Modeling and Analysis of Circulating Currents Among Input-Parallel Output-Parallel Nonisolated Converters

Input-parallel output-parallel (IPOP) nonisolated converters, including dc/dc and dc/ac converters, are effective solutions to increase the power rating and to improve the reliability of the system. For the safe and steady operation of these IPOP nonisolated converters, the circulating currents are the main challenge, whose detailed mathematic model and corresponding characteristics are not well studied. This paper focuses on the modeling and analysis of circulating currents among these IPOP converters by deriving the detailed mathematic model. Through these models, the complicated characteristics of the circulating currents are presented, which shows that there are various types of circulating currents including circulating currents within the single converter and circulating currents among the multiple converters. It is first demonstrated that the circulating currents will cause port degradation of the converter, that is, the positive and negative currents of the converter are unequal, which makes some port-based control methods ineffective and influences the relay protection. Furthermore, the corresponding influence factors, including line resistances, filters, and so on, are analyzed in detail, especially the asymmetry of line resistances will cause large circulating currents even resulting in instability. It is also found that the traditional control methods cannot completely eliminate these circulating currents because of the strong coupling and complexity of IPOP nonisolated converters. All the theoretical analyses are verified by the real-time hardware-in-loop (HIL) tests.