Reduction of the Circulating Current Among Parallel NPC Inverters

In medium/high power applications, including smart transformers, active power filters, and wind turbines, three-level neutral-point-clamped (NPC) inverters proved to be a reliable solution, providing high efficiency and low harmonic distortion. In practice, several NPCs are parallel connected and operated in interleaved manner to further increase the power handling and reduce the line filters size. However, if such configuration has a common dc-link, high-frequency zero-sequence circulating current (HF-ZSCC) arises among the inverters, increasing power losses of the switching devices and propagating the stress on the dc-capacitors. Moreover, the amplitude of the HF-ZSCC is inversely proportional to the filter inductance size, therefore in real applications, it can reach hundreds of amperes even with relatively low output currents. The research on the HF-ZSCC is mostly concentrated on two-level inverters for low voltage grids and traction applications, where the inductance size is relatively big and the HF-ZSCC does not affect the system efficiency. Differently, NPCs provide higher switching degree of freedom and more sophisticated methods can be applied to reduce the HF-ZSCC. This article investigates a double-reference pulsewidth modulation (DRPWM) as solution for diminishing the HF-ZSCC in paralleled NPCs. The performance of DRPWM method is confirmed by both simulation and experiments, performed on a 1.6-MVA system.