Influence of add-assist circulation current at delta side on Y/Delta transformer differential protection

In recent years, some new phenomena have been observed when recording data waveforms, which lead differential protection mal-operations. It has been discovered that one phase of inrush current in differential calculating current is similar to the internal fault current, and the value of its second harmonic component is almost equal to zero, which is very small and low that could cause transformer differential relay mal-operation. In this paper, the causes of this kind of mal-operations are analyzed. It is pointed out that the strategy of angle compensation in differential protection has a great influence on mal-operations. When energizing an unload transformer on YO side, the circulating current in delta side may eliminate or distort inrush current waveforms on YO side. This kind of phenomenon in inrush current is called the affection of add-assist (AA) current in inrush. This paper also studies how AA current affects the performance of differential protection with two different current compensation strategies. The results show that symmetric inrush current will occur on YO side because of AA current on delta side. Compared to the minus-zero-sequence-current compensation method (i_A-i₀), the second harmonic component in differential current only decreases slightly when using the phase-phase compensation method (I_A-i_B) since the former may cause mal-operations in differential protection because of the vast decrease of the 2nd harmonic component. The current compensation strategy with minus-zero-sequence-current compensation method has been used to some extent in power system widely. It is suggested that the scheme of minus-zero-sequence-current compensation method should not be used in transformer differential protection when applying the 2nd harmonic restrained principle. And the best way is to use the phase-phase compensation method in the case of mal-operations caused by energizing inrush if using the 2nd harmonic restrained method.