Performance Analysis of Energy Balancing Methods for Matrix Modular Multilevel Converters

Modular multilevel converters (MMCs) achieve voltage scalability through the series connection of cells, each with its own capacitance. However, this converter structure using floating energy storage elements increases the control complexity significantly. The voltages of each capacitance must be controlled to their respective setpoints to ensure the correct operation of the converter. Relying on the use of circulating currents, which are unobservable at the converter terminals, the energy can be exchanged among the branches. Three control principles resulting in three different implementations are thoroughly explained and their dynamic performance is compared. The results are verified using a hardware-in-the-loop (HIL) platform simulating a hydro power plant using a matrix modular multilevel converter connected between a 6.6 kV grid and a 6 kV synchronous machine.