Study of transformer´s harmonic characteristic in DC magnetic bias

This paper selects one 500 kV mono-phase type transformer as the test object. Its no-load current includes dc component and even harmonics obviously in dc magnetic bias. With the increasing of the neutral dc current, the lower the harmonic order is, the more rapidly the harmonic current increases and the larger its percent on the basic-frequency component is. The dc component keeps 70% of the neutral dc current whether under undervoltage or overvoltage. When the neutral dc current is 4 A, dc magnetic bias comes into being in the transformer even under undervoltage. The research results can be used in the capacitor banks´ relay protection in 500 kV substations. The capacitor banks will be shut off once their 2nd harmonic current or the neutral dc current is over some setting value and time.