Research on Residual Flux Characteristics of Transformer with Single-Phase Four-Limb Core Under Different DC Excitation Current

Residual magnetic flux of core can be generated by DC resistance measurement, no-load test, voltage ratio measurement, and circuit breaker opening of transformer, which is mainly derived from the hysteresis characteristic of ferromagnetic material of core. Among them, DC resistance measurement is one of the main sources of transformer residual flux in practical engineering. Based on the typical core structure of UHV transformer and converter transformer, a single-phase four-limb transformer model and the test platform for residual flux characteristics are designed, and the law of magnetism is analyzed. The results show that, the voltage integration method will introduce the 0th, 1st, 2nd, and even higher-order trend terms, which will distort the core magnetic flux waveform. The integration algorithm based on the magnetic flux automatic correction can better eliminate the linear and non-linear trend terms in the magnetic flux waveform, than to get accurate magnetic flux results. With the increase of DC current excitation, the degree of magnetization of the transformer core increases, and the maximum magnetic induction strength keep a non-linear increasing relationship with the DC current. The residual magnetic characteristics are related to the initial state of the core, the basic law between residual flux and DC current is first increasing and then decreasing when the initial residual flux of core is equal to zero. At the knee point of magnetization curve, that is $I=2A$ , the residual flux becomes the largest. Contrastively, due to the impediment of initial residual flux, the turning of magnetic domains and the displacement of magnetic domain walls become more difficult, the residual flux has nearly no changes with variety of DC current excitation when non-zero initial residual flux existed in the core. The characteristics under different DC current excitation can provide valuable references for the subsequent suppression and elimination of the adverse effects of residual magnetism to the transformer.