ELECTROMAGNETIC AND THERMAL CHARACTERISTICS SIMULATION FOR THREE-PHASE THREE-LIMB TRANSFORMERS UNDER DC MAGNETIC BIAS

Abstract

Direct current (DC) bias is caused by DC intrusion into the transformer. DC bias will not only cause current distortion and damage electric power grid but also will lead to hot spot overheating and increase in the transformer insulation aging. The effect of DC bias on both electromagnetic and thermal characteristics of transformers should be simultaneously considered for transformer design and operation control. In this study, different levels of DC bias were introduced to a three-phase three-limb transformer. Magnetic flux offset and current distortion were obtained by field-circuit coupling simulation. The winding loss and hot spot for different loads were computed with the analytic method and the electromagnetic-thermal-fluid coupling model. The results indicate that the magnetic flux offset is sensitive to the variation of loads, but insensitive to the increase of DC. As the transformer is overloaded, the current distortion increases, and a peaked wave occurs. The harmonic frequencies of distorted current concentrate on even-order. Compared with the rated sinusoidal load, the hot spot temperature increased by 18.59 K with 10 A DC introduction. The hot spot location is transferred from the upper yoke of the core to the HV coil. As the load increases, the hot spot temperature increases first and then decreases. This study is of great significance to the assessment of DC bias effect.