Test and analysis on operating performance of transformer with single-phase three-limb core under DC bias

The impact of DC current in the earth on excitation characteristic of AC transformer under bi-polar asymmetric operation of HVDC power transmission system is analyzed. During the high-power commissioning of two circuit HVDC transmission system from the Three Gorges to Shanghai, the operating performance of single-phase AC transformer with three-limb core located near DC grounding pole at receiving end is measured and tested, and the relationship of vibration and noise of the transformer with such parameters as unbalance degree of bi-polar operation, apparent power of main transformer and oil temperature as well as the influence of DC-bias on operating performance of single-phase AC transformer with three-limb core are analyzed. Measurement and test results show that during bi-polar asymmetric operation of HVDC power transmission system the DC-bias phenomena to different extent appear in transformers inside the AC substation near the grounding pole, however the oil temperature and oil chromatograph are in normal condition, thus such a DC-bias phenomena are sustainable.