Novel Control System of Voltage Source Type AC/DC Converter with Transformer Having Unbalanced Leakage Inductance

Abstract

A voltage source type converter must be linked to a power line through some amount of inductance to restrict the short circuit current. A leakage inductance of a transformer is usually used as that current limiting inductance. The conventional converter control is designed with assuming that the transformer has three-phase balanced equal reactance. This paper proposes novel control system of voltage source type converter, which can manage converter output even if the converter transformer has three phase unbalanced leakage inductance. The proposing control can apply 3 legs type three-phase transformer having three-phase unbalanced leakage inductance to a voltage source type converter, which is cheaper and smaller than three single-phase transformers or precisely well turned 5 legs type three phase transformer having three phase balanced inductance. The proposing control is based on the phase sequence description of the converter system, and the formulization of the system in phase sequence domain is performed in the paper. The control utilizes real time DFT as pre-processing of the control to obtain the state variables in phase sequence domain. The controller indicates the converter to output positive and negative phase sequence voltage in accordance with the transformer impedance, output current reference and AC bus voltage. The theoretical qualitative discussion of the proposing control concept and quantitative evaluation of the proposing control with using computer simulation are presented in the paper.