Managing zero sequence in voltage source converter

Abstract

The 3-phase bridge voltage-source converter (VSC) which has been and continues to be the work-horse of variable-speed AC-motor drives is making penetration into the electric utility environment. In the electric utility environment, there is a greater emphasis on: (i) grounding and (ii) the need of the VSC to survive all conceivable faults. Fault analysis decomposes the unbalanced 3-phase voltages and currents into positive-, negative- and zero-sequence components. The impact of the zero-sequence on the VSC has not been well understood and a common practice is to side-step this issue by excluding the zero-sequence from the VSC at the utility/VSC interface by connecting the transformers in the Y-/spl Delta/ configuration. This paper shows that the Y-/spl Delta/ configuration can lead to destructively high over-voltage or permanent dc voltage unbalance. These problems cease to exist in transformer configurations in which the zero-sequence currents are admitted to the VSC. In order to use these transformer configurations, it is necessary to manage the zero sequence in the VSC. This paper presents the mathematical model of the zero sequence and its equivalent circuit as it affects the voltage source converter.