Enhanced Grid-tie Converter Control Under Unbalanced Conditions with no PLL

Abstract

The ubiquitous Phase-locked loop (PLL) is used to synchronise power converters with the AC system. The limitations of the PLL when the AC system is weak are well known. This paper investigates the modelling and control of Grid-Side Voltage Source Converter (GS-VSC) of a wind-turbine under unbalanced network conditions and connected to a weak system. The proposed control scheme regulates the instantaneous active and reactive power at the PCC. The operation of the GS-VSC under unbalanced voltage condition is analysed and the concept of using different control target is present, which is independent of a PLL. A new current control scheme comprising of a central controller and a compensation controller is proposed. Simulation results using PSCAD/EMTDC are presented to validate the effectiveness of the proposed control scheme and to demonstrate the operation under unbalanced network conditions in a weak grid.