Zero-sequence current dynamics in parallel-connected voltage source converters

Multiple three-phase voltage source converters may be connected in parallel to supply an AC voltage bus or a load in excess of the rating of an individual power converter. Without a circulating path, a zero-sequence current generally does not exist in a single power converter; however, it typically exists in parallel-connected three-phase power converters. The presence of zero-sequence currents in three-phase systems, which may be caused by the circuit asymmetry, introduces further asymmetry of electrical variables and is indicative of abnormal condition of the circuit in which these components are found. This paper presents a detailed small-signal mathematical model that can represent the zero-sequence current dynamics in the parallel-connected voltage source converters and investigates the effects of a variety of factors on the zero-sequence currents. Time-domain simulation studies are performed to verify the controller performance and the effects of the system and controller parameter variations on the zero-sequence currents.