balancing among power buses, i.e., minimizing the power flow from one power bus to the others, whereas the objective of the cloud computing controller is to maximize its own profit with respect to the location-dependent pricing signal. Based on the backward induction method, this paper derives the near-optimal or suboptimal strategies of the two players in Stackelberg game using c- nvex optimization and simulated annealing techniques.

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Description: This paper investigates the use of a virtual synchronous machine (VSM) to support dynamic frequency control in a diesel-hybrid autonomous power system. The proposed VSM entails controlling the grid-interface converter of an energy storage system (ESS) to emulate the inertial response and the damping power of a synchronous generator. In addition, self-tuning algorithms are used to continuously search for optimal parameters during the operation of the VSM in order to minimize the amplitude and rate of change of the frequency variations and the power flow through the ESS. The performances of the proposed self-tuning (ST)-VSM and the constant parameters (CP)-VSM were evaluated by comparing their inertial responses and their damping powers for different scenarios of load variations. For the simulated cases, the ST-VSM achieved a similar performance to that of the CP-VSM, while reducing the power flow through the ESS in up to 58%. Moreover, in all the simulated scenarios, the ST-VSM was found to be more efficient than the CP-VSM in attenuating frequency variations, i.e., it used less energy per Hertz reduced.

Description: In this paper, we present virtual prototyping of the distributed control for a modular multilevel inverter used as a grid-tie interface for photovoltaics. Due to the distributed control and inherent redundancy in the system composed of many panels and inverter modules, the system topology exhibits fault-tolerance capabilities that we study through virtual prototyping. The fault-tolerance is enabled by several distributed algorithms, such as services to identify which, if any, agents controlling inverter modules have failed. A distributed identifier algorithm allows the system to keep track of the number of operating panels to appropriately regulate the dc voltage output of the panels using buck–boost converters and determine appropriate switching times for H-bridges in the grid-tie. We evaluate the distributed inverter, its control strategy, and fault-tolerance through thousands of simulation scenarios in Mathworks Simulink/Stateflow. Our virtual prototyping framework allows for generating multilevel inverters composed of many inverter modules, and we evaluate inverters composed of five to dozens of inverter modules. Our analysis suggests the achievable total harmonic distortion of the modular multilevel inverter may allow for operating solar arrays in spite of failures of the power electronics, control software, and other subcomponents.

Description: This paper proposes an adaptive and distributed secondary voltage control for microgrids with inverter-based distributed generators (DG). The proposed control is fully adaptive and does not require the information of DG parameters. Neural networks are used to compensate for the uncertainties caused by the unknown dynamics of DGs. The controller structure is fully distributed such that each DG only requires its own information and the information of its neighbors on the communication network. Therefore, this secondary control is associated with a sparse communication network. The effectiveness of the proposed methodology is verified for different loading, outage, and islanding scenarios, as well as variable communication structures in a microgrid setup.

Description: Power system operates close to its operational limit to achieve a higher level of utilization of the grid infrastructure. As a result, its transient behavior is of great importance to prevent violation of the operation limits. This paper proposes an internal model control (IMC)-based approach for voltage-sourced converters (VSC) to improve their dynamic behavior. This IMC controller is designed based on a detailed model of the VSC. The proposed control approach has superior performance, i.e., faster step response and less overshoot, and higher axes decoupling than the conventional dq -current control method. The proposed IMC-based controller has a straightforward design procedure, and can be implemented using simple PI and PID controllers. Several case studies are presented to evaluate and compare the transient performance of the IMC-based controller with conventional dq current control subsequent to step changes in the set points of direct and quadrature axes current, single-phase fault, and three-phase fault.

Description: In an islanded ac microgrid with distributed energy storage system (ESS), photovoltaic (PV) generation, and loads, a coordinated active power regulation is required to ensure efficient utilization of renewable energy, while keeping the ESS from overcharge and overdischarge conditions. In this study, an autonomous active power control strategy is proposed for ac-islanded microgrids in order to achieve power management in a decentralized manner. The proposed control algorithm is based on frequency bus-signaling of ESS and uses only local measurements for power distribution among microgrid elements. Moreover, this study also presents a hierarchical control structure for ac microgrids that is able to integrate the ESS, PV systems, and loads. Hereby, basic power management function is realized locally in primary level, while strict frequency regulation can be achieved by using additional secondary controller. Finally, real-time simulation results under various state of charge (SoC) and irradiance conditions are presented in order to prove the validity of the proposed approach.

Description: We formulate the control of reactive power generation by photovoltaic inverters in a power distribution circuit as a constrained optimization that aims to minimize power losses subject to finite inverter capacity and upper and lower voltage limits at all nodes in the circuit. When voltage variations along the circuit are small and losses of both real and reactive powers are small compared with the respective flows, the resulting optimization problem is convex. Moreover, the cost function is separable enabling a distributed online implementation with node-local computations using only local measurements augmented with limited information from the neighboring nodes communicated over cyber channels. Such an approach lies between the fully centralized and local policy approaches previously considered. We explore protocols based on the dual-ascent method and on the alternating direction method of multipliers (ADMMs), and find that the ADMM protocol performs significantly better.

Description: In a dc distribution system, where multiple power sources supply a common bus, current sharing is an important issue. When renewable energy resources are considered, such as photovoltaic (PV), dc/dc converters are needed to decouple the source voltage, which can vary due to operating conditions and maximum power point tracking (MPPT), from the dc bus voltage. Since different sources may have different power delivery capacities that may vary with time, coordination of the interface to the bus is of paramount importance to ensure reliable system operation. Further, since these sources are most likely distributed throughout the system, distributed controls are needed to ensure a robust and fault tolerant control system. This paper presents a model predictive control-based MPPT and model predictive control-based droop current regulator to interface PV in smart dc distribution systems. Back-to-back dc/dc converters control both the input current from the PV module and the droop characteristic of the output current injected into the distribution bus. The predictive controller speeds up both of the control loops, since it predicts and corrects error before the switching signal is applied to the respective converter.

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Description: This paper presents an innovative control scheme for the management of energy consumption in commercial buildings with local energy production, such as photovoltaic panels or wind turbine and an energy storage unit. The presented scheme is based on distributed model predictive controllers, which manage the storage system and the building space heating and cooling. The proposed approach is implemented and tested in SYSLAB, the experimental facility for distributed energy systems at the Technical University of Denmark, Risø Campus. The experimental setup consists of wind and solar renewable sources, a vanadium redox battery system, resistive load, and a point of common coupling to the national grid. Several experiments are carried to assess the performance of the control scheme in managing local energy production and consumption.

Description: Advertisement: The IEEE PES Scholarship Plus Initiative is attracting top engineering candidates to pursue power engineering careers. The continued success of the initiative depends on leaders like you, who can provide financial support, career experience opportunities and mentoring. To find out how you can get involved, visit www.ee-scholarship.org today.

Description: In power electronics-based microgrids, the computational requirements needed to implement an optimized online control strategy can be prohibitive. This paper proposes the derivation of a geometric manifold in the energy–power domain that is based on the a-priori computation of the optimal reactions and trajectories for classes of events in a dc microgrid. The proposed states are the stored energy and power of the dc–dc converter. It is anticipated that calculating a large enough set of dissimilar transient scenarios will also span many scenarios not specifically used to develop the surface. These geometric manifolds will, then, be used as reference surfaces in any type of controller, such as a sliding mode hysteretic controller.

Description: This paper presents a robust nonlinear distributed controller design for islanded operation of microgrids in order to maintain active and reactive power balance. In this paper, microgrids are considered as inverter-dominated networks integrated with renewable energy sources (RESs) and battery energy storage systems (BESSs), where solar photovoltaic generators act as RESs and plug-in hybrid electric vehicles as BESSs to supply power into the grid. The proposed controller is designed by using partial feedback linearization and the robustness of this control scheme is ensured by considering structured uncertainties within the RESs and BESSs. An approach for modeling the uncertainties through the satisfaction of matching conditions is also provided in this paper. The proposed distributed control scheme requires information from local and neighboring generators to communicate with each other and the communication among RESs, BESSs, and control centers is developed by using the concept of the graph theory. Finally, the performance of the proposed robust controller is demonstrated on a test microgrid and simulation results indicate the superiority of the proposed scheme under different operating conditions as compared to a linear-quadratic-regulator-based controller.

Description: This paper presents a distributed hierarchical control framework to ensure reliable operation of dc microgrid (MG) clusters. In this hierarchy, primary control is used to regulate the common bus voltage inside each MG locally. An adaptive droop method is proposed for this level, which determines droop coefficients according to the state-of-charge (SOC) of batteries automatically. A small-signal model is developed to investigate effects of the system parameters, constant power loads, as well as line impedance between the MGs on stability of these systems. In the secondary level, a distributed consensus-based voltage regulator is introduced to eliminate the average voltage deviation over the MGs. This distributed averaging method allows the power flow control between the MGs to be achieved at the same time, as it can be accomplished only at the cost of having voltage deviation inside the system. Another distributed policy is employed then to regulate the power flow among the MGs according to their local SOCs. The proposed distributed controllers on each MG communicate with only the neighbor MGs through a communication infrastructure. Finally, the developed small-signal model is expanded for MG clusters with all the proposed control loops. The effectiveness of the proposed hierarchical scheme is verified through detailed hardware-in-the-loop simulations.

Description: The presence of distributed generators (DGs) such as photovoltaic systems (PVs) is increasing significantly in distribution networks, and in order to accommodate a higher penetration of DGs, technical issues arising from fluctuation and unpredictability of their power output must be addressed. It is beneficial if DGs of high penetration can be dispatched when necessary. To this end, a distributed control and generation estimation approach is developed to dispatch multiple DGs, each of which consists of a PV and a controllable load. A strongly connected digraph with a row stochastic adjacency matrix is a sufficient requirement for the communication topology. A distributed weights adjustment algorithm adaptively makes the adjacency matrix doubly stochastic so that the aggregated power generation capacity can be estimated. Then, the expected consensus operational point of the DGs is calculated by those DGs that can obtain power dispatch command from the supervisory control and data acquisition system and is propagated to the rest of the DGs with a consensus algorithm. With this method, all the DGs operate at the same ratio of available power, while their aggregated power meets the power dispatch command. Simulations in the IEEE standard 34-bus distribution network verify the effectiveness of the proposed approach.

Description: A distributed-adaptive droop mechanism is proposed for secondary/primary control of dc microgrids. The conventional secondary control that adjusts the voltage set point for the local droop mechanism is replaced by a voltage regulator. A current regulator is also added to fine-tune the droop coefficient for different loading conditions. The voltage regulator uses an observer that processes neighbors’ data to estimate the average voltage across the microgrid. This estimation is further used to generate a voltage correction term to adjust the local voltage set point. The current regulator compares the local per-unit current of each converter with the neighbors’ on a communication graph and, accordingly, provides an impedance correction term. This term is then used to update the droop coefficient and synchronize per-unit currents or, equivalently, provide proportional load sharing. The proposed controller precisely accounts for the transmission/distribution line impedances. The controller on each converter exchanges data with only its neighbor converters on a sparse communication graph spanned across the microgrid. Global dynamic model of the microgrid is derived with the proposed controller engaged. A low-voltage dc microgrid prototype is used to verify the controller performance, link-failure resiliency, and the plug-and-play capability.

Description: Wind power (WP) penetration in weak distribution networks is associated with adverse impacts on voltage quality. The installation of an energy storage system (ESS) is a possible voltage quality remedy in such milieus. This paper proposes a supercapacitor ESS for alleviation of voltage flicker resulting from WP integration. The proposed ESS control and management are tailored to that purpose such that the ESS offsets the flicker-producing fluctuations in the generated WP. The proposed power sizing of the ESS is defined by the estimated turbulence intensity and wind speed average at the installation site. A 2 MW wind generator of the doubly fed induction generator type is employed as a source of WP and simulations are conducted on a simplified test system, as well as a detailed 25 kV distribution network on which results are compared with acknowledged reactive power flicker mitigation approaches and verified by prototyping in a real-time simulation platform. The flicker measurement procedure is conducted per IEC Standard 61000-4-15.

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Description: This paper explores control strategies for active power distribution nodes (APDNs) using a hierarchical approach. The proposed APDN consists of multiple bidirectional interface modules and an embedded energy storage that is commonly connected to these modules. Such configuration properties make the APDN attractive as a major interface unit for active power flow control in microgrids and power distribution networks. A two-level hierarchy is considered so that the primary level performs load sharing using droop control, and the secondary level performs embedded energy storage management. The stability of the proposed control framework is studied and the proposed design is verified by an experimental setup.

Description: The increasing installation of distributed energy resources during the last years has lead to a fundamental change of the power system structure. As a consequence, utility operators are faced with new challenges in grid planning and operation. New and intelligent approaches—like smart grids—show promising results in increasing the hosting capacity for distributed and renewable resources. Standardized automation, control, and communication systems are important keys to realize such intelligent methods. In this paper, a standard-based control approach for distributed energy resources is introduced and implemented. It uses the IEC 61850 interoperability approach as well as the IEC 61499 reference model for distributed automation. Elementary implementation guidelines are provided to handle the hierarchical architecture of distributed control applications. In order to show the advantages of the proposed approach, a simulation example and a laboratory test are demonstrated using a prototypical open-source-based implementation.

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Description: Computationally efficient and scalable models that describe droop-controlled inverter dynamics are key to modeling, analysis, and control in islanded microgrids. Typical models developed from first principles in this domain describe detailed dynamics of the power electronics inverters, as well as the network interactions. Consequently, these models are very involved; they offer limited analytical insights and are computationally expensive when applied to investigate the dynamics of large microgrids with many inverters. This calls for the development of reduced-order models that capture the relevant dynamics of higher order models with a lower dimensional state space while not compromising modeling fidelity. To this end, this paper proposes model-reduction methods based on singular perturbation and Kron reduction to reduce large-signal dynamic models of inverter-based islanded microgrids in temporal and spatial aspects, respectively. The reduced-order models are tested in a modified IEEE 37-bus system and verified to accurately describe the original dynamics with lower computational burden. In addition, we demonstrate that Kron reduction isolates the mutual inverter interactions and the equivalent loads that the inverters have to support in the microgrid—this aspect is leveraged in the systematic selection of droop coefficients to minimize power losses and voltage deviations.

Description: DC microgrids (DC-MGs) are becoming popular as an effective means to integrate various renewable energy resources. Conventionally, the droop control is adopted as a decentralized control strategy for proper power sharing without using any communication link. However, the conventional droop control often deteriorates due to the effects of unequal line resistances. In this paper, a control strategy is proposed for a DC-MG to achieve perfect power sharing considering the effects of line resistances. The DC-MG under study consists of a photovoltaic system, two energy storage systems, a grid-connected converter system, and dc loads. The control strategy of the converters is addressed under various operation modes. To obtain prolonged and reliable operation of the DC-MG, the state of charge of the batteries is also considered in the power sharing strategy. Simulation and experimental results are provided to verify the effectiveness and validity of the proposed method.

Description: In multibus islanded microgrids, the power quality requirements for different areas and buses can be different. This paper proposes a hierarchical control to realize optimal unbalance compensation for satisfying the power quality requirements in different areas. Primary and secondary controllers are applied to realize unbalance compensation for critical bus, and at the same time, to make distributed generators (DGs) equally share the compensation efforts. Tertiary control, which inherently is an optimization method, is implemented to adjust the compensating effort of each DG considering the voltage unbalance limits in local buses and DG terminals. This method realizes multipower-quality-level control in a multibus islanded system by optimally utilizing DGs as distributed compensators and saves the investment for additional compensation equipment. Hardware-in-the-loop results demonstrate the effectiveness of the method.

Description: This paper presents the modeling approach for a quasi-2-D tubular anode-supported solid-oxide fuel cell (SOFC) for real-time control implementation. The proposed system considers a multidomain electrochemical, fluidic, and thermal dynamic modeling and it is experimentally validated against a SOFC real world implementation. Implicit iterative algebraic equations have been introduced and implemented in C language in order to have fast real-time execution. The methodology for implementing such an iterative solver is discussed in details and the results demonstrate practical feasibilities in advancing real-time control of SOFC

Description: This paper proposes a noninvasive temperature measurement technique of indirect type, applicable in the permanent magnet synchronous motor (PMSM) drives. The motor temperature is required for protection and monitoring purposes, as well as for updating temperature-dependent control parameters. Direct measurement with dedicated sensors requires peripherals and cabling; hence, it is quite involved and often avoided. Temperature estimation based on test signal injection contributes to torque ripple and often relies on other motor parameters. A solution is proposed here, which makes the use of intrinsic pulse-width modulation excitation and does not use electrical or thermal parameters of the considered PMSM. The temperature of the stator winding is estimated from the motor input impedance $bm Z_{bf IN}$ (ω) calculated over the range of frequencies starting at and going well beyond $bm f_{bf PWM}$ . The paper includes analytical considerations, implementation details and experimental verification obtained with a 4.5-kW PMSM used in battery-supplied propulsion systems.

Description: The performance of batteries as uninterruptable power sources in any industry cannot be taken for granted. The failures in battery systems of safety-related electric systems can lead to performance deterioration, costly replacement, and, more importantly, serious hazards. The possible failures in battery systems are currently determined through periodic maintenance activities. However, it is desirable to be able to detect the underlying degradation and to predict the level of unsatisfactory performance by an online real-time monitoring system to prevent unexpected failures through early fault diagnosis. Such an online fault diagnosis method can also contribute to better maintenance and optimal battery replacement programs. A robust nonlinear estimator-based online condition monitoring method is proposed to determine the state of health of the battery systems online in industry. Real-world experimental data of a modern battery system are used to assess the efficiency of the proposed approach in the existence of parameter uncertainties.

Description: Prediction of acoustic noise distribution generated by electric machines has become an integral part of design and control in noise sensitive applications. This paper presents a fast and precise acoustic noise imaging technique for switched reluctance machines (SRMs). This method is based on distribution of radial vibration in the stator frame of the SRM. Radial vibration of the stator frame, at a network of probing points, is computed using input phase current and phase voltage waveforms. Sequentially, the acceleration of the probing network will be expanded to predict full acceleration on the stator frame surface, using which acoustic noise emission caused by the stator can be calculated using the boundary element method.

Description: An analysis for net power characterization of a proton exchange membrane fuel cell (PEMFC) system is developed with Ballard 1.2-kW Nexa power module. A net power control (NPC) strategy based on the linear matrix inequality (LMI) approach is proposed to achieve optimal oxygen excess ratio (OER). The $H_infty$ suboptimal control problem is solved by LMI and the simulations are carried out under different conditional tests. The results verify that the proposed strategy can track the optimal OER trajectory and reduce the parasitic power as the running process of electrical vehicle is simulated. The comparisons testify that the NPC has better robust performance and less settling time in the presence of large disturbances and environmental noises. Therefore, the proposed strategy will provide a novel approach to increase the net power for the PEMFC system.

Description: This paper proposes a robust adaptive voltage control of three-phase voltage source inverter for a distributed generation system in a standalone operation. First, the state-space model of the load-side inverter, which considers the uncertainties of system parameters, is established. The proposed adaptive voltage control technique combines an adaption control term and a state feedback control term. The former compensates for system uncertainties, while the latter forces the error dynamics to converge to zero. In addition, the proposed algorithm is easy to implement, but it is very robust to system uncertainties and sudden load disturbances. In this paper, a stability analysis is also carried out to show the robustness of the closed-loop control system. The proposed control strategy guarantees excellent voltage regulation performance (i.e., fast transient response, zero steady-state error, and low THD) under various types of loads such as balanced load, unbalanced load, and nonlinear load. The simulation and experimental results are presented under the parameter uncertainties and are compared to the performances of the corresponding nonadaptive voltage controller to validate the effectiveness of the proposed control scheme.

Description: A novel parameters estimation strategy of interior permanent magnet synchronous machine (IPMSM) for accurate torque control is proposed and investigated in this paper. The difference of $q$ -axis inductance and $d$ -axis inductance is estimated by a PI regulator using the deviation between actual output power and reference power. The estimated difference of dq inductance can be used to achieve maximum torque per ampere control and accurate toque control. This strategy is most effective when IPMSM works under high-power high-speed working conditions. It provides an effective way for IPMSM accurate torque control. The effectiveness of the proposed method is verified by both simulation and experimental results.

Description: This paper proposes a new modulation strategy that balances the neutral-point voltage for three-level neutral-clamped inverter systems. The proposed modulation replaces the P-type or N-type small switching states with other switching states that do not affect the neutral-point voltage. The zero and medium switching states are employed to help the neutral-point voltage balancing. This method little bit increases the switching events and output total harmonic distortion. However, this method has a strong balancing ability at all regions. Further, it is very simple to implement in both space vector modulation and carrier-based PWM methods. Simulation and experimental results verify the validity and feasibility of the proposed new modulation strategy

Description: A liquid-cooled eddy current retarder (ECR) with a dual salient poles design is proposed in this paper. Finite element models were developed to perform transient magnetic analysis and steady-state conjugate heat transfer analysis. The braking torque, eddy current field and thermal field of the retarder were each predicted. The retarder was subjected to two sets of tests on a test bench, to obtain the braking torque at various rotational speeds, while utilizing two different magnetomotive forces, respectively, and the torque was also tested while maintaining a constant rotational speed, for a specific amount of time. The good agreement shown between the calculations and measured data of the braking torque served to validate the analysis method. The experiments also showed that the braking torque of the retarder decreased more slowly than the braking torque of conventional ECRs in continuous operation.

Description: Permanent magnet synchronous machines (PMSM) have been widely used in a variety of applications. A strong electromagnetic force exists between the rotor magnets and the stator core in a PMSM. The force directly causes mechanical deformation and vibration of the stator. In small PM motors, mechanical vibration comes mainly from the electromagnetic force of the stator. This paper gives the general expression of the frequency and corresponding mode number of radial force harmonics for a PMSM with a three-phase symmetrical double-layer winding. The paper shows that the lowest mode number of radial force harmonics is the greatest common divisor of pole number and slot number. The force harmonics with a low mode number can induce a high mechanical vibration, especially for PMSMs with a fractional slot combination. This is simulated using a weak-coupling electromagnetic–mechanical finite element model. To illustrate the influence of radial force harmonics with a low mode number on vibration, a method to eliminate the lowest mode number force harmonics for the fractional slot combination motor is proposed. A test rig for the 12-slot 8-pole prototype motor is set up. The experimental and simulated results confirm that force harmonics with low mode number have a major impact on vibration.

Description: This paper introduces a new electromechanical energy conversion concept for use in wind-based power generation systems. Modern wind turbines use frequency converters to meet the speed control requirement. But voltage source-based frequency converters typically have limited overload capability. In addition, they fully or partially decouple the rotating masses of wind energy converter systems, thereby removing altogether or reducing the inertia as a medium of energy storage and modulation from the perspective of the grid during transient conditions. In the proposed concept, a drive system based on electromechanical differential gear including a servo motor is used for generator torque control, while using a synchronous machine as the main generator. The system thus provides the speed variability required for optimal utilization of wind energy combined with the advantages of a directly grid-connected synchronous generator. It as a result permits the supply of high currents during faults to the grid to provide more effective voltage support as there are no electronic circuits in the main power path, which would limit the overload capability. Furthermore, the inertia of the system comes fully to bear, thus contributing to the overall grid inertia. The performance of the system has been studied experimentally on a prototype system and the results of the conceptual analysis have been verified. Using the mathematical model of the system, a grid fault simulation has been performed, and the response during fault and the damping behavior has been studied. The results show the excellent damping behavior due to the active damping control by the differential drive.

Description: A hybrid finite-element–boundary element method is adopted for the analysis of linear induction machines (LIM) in Cartesian coordinates. A mathematical boundary is defined arbitrarily in free space to enclose the system. Finite-element formulation is applied for the interior region and boundary integral formulation is applied on the boundary using the response function excited by a transverse line current. The electromagnetic field problem is solved for the magnetic vector potential, from which all performance calculations such as air gap power, propulsion force, and levitation force can be obtained. It provides an accurate analysis of the LIM in unbounded free space which can be used as a fast evaluation method to approach the proper design. The accuracy of the method is verified through experimental results in the existing literature. The comparison with finite element method is also explored.

Description: This paper develops a doubly-fed induction machine model for steady-state harmonic studies. The model is based on the well-known steady-state induction machine model, considering nonsinusoidal voltage sources on the stator and rotor sides of the machine. The results obtained with the proposed model match those obtained from real lab measurements and with a complete induction machine model for transient studies.

Description: Brushless doubly fed machine (BDFM) shows promising results for wind power and adjustable-speed drive applications. Due to poor rotor magnetic coupling and relatively high value of slip, core loss is an important factor which can affect the steady-state and dynamic performance of BDFM. Specifically, core loss plays a major role when evaluating the efficiency of the machine. By now, there is no comprehensive study on core loss effect on performance characteristics of BDFM in the literature. In this paper, based on the derived relations and applying transformation of parameters from one winding to another one, the steady-state equivalent circuit model of BDFM without core loss is modified in order to take the core loss into account. Experimental results from a BDFM prototype are compared with the predicted results given by the model and the accuracy and effectiveness of the new approach is validated.

Description: A novel power curve monitoring method for wind turbines was developed to prevent a turbine failure in a wind farm. Compared with the existing methods, this algorithm automatically calculates the power curve limits for power curve monitoring, even when a considerable number of abnormal data are included in wind speed-output power data measured at a wind turbine. In addition, the proposed algorithm automatically generates an alarm message when the wind speed–power data measured at the wind turbine deviate from the power curve limits, particularly considering their degree of deviation from the power curve limits and the cases when the measured data hover between the Warning Zones and the Alarm Zones. We confirmed its effectiveness through its field tests.

Description: This paper presents a methodology for obtaining the parameters of the equivalent circuit for an induction motor from the information on the nameplate and manufacturer catalog data sheet. The proposed method is straight and noniterative. The variation in rotor parameters for deep bars or double squirrel-cage rotors is modeled and presented as a function of the slip. The results of remarkable points, such as the starting current, starting and breakdown torques, and rated conditions, are compared with the supplied data, and the comparison showed very good agreement. The methodology was applied to hundreds of motors. A statistical analysis is presented showing that the typical parameters of an equivalent circuit can be readily estimated in the absence of further data.

Description: This paper presents the performance of a self-excited induction generator (SEIG) with a voltage and frequency controller (VFC) in a standalone microhydro power generating system. The VFC consists of a distribution static compensator (DSTATCOM) and a direct torque controlled variable frequency induction motor drive (VFIMD) operating as an electronic load controller (ELC). The DSTATCOM is an insulated-gate bipolar transistor-based three-leg voltage source inverter with a self-sustaining dc bus. The main objective of DSTATCOM is to regulate the system voltage through reactive power compensation. In addition to voltage regulation, the DSTATCOM compensates the harmonics generated by nonlinear loads as well as ELC. The ELC controls the system frequency through the active power balance. The VFIMD of the ELC drives a pump and it consumes the power in excess of consumer load power to maintain constant power generation at the SEIG terminals, which in turn regulates the system frequency. A prototype-proposed VFC is developed and tested with a variety of loads. Experimental results demonstrate that the proposed VFC can effectively control the system voltage and frequency.

Description: This paper presents two short-circuit fault models of a hybrid excitation flux switching permanent magnet machine, i.e., excitation phase short-circuit and interturn short-circuit in excitation windings. Two-dimensional finite element (FE) method is used to calculate the major parameters, such as armature and excitation inductances, which are not only functions of rotor position, but also of armature and excitation currents. Moreover, in case of short-circuits, the variations of armature and excitation currents are often significant. This in turn leads to an important change in winding inductances. Therefore, in order to precisely predict machine performance under short-circuit conditions, the use of inductances versus rotor position and RMS currents is essential. With the obtained FE results, two MATLAB/Simulink-based models are established. Then, the previously mentioned short-circuits have been studied and their influences on electromagnetic performances, such as armature and excitation currents, speed and torque, armature and excitation copper losses, stator and rotor core iron losses, as well as permanent magnet (PM) eddy current losses are analyzed. Experiments have been carried out to validate the simulations.

Description: In brushless excitation systems, the rotating diodes can experience open- or short-circuits. For a three-phase synchronous generator under no-load, we present theoretical development of effects of diode failures on machine output voltage. Thereby, we expect the spectral response faced with each fault condition, and we propose an original algorithm for state monitoring of rotating diodes. Moreover, given experimental observations of the spectral behavior of stray flux, we propose an alternative technique. Laboratory tests have proven the effectiveness of the proposed methods for detection of fault diodes, even when the generator has been fully loaded. However, their ability to distinguish between cases of diodes interrupted and short-circuited, has been limited to the no-load condition, and certain loads of specific natures.

Description: This paper presents an analytical time-domain-based parameter identification method for Thévenin-equivalent circuit-based lithium-ion battery models. The method is based on the analysis of voltage-relaxation characteristics of pulse discharge and pulse charge experiments, and the method can be used for both discharge and charge operation with any number of parallel resistor-capacitor branches. The use of the method is demonstrated for a second-order model and validated with a real-world duty cycle. Experimental results for a commercial lithium-ion battery module are presented.

Description: A unified model is established for virtual vector control schemes of current-regulated single-phase inverters. Based on this model, it is proved that the cross-coupling interaction between the real and virtual orthogonal axes may induce dc positive feedback and destabilize the system. Therefore, a stability criterion is identified to reveal the special characteristics of the virtual vector control system and provide guidelines for the controller design. It is further explicated that the stability issue imposes an extra limitation on the controller parameters so that they may not be sufficiently optimized. To solve this problem, a family of improved control methods is proposed by inserting a high-pass filter in the virtual signal generation stage so that the corresponding dc gain is changed to zero to eliminate the dc positive feedback. As a result, both the steady-state and transient responses can be significantly enhanced. A comprehensive design procedure is provided for the proposed method and the results are compared with conventional solutions, both theoretically and experimentally, to verify the advantages of the improved strategy.

Description: A permanent magnet reluctance generator (PMRG), which is a type of switched reluctance machine (SRM) with magnets on stator yoke, has an extremely simple and robust structure and is suitable for variable speed applications such as wind energy conversion systems (WECSs). A three-phase asymmetric half-bridge (AHB) converter with a torque ripple minimization-assisted maximum power point tracking (MPPT) algorithm presents an effective control for a PMRG-based WECSs, which allows suppressing torque ripples arising from the nature of the PMRG while capturing maximum power from the wind. However, a single package of the converter is not available in the markets and hence, its construction by a user suffers from higher cost and lower reliability. In order to overcome the drawbacks of the AHB converter, this research introduces, for the first time, the use of full-bridge (FB) converter to drive the PMRG-based WECS. The three-phase FB converters, which have widespread use in practice, are commercially available in the markets. The FB converter with torque ripple minimization-assisted MPPT algorithm has been successively employed in the PMRG driven WECS. Performance of the control-converter combination has been experimentally tested on the three-phase 6/4 PMRG.

Description: This paper investigates the optimal arrangement of the coils in a wound-type interphase reactor to avoid the decreased inductance due to local saturation in the core when operated with high dc current. In theory, the direct currents and the related magnetomotive forces in the two ways of an interphase reactor are equal, and this compensation leads one to conclude that the global flux in the yokes is zero. However, the dc currents in the coils produce a significant leakage flux that flows locally in the core. If the coils are not suitably arranged and if the dc current is high, the leakage flux causes local saturation, a drop in the inductance, and noticeable circulation of a triple harmonic current through the reactor. Such a phenomenon is investigated by means of both a simplified field model and three-dimensional magnetostatic simulations. A number of criteria for the arrangement of the coils to improve operation are derived and applied to a real rectification system used in an electrolysis plant. Some in situ operational measurements of both the original and the improved interphase reactor are reported.

Description: This paper presents the design of an ${cal L}_1$ adaptive controller for maximum power point tracking (MPPT) of a small variable speed wind energy conversion system (WECS). The proposed controller generates the optimal torque command for the vector-controlled generator-side converter based on the wind speed estimation. The proposed MPPT control algorithm has a generic structure and can be used for different generator types. In order to verify the efficacy of the proposed ${cal L}_1$ adaptive controller for the MPPT of the WECS, a full converter wind turbine with a squirrel cage induction generator is used to carry out case studies using MATLAB/Simulink. The case study results show that the designed ${cal L}_1$ adaptive controller has good tracking performance even with unmodeled dynamics and in the presence of parameter uncertainties and unknown disturbances.

Description: The sideband current harmonic components would inhere in permanent-magnet (PM) synchronous machine systems driven by a voltage-source inverter with space vector pulsewidth modulation (SVPWM). However, these harmonics could potentially deteriorate the overall performance of the drive system by increasing the resultant losses, torque ripple, and electromagnetic and acoustic noises. The main sideband harmonic voltages and currents in PM synchronous machine driven by voltage-source inverter with SVPWM technique, are analytically derived and expressed in both stator and rotor frame. The experimental results are carried out to underpin the validity of the analytical model. The analytical model could be employed to assess the influencing factors of current harmonics. In addition, it offers insightful guidance to the effective reductions of harmonic losses, torque ripples, and electromagnetic noises.

Description: This paper reports a propulsion motor for a solar-powered aircraft. The motor uses precompressed aluminum stator windings, with a fill factor of greater than 75%, in a permanent magnet synchronous machine. The motor performance is compared empirically to an identical machine with conventionally wound copper windings. It is shown that there are many advantages to using compressed aluminum windings in terms of weight reduction, thermal improvement, and lower cost, for the same loss and electromagnetic performance, provided a sufficiently high slot fill factor can be achieved. The design and manufacture of the compressed coils is also discussed. A modular stator arrangement is used, in the form of a solid coreback with keyed teeth to allow easy assembly of the compressed windings. It is noted that the electromagnetic performance of the machine is unaffected by the modular nature of the magnetic core. Two prototype motors, one wound with conventional copper and the other with precompressed aluminum windings, are constructed and tested.

Description: Paralleled operation of voltage-source inverters (VSIs) is currently achieved by using voltage/frequency droop control techniques which requires the knowledge of the system parameters. Otherwise, centralized control techniques with robust communication among VSIs controllers are also used. This paper presents a new control strategy which allows the load sharing between the power sources of an ac microgrid without centralized controller or any communication among the VSIs; only local measurements of voltage and output current are used. The dispatchable sources (e.g., fuel cells) of the microgrid are operated using voltage control with a direct droop scheme, and the nondispatchables or intermittent ones (e.g., wind turbine generators) are operated using power control with a complementary inverse droop scheme (D-Droop + I-Droop). The number of operating sources can be changed online without any modification needed on the VSI controllers. The proposed VSI controllers are based on the variable frequency adaptive linear neuron with frequency-locked loop for the VSIs system synchronization, voltage/power and signal estimation. Experimental results using field-programmable gate array devices for the implementation of each VSI control in the microgrid test bench demonstrated the validity of the proposition.

Description: This paper describes the magnetic design of an innovative fail-safe clutch based on magnetorheological fluid (MRF). A cylindrical arrangement of permanent magnets (PMs) is used to excite the fluid. The suitable distribution of magnetic field inside the MRF and the transmissible torque is obtained by moving the PMs along the axial direction. The device is designed using a magneto/mechanical FEM model, developed on purpose and based on a three-dimensional (3-D) finite-element code, which takes into account the $Bhbox{–}H$ and $tau hbox{–}H$ functions of the nonlinear materials (e.g., MRF, PM, and ferromagnetic materials). The flux density maps and the shear stress maps inside the fluid are carefully analyzed. Furthermore, in order to validate the FEM model, some preliminary experimental measurements are performed on a prototype. Finally, the magnetic axial force acting on the PM system is investigated.

Description: Axial flux-segmented rotor-switched reluctance motor (SSRM) topology could be a potential candidate for in-wheel electric vehicle application. This topology has the advantage of the increased active surface area for the torque production as compared to the radial flux SSRM for a given volume. To improve the performance of axial flux SSRM (AFSSRM), various stator slot/rotor segment combinations and winding polarities are studied. It is observed that the torque ripple is high for the designed three-phase, 12/8 pole AFSSRM. Therefore, the influence of the stator pole and rotor segment arc angles on the average torque and the torque ripple is studied. In addition, the adjacent rotor segments are displaced with respect to the stator, to reduce the torque dips in the phase commutation region. The proposed arrangement is analyzed using the quasi-3-D finite-element method-based simulation study and it is found that the torque ripple can be reduced by 38%. Furthermore, the low-frequency harmonic content in the torque output is analyzed and compared. The variation of the axial electromagnetic attractive force with displaced rotor segments is discussed. The effectiveness of the proposed technique is verified experimentally.

Description: This paper presents feasible slot/pole number combinations of machines with two slot-pitches. They are becoming popular since they not only exhibit the low torque ripple but also the low harmonic content of the magnetomotive force. Some examples of such windings are reported in the literature, but they are limited to a specific slot/pole combination. On the contrary, a general theory is presented in this paper. The main rules for the design of such windings are illustrated and the relationship of slot $(N_{s})$ and pole (2 $p$ ) number, and winding topologies for machines with two slot-pitches is derived. Then, the average torque, torque ripple, eddy current magnet loss, and saliency ratio of machines with slot/pole combinations $N_{s} = 2(2p pm 1)$ and $N_{s} = 2(2p pm 2)$ are investigated by the finite-element method and compared with the conventional one. The $N_{s} = 2(2p pm 1)$ machines have the high average torque and the low-torque ripple compared with $N_{s} = 2(2p pm 2)$ machines. In addition, the eddy current magnet losses for both machines are limited, and the saliency ratio of the interior permanent magnet (IPM) machine is improved compared with the conventional slot/pole number combinations. Finally, the experimental results on surface-mounted permanent magnet and IPM machines are given for validation of theoretical analyses and finite element (FE) results.

Description: The leading operation of large generators is an effective method to adjust the electric reactive power surplus, and the leading-power factor was restricted by the temperature rise of the end parts. In this paper, a three-dimensional (3-D) electromagnetic field of a 330-MW water–hydrogen–hydrogen-cooled turbogenerator in the end region was investigated, and the effect of end leakage magnetic flux on stator step packets was considered in the calculation process. The magnetic flux density distribution was analyzed, and the eddy current losses of metal parts in the end region and core losses of stator step packets were calculated under lagging and leading operations. The influences of the leading-power factor on the losses of end components were analyzed, which could provide a theoretical basis for the safety of the leading operation.

Description: This paper presents a robust controller design method for wind turbine generators using the concepts of fractional calculus. It also compares features of fractional order control systems with those of classic integer order controllers. The proposed method uses isodamping feature, which desensitizes the phase-frequency variations about a gain crossover frequency. This increases the robustness of a fractional order control system against uncertainties. In conventional integer order control systems, realization of isodamping feature requires a controller with very high-order transfer function whereas a fractional order system can readily realize this feature in a compact form. The proposed method is applied to a study system consisting of a permanent magnet wind turbine generator. The test system investigates the tracking performance of the control system considering the backlash and aging phenomenon within the dynamic model of the wind turbine generator. The study results based on a time-domain simulation show the superior capabilities of fractional order controller compared with classic controllers in the presence of model uncertainties. It has been shown that the concept of fractional calculus can be used as a promising robust control approach for the future high performance feedback control systems with application to wind energy systems.

Description: This paper presents optimal third harmonics in both permanent magnet (PM) shaping and current waveform to improve the output torque of five-phase surface-mounted PM (SPM) machines. The optimal value of third harmonic injected into the sinusoidal PM shape and current waveform for maximum torque improvement is analytically derived and validated by both finite element (FE) analyses and experiments. It is found that the optimal third harmonic is 1/6 of the fundamental one for both PM shape and current waveform. For the five-phase SPM machines having rotors without shaping, Sine shaping, or Sine shaping with third harmonic injected, the electromagnetic performance including the back EMF waveform, cogging torque, average torque, torque ripple, copper loss, iron loss, impact on power inverter, and demagnetization withstand capability are compared. It is demonstrated that, although the copper loss and iron loss increase due to additional third harmonic in the winding current and magnet shape, the average torque with optimal third harmonics injected in PM shaping and current waveform can be improved by >30% while the torque ripple and remains similar to that of the one with Sine shaping. In addition, this will reduce the dc bus voltage while maintaining the machine torque density. Furthermore, the machine with Sine and Sine+3rd rotors presents much better demagnetization withstand capability performance than conventional rotor.

Description: A new state-space constant-parameter voltage-behind-reactance synchronous machine model has recently been proposed. Several efficient explicit formulations were derived using continuous- and discrete-time approximations. This letter complements the previous publication and presents a simple and flexible procedure for selecting the parameters of the continuous-time filters. A case study is included to demonstrate the proposed procedure.

Description: This paper describes a study of the interbar current losses in induction motors under no-load condition by comparing the measured and calculated results. The interbar resistances and losses are measured on the machine of the rotor with end rings removed. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The losses are calculated with three-dimensional (3-D) magnetic field analysis on finite-element model reflecting the measured interbar resistance. The calculated losses agree well with the dependence of the measured losses on the interbar resistances. Moreover, it is confirmed that the core heating treatment caused the loss reduction and the increase of the high-frequency impedances form measurement results.

Description: This paper mainly proposes a general model of eccentric squirrel-cage induction motor (SCIM), which includes the stator and rotor slotting effect as well as their individual tooth reluctance effect. Considering open slots on the stator and rotor, their rather exact shape is included in the machine's air gap function. Then, the resulting air gap function is used for evaluating various inductances of SCIM by the winding function approach (WFA). A simple general technique is used for adding eccentricity effect, including static, dynamic, and mixed eccentricities, on the air gap function and, therefore, on the inductances. Using a novel technique, saturable reluctance of each tooth of the stator and rotor is evaluated in every simulation step. Then, these reluctances are added to the model by equivalent increase of the air gap in front of the teeth; this reflects the teeth saturable reluctance effect on the inductances of the SCIM. In effect, dynamic simulation of SCIM with all aforementioned effects using WFA becomes possible. Simulation results compared to corresponding experimental results show accuracy and exactness of the proposed model.

Description: In this paper, a short-circuit current-based adaptive perturb and observe maximum power point tracking algorithm is proposed to extract the maximum power from photovoltaic (PV) panel under sudden changes in the irradiance. This scheme is divided into two algorithms: 1) current perturbation algorithm; and 2) adaptive control algorithm. The current perturbation algorithm makes the PV panel operate at maximum power point. The adaptive control algorithm identifies the operating limit violation and sets a new operating point nearer to maximum power point. These limits are derived in terms of changes in the irradiance and current. The new operating point is set by estimating the short-circuit current. This algorithm proposes variable current perturbation, which varies continuously with the irradiance. A boost converter is used to realize the proposed algorithm. The proposed algorithm is compared with a conventional algorithm and validated for sudden changes in the irradiance through the experimental results.

Description: This paper presents analytical methods to calculate the equivalent circuit parameters for large-scale brushless doubly fed machines (BDFMs) with magnetic wedges utilized for closing stator open slots. The use of magnetic wedges reduces the magnetizing currents in the machine, reflected in the values of magnetizing inductances, but also increases leakage fluxes affecting the value of series inductances in the equivalent circuit. Though such effects can be modeled by numerical models, the proposed analytical methods are particularly helpful in optimizing machine design, inverter rating, reactive power management, and grid low-voltage ride-through performance. The conventional analytical methods cannot be readily applied to the BDFM due to its complex magnetic field distribution; this paper presents analytical methods to calculate the magnetizing and leakage inductances for the BDFM with magnetic wedges used in the stator slots. The proposed methods are assessed by experimentally verified finite-element models for a 250 kW BDFM.

Description: In order to simplify manufacture processes and improve fault-tolerant capabilities, modular electrical machines, especially the ones with segmented stators, are increasingly employed. However, flux gaps between segments are often inevitable. In this paper, to take advantage of these flux gaps to enhance the machine performance, novel modular permanent magnet machines with different slot/pole combinations have been proposed. The influence of these flux gaps on the electromagnetic performance of modular PM machines, such as winding factor, open-circuit air-gap flux densities, back-EMFs, cogging torque, on-load torque, inductances, magnetic saturation and copper losses, are comprehensively investigated and general rules have been established. It is found that for modular machines having slot number higher than pole number, the flux gaps between stator segments degrade the electromagnetic performance due to the lower winding factor and the flux defocusing effect. However, for modular machines having slot number lower than pole number, the electromagnetic performances can be significantly improved using proper flux gap width due to the higher winding factor and the flux focusing effect. The finite element results are validated by experiments using two prototype modular machines.

Description: It is important for fuel cell to become flexible in order to accommodate the situations of application areas for reducing costs and expanding coverage. Scale ups in series and parallel to 1-kW class proton exchange membrane (PEM) fuel cell stack are characterized experimentally with oxidant as air and oxygen. Through a serial scaled-up 4-cell stack with an active area of 25 cm 2 and a parallel scaled-up single cell with an active area of 100 cm 2 , respectively, the 1-kW class 20-cell stack with an active area of 150 cm 2 per cell is scaled up from the basic unit cell with maintaining their performances. The polarization and power curves of the 1-kW class PEM fuel cell stack with the reactants H 2 /air and H 2 /O 2 are evaluated. Sufficient power of the PEM fuel cell can operate at flexible nominal current of 40–80 A and nominal voltage of 12–15 V for providing durability and balance between cells. The 1-kW class PEM fuel cell stack is characterized through the comparison of cell voltage differences utilizing H 2 /air and H 2 /O 2 . Both cell voltages with H 2 /air and H 2 /O 2 follow well the one-twentieth of the responding stack voltages with the small difference.

Description: Cascaded brushless doubly fed induction machines have attracted much attention because of the elimination of slip rings and brushes. However, only limited works have been reported on the control strategies due to the complex machine modeling. Further developments of these kinds of machines are still under investigation, especially for wind energy applications. This paper proposes a new control strategy for cascaded brushless doubly fed induction generators (CBDFIGs) based on direct power control. The effects of voltage vectors on the output active and reactive powers are first investigated. A vector selection strategy is then proposed to achieve flexible power regulation. In addition, the behaviors of the CBDFIG under unbalanced grid voltage conditions are studied and a power compensation scheme is developed to improve the power quality. The effectiveness of the proposed control strategy is validated experimentally on a laboratory prototype.

Description: Based on the theory of linear induction motor (LIM), this paper presents a strategy for driving vehicles climbing on erect steel plates. The novel strategy makes use of the steel plate as partial magnetic circuit of the LIM, which can provide thrust and attractive force simultaneously, and consequently, the vehicle structure is simplified. Experiment result obtained from a prototype shows the validity of the presented strategy.

Description: Provides a listing of current staff, committee members and society officers.

Description: A novel high-efficiency parallel-winding connection for a three-phase induction motor fed by a single-phase power supply is presented in this paper. The proposed connection uses two sets of capacitors to achieve approximately symmetrical winding currents in the motor. A 2-D finite-element magnetodynamic model of the induction motor is presented. The Kriging surrogate model along with a global optimization method was used to find the optimal operating conditions of the proposed motor. Based on the conditions for the best efficiency at full load and the best starting torque of the motor, a straightforward calculation is proposed to select the capacitors. To demonstrate the validity of the proposed connection, experimental tests are performed in three induction motors. It is demonstrated that the rated load efficiency is the same and the starting torque is close when the motor is fed by a single-phase and three-phase power supply. Additionally, the rated power factor of the motor is near to unity. Our proposed motor connection has higher efficiency and higher power factor than a conventional single-phase motor.

Description: Condition monitoring helps reduce the operations and maintenance costs by providing information about the physical condition of wind power systems. This study proposes to use a statistical method for effective condition monitoring. The turbine operation is significantly affected by external weather conditions. We model the wind turbine response as a function of weather variables, using a nonparametric regression method named least squares support vector regression. In practice, online condition monitoring of wind power systems often relies on datasets contaminated with outliers. This study proposes to use a weighted version of least squares support vector regression that provides a formal procedure for removing the outlier effects. We determine the decision boundaries to distinguish faulty conditions from normal conditions by examining the variations in the operational responses that are significantly affected by external weather. The results show that the proposed method effectively detects anomalies.

Description: The dual-primal finite-element tearing and interconnecting (FETI-DP) method is combined with the Newton–Raphson method to expand the capability and improve the efficiency of 3-D finite-element analysis (FEA) of nonlinear electromechanical problems. Despite its modeling capability and high degree of accuracy, FEA has high computational complexity, especially for nonlinear analysis. The FETI-DP method is a robust domain decomposition method, which has been enhanced and applied to solve electromechanical problems involving linear materials. In this paper, the FETI-DP method is extended with the Newton–Raphson method to address problems involving nonlinearity and saturation. Using parallel computing techniques, the total computation time is reduced significantly. Linear and nonlinear regions are separated using the FETI-DP method. This further improves simulation efficiency and flexibility. Cubic splines and relaxation techniques are adopted to ensure stable and fast convergence of the Newton–Raphson method. The performance of the proposed method is compared with infolytica's MagNet, a commercial 3-D FEA solver.

Description: This paper presents an electric circuit-based battery and a capacity fade model suitable for electric vehicles (EVs) in vehicle-to-grid applications. The circuit parameters of the battery model (BM) are extracted using genetic algorithm-based optimization method. A control algorithm has been developed for the battery, which calculates the processed energy, charge or discharge rate, and state of charge limits of the battery in order to satisfy the future requirements of EVs. A complete capacity fade analysis has been carried out to quantify the capacity loss with respect to processed energy and cycling. The BM is tested by simulation and its characteristics such as charge and discharge voltage, available and stored energy, battery power, and its capacity loss are extracted. The propriety of the proposed model is validated by superimposing the results with four typical manufacturers’ data. The battery profiles of different manufacturers’ like EIG, Sony, Panasonic, and Sanyo have been taken and their characteristics are compared with proposed models. The obtained battery characteristics are in close agreement with the measured (manufacturers’ catalogue) characteristics.

Description: During nonsevere fault conditions, the crowbar protection is not activated and the rotor windings of a doubly fed induction generator (DFIG) are still excited by the ac/dc/ac converter. In these cases, the dynamic response of a rotor-side converter (RSC) has a large influence on the fault current characteristics of the DFIG. In this paper, an analysis method for the fault current characteristics of the DFIG under nonsevere fault conditions is proposed. First, the dynamic response of the RSC is analyzed on condition that the external power control loop is shut down and the reference signals of the inner rotor current control loop are kept constant. Then, the simplified calculation models of the rotor fault current are established according to different design principles of the inner rotor current controller. Based on it, the fault characteristics of the stator current are studied and the analytical expressions of the stator fault current are obtained. Finally, digital simulation results validate the analytical results. The proposed analysis method is applicable for the study of fault current characteristics of DFIG with different control strategies for low-voltage ride through. The research results are helpful to the construction of adequate relaying protection for the power grid with penetration of DFIGs.

Description: In this paper, a novel configuration of a three-level neutral-point-clamped (NPC) inverter that can integrate solar photovoltaic (PV) with battery storage in a grid-connected system is proposed. The strength of the proposed topology lies in a novel, extended unbalance three-level vector modulation technique that can generate the correct ac voltage under unbalanced dc voltage conditions. This paper presents the design philosophy of the proposed configuration and the theoretical framework of the proposed modulation technique. A new control algorithm for the proposed system is also presented in order to control the power delivery between the solar PV, battery, and grid, which simultaneously provides maximum power point tracking (MPPT) operation for the solar PV. The effectiveness of the proposed methodology is investigated by the simulation of several scenarios, including battery charging and discharging with different levels of solar irradiation. The proposed methodology and topology is further validated using an experimental setup in the laboratory.

Description: A new method for determination of the steady-state equivalent circuit parameters of wound-rotor induction motors using experimental data from starting transient measurements is presented. The algorithm data are the stator currents and voltages and mechanical speed. The algorithm uses the least-square method and motor dynamic equations in the synchronous reference frame. Moreover, an approximation of the rotor flux that improves the accuracy of the estimation method, as well as a detailed study of errors, is included. The estimation method is applied to starting transient measurement data of a 2 kW wound rotor induction motor, and the accuracy of the obtained parameters is verified by comparing steady-state torque- and current-slip curves calculated with the estimated parameters and those measured in the laboratory.

Description: In this paper, a new method for modeling converter-based power generators in ac-distributed systems is proposed. It is based on the concept of electrostatic synchronous machines. With this new concept, it is possible to establish a simple relationship between the dc and ac side and to study stability in both the small and large signals of the microgrid by considering a dc-link dynamic and high variation in the power supplied. Also, for the purpose of illustration, a mathematical and electrical simulation is presented, based on MATLAB and PSCAD software. Finally, an experimental test is performed in order to validate the new model.

Description: Wind turbines are required to meet grid interconnection codes for voltage ride-through, which mandate that they should not disconnect during a specified range of grid disturbances. One of the tasks of a turbine’s design verification process is to ensure that it will be able to withstand any such event. This paper sets forth a powerful computational technique for design verification of Type-4 permanent-magnet synchronous generator-based turbines with fully-rated grid-side converters. The proposed method can provide assurance as to whether the currents and voltages will remain within acceptable bounds. To this end, reachability theory is employed to find sets that enclose all possible trajectories for the system states caused by a given set of unknown-but-bounded disturbances. The computation of the reachable sets is carried out by using on-the-fly linearization of the nonlinear system dynamics, zonotopes, and interval analysis.

Description: This paper presents single-phase power generation using a three-phase self-excited induction generator (SEIG) working in conjunction with a three-phase static synchronous compensator (STATCOM). The STATCOM is employed to compensate the unbalanced currents caused by single-phase loads that are connected across the two terminals of the three-phase SEIG. Therefore, the SEIG is capable of feeding single-phase loads up to its rated power. Moreover, the STATCOM regulates the SEIG terminal voltage through reactive power compensation and also suppresses the harmonics injected by consumer loads. A single-phase synchronous D-Q frame theory-based control algorithm is used to generate gating pulses to the three-phase STATCOM. The proposed method of single-phase power generation from the three-phase SEIG is investigated experimentally on a 3.7-kW, 230-V, Y-connected induction machine. The performance of the SEIG–STATCOM system is evaluated for both linear and nonlinear single-phase loads. Furthermore, the performance of the SEIG at different terminal voltages is investigated and the terminal voltage corresponding to the maximum power output is identified.

Description: In this paper, a novel magnetic configuration inter phase zero sequence blocking transformer (IP-ZSBT) is presented. The IP-ZSBT is suitable for the parallelization of two three-phase converters, avoiding the circulation of any zero sequence current. The configuration assures an equated current sharing between the converters, providing the functionality of an Inter-Phase Transformer. In addition, it imposes a high impedance for zero sequence currents, providing also the functionality of a zero sequence blocking transformer. A simulation of the proposed configuration has been carried out by using the Saber simulation tool and it has been validated by experimental results.

Description: In single-speed three-phase squirrel-cage induction motors with rated power above 3 kW, although the use of electronic soft-starters is increasing fast, the conventional star-delta starters are still the most used, requiring three contactors. In this paper, motor soft starting and/or magnetizing flux adjustment solutions based on two-flux-level, delta- and star-connected, tapped windings are proposed, providing an alternative to the conventional star/delta windings. The motor flux adjustment in the proposed tapped winding is based on the change of the winding factor and/or per-phase number of turns. The connection-mode change system for the proposed tapped windings requires only two contactors, and no extra cost is expected for the motor manufacturing or rewinding. For the proposed tapped winding arrangements, a theoretical analysis is made, and simulated and experimental results are presented. The presented solutions can be used to improve the part-load efficiency and power factor of fixed-speed variable-load motors, particularly if they operate long periods with load lower than 35%–45% and short periods with load close to 100%, and an automatic load-based winding connection-mode management system is used.

Description: Module parallel connection for three-phase VSC can increase the system level effectively. However, the circulating currents problem will occur. The circulating currents will distort the three-phase currents, increase the power loss and decrease the system efficiency. A novel deadbeat circuiting currents control method is proposed in this paper. Based on the analysis of the average model of parallel system, a circuiting currents deadbeat controller is designed while presenting the design method. The control strategy is realized by adjusting the voltage zero vector of space-vector pulse-width modulation in each paralleled module. No additional hardware is needed through the method. Fast dynamic response can be achieved and the performance of circulating currents is better compared with conventional PI controller. This method can be applied to common dc-link double parallel three-phase voltage converters with communication line. The validity of proposed theory was verified by simulation and experimental results. It is shown that the parallel converters can operate with different line inductor or different line currents by using this novel control strategy.

Description: Present energy need heavily relies on the conventional sources. But the limited availability and steady increase in the price of conventional sources has shifted the focus toward renewable sources of energy. Of the available alternative sources of energy, wind energy is considered to be one of the proven technologies. With a competitive cost for electricity generation, wind energy conversion system (WECS) is nowadays deployed for meeting both grid-connected and stand-alone load demands. However, wind flow by nature is intermittent. In order to ensure continuous supply of power suitable storage technology is used as backup. In this paper, the sustainability of a 4-kW hybrid of wind and battery system is investigated for meeting the requirements of a 3-kW stand-alone dc load representing a base telecom station. A charge controller for battery bank based on turbine maximum power point tracking and battery state of charge is developed to ensure controlled charging and discharging of battery. The mechanical safety of the WECS is assured by means of pitch control technique. Both the control schemes are integrated and the efficacy is validated by testing it with various load and wind profiles in MATLAB/SIMULNIK.

Description: This paper reports the development of a maximum power-point tracking (MPPT) method for photovoltaic (PV) systems under partially shaded conditions using firefly algorithm. The major advantages of the proposed method are simple computational steps, faster convergence, and its implementation on a low-cost microcontroller. The proposed scheme is studied for two different configurations of PV arrays under partial shaded conditions and its tracking performance is compared with traditional perturb and observe (P&O) method and particle swarm optimization (PSO) method under identical conditions. The improved performance of the algorithm in terms of tracking efficiency and tracking speed is validated through simulation and experimental studies.

Description: As penetration of distributed generation increases, the electrical distribution networks may encounter several challenges mainly related to voltage control. The situation may deteriorate in case of a weak grid connected to an intermittent source such as photovoltaic (PV) generation. Fast varying solar irradiance can cause unacceptable voltage variations that may not be easily compensated by slow-responding utility equipment. The PV inverter can be used to control the grid voltage by injecting/absorbing reactive power. A small-signal model is derived in order to study the stability of a PV inverter exchanging reactive power with the grid. This paper also proposes a method that utilizes the available capacity of the PV inverter to support the grid voltage without violating the rating of the inverter and the maximum voltage that the inverter's switching device can withstand. The method proposed in this paper is validated using PSCAD/EMTDC simulations.

Description: Analytical description of a constant power fed supercapacitor behavior is revealed in the paper. The derivation is based on a simple RC model with parameters taken from a manufacturer datasheet. Different power and energy-related figures of merit are obtained using the derived expressions. It is shown that some of the performance figures used in datasheets are strictly theoretical and cannot be achieved in practice. The process of Ragone plot construction based on the proposed method is described as well. Moreover, it is shown that the upper limit of a supercapacitor voltage imposes certain limits on power and energy capabilities of the device. Extended simulation and experimental results are provided in order to reinforce the proposed method and justify the selected model for describing supercapacitor performance. Appropriate comparison of simulations and experiments shows that the simple first-order model may be utilized to predict supercapacitor behavior with reasonable accuracy to perform an initial design.

Description: In this paper, an inner loop for the control and frequency regulation of the doubly fed induction generator connected to a dc link through a diode bridge on the stator is presented. In this system, the stator is directly connected to the dc link using a diode bridge, and the rotor is fed by only a pulsewidth-modulated (PWM) converter. If compared to the DFIG connected to an ac grid, this system uses one PWM inverter less and a much less expensive diode bridge. Thus, the cost of power electronics is reduced. The application in mind is for dc networks such as dispersed generation grids and microgrids. These networks use several elements that should work together. Usually, these elements are connected with each other by power electronic devices in a common dc link. This paper presents a control system for the inner control loop in order to regulate the torque and the stator frequency. Simulation and experimental results show that the system works properly and is able to keep the stator frequency near the rated value of the machine.

Description: The variability of solar irradiance with a high ramp-rate, caused by cloud passing, can create fluctuation in the PV output. In a weak distribution grid with a high PV penetration, this can create significant voltage fluctuations. Energy storage devices are used to smooth out the fluctuation using traditional moving average control. However, moving average does not control the ramp-rate directly; rather the ramp-rate depends on previous values of PV output. This paper proposes a strategy where the ramp-rate of PV panel output is used to control the PV inverter ramp-rate to a desired level by deploying energy storage (which can be available for other purposes, such as storing surplus power, countering voltage rise, etc.). During the ramping event, the desired ramp-rate is governed by controlling the energy storage based on an inverse relationship with the PV panel output ramp-rate to improve the fluctuation mitigation performance. In contrast to the moving average method, the proposed strategy is able to control the desired ramp-rate independent of the past history of the PV panel output. A dynamic model of the PV-storage integrated system is developed to verify the proposed strategy in the presence of physical device time lags. The proposed strategy is verified using simulation results based on an Australian distribution system. A laboratory experiment is also conducted to validate the concept of the proposed control strategy.

Description: The third harmonic current produced by salient pole synchronous generator has always been associated with high neutral current causing problem to neutral earthing resistor, telecommunication system, and earthfault protection. In this paper, the third harmonic model for salient pole synchronous generator in symmetrical component domain is proposed that comprise of generator parameter and generated voltage. The new computational technique in determining the third harmonic zero-sequence synchronous impedance is introduced. In completing the model, the method to characterize generated third harmonic zero-sequence voltage under balanced combine resistive and inductive load is described. In order to study the third harmonic current flowing in the network, the propagation model is also established. The third harmonic synchronous generator parameters are measured and comparatively validated with laboratory experiment results. In addition to that, the third harmonic generated voltage is characterized and validated experimentally.

Description: Fault ride-through (FRT) is a common requirement to abide by grid code all over the world. In this paper, to enhance the FRT capability of fixed speed wind generator system, a modified configuration of bridge-type fault current limiter (BFCL) is proposed. To check the effectiveness of the proposed BFCL, its performance is compared with that of the series dynamic braking resistor (SDBR). A harmonic performance improvement by the proposed method is also analyzed. A three-phase-to-ground (3LG) fault was applied to one of the double circuit transmission lines connected to the wind generator system. Simulation was carried out by using MATLAB/Simulink software. Simulation results show that the proposed BFCL is a very effective device to achieve the FRT and suppress fault current that eliminates the need for circuit breaker replacement. Also, the BFCL improves the harmonic performance and helps follow harmonic grid code. Moreover, it was found that the BFCL works better than the SDBR and has some distinct advantages over the SDBR.

Description: An interior permanent-magnet (PM) motor is modeled by a combined analytical–numerical approach, in which the relationships between the stator currents and flux linkages are identified with static finite-element (FE) analysis. In addition to the previous approaches using the current space vector as the state variable, new models are also developed using the flux-linkage space vector, which leads to more convenient time-integration of the voltage equations. In order to account for the zero-sequence effects in delta connection, the models also include either the zero-sequence flux or current as an additional state variable. Finally, the possibilities of deriving the required quantities as partial derivatives of the magnetic field energy are discussed. The energy-based approaches avoid inaccuracies related to torque computation and thus allow better satisfying the power balance in the state-space model. We show the ability of the developed state-space models to predict the currents and torque equally to a nonlinear time-stepping FE model with much less computational burden. The results are validated by means of measurements for a prototype machine in both star and delta connections. In addition, we also demonstrate the effect of the zero-sequence current on the torque ripple in case of a delta-connected stator winding.

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Description: Predictive control represents an optimization-oriented alternative for the control of power converters and drives. Predictive torque control of induction machines has been shown to achieve excellent initial results. The objective of this paper is to help develop this promising control approach by introducing new elements to improve its performance. The resulting algorithm improves the efficiency of the converter from $hbox{91.1}%$ to $hbox{92.6}%$ and achieves a common-mode voltage mitigation of $hbox{50}%$ , compared to the basic control method. A tradeoff is observed in the power quality. The algorithm gives the designer the ability to select the best operating point for each particular application and to optimize the converter’s performance. Experimental results are presented to validate the proposals.