Research Of Sub-synchronous Oscillation When Large-scale Doubly Fed Induction Generators Connected To Grid By Series Compensation

With the development of China's large-scale wind power, some problems of wind power connecting to grid become increasingly prominent. Because China's wind power production centers is far away from the load centers, the wind power sent out needs to rely on long-distance transmission. Installing of series compensation device for transmission lines can significantly improve system performance, improve power system‘s stability and capability of transmission lines, and can improve the economic efficiency of the system. Doubly fed induction generators(DFIG) have superior performance. So they are one of the most widely used generators currently. But because of their special nature of its structure and control system, sub-synchronous oscillation(SSO) can be easily caused when they send out wind power by series compensation transmissions. Recent years, sub-synchronous oscillation problems occur frequently in an area of China. This area has large-scale wind farms. This problem results in a large number of generators off the net, seriously affects the sent out of wind power, limits the wind power‘s connection to grid, and causes large economic losses.This paper bases on actual grid model of the area where sub-synchronous oscillations occur frequently. It establishes a kind of large-scale wind farm aggregation model to analyze the effect factors of sub-synchronous oscillations when doubly-fed wind farms send out wind power through series compensation equipment. It also proposes some measures to mitigate the oscillation.First of all, according to the research purpose, it selects the electromagnetic transient analysis to establish the mathematical models of the various parts of the wind power generation systems, including wind speed model, wind turbines and driveline model and DFIG internal control model. In the establishment of wind speed model, it takes full consideration of wake effects to modify wind speed. In the establishment of the wind turbines and driveline transmission model, it uses the maximum wind power control, ignoring the secondary factors, and use single characterization method to aggregate several turbines. When aggregate model of DFIG, it establishes detailed control models of stator and rotor converters, and analyzes the variation regulation of converter parameters between aggregating generator and a single generator.Then, it builds a model which aggregating generators accessed to real grid in EMTDC / PSCAD simulation platform. The simulation takes connecting process into consideration. Firstly it verifies the consistency between the aggregation model and variable metric model and the actual data. Then it researches affecting factors of sub-synchronous oscillations when DFIG send out wind powers through series compensation transmissions by variable control study. The simulation shows that wind speed, series compensation degree, the number of connected DFIG and rotor-side converter parameters have a great influence on the generation of sub-synchronous oscillations.Finally, according to the obtained sub-synchronous oscillation factors, it proposes some measures to suppress sub-synchronous oscillations and prevent further incidents including cutting off some generators, changing series compensation degree when wind speed changes, changing the rotor-side converter parameters according to the wind speed and designing a kind of damping controller.