Modular Multilevel Converters—Part I: Modeling Based on State-Plane Analysis

Modular multilevel converter is deemed the solution for high-voltage high-power applications. The introduction of capacitor to each arm leads to complicated power flow and thus complicated control scheme. Without a proper control, this often results in large circulating energy. The state plane analysis identified the two types of circulating energies, one related to the power difference between input and output, and the other related to the internal power swapping. Due to their orthogonal nature, a coordinate transformation is proposed, which leads to a decoupled equivalent circuit model. The proposed model suggests two control laws, aiming at eliminating both types of circulating energy.