Finite Element Simulation Analysis of Three-phase Bridge Arm Reactor of Modular Multilevel Converter

As an important part of modular multilevel converter (MMC), the reactor at the bridge arm of the converter is an essential component in its topology. During the operation of MMC, because the capacitor voltage on each phase bridge arm is not equal, it is easy to cause the unbalanced voltage of bridge arm, and finally lead to the unequal DC current component in the three-phase bridge arm. The unbalance of DC component can easily lead to excessive magnetic bias of one phase in three-phase reactor, and then cause the unbalance of AC component in three-phase bridge arm, and finally lead to zero sequence circulating current inside the bridge arm. The zero sequence circulating current enters the DC side and intensifies the fluctuation of voltage, current and power at DC side. In this paper, the finite element method is used to simulate and analyze the suppression effect of three-phase bridge arm reactor on the second harmonic circulating current under the condition of unbalanced DC component, which provides a theoretical basis for the design and selection of MMC bridge arm reactor.