Control Strategy of Hybrid Energy Storage System for High-Dynamic Load Changes

During the last few years, the idea of a single battery system for DC load supply has been expanded to provide the possibility to parallel work of two different energy storage types: one with high power density e.g., supercapacitor (SC), and a second with high energy density – usually electrochemical Lead Acid or Li-Ion batteries. A system composed of both kinds of energy storage is known as a hybrid energy storage system (HESS). It ensures high power capacity during load changes, which helps decrease the DC bus voltage error in transient. Simultaneously the HESS delivers the energy to the load from the source characterized by high energy density, which provides the power balance for a long time in a steady state. The advanced hybrid energy storage system and different characteristics of utilized energy sources required a novel control method, which improves energy management inside the HESS. It necessitates independent energy management for high-power density and high-energy density storage. Due to the utilization of energy collected in the SC under transients, the control algorithm of the HESS should provide its stored energy in order to keep the voltage of the DC bus constant, which results in the need to recharge the SC after load change. In the paper, a novel control method has been described, which provides the constant DC bus voltage as well as the constant voltage of the SC in the steady state of HESS. As a result, the fully charged SC can be utilized in each dynamic state as well as during the short battery overload, when the current of the battery is limited.