直驱风电场并网对火电机组次同步谐振影响

Influence of Grid Connected DirectDrive Wind Farm on Subsynchronous Resonance of Thermal Power Units

直驱风电场(D-DWF)接入火电经串补外送系统后,其对火电机组的次同步谐振(SSR)影响机理尚不明确。针对风火打捆经串补点对网型系统,采用适用于模块化建模的状态变量消去方法,建立模块化的耦合小信号模型;利用特征值分析法研究D-DWF接入后系统的SSR情况;结合特征值分析结果阐释SSR机理,并针对D-DWF及串补线路参数进行阻尼分析。研究结果表明:D-DWF会引入与SSR频率相近并引起“共振”的一个新模态,使得“机电谐振”与“模态频率接近”的振荡机理耦合共存。在跟网型风机的控制策略及参数下,D-DWF并入会导致SSR模态阻尼大幅降低,恶化系统的SSR。随着D-DWF的容量、风速和系统串补度降低,系统阻尼将提高,SSR的风险将降低。基于PSCAD/EMTDC平台的电磁暂态仿真,验证了机理分析的正确性。

The direct-drive wind farm(D-DWF)and thermal power bundled point-to-network system sent through series supply lines have become the main form of energy consumption in recent years because of its advantages of large transmission capacity and mature and reliable technology.However,there is a subsynchronous resonance problem in the thermal power series compensation system,and the direct drive wind farm system itself also has the risk of subsynchronous oscillation.Therefore,the oscillation problem of the baling system cannot be ignored.After D-DWF is connected to the thermal power series replenishment system,its influence mechanism on the subsynchronous resonance(SSR)of thermal power units is not clear.Firstly,based on the linearization mathematical model of point-to-mesh system with D-DWF and thermal power bundled and sent by series supplementation,the state variable elimination method suitable for modular modeling is used to establish a system small signal model based on modular modeling idea in MATLAB/Simulink,and its reliability is verified by step response.Secondly,the small signal model of the D-DWF system before and after grid connection is solved,and the SSR characteristics are analyzed.Then,the SSR mechanism of the D-DWF system after access is explained,and the SSR mechanism is verified by time domain simulation.Finally,taking the capacity,wind speed and string complement of D-DWF as the research objects,the risk of SSR in the system is analyzed from the perspective of damping characteristics and time-domain response.When the series complement is 23.5%,the corresponding resonant frequency is about 29 Hz,which is complementary to the frequency of SSR mode(corresponding to thermal shafting mode 4)in the system.At this time,when the system is disturbed,the subsynchronous component acts on the thermal power unit through the point of common coupling(PCC),inducing SSR.When there is a mode of about 32 Hz in D-DWF,after the disturbance occurs,the subsynchronous component will also act on the thermal power unit through PCC,which will excite the SSR in the system and intensify the oscillation.After the thermal power unit generates SSR,the thermal power unit feeds the subsynchronous component back to the PCC through the speed signal,and the subsynchronous component flows through the closed loop.The main conclusions of this paper are as follows:(1)Through step response,the reliability and accuracy of the small signal model of wind-fire bundle warp supply system are verified.(2)Before and after D-DWF access,the system has four negative damping torsional vibration modes,including an SSR mode.After D-DWF is connected,it will form a subsynchronous excitation loop with series lines and thermal power units.The oscillation mechanism shows the coupling phenomenon of"electromechanical resonance"and"modal frequency approach";SSR mode damping is reduced,the risk of system SSR occurrence is increased,and the torsional vibration mode damping of other negative damping is improved.(3)The reduction of D-DWF capacity,wind speed decrease,and series complement decrease will increase the damping of SSR mode and reduce the risk of SSR.The string complement degree after D-DWF is incorporated is still the main influencing factor of SSR in the system,and the influence effect of D-DWF is limited.