Analysis on Deblocking and Blocking Characteristics of UHVDC Commutation Units Based on Real-Time Digital Simulator

By means of real time digital simulator (RTDS), the control strategies for two conditions are researched, i.e., another commutation unit is deblocked in on-line way while one single commutation unit in a pole of UHVDC project is running or one of two single commutation units is blocked in on-line way while two single commutation units in a pole of UHVDC project are running. Research results show that both zero power deblocking mode and small firing angle mode are feasible, the firing angle under zero power deblocking mode is about 85° and the firing angle should be not less than about 70°; When a set of commutation units are deblocked by on-line way, so long as the coordination is appropriate, both deblocking modes are available, i.e., the deblocking is firstly implemented at rectifier side then at inverter side or adopting the contrary operating order. Considering security and reliability as well as smaller disturbances in DC and AC system, when a set of commutation units is deblocked in on-line way, the zero power deblocking mode can be used while the deblocking is firstly executed at rectifier side; when a set of commutation units are blocked in on-line way, in order to be favorable to the deblocking of whole commutation set, the inverter should be firstly seceded from the commutation unit.