Introduction to Electromagnetic Compatibility

Doubly fed induction generators (DFIGs), often organized in wind parks, are the most important generators used for variable-speed wind energy generation. This paper reviews the control systems for the operation of DFIGs and brushless DFIGs in wind energy applications. Control systems for stand-alone operation, connection to balanced or unbalanced grids, sensorless control, and frequency support from DFIGs and low-voltage ride-through issues are discussed.

Overview of Control Systems for the Operation of DFIGs in Wind Energy Applications

The parallel operation of inverters in microgrids is mainly based on the droop method. The conventional voltage droop method consists of adjusting the output voltage frequency and amplitude to achieve autonomous power sharing without control wire interconnections. Nevertheless, the conventional voltage droop method shows several drawbacks, such as complicated inner multiloop feedback control, and most importantly, frequency and voltage deviations. This paper proposes a new control strategy in microgrid applications by drooping the virtual flux instead of the inverter output voltage. First, the relationship between the inverter virtual flux and the active and reactive powers is mathematically obtained. This is used to develop a new flux droop method. In addition, a small-signal model is developed in order to design the main control parameters and study the system dynamics and stability. Furthermore, a direct flux control algorithm is employed to regulate the virtual flux according to the droop controller, which avoids the use of proportional-integral controllers and pulse-width modulation modulators. Both the simulation and experimental results show that the proposed flux droop strategy can achieve active and reactive power sharing with much lower frequency deviation than the conventional voltage droop method, thus highlighting the potential use in microgrid applications.

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Virtual Flux Droop Method—A New Control Strategy of Inverters in Microgrids

This paper presents a model predictive direct power control strategy for a grid-connected inverter used in a photovoltaic system as found in many distributed generating installations. The controller uses a system model to predict the system behavior at each sampling instant. The voltage vector that generates the least power ripple is selected using a cost function and applied during the next sampling period; thus, flexible power regulation can be achieved. In addition, the influence of a one-step delay in the digital implementation is investigated and compensated for using a model-based prediction scheme. Furthermore, a two-step horizon prediction algorithm is developed to reduce the switching frequency, which is a significant advantage in higher power applications. The effectiveness of the proposed model predictive control strategy was verified numerically by using MATLAB/Simulink and validated experimentally using a laboratory prototype.

Model Predictive Control of Grid-Connected Inverters for PV Systems With Flexible Power Regulation and Switching Frequency Reduction

In this paper, a combined vector and direct power control (CVDPC) is proposed for the rotor side converter (RSC) of doubly fed induction generators (DFIGs). The control system is based on a direct current control by selecting appropriate voltage vectors from a switching table. In fact, the proposed CVDPC enjoys the benefits of vector control (VC) and direct power control (DPC) in a compacted control system. Its benefits in comparison with VC include fast dynamic response, robustness against the machine parameters variation, lower computation, and simple implementation. On the other hand, it has benefits in comparison with DPC, including less harmonic distortion and lower power ripple. An extensive simulation study, using MATLAB/Simulink, is conducted on a 9-MW wind farm composed of six 1.5-MW DFIG-based wind turbines. The performance of the proposed CVDPC method is compared with both VC and DPC under steady-state and transient conditions. Simulation results confirm the superiority of the CVDPC over either VC or DPC.

A Combined Vector and Direct Power Control for DFIG-Based Wind Turbines

Predictive direct torque control of the electric motors has been well developed. It is simple and has excellent steady state and transient performance. However, further developments are still under investigation for applications in the field of power generation. This paper presents a predictive direct virtual torque and power control strategy for a doubly fed induction generator, which allows fast and smooth grid synchronization, and flexible active and reactive power regulation. In the no-load mode, predictive direct virtual torque control is employed to meet the grid synchronization conditions. In the grid-connected mode, predictive direct power control is utilized to achieve flexible active and reactive power regulation. To simplify the control system structure and improve the reliability, a sensorless rotor position scheme is proposed. Furthermore, a model-based predictive scheme is introduced to compensate for a one-step delay in the digital implementation. The proposed control strategy is very simple and robust. There is constant switching frequency, while the requirement of smooth and fast grid synchronization is fulfilled. The transition from no load to flexible power regulation is achieved without changing the switching table. The proposed control strategy was tested by simulation using MATLAB/Simulink and experimentally validated on a 20-kW laboratory prototype.

Predictive Direct Virtual Torque and Power Control of Doubly Fed Induction Generators for Fast and Smooth Grid Synchronization and Flexible Power Regulation

The abrupt change in current or voltage caused by the rapid switching action of power semiconductor devices will generate high-frequency electromagnetic interference (EMI) and needs to be located as the primary interference sources in power electronics. This paper deals with interference suppression on the basis of the study of shaped switching transients in switching waveforms and closed-loop gate drive method for power MOSFETs. The characteristics of switching waveforms with arbitrary switching transients are investigated in both time- and frequency-domain. In addition, conditions of constructing a switching waveform with optimized switching transients are put forward. An optimized Gaussian switching waveform with infinite successive derivatives is proposed which provides the deepest decreasing rate in the interference source spectrum for a given switching time. Compared with a trapezoidal waveform with single-slope transients, the proposed switching waveform has better interference suppression at high frequencies. Meanwhile, a closed-loop gate control prototype with on-state resistance compensation is presented to shape the switching transients of power MOSFETs. The simulated and experimental results show that the proposed controller is successfully implemented to shape the drain–source voltage into an optimized Gaussian reference. Spectrum comparisons show that the EMI generation is effectively suppressed as expected.

Analysis and Optimization of Power MOSFETs Shaped Switching Transients for Reduced EMI Generation

The rate at which large variable speed wind farms are being developed had increased rapidly as the demand for green energy has grown. With this increase in distributed energy penetration into power networks, the main focus must be on power quality (Voltage Quality and Supply Reliability). However, new grid codes require wind energy generating systems to ride through grid faults, and at the same time inject reactive current into the grid. This paper presents a comprehensive survey of the control strategies for doubly-fed induction generator wind systems, including grid synchronization, maximum power point tracking (MPPT), active and reactive power regulation, and fault ride-through. From the assessment of these control techniques, conclusions about the trend of wind power generation technologies are drawn.

Control strategies of variable-speed wind system under new grid code requirement — A survey

For conventional direct torque control (CDTC) methods, there are usually undesired torque and flux ripples mainly for two reasons. First, the vectors selected are not necessary the best. Secondly, one-step delay influence in digital implementation causes additional torque and flux ripples. This paper proposes a novel predictive direct torque control (PDTC) strategy of the doubly fed induction generator (DFIG). The proposed strategy aims to reduce torque and flux ripples effectively at low constant switching frequency by appropriately arranging two active vectors followed by one zero vector within one control period. Furthermore, one-step delay is compensated using a mode-based prediction scheme. Finally, the control system is simplified through further analysis of the transient slope of torque and flux without performance degradation. Simulation results validate the proposed strategy with excellent steady-state and transient performance, which makes it very suitable for wind power generation.

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Simple and robust predictive direct control of DFIG with low constant switching frequency and reduced torque and flux ripples

A novel operation of controlling the phase currents to minimize the torque ripple of switched reluctance motor is proposed in this paper. By changing the slops of the supplied currents appropriately, the torque ripple can be reduced significantly. The dynamic simulation model is built based on the Matlab/Simulink. The results show that the torque ripple can be minimized by the proper shaping of currents during commutation, demonstrating the effectiveness of the proposed control method.

Qishuang Ma

Papers

Predictive Direct Virtual Torque and Power Control of Doubly Fed Induction Generators for Fast and Smooth Grid Synchronization and Flexible Power Regulation

Analysis and Optimization of Power MOSFETs Shaped Switching Transients for Reduced EMI Generation

Control strategies of variable-speed wind system under new grid code requirement — A survey

Simple and robust predictive direct control of DFIG with low constant switching frequency and reduced torque and flux ripples

Torque ripple minimization of switched reluctance motors by controlling the phase currents during commutation