Modelling of battery energy storage systems under faulted conditions: Assessment of protection systems behaviour

The paper present a preliminary study of the protective relays of a 10 MW sized battery energy storage system (BESS) recently installed by TERNA, the Italian TSO. A complete model of the system, including inverters, has been implemented in ATP-EMTP in order to assess the behaviour of protective relays during symmetric and asymmetric faults. Results show that ground fault protection of MV feeders can be performed by zero-sequence non-directional overcurrent relays, obtaining a correct fault selection even for fault resistances in the kΩ range. Phase-to-phase and three-phase fault selection can be performed by means of overcurrent relays: an inverter current limiting control under fault conditions has to be implemented to avoid the system shutdown. Lastly, HV-level faults have been simulated during BESS-supplied, islanded operation, evidencing very low fault current contributions, depending on the actual share of battery modules operating at the time of fault, with less modules giving worst results.