Comparison of modular multilevel and neutral-point-clamped converters for medium-voltage grid-connected applications

Abstract

Battery energy storage systems (BESS) are expected to increase in the number and capacity because of the increasing share of intermittent renewable energy resources in the electrical grid. Due to the cost and nature of the BESS, they are well suited for the primary reserve to balance load fluctuations in the grid. Power-electronic converters are employed for the power exchange between the transmission or distribution grid and the BESS. Usually, the voltage of such BESS is in the low-voltage range. This dc voltage can be stepped-up to medium voltage (MV) using a high-power dc-dc converter. The MV dc voltage is then inverted using an MV inverter to couple the BESS to the MV grid. In this work two converters, namely, the modular multilevel converter (MMC) and the three-level neutral-point-clamped (3L-NPC) converter, are compared and benchmarked for their application in BESS as MV inverter. Both converters are rated for 5 MVA and are to be connected to the 10 kV distribution grid. The converters are compared in terms of the components such as the number and ratings of the semiconductor devices, the filter elements; the operational characteristics such as the components losses, the system efficiency, the harmonic content of the terminal quantities; and in terms of the investment cost.