Control of parallel and multiple inverters for traction system of maglev

The paper describes math model and the equivalent circuits of parallel and multiple inverters. When inverters are paralleled through the midpoint of transformers, current which flows between two inverters is called as circulating current. On the condition of different DC power source, the account and paths of circulating current are different, this paper describes the differences between them. When inverters are multiple through transformers´ primary side, there are also the different characteristics on the condition of different DC power source. In order to lower harmonic, the multilevel carrier phase shift is applied. The paper compares two control strategies for eliminating circulating current. On this basis, a 150KVA unit composed of two 75KVA NPC inverters was set up. As shown in experiment result, good performance is attained even if the switching frequency is only a few hundred Hertz.