The d and q Axes Technique for Suppression Zero-Sequence Circulating Current in Directly Parallel Three-Phase PWM Converters

The directly parallel pulse width modulated converter (DPC) method is used to construct large-scale converter systems to increase system performance and reduce costs. However, circulating currents arise when the converters supply unbalanced currents or have different converter parameters. Circulating currents cause distortions in the current, increase the current coming from each converter, and degrade the system performance. This paper proposes a circulating current control (CCC) methodology to eliminate circulating currents through the analysis and approximation of the circulating current of the fundamental harmonic current at 150 Hz. By using the phase shift technique, this new model of the circulating current in the 0-axis was extended to the 0d- and 0q-axes. The proposed CCC was then designed according to the new circulating current model and added to the conventional controller. The performance of the proposed CCC converter was verified via simulation and experimentally with three directly parallel 5.5 kVA converters. The proposed CCC enhanced controller and conventional controller were tested with various current values, and all resulting circulating currents were recorded. The experimental results showed that the proposed CCC eliminated nearly all the circulating currents and that it had a fast, dynamic response to step-change currents. Using ramp current sharing, the experimental results were confirmed. Additionally, the efficiency of the proposed CCC in the DPC was significantly higher than the conventional controller, and the system's efficiency was the same as that of a single converter connection. Thus, using the proposed CCC in DPC systems is economical, highly efficient, modular, and flexible.