Research On Mechanism Of Subsynchronous Resonance And Risk Assessment Of The Doubly-fed Induction Generators

Most of the wind energy resources in our country are far away from the load center, large capacity and long-distance transmission are the main way to realize the full utilization of wind energy resources and rational allocation of renewable energy resources. In order to improve the capacity of wind power transmission, the series compensated transmission technology has been widely used in wind farms, that leads to the problem of the subsynchronous resonance(SSR) of wind power in our country, which seriously endanger the safe and stable operation of the whole power system, such as serious subsynchronous resonance phenomena occurred in the doubly-fed wind farm in Guyuan, northern Hebei Province in 2012. The rotating speed of the doubly-fed generator is changing with the wind speed because of the maximum power point tracking(MPPT), therefore, wind speed is a significant factor that affects the variation of the risk area of the sub synchronous resonance of the doubly-fed induction generator and the safety risk. To solve this problem, this paper mainly carried out the following two aspects of research work.(1) Aiming at the problem of the influence of the wind speed on the dangerous area of the SSR of the doubly-fed generator, this paper reveals the rule that the equivalent resistance varies with the rotor speed of doubly-fed generators and the system natural resonant frequencies. A new method is developed for investigating the induction generator effect of doubly-fed wind farms using the equivalent resistance. The analysis results show that the risk region for the induction generator effect of doubly-fed generators is located on the right region of the rotor speed of doubly-fed generators corresponding to the system natural resonant frequencies. It is also observed that the risk region for the induction generator effect of doubly-fed generators moves to the higher wind speed regions with the increasing of the series compensation level. Furthermore, the induction generator effect gets more and more severe when the minimum of the equivalent resistance of the system decreases with the increasing of the series compensation level.(2) The security assessment of SSR caused by wind power generation mainly adopts deterministic assessment methods which can not fully reflect the effect of wind speed random variation on subsynchronous resonance risk of the doubly-fed induction generator. To solve this problem, this paper defines a risk matrix and a risk index, and develops a SSR-oriented security risk assessment method using probabilistic collocation method. The proposed method is used to assess the SSR security risk of a wind farm including the impact of the uncertainty of wind speeds on SSR. The results show that the SSR security risk of the system with the lower level of series compensation decreases with the increasing of the average wind speeds, while the SSR security risk of the system with the higher level of series compensation rises up with the increasing of the average wind speeds. Under the same wind speed distribution, with the increasing of the series compensation level, the SSR security risk of the system increases at first and then decreases.