A Novel Commutation Failure Suppression Method For Series Voltage Commutation Converters

Line Commutated Converter based High Voltage Direct Current(LCC-HVDC)has a series of advantages such as large transmission capacity long transmission distance,Flexible power regulation and has been widely used in cross-regional power transmission in my country.It is an important supporting technology for my country’s "West-to-East Power Transmission" strategy.Commutation failure is a common failure in the operation of the LCC-HVDC system.After the first commutation failure of the DC system,improper control and protection adjustment may cause continuous commutation failure and even DC blockage,which seriously endangers the stable operation of the AC and DC system.Therefore,it is urgent to adopt effective suppression methods to avoid the harm caused by commutation failure.The existing commutation failure suppression methods mainly include DC system control and protection optimization,additional reactive power compensa tion devices,and converter topology transformation.Among them,the converter topology reformation improves the commutation structure of the system and has a better suppression effect of commutation failure under severe AC faults.Starting from the idea o f converter topology transformation,this article focuses on the topology,mathematical model,and new control methods of the Series Voltage Commutated Converter(SVCC),and proposes a new type of commutation failure suitable for SVCC.The suppression me thod improves the SVCC’s resistance to commutation failure and operation stability,and provides a new solution for the prevention of commutation failure.The main research contents include:(1)To explore the basic operating characteristics and commutatio n failure resistance of SVCC.First,the basic topology of the SVCC converter chain is studied,and according to the actual operating conditions of the SVCC converter chain,IGCT is determined as the power device of the SVCC converter chain;then the tradi tional control method of SVCC is studied,combined with the SVCC converter chain.The mathematical model of SVCC is established in the process of capacitor switching.Finally,according to the SVCC mathematical model,the SVCC commutation failure resistanc e capability is evaluated,and the capacitor charging and discharging imbalance problem existing in the SVCC operation is revealed.(2)Aiming at the problems of unbalanced capacitor charging and discharging and insufficient flexibility in the operation of SVCC,a flexible capacitor voltage adjustment control method suitable for SVCC is proposed,which includes a capacitor voltage control strategy based on a proportional controller and a fast fault degree detection..This method flexibly controls the voltage value of the capacitor voltage of the converter chain connected to the system according to the degree of AC failure.By cooperating with the commutation process of the valve group,the converter chain is constructed to trigger the pulse signal in advance,which solves the imbalance of capacitor charging and discharging during the operation of the SVCC converter chain.The problem is to improve the system’s resistance to commutation failure and reduce the reactive power demand of the inverter on the receiving-end power grid.Based on PSCAD/EMTDC simulation,the effectiveness of the flexible capacitor voltage adjustment control method is verified.(3)Aiming at the difficult problems of SVCC control such as large detection volume and complex logic processing,a SVCC fixed-time control method suitable for engineering application is proposed.This method determines the SVCC variable current by solving the SVCC variable current chain capacitor charge and discharge equation under fault conditions.The chain switching time avoids complicated signal detection and logic processing,and improves the stability of SVCC control.Simulation based on PSCAD/EMTDC verifies the effectiveness of the fixed-time control method.