Effect Of Grid-integrated DFIG-based Wind Turbines On Small-signal Stability Of Power System

The environmental pollution and energy shortage caused by fossil energy usage are increasingly serious. The wind power generation, as a kind of renewable energy with mature technology, plays an important role in reducing environmental pollution and adjusting energy structure. Wind energy resources are rich in our country, but mainly concentrated in the "three norths" and the coastal areas which have lighter local load and are far from eastern load centers. Therefore, the wind power needs to be transimitted in a long distance, which makes the system small signal stability issues more prominent.Due to its higher energy conversion efficiency and flexible control, the doubly-fed induction generator(DFIG) has become the mainstream in wind farms. With the increasing penetration of wind power, it is necessary to study the influence of DFIG when it comes to system small-signal stability. In this paper, the main contributions are as follows:(1) The influence on system damping of DFIG under different working conditions is analyzed in this paper, which includes wind penetration, installation location, exiting ways of conventional generators. Based on the eigenvalue analysis in two-area four-machine system, it is found that the DFIG influences system damping mainly through changing the working point of adjacent synchronous generators(SGs).(2) The parameters change of phase-locked loop(PLL) has influence on system small signal stability. PLL is used to realize the synchronization between DFIG and the grid. When the control parameters of PLL are decreased, there is a phase error between the output signal of PLL and the actual terminal voltage, which results in the power change of DFIG. From the perspective of internal voltage, this paper analyzes the phase relationship between the internal voltage and terminal voltage of DFIG under different PLL control parameters, which accounts for the mechanism of PLL affecting system damping. The conclusion validates by time-domain simulation and frequency-domain simulation.(3) The voltage control of DFIG influencing system small-signal stability is also studied in this paper. In order to maintain the grid voltage, the wind farms need to be equipped with voltage control system. The influence of voltage control of DFIG on the neighboring SGs is emphasized in this paper. The simplified DFIG model based on the internal voltage is firstly estabilished. Based on it, the block diagram of voltage control of DFIG affecting the SG is deduced. And then, it can be derived that the influcing approach of voltage control is excitation system of adjcacent SG. Both the system topology change due to the integration of wind power and the voltage control of DFIG have effects on system damping, and the influences of them are opposite. Voltage control parameter and control methods are important factors affecting capacity of DFIG providing damping for the grid. The conclusion derived in this paper is also suitable for the other power electronic equipments with voltage control system..