The Open-loop Modal Resonance Mechanism Of SSOs In Power Systems Induced By Grid-connected DFIG Wind Farms

In recent years,several sub-synchronous oscillation(SSO)incidents caused by grid-connected doubly-fed induction generator(DFIG)wind farms occur in practical power systems.The SSO incidents seriously threaten the safe and reliable operation of power systems.For the series-compensated transmission systems with grid-connected DFIG wind farms,an extensive study has reached the well-accepted conclusion that the mechanism of SSOs caused by grid-connected DFIG wind farms can be attributed to the induction generator effect participated by the converter control system.In addition,the series compensation capacitor is a vital factor.However,on July 1st 2015,a serious SSO incident caused by grid-connected wind farms occurred in the Hami power system in West China where there are no series compensated transmission lines.During the incident,torsional stress relays of three large-scale thermal generation units in a power plant,which was 300 km away,were triggered and the units were tripped-off due to the torsional oscillations.Presently,the mechanism of SSOs caused by grid-connected wind farms in power systems without series compensated lines is not clear.Therefore,this dissertation investigates the mechanism of SSOs caused by grid-connected DFIG wind farms in power systems without series compensated lines from the perspective of modal resonance.The research works of this dissertation are listed as follows.(1)Based on the open-loop modal resonance theory,the mechanisms of sub-synchronous torsional oscillations(SSTOs)and sub-synchronous control interactions(SSCIs)caused by grid-connected DFIG wind farms in power systems without series compensated lines are investigated.By taking the grid-connected DFIG wind farm as the feedback loop and the rest of the power system as the forward loop,the single-input single-output closed-loop interconnected model of the power system with grid-connected DFIG wind farms is established which is applicable to the SSO stability analysis.Based on the established model,the impact of the dynamic interactions induced by the grid-connected DFIG wind farm on the SSO stability of the power system is examined.The impact can be evaluated by the difference between open-loop SSO modes and closed-loop SSO modes.Under the condition of open-loop modal resonance,strong dynamic interactions between the grid-connected DFIG wind farm and the rest of the power system are exhibited.Due to the strong dynamic interactions,the corresponding closed-loop SSO modes are located in the opposite positions with respect to those of open-loop SSO modes on the complex plane.As a consequence,the SSO stability of the power system is deteriorated.The study indicates that,when the parameters of the converter control system are inappropriately tuned,the open-loop modal resonance may occur between the DFIG wind farm and the synchronous generator(SG)in the rest of the power system.Due to the open-loop modal resonance,the closed-loop shaft torsional mode of the SG can be located on the right half plane which will cause SSTOs in the power system.In addition,as a result of the open-loop modal resonance between two DFIG wind farms,the modal damping of the closed-loop SSO mode corresponding to the converter control system can become negative which will cause SSCIs in the power system.(2)The mechanism of SSOs caused by grid-connected DFIG wind farms in power systems without series compensated lines under the condition of near strong open-loop modal resonance is investigated.The multi-input multi-output closed-loop interconnected model of the power system with grid-connected DFIG wind farms is firstly established.Based on the established model,it is demonstrated that the open-loop modal resonance and near strong modal resonance are the same modal condition.In specific,the open-loop modal resonance is a special case of near strong modal resonance which is defined as the near strong open-loop modal resonance.Based on the eigenvalue sensitivity theory of the state-space matrix,an analysis method is proposed for the examination of SSOs caused by grid-connected DFIG wind farms in power systems under the condition of near strong open-loop modal resonance.The proposed method connects the SSO stability of the closed-loop interconnected system to the dynamic interactions between open-loop subsystems in which way the physical significance of the near strong modal resonance is clarified,i.e.the dynamic interactions induced by the grid-connected DFIG wind farm.Moreover,as the proposed method is based on the eigenvalue sensitivity theory of the state-space matrix,the method provides a rigorous theoretical basis for the open-loop modal resonance theory based research works on SSOs caused by grid-connected DFIG wind farms in power systems.Furthermore,the applicability of the open-loop modal analysis method is extended to the multi-input multi-output closed-loop interconnected system.(3)The mechanism of SSOs caused by dynamic interactions among wind clusters within the grid-connected DFIG wind farm in power systems without series compensated lines under the condition of multi-modal resonance is investigated.The linearized state-space model of the power system with the grid-connected DFIG wind farm which consists of multiple wind clustsers is established.Based on the established model,the multi-modal resonance theory is proposed to evaluate the impact of dynamic interactions among wind clusters within the grid-connected DFIG wind farm on the SSO stability of the power system.The study shows that multi-modal resonance will occur when open-loop SSO modes of N wind clusters are close to each other on the complex plane.In consequence,the corresponding N-1 fixed closed-loop SSO modes and one varied closed-loop SSO mode will be located approximately in the opposite positions with respect to the multi-modal resonance point where the SSO stability of the power system is deteriorated.In addition,the frequency drift of SSOs is examined based on the proposed multi-modal resonance theory.When the SSO caused by multi-modal resonance among wind clusters within the grid-connected DFIG wind farm occurs in the power system,the oscillation frequency of the varied closed-loop SSO mode will be changed with respect to the number of in service wind clusters.Therefore,if the varied closed-loop SSO mode is the dominant SSO mode of the power system,the frequency drift of the SSO will occur when the number of in service wind clusters is different.