The Small Signal Stability Analysis Of MMC For Wind Farm Integration

High voltage direct current(HVDC)transmission based on modular multilevel converter(MMC)has a series of advantages such as low output harmonics,low switching losses,no reactive power compensation,and black start capability,and therefore is widely applied in grid integration of newly commissioned and under construction wind farms.However,both the wind farm and the sending-terminal MMC are complex power electronic devices,and their interconnected system is prone to instability and resonance due to the mismatch of power electronic characteristics,significantly restricting the capability of wind power transmission.The impedance-based stability analysis method is an effective way to study the small signal stability of interconnected systems,and it is applied in this paper to study the small signal stability of interconnected system between the wind farm and the MMC.1.Based on the multi-harmonic linearization method,the small signal impedance model of sending-terminal MMC with double closed-loop AC voltage control and circulating current suppression control is developed.Based on the derived model,the impedance characteristics of sending-terminal MMC are analyzed,and the influences of factors,such as circuit parameters,bandwidth of voltage control loop,bandwidth of control current loop on MMC impedance characteristics are studied.2.The frequency coupling mechanism of MMC is revealed by analyzing the interaction between perturbation and steady-state harmonics.Based on the derived impedance model of sending-terminal MMC,the frequency coupling characteristics are further considered,and the frequency coupling model of MMC with AC voltage control and circulating current suppression control is developed.Based on the derived model,the main influencing factors of MMC frequency coupling characteristics in different frequency bands are studied.3.Based on the derived MMC impedance model and impedance-based stability analysis method,the small-signal stability of interconnected system between the wind farm and the sending-terminal MMC is analyzed.The instability problems of this system caused by unsuitable bandwidth of MMC voltage loop and current loop are revealed,which provides a basis for optimization of voltage loop and current loop parameters.With the MMC frequency coupling model and the generalized Nyquist stability criterion,the influence of MMC frequency coupling characteristics on the stability judgment of interconnected system is studied.The simulation system based on Matlab / Simulink verifies the accuracy of derived model and stability analysis results.