Distributed sliding mode control for multi-module battery energy storage system state of charge balancing

Abstract

This paper presents a hierarchical control strategy for a multi-module battery energy storage system, with distributed sliding mode controllers for state of charge balancing between modules. Under the proposed control strategy, a central controller generates a nominal module output power reference to regulate the total battery energy storage system output power. The nominal output power reference is sent to distributed battery module controllers, which use local state of charge information and neighbour to neighbour communication over a sparse network to coordinate their output powers so that they move towards a balanced state of charge. Unlike a linear control approach, the proposed control strategy ensures that the battery modules are either all charged or all discharged, depending on whether the total power demand is negative or positive. This eliminates circulating currents, increasing efficiency and reducing battery lifetime degradation. As the state of charge of a battery module approaches a boundary layers around the average of its neighbours' state of charge, the sliding mode control swaps to a linear interpolation that prevents chattering. State of charge synchronisation under the proposed control strategy is achieved for very general assumptions on the nonlinear battery dynamics. The performance of the proposed control strategy was verified with an RTDS Technologies real-time digital simulator, using switching converter models and nonlinear lead-acid battery models.