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The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.Index Terms-Battery energy storage system, dc-ac converter, dc-dc converter, power conversion, power electronics I. INTRODUCTIONRenewable energy sources (RES), including wind turbines and solar PV systems, have been installed at a fast pace globally in recent years [1], [2]. The intermittent nature of output power from RES becomes a serious concern for the stability of the grid, particularly with increased RES penetration and at times when a high percentage of instantaneous demand is supplied by RES. In the case of Germany where 80% of instantaneous demand was supplied by RES on the 23 rd August 2015 [3], significant operating reserves were required to meet the demand in case of a sudden decrease in the output of RES, thus causing an increase in the operational cost of the electricity network. Utility-scale battery energy storage systems (BESS) featuring fast response characteristics can provide an economic and promising alternative to smooth the output power of RES [4] and provide operating reserves [5], as there is virtually no cost to the system when BESS are in reserve state i.e. not providing power [6].

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