| According to the characteristics of reverse distribution of load centers and wind energy resources in China,in order to effectively utilize wind energy and promote the transformation of energy structure,it is necessary to carry out long-distance transmission of wind power.However,the inductance of the line will affect the transmission capacity of the power system.Therefore,it is necessary to configure the series compensation capacitor in the high-voltage transmission line.The existence of the series compensation capacitor will change the electrical damping characteristics of the system,which may induce the sub-synchronous oscillation(SSO)of the power system,and seriously threaten the safe and stable operation of the power system.Aiming at the problems of SSO in power system with doubly-fed induction generator(DFIG)-based wind farm,this paper studies the generation mechanism of SSO.Based on the eigen-structure analysis method,the influence of different system parameters on the SSO is analyzed.What’s more,from the perspective of destroying the electrical resonance and changing the control structure of the unit,the corresponding SSO suppression strategy is explored.The main work contents of this paper include:Firstly,the mathematical models of turbo generator,DFIG-based wind farm and alternate current(AC)power grid suitable for analyzing SSO of power system are established.On this basis,the linearized models of the turbo generator,DFIG-based wind farm and AC power grid are derived.The influence of series compensation level on the SSO of different power equipment is analyzed by using the eigen-structure analysis method.Secondly,based on the coordinate transformation and Kirchhoff’s theorem,the small-signal model of the power system with DFIG-based wind farm is extended.By using the eigen-structure analysis method,the SSO modes of the system are determined.Combined with the electrical damping and equivalent resistance,the generation mechanism of the SSO is studied.On this basis,the influence of the phase-locked loop,connecting line and active power characteristics on the SSO of the system is analyzed,and the validity of the analysis results is further verified by time-domain simulation.Finally,according to the cause of SSO,two kinds of SSO suppression strategies are studied,respectively.The first one is based on the bypass damping filter,which can suppress the SSO by changing the electrical damping characteristics of the system and destroying the electrical resonance.The second one is an improved additional damping control strategy,the sub-synchronous current component is injected into the current loop to suppress the SSO by using the regulating ability of the DFIG unit itself.The validity of the two control strategies are verified by time domain simulation.The research results obtained in this paper have important reference value for analyzing the generation mechanism of SSO in power system with DFIG-based wind farm.Meanwhile,it provides theoretical support for the design of SSO suppression strategy for power system with DFIG-based wind farm,which can ensure that the power system operate safely and stably. |
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