High Frequency Resonance Stability Analysis And Additional Damping Control At AC Side Of VSC-HVDC Transmission System

In order to meet the needs of clean energy transmission,load center power supply,energy conservation and emission reduction,China vigorously develops ultra / ultra high voltage DC transmission to realize long-distance and large capacity transmission of energy resources.modular multilevel converter based high voltage direct current(MMC-HVDC)transmission technology has unique advantages and has been booming.In recent years,high frequency resonance events have occurred in many MMC-HVDC transmission projects in China.High frequency oscillation will distort the ac side voltage and current,cause large fluctuations of voltage and current,and may trigger over-voltage and over-current protection device action,resulting in system shutdown and serious harm to system stability.Therefore,in-depth study on the mechanism of high-frequency resonance in MMC-HVDC transmission system is of great significance to suppress the phenomenon of high-frequency resonance and improve the stability of the system.Firstly,the high frequency resonance mechanism of MMC-HVDC transmission system based on impedance analysis method is introduced.The stability analysis in the impedance analysis method is based on the generalized Nyquist stability criterion.The condition of harmonic resonance at a certain frequency point of MMC-HVDC transmission system is that the amplitude of grid impedance and MMC impedance is equal at that frequency point,and the phase angle difference between grid impedance and MMC impedance is greater than 180°.Then,based on the harmonic linearization method,the MMC detailed impedance model is established,which considers the control of current inner loop,power outer loop and phase-locked loop.In order to verify the correctness of the impedance model,based on the electromagnetic transient simulation model of Guangxi side of Luxi back-to-back HVDC project,the impedance characteristics of MMC are obtained by impedance scanning method,and compared with the MMC impedance model established in MATLAB.The results show that the impedance characteristics of the two models are basically the same.Next,based on the equivalent impedance model of MMC,the main influencing factors of MMC impedance characteristics in high frequency band are analyzed,and the influence of VSC-HVDC link delay,phase-locked loop control,positive and negative sequence current inner loop control and power outer loop controller parameters on MMC impedance characteristics is analyzed.As the link delay is the main factor that influences the negative damping of MMC in high frequency band,a high frequency resonance suppression strategy is proposed by adding a filter in the voltage feedforward link to reduce the influence of link delay.The improvement effects of adding low-pass filter,band stop filter and a novel nonlinear filter on MMC impedance characteristics are compared and analyzed.Then,three kinds of additional damping control strategies are proposed,which feedback ac side voltage or ac side current to the current inner loop control link to further improve the MMC impedance characteristics.The improvement effect of additional damping controller parameters on MMC impedance characteristics is analyzed,and the parameter optimization method of additional damping controller is proposed.The effectiveness of the additional damping control and the fault ride through capability of the system are verified by changing delay,setting single-phase metal ground fault and three-phase metal ground fault.Finally,the above researches are summarized,and further research works about high frequency resonance of VSC-HVDC system is prospected.