Design and test of a new droop control algorithm for a SMES/battery hybrid energy storage system

High capacity energy storage units are desirable to maintain power system stability in the presence of power disturbances produced by renewable energy sources and fluctuating load profiles. Battery energy storage systems may be used to smooth power flow, however, the frequent, deep charge and discharge cycling required dramatically reduces battery service life. A hybrid energy storage system (HESS) using battery energy storage with superconducting magnetic energy storage (SMES) is proposed to mitigate battery cycling while smoothing power flow. A HESS power sharing control method based on the novel use of droop control is proposed. This is able to control charge/discharge prioritization and hence protect the battery from high power demand and rapid transient cycling. A sizing strategy is proposed for the battery and SMES which overcomes the oversizing problem. A hardware implementation is used to assess the control and SMES sizing methods for short time scale HESS operation. A dynamic off-grid sea-wave energy conversion system is simulated to assess the performance of the HESS over a longer time scale. A battery lifetime model which takes into account both battery life cycles and discharge current rate is used to estimate battery lifetime extension. A lifetime increase of 26% is obtained for the HESS design example investigated.