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

Space Vector Modulation Technique for Reducing Harmonics in Current with Zero Common-Mode Voltage for Two-Parallel Three-Level Converters

A zero common-mode voltage (ZCMV) modulation has the advantage of reducing electromagnetic interference (EMI) and a feature that hardly generates a zero- sequence circulating current (ZSCC) in converters operating in parallel. However, this modulation has a critical issue related to the increase in harmonics in the phase current due to the limitation of using voltage vectors generating ZCMV. Thus, this paper proposes an optimal space vector modulation (SVM) technique for two-parallel three-level converters to reduce the harmonics increased by using the ZCMV PWM. The creation of virtual voltage vectors (VVVs) using vector synthesis based on the ZCMV PWM is addressed. Accordingly, new small regions in each sector of the SVM are defined in consideration with the nearest three voltage vectors, including the virtual vectors. In addition, PWM sequences for each region and dwell time for each voltage vector are also determined. Optimal vector utilization of the proposed ZCMV SVM can further reduce the current harmonics. The performance comparison between the proposed ZCMV SVM and existing methods are presented in simulation and experimental results