Selection of converter topologies for distributed energy resources

Distributed energy resources (DER) are becoming increasingly common on the electrical grid. Depending on the operating conditions of the DER, which depend on the application, different topologies need to be selected in order to achieve the maximum efficiency of each DER. Complicating the selection is the fact that operating conditions vary over time. For example, the voltage and current drawn from a PV panel varies over the course of a day. To calculate the overall efficiency, the efficiency of a topology at each operating point and the amount of time spent at that operating point must be considered. This work extends existing analytical methods for loss calculations by taking this into account. The specific DER applications considered are a three-phase ultracapacitor energy storage unit (UC-ESU), battery energy storage unit (B-ESU), and photovoltaic array (PV). This work determines for each application if an inverter-only (single-stage) or an inverter plus boost converter (double-stage) topology is more efficient. The results show that a single-stage topology is better for the B-ESU and PV, while the double-stage topology is better for the UC-ESU. The method is applicable to other DER types, including wind turbines, micro-hydro generators, variable-speed gensets, and microturbines.