There are many advantages for modular multilevel
converter (MMC), such as modular structure, scalability and reduced harmonics,
which makes it
become a potential converter technology
for high voltage direct current (HVDC) transmission system. The basic topology structure
and working principle are introduced, capacitor voltage fluctuation and
circulation current are analyzed, and point out the necessity of controlling.
Capacitor voltage balancing control based on circulation current suppression
strategy are applied to keep voltage stability of
sub-module(SM) capacitor and suppress circulation current. Circulation current suppression
controller based on low pass filter and quasi-proportional resonant controller
is designed and added to additional capacitor voltage balancing control
strategy. The effectiveness and
feasibility of the proposed
method is proved by Matlab/Simulink, the simulation results show that the fluctuation of capacitor
voltages as well as the circulating current can be confined into acceptable
range.