A dual-functional medium voltage level DVR to limit downstream fault currents

Abstract

Dynamic Voltage Restorer (DVR) is a modem custom power device used in power distribution networks to protect consumers from sudden sags (and swells) in grid voltage. Implemented at medium voltage level, the DVR can be used to protect a group of medium voltage or low voltage consumers. With the implementation at medium voltage level the DVR would also suffer from faults those involving downstream loads. Large fault currents would flow through the DVR during a downstream fault before the opening of a circuit breaker. Large fault currents will cause the voltage at Point of Common Coupling (PCC) to drop, which would affect the loads on the other parallel feeders connected to PCC. Furthermore, if not controlled properly, the DVR might also contribute to this PCC voltage sag in the process of compensating the missing voltage thus further worsening the fault situation. To limit the flow of large line currents, and therefore restore the PCC voltage as well as protect the DVR system components, a downstream fault limiting function is proposed and integrated in the DVR operation. A flux-charge-model feedback algorithm is implemented so that the DVR would act as a large virtual inductance in series with the distribution feeder in fault situations. Controlling the DVR as a virtual inductor would also ensure zero real power absorption during the DVR compensation and thus minimize the stress in the DC link. Finally the proposed fault current limiting algorithm has been tested experimentally on a medium voltage level DVR system with symmetrical and unsymmetrical downstream fault conditions with the results verifying the effectiveness of the proposed strategy for limiting the downstream fault conditions.