Virtual synchronous machine control with virtual resistor for enhanced short circuit capability

Increasing shares of power electronic converters in renewable generation challenge power system stability. For very high shares the state of the art current-control approach should be replaced by grid forming control schemes. The virtual synchronous machine (VSM) approach is grid forming and fully compatible to conventional synchronous-machine-based power plants. The high fault currents of synchronous machines are also replicated and lead to excessive current ratings of the semiconductors. Current control could be applied again during faults, however the grid-forming property is lost when it is most needed. We propose a VSM with a virtual resistor that reduces fault currents while the grid forming functionality is maintained. For the most severe fault the peak current amplitude is reduced to 2.5 times the nominal current. Swing mode and high-frequency stability is investigated in detail. The results have been confirmed by experiments. The over-current requirements and thus component cost can be reduced drastically through the application of a virtual resistor. Nonetheless, an increased current rating of the hardware is required to handle severe faults. System studies should be performed to confirm the method on system level.