Showing papers in "IEEE Transactions on Energy Conversion in 2014"

Distributed Adaptive Droop Control for DC Distribution Systems

Abstract: 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.

Hierarchical Control for Multiple DC-Microgrids Clusters

Abstract: 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.

MPPT of PV Systems Under Partial Shaded Conditions Through a Colony of Flashing Fireflies

Abstract: 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.

Self-Tuning Virtual Synchronous Machine: A Control Strategy for Energy Storage Systems to Support Dynamic Frequency Control

Abstract: 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.

A Novel Approach for Ramp-Rate Control of Solar PV Using Energy Storage to Mitigate Output Fluctuations Caused by Cloud Passing

Abstract: 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.

Optimal Distributed Control of Reactive Power Via the Alternating Direction Method of Multipliers

Abstract: 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.

A Novel Adaptive P&O MPPT Algorithm Considering Sudden Changes in the Irradiance

Abstract: 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.

Autonomous Active Power Control for Islanded AC Microgrids With Photovoltaic Generation and Energy Storage System

Abstract: 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.

A Decentralized Control Method for a Low-Voltage DC Microgrid

Abstract: 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.

Model Predictive Control of PV Sources in a Smart DC Distribution System: Maximum Power Point Tracking and Droop Control

Abstract: 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.

Time-Domain Parameter Extraction Method for Thévenin-Equivalent Circuit Battery Models

Abstract: This paper presents an analytical time-domain-based parameter identification method for Thevenin-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.

Influence of Radial Force Harmonics With Low Mode Number on Electromagnetic Vibration of PMSM

Abstract: 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.

Deadbeat Control Strategy of Circulating Currents in Parallel Connection System of Three-Phase PWM Converter

Abstract: 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.

New Modulation Strategy to Balance the Neutral-Point Voltage for Three-Level Neutral-Clamped Inverter Systems

Abstract: 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.

Spatiotemporal Model Reduction of Inverter-Based Islanded Microgrids

Abstract: 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

Solar PV and Battery Storage Integration using a New Configuration of a Three-Level NPC Inverter With Advanced Control Strategy

Abstract: 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.

Decentralized Optimal Dispatch of Photovoltaic Inverters in Residential Distribution Systems

Abstract: 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.

Control of a Flywheel Energy Storage System for Power Smoothing in Wind Power Plants

Abstract: This paper deals with the design and the experimental validation in scale-lab test benches of an energy management algorithm based on feedback control techniques for a flywheel energy storage device. The aim of the flywheel is to smooth the net power injected to the grid by a wind turbine or by a wind power plant. In particular, the objective is to compensate the power disturbances produced by the cycling torque disturbances of the wind turbines due to the airflow deviation through its tower section. This paper describes the control design, its tuning, as well as the description of the experimental setup, and the methods for the experimental validation of the proposed concepts. Results show that the fast wind power fluctuations can be mostly compensated through the flywheel support.

Design of a Low-Harmonic-Content Wound Rotor for the Brushless Doubly Fed Generator

Abstract: The rotor structure is one of the key factors affecting the performance of the brushless doubly fed machine (BDFM). This paper presents a “double-sine” wound rotor for the brushless doubly fed generator to reduce the harmonic contents of the rotor winding. Ideally, so-called double sine is to make the magnetomotive force (MMF) space vector in one set of three-phase symmetric windings only containing two sinusoidal-waveform magnetic fields of different pole pairs. According to the principles of tooth harmonic and sinusoidal winding, the double-sine winding is designed to be constituted by double-layer unequal-turn coils. The design procedure and rules are given in detailed with the design example. The steady-state performance of a 60 kW prototype generator with double-sine wound rotor is analyzed by the simulation and experimental test, and the validity of the design method for proposed rotor structure is verified.

Analytical Modeling of Current Harmonic Components in PMSM Drive With Voltage-Source Inverter by SVPWM Technique

Abstract: 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.

Distributed Adaptive Voltage Control of Inverter-Based Microgrids

Abstract: 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.

Mathematical Modeling of Li-Ion Battery Using Genetic Algorithm Approach for V2G Applications

Abstract: 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.

Enhanced Position Observer Using Second-Order Generalized Integrator for Sensorless Interior Permanent Magnet Synchronous Motor Drives

Abstract: For the purpose of improving the performance of sensorless interior permanent magnet synchronous motor (IPMSM) drives, an enhanced electromotive-force (EMF)-based position observer adopting second-order generalized integrator (SOGI) to eliminate the harmonic error is proposed. The inverter nonlinearities and flux spatial harmonics that give rise to the harmonic error of position estimation are analyzed. A harmonic decoupling network consisting of multiple SOGIs based adaptive filters is adopted to achieve the multiple selective EMF harmonic elimination (MSEHE). The application of the frequency-locked loop ensures the SOGI resonance frequency adaptive, which is critical for the variable-speed operation in IPMSM drive applications. Using the SOGI-MSEHE in the position tracking estimator, the low-order harmonics in the estimated EMF can be effectively eliminated, contributing to cancelling the position estimation harmonic error. The effectiveness of the proposed enhanced position observer is verified by the experimental results at a 2.2-kW IPMSM sensorless drive.

Nonlinear Modeling of Eddy-Current Couplers

Abstract: Analytical models play an important role in the design of electromagnetic devices by providing computationally efficient solutions. In this paper, by combining magnetic equivalent circuit approaches and Faraday's and Ampere's laws, a model for radial-flux eddy-current couplers is developed, which can easily handle complex geometries as well as account for iron saturation, all material properties, and three-dimensional (3-D) parameters. The characteristics and the design considerations of a surface-mounted permanent-magnet structure are presented. Also, a procedure aimed at an optimal design of the yoke thicknesses is utilized. Moreover, 2-D and 3-D finite-element methods are employed in the analyses and evaluation of the model. Finally, sensitivity analysis is performed to explore the impacts of the machine parameters on the device performance.

Tertiary Control of Voltage Unbalance Compensation for Optimal Power Quality in Islanded Microgrids

Abstract: 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.

Robust Nonlinear Distributed Controller Design for Active and Reactive Power Sharing in Islanded Microgrids

Abstract: 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.

Application of Fractional Calculus Theory to Robust Controller Design for Wind Turbine Generators

Abstract: 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.

Study of Fault Current Characteristics of the DFIG Considering Dynamic Response of the RSC

Abstract: 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

Torque Improvement of Five-Phase Surface-Mounted Permanent Magnet Machine Using Third-Order Harmonic

Abstract: 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.

Predictive Control of an Induction Machine Fed by a Matrix Converter With Increased Efficiency and Reduced Common-Mode Voltage

Abstract: 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.