PNNL

Abstract

The role of energy storage systems (ESSs) is becoming increasingly important for today’s electric power systems. Unavailability of an ESS assigned to critical grid services may cause unwanted disruption of those services and hence, may have a significant techno-economic impact. Like any physical equipment, an ESS is vulnerable to various types of faults. Failure of one or more strings in a multistring ESS does not have to be the cause of shutting down the entire ESS. It can still operate with partial number of strings and continue providing critical services to the grid, if there are no reliability or safety issues and is acceptable under applicable standards. However, it is important to make sure that the control strategies are adaptable to the changes in ESS capacity caused by failed strings. Also, depending on the previous operation and type of failure, the reallocation of duty cycle burden among available strings could be non-uniform. These complexities suggest that mitigation of the impact of partial failure in multistring ESSs is not trivial and will need careful consideration. This paper will investigate the impact of partial failure of large multistring ESSs on the assigned services and will develop strategies to adjust the ESS control duty cycles for reducing such impacts. Illustrations provided in this report are based on frequency regulation use case, which is a common application for many utility scale ESSs. A 750 kilowatt (kW)/1500 kilowatt-hour (kWh) 3-string ESS will be used for the demonstration in this work. The strategy can be scaled up for larger ESSs.