Optimum Design of MMC-Based ES-STATCOM Systems: The Role of the Submodule Reference Voltage

In recent years, the combination of battery energy storage (ES) systems and static synchronous compensators (STATCOM) in a single device, known as ES-STATCOM, has been studied by both academia and industry. The modular multilevel converter is an interesting candidate for ES-STATCOM realization. The integration of batteries based on dc/dc converter presents advantages in terms of design flexibility. However, there is a gap in the literature in terms of design and cost optimization of ES-STATCOM systems. To fill this void, this article provides a methodology to support engineers to design and perform cost optimization of ES-STATCOM systems. Both capital expenditures and operational expenditures due to energy losses and battery replacement are taken into account. The methodology identifies what is the optimum submodule voltage reference, which minimizes the ES-STATCOM total cost for a given target of operating time. A lifetime model is employed to predict if battery replacement is required. This model computes the battery capacity fade caused by the operation mission profile considering both calendar and cycling aging mechanisms. The case study is based on a 25 MVA/75 MWh ES-STATCOM connected to a 34.5 kV grid, which provides ancillary services for a photovoltaic power plant. Different rated blocking voltages are assumed for the semiconductor devices. Moreover, different maximum and minimum allowed state-of-charge are adopted in the converter design. The proposed methodology can also be used to find the optimum combination of the battery and insulated gate bipolar transistor (IGBT) part-numbers based on a given initial set.