Rationalisation and validation of dc power transfer limits for voltage sourced converter based high voltage DC transmission

Maximum available power (MAP) is an index for analyzing power-voltage stability in high-voltage direct current (HVdc) converters. Earlier work on calculating MAP for voltage sourced converters (VSC) was not consistent with the methods used for line commutated converters (LCC). The Thévenin voltage source used in VSC-HVdc analysis was set to 1.0 pu, but for LCC-HVdc, it was calculated so as to provide rated conditions at the converter busbar. This study attempts to rationalize the approaches used for VSC-HVdc and LCC-HVdc calculations by making them mutually consistent. However, as the VSC can operate at much lower short-circuit ratio (SCR) than can the LCC, the Thévenin equivalent voltage at low SCRs can be significantly larger than 1.0 pu. This would require an unreasonably large tap changer setting. As an alternative, the paper recommends that for calculating MAP for a VSC, an ac network equivalent be used where some reactive power is locally supplied by shunt capacitors. This renders a Thévenin voltage close to 1.0 pu. This study also considers the impact of the VSC’s internal voltage limits on the MAP. The key results are validated using electromagnetic transients simulation.