Design/test of a hybrid energy storage system for primary frequency control using a dynamic droop method in an isolated microgrid power system

Frequency dynamics, occurring due to the high penetration of the renewable energy in the microgrid (MG) are of great concern to the system dynamic stability. The battery energy storage systems are reported to have a good frequency regulating ability in the off-grid microgrid systems. However, to compensate the power irregularities, the battery is needed to charge and discharge at a high frequency, which degrades its lifetime significantly. In addition, in the primary frequency control (PFC) the battery needs to deal with the abrupt power changes, which will also accelerate the battery degradation process. In this regard, this paper presents a new concept of primary frequency control by integrating the superconducting magnetic energy storage (SMES) with battery, thus achieving the ability of not only performing a good frequency regulating function but also extending the battery service time. A novel power sharing method using the dynamic droop factors to control charge/discharge prioritization between the SMES and the battery is proposed and has been proved to have a better operation than the preceding droop control. A microgrid system based on the case of Uligam Island of Maldives is developed in the PSCAD, verifying the performance of PFC with the hybrid energy storage system (HESS) using the dynamic droop control. The results show that the HESSs have a better frequency regulating ability and the proposed dynamic droop control is capable of exploiting the different characteristics of both SMES and battery, forming a kind of complementary hybrid energy storage system. Moreover, the battery in the new control scheme is better protected from the short-term frequent cycles and abrupt currents, hence has been proved to have a longer lifetime extension.