Small Signal Oscillation Stability Of The Power System With Grid-connected DFIGs

In recent years,there have been many oscillation accidents in power systems caused by grid-connected wind farms at home and abroad.In these accidents,typical system structures includes wind farm-series compensation transmission system,wind farm-weak AC grid system,wind farm-VSC HVDC system,etc..The oscillation instability originates from the unfavorable dynamic interaction between the wind farm and the other dynamic components in the power system.And the oscillation characteristics are fast divergence speed and wide-ranging time-varying.Due to the large number of wind turbines generators in the wind farm,the coupling between wind turbine generators,and between wind turbine generator and power systems is complex.At present,the mechanism of oscillation instability caused by grid-connected wind farms is not clear.Therefore,in this dissertation,a series of work is carried out on the small signal oscillation stability of power system with grid-connected DFIGs,in which the subsynchronous oscillation stability is focused.The main research contents and contributions are as follows:(1)The mechanism commonality between the sub-synchronous oscillation(SSO)problem of the DFIG-series compensation transmission system and the traditional sub-synchronous resonance problem is studied.Firstly,from the perspective of open-loop modal coupling and closed-loop modal repulsion,a new explanation of the torsional vibration is introduced,that is,when the open-loop RLC oscillation mode dominated by the series-compensated transmission line coincides with the open-loop torsional vibration mode dominated by the shaft of the synchronous generator,the closed-loop torsional vibration mode and the closed-loop RLC oscillation mode repulsed each other,and the damping of one of the closed-loop oscillation modes is reduced.This idea is further extended to the study of the mechanism of the SSO in series-compensated transmission system integrated with a DFIG wind farm.A closed-loop interconnection model is established,wherein the DFIG wind farm and the series-compensated transmission line are two interconnected open-loop subsystems.The open-loop modal coupling and the closed-loop modal repulsion between the RLC oscillation mode dominated by the series-compensated transmission line and the SSO mode dominated by the converter control of the DFIG is examined.Further,the connection between the two mechanisms of open-loop modal coupling and the induction generator effect is studied,and it is pointed out that the two mechanisms can complement each other,which makes the mechanism explanation of the SSO in series-compensated transmission system integrated with a DFIG wind farm more perfect.Finally,based on the impedance analysis method and the open-loop modal analysis method,the effects of impedance interaction and modal interaction between the two subsystems which are DFIG and the series-compensated transmission line on the SSO stability are analyzed.(2)For the SSO frequency drift problem of the power system integrated with wind farm,the mechanism is investigated from the perspective that the variation of wind speed affects the open-loop modal coupling condition.The open-loop SSO modes and the corresponding residues of wind turbine generators affected by wind speed are derived.It is pointed out that the variation of wind speed may bring about two effects.One is that the mode pair that meets the open-loop mode coupling condition changes.This results in the repulsion of closed-loop modes in different frequency ranges.The other is that it affects the degree of closed-loop modal repulsion under the condition of open-loop modal coupling.Thus,the variation of wind speed leads to the change of damping and frequency of the dominant oscillation mode.The theoretical analysis results are examined in two example systems,each is power system integrated with double-machine equivalent DFIG wind farm or power system integrated with hybrid wind farm.(3)For the problem of SSO of a multi-machine power system with multiple wind farms,a multi-input multi-output closed-loop interconnection model is established first.Based on the open-loop modal coupling theory of the multivarable system,combined with the structural characteristics of the mathematic model of multiple wind farm subsystem,an equivalent reduced-order open-loop modal analysis method is proposed.The effects of the number of grid-connected wind turbine generators,the output active power of the wind turbine generators,and the position of the wind farm on the SSO stability are analyzed.And a method of suppressing the SSO is proposed from the perspective of optimizing the converter control parameters to eliminate the open-loop modal coupling condition.(4)The effect of the dynamic interaction between multiple DFIGs on the small-signal oscillation stability of the power system is studied.The stability analysis method in the consensus control theory of homogeneous system is applied to the multiple DFIGs interconnected power system.The decoupling analysis of dynamic interaction was realized.And the results demonstrated that the influencing factors of the SSO stability can be divided into two aspects:1)The dynamic characteristics of a single DFIG,which determine the nature of the impact of dynamic interaction on stability;2)The topology of the interconnected power network,which affects the degree of impact of dynamic interaction on stability.