Comparative Analysis of Unbalanced Three-Phase Grid Current Compensation Methods for the Scott Transformer Based Grid

This paper proposes two control algorithms to solve the three-phase power unbalance problem in the Scott transformer based grid, and two algorithms are power-based and current-based compensation method. The power-based compensation method uses the active/reactive power at the load side for control, and the current-based compensation method utilizes the Scott transformer secondary current directly to eliminate an effect of active/reactive power loss through the power supply chain. In addition, voltage notch effect generated by the low switching frequency and weak grid condition caused by the Scott transformer is compensated by using an observer. Performance comparisons for the two compensation methods are carried out using a real-time simulator and a 3-level back-to-back converter prototype with a 5kVA load. From the experimental results, recorded 34.2% of unbalance rate of the three-phase grid current before compensation is reduced to 2.8% by the power-based compensation method, and 1.2% by the current-based compensation method. This study provides a new insight into developing the advanced algorithms which improves the unbalance of three-phase grid current in electric railway transportation system.