Research On Suppression Strategy For Subsequent Commutation Failure Of HVDC Transmission System

The successive commissioning of domestic UHVDC transmission projects has effectively solved the current outstanding problems with the development of energy and power,meeting the development and transmission of large-scale energy bases in the western and northern regions and the power needs of the eastern and central regions.Conventional HVDC systems use ordinary thyristors with no self-shut-off capability as commutation elements,and therefore there is a risk of commutation failure in HVDC systems.Among them,the subsequent commutation failure is particularly harmful to the AC and DC systems.The paper takes the subsequent commutation failure as the research object,analyzes its mechanism,and proposes corresponding suppression strategies,which are of great significance for the safe operation of AC/DC hybrid power grids.First of all,analyze the occurrence mechanism of subsequent commutation failure.From the perspective of the response change of each control link at the inverter side of the HVDC system during a fault,the interaction mechanism and action law of each control link are sorted out and analyzed.It is pointed out that the large and severe fluctuation of the output current command of the voltage dependent current order limiter(VDCOL)is the key cause of the subsequent commutation failure.Then,a non-linear Logistic function VDCOL control method is proposed based on the original VDCOL from the perspective of VDCOL itself.This method can effectively limit the fluctuation range and steepness of the VDCOL output current command,thereby achieving suppression of the subsequent commutation failure.The control method is implemented by the CIGRE HVDC benchmark model,and its effectiveness is verified.The suppression effect of this control method is more significant under the condition of three-phase ground fault.Finally,seen from the input link of VDCOL,a virtual-capacitance control method that suppresses the drastic fluctuation of VDCOL input voltage is proposed.This method effectively improves the large and severe fluctuations of the VDCOL output current command during a fault and reduces the probability of the subsequent commutation failure.This control method is implemented by the CIGRE HVDC benchmark model,and its effectiveness is verified.Compared with other methods,this control method has certain advantages in the suppression of the subsequent commutation failure and system recovery characteristics.