This paper reports on an investigation of the impact of a two-stage battery energy storage system (BESS) on the voltage and frequency stability of a power transmission system. The BESS's two stages are: a DC-DC converter and a DC-AC converter connected in tandem. System stability is a major concern in power system transmission and it has long been acknowledged that battery energy storage would be a possible way forward to improve power system stability, but the battery technology and the ancillary power electronics have not been developed sufficiently, until quite recently, to meet such expectations. The two-stage BESS is able to mimic the operation of a conventional synchronous generator plant but with a faster speed of response because of its lack of rotating parts - it is in fact an electronic power plant. The results show that the BESS does improve power system frequency stability very significantly.