Analysis and Suppression of Zero-Sequence Circulating Current in Multi-Parallel Converters

The use of a multi-parallel converter system has many advantages in increased scalability, better maintenance, scheduling, and improved output current quality. However, a periodic zero-sequence circulating current (ZSCC) may occur due to the asymmetry of parallel-connected converters. ZSCC produces additional losses and possible instability of the system. Therefore, proper control must be applied to suppress this harmful ZSCC. In order to design an effective controller for suppressing ZSCC, it is necessary to analyze the cause of the circulating current generation. However, most of the existing studies have applied the controller without detailed analysis. Therefore, this paper mathematically analyzes the ZSCC spectrum using the Fourier series to identify which harmonics are included in ZSCC. From the analysis results, the necessity of multi-resonant controllers to suppress the ZSCC at specific harmonics is demonstrated. Simulation and experiments are conducted to validate the analysis results and the necessity of multi-resonant controllers.