Functioning of battery energy storage system converters during a fault in electricity network

Abstract

MW-scale battery energy storage system (BESS) connections to medium-voltage (MV) distribution networks are increasing, raising concerns for distribution system operators (DSOs) about fault currents and protection performance. Because BESS plants are interfaced through power conversion systems (PCS), their fault response is primarily shaped by converter control and semiconductor current limits rather than by synchronous-machine dynamics. This thesis examines how grid-connected BESS converters behave during MV faults and how their behaviour alters protection-relevant quantities at the point of connection (PoC) in a 20 kV compensated-neutral MV network. An EMT simulation framework compares a validated baseline MV feeder against 1 MW, 5 MW, and 10 MW BESS cases under strong- and weak-grid conditions defined by different short-circuit strengths (fixed X/R = 7). Single-line-to-ground faults are analyzed with 0.5 Ω and 5 Ω fault resistances, while line-to-line, double-line-to-ground, and three-phase faults are analyzed with 0.5 Ω, using a grid-following, current-limited converter model with reactive-priority response (1.2 pu limit). Results based on RMS currents and voltages and fundamental-frequency active and reactive power show that, for phase faults, fault-current magnitude is dominated mainly by upstream short-circuit strength, while the BESS contribution remains bounded by current limits and does not scale like a synchronous source. In compensated-neutral SLG faults, the baseline response is governed mainly by the earthing method, while the BESS modifies the relay-facing current and voltage pattern through controlled current injection. The main protection implications are changes in voltage depression, current distribution between phases, sequence-related behaviour, and reactive power exchange at the PoC, most pronounced in weak-grid conditions and unbalanced faults.