Protection Strategies For Integrating Battery Energy Storage Systems In Transmission Networks

Integrating Battery Energy Storage Systems (BESS) into transmission systems presents unique system modelling and protection challenges. BESS acts as an electricity buffer, absorbing the excessive power from the grid and sending it back whenever it is needed. It connects to the grid through inverters, necessitating careful consideration of short-circuit modelling and protection schemes. This paper will focus on modelling the 115kV transmission system of 180 MW BESS in commercially available and widely used short circuit simulator software. BESS is integrated via a 115 /34.5 kV wye grounded/wye-grounded and a delta tertiary. The paper shows the system and protection benefits of the transformer configuration over other configurations. Issues arise during unbalanced faults due to low or unpredictable negative sequence currents from the inverter, impacting the reliability of directional elements in relay protection, particularly in distance and directional ground overcurrent protection of transmission lines. Two protection schemes, Directional Comparison Blocking (DCB) and Permissive Overreaching Transfer Trip (POTT), are evaluated in terms of their effectiveness and limitations. Torque control equations of overcurrent relays are adjusted for both phase and ground faults, supplemented by under-voltage relays to provide backup protection in case of tele-protection loss. The BESS comprises seventeen pairs of 480 V DC cells, each connected through an inverter and 480 V/34.5 kV transformers to a 34.5 kV collector station. The transformers are ungrounded on the 34.5 kV side. The ungrounded collector cables linking transformers to the bus are protected by expulsion (EXP) fuses with a backup current-limiting (PRCL) fuse arrangement. The paper highlights challenges in protecting the collector cables sourced by the ungrounded inverter system. Furthermore, the paper reviews anti-islanding protection mechanisms, where the inverter disconnects from the grid to prevent islanded operation of the BESS, ensuring loads are not exposed to abnormal frequencies and voltages. Anti-islanding protection must operate swiftly while adhering to regulatory requirements such as NERC PRC-024, which mandates compliance with ride-through requirements for external system disturbances. Challenges arise as these requirements apply to the point of interconnection at 115 kV. At the same time, anti-islanding protection is embedded within the inverter control system operating at 480 V. Since the BESS is 180 MW, compliance with PRC-025 is required for the inverter protection system. The paper addresses these challenges and discusses compliance with regulatory mandates.