A study on energy balancing and control of Modular Multi-Level Converters

摘要

This paper explains internal capacitor energy balancing and circulating current control of Modular Multi-level Converters (MMCs). MMCs are considered as one of the promising multilevel converter topologies especially in high voltage applications such as HVDC transmission system. Circulating current is a unique feature in MMC, it arises from capacitor voltage differences between the three phases. The idea behind MMC is to use floating capacitors as a voltage sources, they are inserted or bypassed using a switching circuitry to generate the required output voltage level. To ensure a stable operation, capacitor voltages should be kept in the same level and the total converter energy should be uniformly distributed between the three phases. This can be achieved by controlling the current that circulates between the three phases. In this paper a three phase grid connected MMC is considered. An analytical study has been done, operating principles, mathematical model and control system have been described. Also, the system performance has been verified using simulation with Matlab-Simulink, simulation results were satisfactory and in conformity with the analytical study.