Yipeng Song

High-Frequency Resonance Damping of DFIG-Based Wind Power System Under Weak Network

When operating in a micro or weak grid which has a relatively large network impedance, the doubly fed induction generator (DFIG)-based wind power generation system is prone to suffer high-frequency resonance due to the impedance interaction between the DFIG system and the parallel compensated network (series RL + shunt C). In order to improve the performance of the DFIG system as well as other units and loads connected to the weak grid, the high-frequency resonance needs to be effectively damped. In this paper, the proposed active damping control strategy is able to implement effective damping either in the rotor-side converter or in the grid-side converter, through the introduction of virtual positive capacitor or virtual negative inductor to reshape the DFIG system impedance and mitigate the high-frequency resonance. A detailed theoretical explanation on the virtual positive capacitor or virtual negative inductor has been given, and their parameters are also optimally designed. The proposed DFIG system damping control strategy has been validated by experimental results.

Impedance-Based High-Frequency Resonance Analysis of DFIG System in Weak Grids

The impedance-based model of doubly fed induction generator (DFIG) systems, including the rotor part (rotor side converter (RSC) and induction machine), and the grid part (grid side converter (GSC) and its output filter), has been developed for analysis and mitigation of the subsynchronous resonance (SSR). However, the high-frequency resonance (HFR) of DFIG systems due to the impedance interaction between the DFIG system and parallel compensated weak network is often overlooked. This paper, thus, investigates the impedance characteristics of DFIG systems for the analysis of HFR. The influences of the rotor speed variation, the machine mutual inductance and the digital control delay are evaluated. Two resonances phenomena are revealed, i.e., 1) the series HFR between the DFIG system and weak power grid; 2) the parallel HFR between the rotor part and the grid part of DFIG system. The impedance modeling of DFIG system and weak grid network, as well as the series HFR between DFIG system and parallel compensated weak network has been validated by experimental results.

Overview of DFIG-Based Wind Power System Resonances Under Weak Networks

The wind power generation techniques are continuing to develop and increasing numbers of doubly fed induction generator (DFIG)-based wind power systems are connecting to the on-shore and off-shore grids, local standalone weak networks, and microgrid applications. The impedances of the weak networks are too large to be neglected and require careful attention. Due to the impedance interaction between the weak network and the DFIG system, both subsynchronous resonance (SSR) and high-frequency resonance (HFR) may occur when the DFIG system is connected to the series or parallel compensated weak network. This paper will discuss the SSR and the HFR phenomena based on the impedance modeling of the DFIG system and the weak networks, and the cause of these two resonances will be explained in details. The following factors will be discussed in this paper: 1) transformer configuration; 2) different power scale of DFIG system with different parameters; 3) L or LCL filter adopted in the grid side converter (GSC); 4) rotor speed; 5) current closed-loop controller parameters; and 6) digital control delay. On the basis of the analysis, active damping strategies for HFR using virtual impedance concept will be proposed.

Wide Frequency Band Active Damping Strategy for DFIG System High Frequency Resonance

As a popular renewable power generation solution, the Doubly Fed Induction Generator (DFIG)-based wind power system may suffer from High Frequency Resonance (HFR) caused by the impedance interaction between the DFIG system and the parallel compensated weak network. A wide frequency band active damping strategy for DFIG system HFR, including a high-pass filter and a virtual resistance, is proposed in this paper. The advantages of this active damping strategy are: 1) no resonance frequency detection unit is required; thus the control complexity can be decreased; and 2) no active damping parameters adjustment is needed within certain wide frequency band; thus the robustness of the proposed active damping strategy can be improved. The parameter design of the high-pass filter cutoff frequency and the virtual resistance are theoretically analyzed with the purpose of satisfactory active damping. A 7.5-kW down-scaled experimental setup is built up for the experimental validation of the proposed active damping method.

Doubly Fed Induction Generator System Resonance Active Damping Through Stator Virtual Impedance

The penetration of wind power has been increasing in the past few decades all over the world. Under certain nonideal situations where the wind power generation system is connected to the weak grid, the doubly fed induction generator (DFIG)-based wind power generation system may suffer high-frequency resonance (HFR) due to the impedance interaction between the DFIG system and the weak grid network whose impedance is comparatively large. Thus, it is important to implement an active damping for the HFR in order to ensure a safe and reliable operation of both the DFIG system and the grid-connected converters/loads. This paper analyzes and explains first the HFR phenomenon between the DFIG system and a parallel compensated weak network (series RL + shunt C). Then, on the basis of the DFIG system impedance modeling, an active damping control strategy is introduced by inserting a virtual impedance (positive capacitor or negative inductor) into the stator branch through stator current feedforward control. The effectiveness of the DFIG system active damping control is verified by a 7.5 kW experimental downscaled DFIG system, and simulation results of a commercial 2 MW DFIG system is provided as well.

Analysis and Active Damping of Multiple High Frequency Resonances in DFIG System

As the wind power generation develops, the doubly fed induction generator (DFIG) based wind power system is more and more likely to operate in the emerging weak network rather than in the conventional stiff network. Due to the comparatively large impedance of the weak network than the stiff grid, the DFIG system may be subject to the resonances due to the impedance interaction between the DFIG system and the weak network. Especially, when connected to a series π sections weak network, the multiple high-frequency resonances (MHFR) may occur and require careful studies. The impedance modeling of the DFIG system and the series π sections weak network is first demonstrated in this paper. Then, due to the multiple magnitude peaks of the series π sections of the weak network, the MHFR will be produced and can be theoretically explained based on the impedance modeling results. For the purpose of mitigating the MHFR, an active damping strategy which introduces a virtual impedance, including a phase leading compensation unit and a virtual positive resistance, is proposed and demonstrated. Simulations are conducted to validate the DFIG system MHFR as well as the proposed active damping strategy.

Chapter 10 - Control of Wind Turbine System

Abstract Wind power is a main pillar of renewable energy supply, as it generates clean and climate-friendly electricity. Among the mainstream wind turbine systems, the configurations of the doubly fed induction generator (DFIG) and the permanent-magnet synchronous generator (PMSG) are dominating and important in the current market. In this chapter, it starts with the configurations of the wind energy conversion system and their state-of-art control solutions. The common used control schemes (e.g., the vector control and direct control) are comprehensively evaluated based on the modeling of the generator and the power converter. The main control tasks—generator-side active power control, grid-side reactive power control, and DC-link voltage control are analyzed and demonstrated by case studies.

Modeling and Control of Three-Phase AC/DC Converter Including Phase-Locked Loop

Abstract In this chapter, a mathematical model of the power circuit of a three-phase AC/DC converter is developed in the stationary and synchronous reference frames. Then, the operation principle of the phasor locked loop is addressed to exact the angle information of the power grid to realize the accurate control synchronized with the power grid. Afterward, based on the modeling of the three-phase grid-tied converter, the controller design of the PI (Proportion + Integral) and PR (Proportion + Resonant) is implemented under the synchronous reference frame and stationary reference frame, respectively. It is realized by the desired bandwidth and its corresponding phase margin with the aid of Bode plot. It is simulation proven that the step response of the inner grid current and outer DC voltage matches their desired bandwidth.

Chapter 5: Modeling and Control of Three-Phase AC/DC Converter Including Phase-Locked Loop

Abstract In this chapter, a mathematical model of the power circuit of a three-phase AC/DC converter is developed in the stationary and synchronous reference frames. Then, the operation principle of the phasor locked loop is addressed to exact the angle information of the power grid to realize the accurate control synchronized with the power grid. Afterward, based on the modeling of the three-phase grid-tied converter, the controller design of the PI (Proportion + Integral) and PR (Proportion + Resonant) is implemented under the synchronous reference frame and stationary reference frame, respectively. It is realized by the desired bandwidth and its corresponding phase margin with the aid of Bode plot. It is simulation proven that the step response of the inner grid current and outer DC voltage matches their desired bandwidth.

Analysis of High-Frequency Resonance in DFIG-Based Offshore Wind Farm via Long Transmission Cable

During the past two decades, the doubly fed induction generator (DFIG) based wind farm has been under rapid growth, and the increasing wind power penetration has been seen. Practically, these wind farms are connected to the three-phase ac grid through long transmission cable which can be modeled as several Π units. The impedance of this cable cannot be neglected and requires careful investigation due to its long distance. As a result, the impedance interaction between the DFIG-based wind farm and the long cable is inevitable, and may produce high-frequency resonance (HFR) in the wind farm. This paper discusses the HFR of the large-scale DFIG-based wind farm connected to the long cable. Several influencing factors, including 1) the length of the cable, 2) the output active power, and 3) the rotor speed, are investigated. The transformer leakage inductances in the transmission system are taken into consideration when investigating the HFR. Simulation validations using MATLAB/Simulink have been conducted to verify the theoretical analysis.