Comparison of AC and DC Bus Interconnections of Energy Storage Systems in PV Power Plants with Oversized PV Generator
Markku Järvelä& Seppo Valkealahti
Tampere University, Tampere, Finland

Oversizing the photovoltaic (PV) generator increases the energy yield and improves the profitability of PV power systems. Downside of oversizing is energy losses due to power clipping. If an energy storage system (ESS) is needed to comply with grid codes, connecting it on the DC bus of the inverter enables the utilization of otherwise clipped energy. We created a simulation model to compare the AC and DC bus interconnections of ESS in a typical building block of megawatt-scale PV power plants. We used real irradiance measurement data to determine the output power of the PV generator and analyzed different combinations of DC-to-AC power ratios and ESS energy capacities. To make the analysis universally more applicable, we categorized the results by daily production profile types. The DC bus interconnection increases the energy yield compared to the AC bus interconnection. Defining the optimum ratio between the DC to AC power ratio and ESS energy capacity is straightforward on clear-sky days. However, ESSs with smaller energy capacities perform better on partially cloudy days, which makes the determination of optimum PV generator power to ESS energy capacity ratio difficult.