Flexible on-load voltage regulating transformer based on common-capacity-midpoint two-half-bridge converter

Abstract

Traditional on-load voltage regulating transformer are composed of segmented windings controlled by tap changers and power frequency transformers. The tap changer can only perform ‘step-wise voltage regulation’ and is slow in response. A topology of flexible on-load voltage regulating transformer is presented in this paper, which only requires connecting a power electronic converter at the tap changer of the on-load voltage regulating transformer. PEC can fully combine the existing on-load tap changer to achieve continuous stepless voltage regulation function. This paper conducts modelling of the switch cycle average and analyses the voltage regulation range for the proposed topology. It also performs stability analysis of the small signal model for the introduced converter. A multilayer integrated control strategy is proposed based on the degree of voltage fluctuation in the power grid and the current state of the on-load tap changer. In addition, in the closed-loop control strategy of the converter, the quasi proportional-resonance controller is introduced to improve the tracking performance of AC signal, and the voltage balance control is introduced to reduce the capacitor voltage difference on the DC side of the converter. Finally, simulation and experimental validation confirm the rationality and effectiveness of the proposed topology.