Navid R. Zargari

Power Conversion and Control of Wind Energy Systems: Wu/Power

Preface. List of Symbols. Acronyms and Abbreviations. 1. Introduction. 1.1 Introduction. 1.2 Overview of Wind Energy Conversion Systems. 1.3 Wind Turbine Technology. 1.4 Wind Energy Conversion System Configurations. 1.5 Grid Code. 1.6 Summary. 2. Fundamentals of Wind Energy Conversion System Control. 2.1 Introduction. 2.2 Wind Turbine Components. 2.3 Wind Turbine Aerodynamics. 2.4 Maximum Power Point Tracking (MPPT) Control. 2.5 Summary. 3. Wind Generators and Modeling. 3.1 Introduction. 3.2 Reference Frame Transformation. 3.3 Induction Generator Models. 3.4 Synchronous Generators. 3.5 Summary. 4. Power Converters in Wind Energy Conversion Systems. 4.1 Introduction. 4.2 AC Voltage Controllers (Soft Starters). 4.3 Interleaved Boost Converters. 4.4 Two-Level Voltage Source Converters. 4.5 Three-Level Neutral Point Clamped Converters. 4.6 PWM Current Source Converters. 4.7 Control of Grid-Connected Inverter. 4.8 Summary. 5. Wind Energy System Configurations. 5.1 Introduction. 5.2 Fixed Speed WECS. 5.3 Variable Speed Induction Generator WECS. 5.4 Variable-speed Synchronous Generator WECS. 5.5 Summary. 6. Fixed-Speed Induction Generator WECS. 6.1 Introduction. 6.2 Configuration of Fixed-Speed Wind Energy Systems. 6.3 Operation Principle. 6.4 Grid Connection with Soft Starter. 6.5 Reactive Power Compensation. 6.6 Summary. 7. Variable-Speed Wind Energy Systems with Squirrel Cage Induction Generators. 7.1 Introduction. 7.2 Direct Field Oriented Control. 7.3 Indirect Field Oriented Control. 7.4 Direct Torque Control. 7.5 Control of Current Source Converter Interfaced WECS. 7.6 Summary. 8. Doubly-Fed Induction Generator Based WECS. 8.1 Introduction. 8.2 Super- and Sub-synchronous Operation of DFIG. 8.3 Unity Power Factor Operation of DFIG. 8.4 Leading and Lagging Power Factor Operation. 8.5 A Steady-State Performance of DFIG WECS. 8.6 DFIG WECS Start-up and Experiments. 8.7 Summary. 9. Variable-Speed Wind Energy Systems with Synchronous Generators. 9.1 Introduction. 9.2 System Configuration. 9.3 Control of Synchronous Generators. 9.4 SG Wind Energy System with Back-to-back VSC. 9.5 DC/DC Boost Converter Interfaced SG Wind Energy Systems. 9.6 Reactive Power Control of SG WECS. 9.7 Current Source Converter Based SG Wind Energy Systems. 9.8 Summary. Appendix A. Per Unit System. Appendix B. Generator Parameters. Appendix C. Problems and Answers Manual.

Performance investigation of a current-controlled voltage-regulated PWM rectifier in rotating and stationary frames

Active front-end rectifiers with reduced input current harmonics and high input power factor will be required in the near future for utility interfaced applications. In order to meet the new and more stringent regulations with force-commutated switches, the voltage source inverter approach is superior to the conventional current source approach, in terms of number of components and control options. However, the straightforward power angle control of the rectifier is characterized by a slow response and potential stability problems. This paper proposes a current-controlled PWM rectifier as an alternative. It provides near sinusoidal input currents with unity power factor and a low output voltage ripple. Moreover, it produces a well-defined input current harmonic spectrum, exhibits fast transient response to load voltage variations, and is capable of regenerative operation. PWM pattern generation is based on a carrier technique and the current controller is implemented in the: (a) stationary (abc) frame; and (b) rotating (dqo) frame. The design and the performance of the two controller options are investigated and compared. >

Space Vector Sequence Investigation and Synchronization Methods for Active Front-End Rectifiers in High-Power Current-Source Drives

Space vector pulsewidth modulation (PWM) schemes for the active front end of a high-power drive normally produce low-order and suborder harmonics due to the low switching frequency and the drifting of synchronization between the PWM waveform and the rectifier input frequency. To provide a synchronized PWM and achieve the best harmonic performance, different space vector sequences suitable for a current-source converter are investigated in this paper. Details on how to achieve the waveform symmetries with a minimum switching frequency for each sequence are discussed. A thorough comparison of the harmonic performance of different space vector sequences is carried out. An optimum space vector modulation method by switching between two best sequences is proposed to achieve the best line-current total harmonic distortion with reduced switching losses. In addition, two synchronization methods, namely a PWM frame regulation method and a direct digital phase-locked loop synchronization method, are proposed. Both methods are equally effective in providing tight synchronization of the PWM waveform with the rectifier input frequency. The work has been verified in simulation and experiment.

Unified DC-Link Current Control for Low-Voltage Ride-Through in Current-Source-Converter-Based Wind Energy Conversion Systems

The increased penetration of wind power into utility grid brings challenges to power converter design in wind energy conversion systems (WECSs). Among all, low-voltage ride-through has been enforced in the field, which is one of the major challenges for WECS. It is necessary to design an integrated controller to protect the converter from overvoltage/overcurrent and to support the grid voltage during faults and recoveries. In this paper, a unified dc-link current control scheme for current-source-converter-based WECS is proposed. The controllers for generator- and grid-side converters are coordinated to provide fault ride-through capability. In normal operations, the proposed control scheme can also smooth the real power flow while keeping the fast dynamic performance of the dc-link current control. Simulation and experimental results are provided to verify the proposed control scheme.

A novel nine-switch PWM rectifier-inverter topology for three-phase UPS applications

A novel three-phase PWM rectifier-inverter topology for UPS applications is proposed in this paper. The topology uses only nine IGBT devices for AC/AC conversion through a quasi DC link circuit. This converter topology features sinusoidal inputs and outputs, unity input power factor, and more importantly, low manufacturing cost. The operating principle of the converter is elaborated and a novel modulation scheme is presented. The performance of the proposed converter is verified by experiments on a 5 kVA prototyping UPS system.

DC-Link Current Minimization for High-Power Current-Source Motor Drives

In this paper, a loss reduction and DC link current minimization strategy for a high power current source inverter (CSI) fed drive is proposed. The proposed strategy consists of an inverter maximum modulation index control scheme and a flux optimization algorithm. Specifically, in the inverter modulation index control, the CSI modulation index is kept at the maximum value while the current source rectifier (CSR) is used to regulate a reduced variable DC link current and therefore to control the motor current magnitude. This control scheme can effectively reduce the dc link current and at the same time improve the line side and motor side harmonics. On the other hand, for the optimized flux control, the relationship between the rotor flux and the dc link current is first thoroughly investigated. Based on this analysis, the DC link current from the maximum inverter modulation index control can be further minimized by optimizing the rotor flux according to system variables such as the motor speed, the applied torque and the motor side capacitor size. With the proposed dc current minimization strategy, the drive current rating, semiconductor device losses and the drivepsilas DC link losses can be reduced. Both simulation and experimental results on a 4.16 kV 600 hp CSI fed drive system are obtained to verify the effectiveness of the proposed strategy.

A New Nested Neutral Point-Clamped (NNPC) Converter for Medium-Voltage (MV) Power Conversion

In this paper, a new voltage source converter for medium voltage applications is presented which can operate over a wide range of voltages (2.4-7.2 kV) without the need for connecting the power semiconductor in series. The operation of the proposed converter is studied and analyzed. In order to control the proposed converter, a space-vector modulation (SVM) strategy with redundant switching states has been proposed. SVM usually has redundant switching state anyways. What is the main point we are trying to get to? These redundant switching states help to control the output voltage and balance voltages of the flying capacitors in the proposed converter. The performance of the converter under different operating conditions is investigated in MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally on a 5-kVA prototype.

Selective Harmonic Compensation (SHC) PWM for Grid-Interfacing High-Power Converters

Compensating the grid background harmonics in a grid-interfacing converter system, such as a drive systems active-front-end rectifier or a grid-connected inverter in a distributed generation system, is an effective method of reducing line side current harmonics. However, this harmonic compensation is particularly challenging in medium-voltage high-power applications (>1 MVA). This is mainly due to the low-switching frequency operation of high-power converters (300-800 Hz) to maintain low power loss. Therefore, the traditional tasks of active power filters with relatively high-switching frequency cannot be easily realized here. This paper proposes a new pulse width modulation technique, named selective harmonic compensation (SHC), which actively compensates the power system background harmonics, but still operates at very low-switching frequencies. Details of the proposed SHC are presented. An SHC application example on a high-power current-source rectifier is provided in this paper. The simulations and experiments show that the proposed SHC scheme can effectively compensate the system background harmonics and improve the line current harmonic performance.

An Input Power Factor Control Strategy for High-Power Current-Source Induction Motor Drive With Active Front-End

This paper proposes an input power factor control strategy for a current-source drive with active front-end. The proposed strategy is realized without modification of the drives pulsewidth modulation and speed control schemes through the collaborative use of two methods: a modulation index regulation method and a flux adjustment method. Specifically, the modulation index regulation method functions to directly control the dc-link current and voltage, and it can effectively correct a leading input power factor as long as the space vector modulation is used for the inverter. On the other hand, the flux adjustment method can improve the power factor when the power factor is lagging or when the selective harmonic elimination modulation is used for the inverter at high motor speeds. When implemented together, the two methods will complement each others functionalities and improve the overall compensation performance. Experimental results are obtained from a 600-hp and an 1100-hp drive system.

Common-Mode Voltage Reduction Methods for Current-Source Converters in Medium-Voltage Drives

Common-mode voltages (CMVs) can lead to premature failure of the motor insulation system in medium-voltage current-source-fed drives. By analyzing the CMV values at all switching states under different operating conditions of a current-source-inverter (CSI)-based motor drive, this paper first indicates that the CMV peaks are produced by the zero states in most of the cases. The nonzero-state (NZS) modulation techniques employed in voltage-source converters are adapted for use in a space-vector-modulated current-source converter (CSC) to reduce the CMV magnitude. For NZS modulation in CSCs, the nearest three-state (NTS) modulation sequences are designed with good low-order harmonic performances in their linear modulation region of ma ≥ 0.67 and with no increase in the device switching frequency. A combined active-zero-state (AZS) modulation technique is also proposed as compensation, for a lower modulation index in the range of 0.4-0.67, when a compromise is made between the dc-link current minimization and high input power factor control. The simulation and experimental results are provided to validate the CMV reduction effects and harmonic performances of the NTS and combined AZS modulation methods in CSI-fed drives.