Reliability evaluation of an aggregate battery energy storage system in microgrids under dynamic operation

A microgrid (MG) which mainly consists of distributed generators (DGs) and energy storage systems (ESSs), is deployed to supply power for local loads. Distributed generators are mostly renewable energy sources. An aggregate system with multiple battery energy storage devices that should be used to improve the reliability of power supply from these renewable energy sources in the MG, is defined as an aggregate battery energy storage system (ABESS). The ABESS is used to control the source-load power balance so that the MG can operate at high stability and reliability to supply electricity for different customers. To demonstrate the significance of the ABESS in the MG, its operation reliability will be analyzed in this paper. A systematic method is proposed to evaluate reliability performance of the ABESS under different dynamic operation cases in the MG. Specifically, an analytic approach based on Markov models is developed to assess the operation reliability of the whole ABESS. Besides the used-time-dependent failure rates, voltage-fluctuation and power-loss dependent failure rates (VF-PL DFR) of critical components of the ABESS such as bidirectional DC/DC converters, DC/AC inverters, switching and protective devices, battery modules, and battery charger/controller are also formulated and incorporated in the reliability evaluation. According to differently dynamic operation cases of a microgrid with the ABESS and photovoltaic (PV) generation systems, the VF-PL DFR of the ABESS will be variously affected. Randomly dynamic operation scenarios of the MG are analyzed including: change in load power, the intermittent and unstable operation of PV sources, on/off-grid operation modes of the microgrid, and discharging and charging states of the ABESS. Simulation test results are presented and discussed to validate that the operation reliability of the ABESS in the microgrid significantly depends on its differently dynamic operation strategies along with the applied voltage stress.