The Research Of Impact Of Wind Farms On Power System Dynamic Characteristics

Nowadays, development of wind power is transformed from distribution, small-scale, local consumption to large-scale, concentrated exploitation, long distance and high voltage. The combining system of local wind and thermal plants that deliver power can effectively reduce power fluctuations of line, and has good impact on stable operation of power system. However, this hybrid system that exports high capacity power through long distance can weaken damping characteristics. Due to fault or wind power fluctuations, low frequency oscillations may happen to endanger security and stability of power grid. To improve transmission efficiency of power system, serial compensation devices are installed, which may result in Subsynchronous Resonance(SSR).Electromagnetic transient model of double fed induction generator(DFIG) is established, whose control is based on the track of optimal wind power and stator flux oriented in PSCAD/EMTDC. A equivalent simulation model of DFIG whose converters are simulated by controlled source, is proposed. And a method is put forward that large-scale wind farm is polymerized by single wind turbine plus ideal controlled current source. These are basis of the following study.Under normal circumstances, the power of wind turbine is lower than optimal power, part of wind is abandoned to keep reserve margin.Large-scale wind farm is introduced to the typical simulation system with four generators in two areas; By model identification methods, additional damping controller is designed based on DFIG. Under the same outgoing power conditions, the damping controller regulates DFIG’s output by the message of power oscillation, which can act as a "load shifting" role and enhance system damping. When power of wind farm is lower than optimal power, speed of suppressing oscillation is fast and results are good.Under the same outgoing scale, wind farms can improve SSR characteristics of thermal power units. The greater wind farm output, the better improving results; but it can not eliminate the problem of SSR. Additional damping controller of DFIG based on the thesis of phase compensation is designed, which provides positive damping in subsynchronous domain to eliminate SSR. Due to decoupling control of active and reactive power of DFIG, when DFIG improve SSR characteristics by regulating reactive output, its active output is not affected.