Honglin Zhou

An LVRT Control Strategy Based on Flux Linkage Tracking for DFIG-Based WECS

For doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs), large electromotive force will be induced in the rotor circuit during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, which may even destroy the rotor-side converter (RSC). To mitigate this problem, a new flux-linkage-tracking-based low-voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5-MW DFIG-based WECS is carried out by simulation using the full-order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.

Modeling, Analysis, and Control for the Rectifier of Hybrid HVdc Systems for DFIG-Based Wind Farms

In order to improve the dynamic performance of the hybrid HVdc system for doubly fed induction generator wind farms, this paper presents the modeling, analysis, and control methods for its line-commutated rectifier. First, the state variable model of the rectifier subsystem on the reference frame is derived considering different control modes of the current source inverter subsystem. Then, the ac current dynamic of the rectifier subsystem is quantitatively analyzed based on the eigenvalue analysis method. According to the analysis results, a double loop control scheme is designed: the inner loop utilizes the inverse system control technique plus a switchable phase-lead compensator, and the outer loop implements an ordinary propotional-integral controller. Finally, the validity of the quantitative analysis method and the superiority of the proposed control scheme are verified by Simulink/SimPowerSystems simulations.

Short circuit current analysis of DFIG wind turbines with crowbar protection

For DFIG (Doubly Fed Induction Generator)- based wind turbines, crowbar is a commonly used protection method against surge current caused by sudden drop of the grid voltage during the period of LVRT (Low Voltage Ride-Through). Based on detailed analysis of the transient flux characteristics of DFIG with the crowbar fired, a simplified model is proposed to analyze and approximate the short circuit current accurately. Meanwhile the maximum short circuit current and the LVRT capability are evaluated. After that the dc-link voltage clamp effect is pointed out and a crowbar design method based on the dc-link voltage is proposed accordingly. A typical 1.5 MW DFIG wind turbine is used for case study. The analysis and simulation results show that, when the crowbar resistance is small or the fault is not severe, the dclink clamp effect does not occur so the simplified model can give a satisfactory approximation of the short circuit current; otherwise, the dc-link clamp effect must be taken into account in determining the short circuit current. In this case, it is found that excessive high crowbar resistance will not improve the LVRT capability substantially, but put more stress on the fly-wheel diodes of the rotor side converter.

A LVRT control strategy based on flux tracking for DFIG-based wind power systems

For doubly-fed induction generator (DFIG)-based wind power systems, when a grid voltage dip happens, a large electromotive force (EMF) will be induced in the rotor circuit. Without proper control for protection, over-current will appear in the rotor circuit, and it may lead to the destruction of the rotor-side converter (RSC). To mitigate this problem, a new low voltage ride-through (LVRT) control strategy based on flux tracking is proposed. With the proposed control strategy, once the voltage dip is detected, the rotor flux is controlled to track the changing stator flux by controlling the RSC. The validity of the proposed method is verified using the full order model of a typical 1.5 MW DFIG in the SIMULINK/SimPower Systems simulation environment. The proposed control strategy is proven to be able to effectively reduce the rotor current during both symmetrical and asymmetrical grid faults without any additional hardware circuit.

Control of DFIG-based wind farms with hybrid HVDC connection

This paper presents the control methods for large Doubly Fed Induction Generator (DFIG)-based wind farms with hybrid HVDC (High-Voltage dc) connection. The sending end of the hybrid HVDC topology is a Line-Commutated Converter (LCC) with a STATCOM (Static Synchronous Compensator) while the receiving end is a Current Source Inverter (CSI) using forced-commutated devices. The STATCOM provides stator voltage support for DFIGs, as well as commutation voltage for the LCC. Taking into account the nonlinear and coupling characteristics of each subsystem, a series of control schemes are provided. Simulations under both normal and fault conditions are carried out, which verify that, incorporating the advantages of existing LCC-HVDC and VSC (Voltage Source Converter)-HVDC technologies, the proposed system has the black-start capability, and can provide flexible reactive power support to the grid. Moreover it is able to separate the wind farm from the ac fault on the inverter side, thus enhancing the reliability of the overall wind energy system.

Modeling and design of a new bi-directional buck-boost cascade inverter

This paper concentrates on the modeling and design of a new bi-directional buck-boost cascade inverter. The proposed inverter can be seen as the cascade of a Buck converter and a Boost converter, both with bipolar outputs. First, The operation principle of the proposed inverter is explained. With detailed analysis, the switching function model is established. Then, the averaged model for control purpose is given. Afterwards, a decoupled control scheme is presented. Device-level simulations and experiments verify that the proposed system can not only perform bi-directional operation with bipolar buck/boost output voltage, but also achieve good steady-state and dynamic performance.

Performance verification of the inverse-system controller for the variable-speed variable-pitch wind generation system

This paper presents the performance verification results of the inverse-system controller (ISC) which was proposed by us recently for the pitch control of a variable-speed variable-pitch wind generation system (VSVP-WGS). Based on multi-time-scale analysis, the motion of pitch is reduced to a first-order model with non-affine and nonlinear characteristics. Using the inverse-system method, the nonlinear model is further converted to a pseudo-linear one and then can be synthesized simply by linear system approach. A hardware-in-the-loop experimental platform of VSVP-WGS is established for the pitch control study. The wind turbine is emulated by an induction motor with torque control mode. The models of wind speed and pitch regulator are implemented in MATLAB/SIMULINK real-time environment. Experimental results show that, compared with conventional proportional-integral (PI) controller, ISC is simpler to be designed, easier to be stabilized. Also, it can smooth the output power of the VSVP-WGS and reduce the mechanical fatigue of the pitch regulator.