Reliability evaluation to converter transformers of 2×12-pulse UHVDC transmission system

The converter transformers (CTs) between the AC system and the converters are necessary for reliable operation of the ultra high voltage DC (UHVDC) system. Compared with the traditional HVDC system, 2×12 pulse UHVDC system has more complicated configuration and more capacity levels; the CTs with Y/Y and Y/Δ connections can´t be replaced by each other, adding difficulty to the reliability evaluation, which has not be fully investigated yet. In this paper, reliability of the CTs for UHVDC system is quantified using the State Space (SS) and Frequency & Duration (F&D) methods. The state spaces of the single-phase CTs with three- and two-windings are proposed. Six operating modes are defined for the CTs. The spare for the three- and two- winding CTs is considered. The two-winding CTs with Y/Y and Y/Δ connections are differentiated and modelled separately to reduce number of the states. The numerical results are compared with the existing literatures to show accuracy and advantage of the proposed model.