Impact Of The Grid Connection Of Wind Power On Power System Oscillations

This paper investigates the impact mechanism of grid-connected wind power on power system low-frequency and sub-synchronous oscillations.The reason that grid-connected wind power brings about power system oscillations is twofold,either by changing the initial power flow conditions or by interacting dynamically with the power system.This paper focuses on the latter one,i.e.,the inherent mechanism that dynamic interactions between wind turbine generators(WTGs)and power systems induce power system oscillations.To carry out the research,an interconnected closed-loop model is established based on the linearized model of the power sysetem,wherein the WTG or phase-locked loop(PLL)is in the feedback path and the remainder of the power system is in the forward path.By doing so,the impact of dynamic interactions between the sub-systems can be described by the open-loop and closed-loop modal difference.Based on this model,the paper examines respectively the impact of dynamic interactions between the WTG or PLL and the power system on power system oscillation modes under the condition of weak interactions or strong interactions.The main works are presented as follow.(1)The paper discusses the impact of dynamic interactions between the doubly-fed induction generator(DFIG)and the power system on power system low-frequency oscillation modes.Two indice,index of dynamic interactions(IDI)and index of reactive power control(IRPC),are proposed to assess the impact.IDI can indicate quantitatively the impact of dynamic interactions on power system low-frequency oscillation modes.IRPC can suggest the difference of impact on power system oscillation modes under different reactive power control mode(reactive power control mode or voltage conrol mode)of rotor-side converter(RSC)of DFIG.(2)General perspective has it that dynamic interactions between the WTG and the power system is too weak to bring about any impact on power system low-frequency oscillations.However,the simulation results reported in some references suggest that strong interactions are possible under certain conditions and will lead to the damping degradation of low-frequency oscillation modes.This paper investigates the mechanism of the presence of strong interactions and points out the modal condition when strong dynamic interactions occur,i.e.open-loop modal resonance(OLMR).It is found that when the open-loop mode of the WTG or PLL coincides with the open-loop low-frequency oscillation mode of the power system,the coresponding closed-loop modes will be located on the opposite sides of the open-loop modes.Hence,one of the closed-loop modes will be located on the right side with respect to the open-loop modes,thus triggering power system low-frequency oscillations.The occurrence of low-frequency oscillations is due to the nearness of the open-loop modes of the sub-systems,therefore the modal conditon is named open-loop modal resonance.In addition,an index is derived to evaluate the impact of OLMR on power system stability.(3)The paper extends the application of the theory of OLMR to the area of power system sub-synchronous oscillations and proposes a method to examine the stability of sub-synchronous oscillation modes of a multi-machine power system integrated with WTGs.Not only can this approach find out whether or not the OLMR between the WTG and the remainder of the power system will trigger sub-synchronous oscillations,but also can be used to identify the trouble-making generators.