Inverter turn-off angle control optimization of UHVDC with hierarchical connection to AC grid

Advanced DC transmission technology is an important basis for building global energy Internet in the future. It is the main scheme to solve the problem of large power capacity long distance transnational or intercontinental transmission. In order to improve the receiving system of large scale power consumptive ability, The usage of UHVDC with hierarchical connection to AC grid technology has been extended in a wide range. However, due to the coupling relationship between the hierarchical DC inverter side, the traditional control strategy in inverter side easily leads to an commutation failure of converter valves on non-fault level when AC power system failure occurs on the other voltage level. This single fault causes fast conduction in different voltage levels through DC system, and the power grid will show fault characteristics in different voltage levels simultaneously, which expands the influence scope. This paper presents an improved inverter turn-off angle control strategy. Commutation failure preventive control parameters in logic were optimized through formula derivation and simulation analysis methods, which can ensure the stable operation of converter valve on non-fault level, and avoids the occurrence of simultaneous commutation failure on both voltage level. Electromechanical and electromagnetic hybrid simulation with practical planning power system is taken based on ADPSS software and the effectiveness is proved.