Circulating Current Reduction in Parallel Grid Connected Two Level VSI using Hybrid Carrier Based Modulation

The power handling capacity of a grid connected converter system can be increased by connecting inverters in parallel. It offers advantages such as modularity, reliability and efficiency. The voltage difference between parallel connected inverters will cause a circulating current to flow between the inverters. This voltage difference occurs (i) when the carriers of both the inverters are phase shifted for interleaved PWM, (ii) when operating both the inverters with different power levels and (iii) due to hardware issues like device tolerances, dead time differences etc. Due to circulating current, the inverters conduct more current, which will effect the performance of parallel connected system and also result in additional conduction losses. In this paper, a hybrid carrier based modulation technique is proposed to reduce the circulating current when the parallel connected inverters (equal power sharing) are operated with interleaved PWM. The circulating current is directly related to the difference of common-mode voltages of the parallel connected inverters. The proposed technique reduces this difference by using a novel hybrid PWM method, where the carrier frequency is changed for some duration to reduce the overlap time of common mode voltage of two converters. Mathematical model and simulation results are presented to validate the proposed technique.