Analytical Approach to Circulating Current Mitigation in Hexagram Converter-Based Grid-Connected Photovoltaic Systems Using Multiwinding Coupled Inductors

The hexagram multilevel converter (HMC) is composed of six conventional two-level voltage source converters (VSCs), where each VSC module is connected to a string of PV arrays. The VSC modules are connected through inductors, which are essential to minimize the circulating current. Selecting inductors with suitable inductance is no simple process, where the inductance value should be large to minimize the circulating current as well as small to reduce an extra voltage drop. This paper analyzes the utilization of a multiwinding (e.g., two, three, and six windings) coupled inductor to interconnect the six VSC modules instead of six single inductors, to minimize the circulating current inside the HMC. Then, a theoretical relationship between the total impedance to the circulating current, the number of coupled inductor windings, and the magnetizing inductance is derived. Owing to the coupled inductors, the impedance on the circulating current path is a multiple of six times the magnetizing inductance, whereas the terminal voltage is slightly affected by the leakage inductance. The HMC is controlled to work under variable solar radiation, providing active power to the grid. Additional functions such as DSTATCOM, during daytime, are also demonstrated. The controller performance is found to be satisfactory for both active and reactive power supplies.