Improved Residual Flux Measurement Method based on Transient Current Characteristics used in Single-Phase Power Transformer

Abstract

With the development of AC/DC UHV grid, large capacity transformer is widely used in power grid. After DC resistance measurement, no-load test and other operations of power transformer, mainly due to hysteresis phenomenon of ferromagnetic materials, a certain magnetic flux is left in the core, which is called residual flux. When transformer switching on without load, residual flux will lead to inrush current which is several times of the rated current. Inrush current endangers the mechanical stability and insulation strength of power transformer windings, and affects the normal operation of power system, resulting in many adverse effects. In this paper, an improved method is proposed to establish the relationship between residual flux and transient current characteristics by analyzing the transient current waveform of winding under different residual flux with different polarity DC voltage. DC voltage amplitude is determined by a criterion that the residual flux in the core should not change over 10% after the DC voltage unloaded. Based on the previous studies on positive and negative transient current, the relationship between transient characteristics and residual flux is further studied and analyzed. Combined with theory and experiment, the form of empirical formula and parameter calculation flow are determined. Compared with directly using transient current amplitude at a certain moment to calculate, using waveform characteristics to calculate residual flux is more accurate and less susceptible to measurement error and noise.