Research On Low Voltage Ride-through Of DFIG Based On Dynamic Feed-forward Compensation Of EMF

With the increasingly severe energy crisis and environmental problems, clean energy for sustainable development has attracted more and more attention. As the most promising new energy for large-scale development and commercial application, wind energy-related industry is on the rise at a fast pace. For the last decade, the proportion of the installed capacity of wind turbines in the power system has been growing rapidly, thus the safe and stable operation of wind farm becomes very crucial. Different countries have put forward wind farm grid specifications. Low voltage ride-through( LVRT) capability is one of the key requirements for wind turbine under the grid fault. Because the capacity of the inverter is relatively small, and the active and reactive power can be decoupled controlled independently, doubly fed induction generator( DFIG) becomes mainstream models of wind turbine. However, the stator of DFIG is directly connected to the grid side so that it is greatly affected by the grid fault. Thus improving the LVRT capability of DFIG is quite important, and it becomes a hot topic. Existing LVRT technologies are mainly divided into two types: additional hardware circuitry and improved excitation control strategy. Excitation control strategy, with no additional hardware, controls the transient process during the grid fault to reduce output current and torque ripple of the rotor side. For a limited capacity of the rotor converter, the rotor current command is designed to offset DC and negative sequence component of transient flux, which reduces the excitation voltage requirements, and meets the capacity constraints of rotor side to go through grid fault. Therefore this excitation control strategy is favored in LVRT technology research.However, the excitation control strategy will lose effectiveness if the controller lacks tracking ability. This paper focuses on the realization of improved excitation control strategy for high-power wind turbine. Firstly the DFIG transient process is analyzed to find out the reason for the rotor-side converter over-voltage and over-current. Then the typical excitation control strategies are studied. It shows that most of the excitation control strategies that are beneficial for LVRT aim at the rotor current control, essentially. In the dq rotating coordinate system, the rotor current command contains fundamental and the second harmonic component to eliminate the DC and negative sequence component of the flux during the voltage dropping. In the face of AC component in rotor current command and EMF disturbance during the transient process, the low switching frequency of high-power wind turbines, and the limited ability to increase the bandwidth of the controller, traceability and anti-disturbance of the current loop are restricted with traditional PI regulator. Based on transient characteristic of excitation control strategy analyzed above, we will find that there is a certain relationship between the induced electromotive force and the rotor current command. An EMF dynamic feed-forward compensation scheme is proposed in this paper. This scheme improves tracking ability of AC component in the rotor current command and disturbance rejection of AC component in the induced electromotive force. The design process of the dynamic feed-forward coefficients is given.Two typical excitation control strategies are selected to verify the scheme in this paper. Simulation under high-power fan system and experiment under low-power fan platform verify that the EMF dynamic feed-forward compensation method is feasible and effective. Simulation results show that the scheme in the current anti-tracking control strategy reduces the electromagnetic torque ripple, and the scheme in the demagnitizing control strategy accelerates the decay rate of the rotor currents. The former is also verified in the experiment. The EMF dynamic feed-forward compensation scheme can improve tracking ability of the AC component in the rotor current command and disturbance rejection of AC component in the induced electromotive force to achieve control objectives of the excitation control strategy and give full play of their own advantages, which can be seen from the simulation results and experimental phenomena.