Xie Zhen

Study on control strategy of maximum power capture for DFIG in wind turbine system

This paper analyzes the stator flux oriented vector control theory for doubly fed induction generator, control strategy of grid connected based on speed feedback and the arithmetic of tracking the peak power independent of the turbine characteristics and air density are used in this system. a principle of power characteristic for variable speed wind power generation systems is first explained. To optimize the output power, a method of MPPT which is based on the variable step of the velocity is researched and analysed. According to the error of the output power, the reference of the speed for turbine is calculated and adjusted by the PI controller. At last the article gives the simulation result of this method, the result shows the validity. 110kw VSCF doubly fed induction generator wind power system is constructed in the laboratory, which realizes the decoupled control of the active power and reactive power for stator output and the control of maximum power point tracking.

A SCR crowbar commutated with rotor-side converter for doubly fed wind turbines

Doubly fed induction generator (DFIG) based wind turbines have been dominating wind market in the past decade, due to the better balance between variable speed operation and the drive converter size in terms of the unique drive topology. However, the difficulties of low voltage ride through (LVRT) fulfillment are threatening this type of turbine technology, out of the limits of current and voltage to the partial rated converter. Active crowbar at present is a relatively mature solution for severe voltage dips, but the requisite self-turn-off devices, typically IGBTs, have made the cost-effectiveness of this type of turbines damaged. Meanwhile, once the crowbar is activated, the rotor-side converter (RSC), along with all its six valuable self-turn-off IGBTs, would be idle temporarily. In this paper, a delta-connected SCR circuit engaged with the rotor via a set of by-pass resistors is designed as a crowbar to divert the outrush currents, while the idle RSC is in charge of commutating the semi-controlled devices at the desired deactivation moment. With this design, an active crowbar function is achieved with SCRs, featured with low cost and high instantaneous overload capacity, benefiting the cost-performance ratio of the preferable wind turbine concept. Meanwhile, the harmful effects of the harmonics produced by the diode rectifier in the widely used IGBT crowbar topology are eliminated. The validity of the crowbar technique is demonstrated with simulations and experiments.

An analytical method for the response of DFIG under voltage dips

Electromagnetic transients resulting from voltage dips complicate the low-voltage ride-through (LVRT) of doubly fed induction generator (DFIG) based wind turbines (WTs). To study the transients deeply, an analytical method to solve the dynamic response is presented in this paper. Although some attempts have been made in literature, the accuracy cannot be ensured, or the physical significance implied in the transients is not feasible to be revealed without a proper method to this complex process. To clarify the response, a concept of equivalent fault source is defined in the stator and rotor circuit respectively and the overall behavior is decomposed into a stator-fault-sourced response and a rotor-fault-sourced response. The former corresponds to the dips of the stator voltage, while the latter deals with the variations in the rotor resistance and the terminal voltage due to activation of the crowbar. Adding these two zero-state responses to the healthy pre-fault conditions, the overall response can be constructed accurately and each of the terms has its apparent significance. With the analytical solution, the behavior is discussed and some important features are presented. The accuracy of the solution is verified by close agreement with the simulative results from a MATLAB/Simulink model of a 2MW DFIG and the experimental records on an 11kW laboratory-scale test rig.