AC system’s strength evaluation of UHVDC system under hierarchical connection mode

The inverter’s end of ultra high voltage DC (UHVDC) system under hierarchical connection mode (HCM) is connected to 500 and 1000 kV AC buses through high end converter and low end converter transformers, respectively. The two AC buses are interconnected via coupling transformer and the tie line. This complex structure of interconnection at inverter’s end can make the understanding unclear about each AC system’s strength of UHVDC-HCM. Therefore, this paper covers the system’s strength evaluation by developing the “Hierarchical mode effective short circuit ratio” (HMESCR) index. The HMESCR incorporates the impact of neighborhood AC networks, impedance of coupling transformer, the tie line and strength of the objective AC network. Based on the proposed index, an equivalent model for UHVDC-HCM system is developed. An equivalent model is verified under the steady state conditions such as maximum available power and the transient conditions such as commutation failure immunity index under AC faults. In addition, three different cases are considered in order to get the in-depth and comparative performance analysis of an equivalent model with UHVDC-HCM system. MATLAB and PSCAD/EMTDC are used as analysis tools for illustrating the steady state and transient state conditions. The analysis indicates that the presented equivalent model has similar behavior as UHVDC-HCM system under both steady state and transient state conditions, which verifies the accuracy of proposed index in assessing the strength of both AC systems of UHVDC-HCM.