A smooth switch method for battery energy storage systems between Vf mode and PQ mode by utilizing electromagnetic relay

Battery energy storage systems (BESS) plays a significant role in micro-grids which consist of renewable energy systems. As the interface between storage components and grids, converter takes a big cost in ESS. Cost abatement of each part in converters is an important part in economic consideration. For grid connection, solid state relays (SSR) and electromagnetic relays (EMR) are two common choice. SSR owns a quick response (less than few ms) while EMR is much cheaper and with low conduct loss, but suffers a much slower response (5 to 50 ms) due to its excitation and mechanical movement. For fast response (milliseconds) of converters, the switch that connects ESS and grid is ordinarily SSR. Grid connection relay is not one of the most expensive components in converters, but assuming that thousands of SSRs in ESS being replaced by EMRs, a huge cost would be saved. This paper mainly discuss a new smooth switch method between Grid-connected and off-grid states based on Vf and PQ control, which allows electromagnetic relay takes the place of solid state relay, so as to reduce the cost.