Theoretical aspects of fault isolation on DC lines using resonant DC/DC converters

This study investigates direct current (DC) fault current limiting and interrupting capability of a multi-megawatt resonant DC/DC converter, which is proposed for applications with high-voltage direct current (HVDC) systems. The converter can be used as a DC circuit breaker (CB) or as a multi-functional unit: DC transformer, regulating element and DC CB. The study is primarily concerned with the inherent converter responses, in the first several milliseconds after the fault, and prior to any controller action. A detailed converter design is given to prevent fault propagation through the converter even for most severe faults on low-voltage or high-voltage terminals. The analytical modelling proves that the converter will internally reduce power transfer during fault conditions. The simulation on PSCAD/EMTDC shows that converter operates uninterrupted through most severe faults, the switch turn-off time is not violated and inherent stabilising properties prevent any overvoltage or extreme currents. The converter can respond like high-impedance circuit on the unfaulted terminals. The impact of unbalanced DC line faults on a bipolar DC system is also discussed. The detailed PSCAD tests with a 200 MW DC/DC converter interconnecting +44 and +250 kV DC lines demonstrate the ability to prevent fault propagation even for most severe DC line faults. The interaction with other HVDC converters in case of faults is also demonstrated.