Effects and suppression methods for inrush-caused unbalanced third harmonic on transformer delta-side

Under the steady state condition, the third harmonic is in general thought to have the same features as those of the zero-sequence component, which cannot be coupled to the delta-side device of the transformer. However, field test results show that the unbalanced third harmonic caused by the sympathetic inrush can flow into the low-voltage side of the transformer. Hence, the third harmonic will form a loop with the capacitor on the low voltage side and may damage the capacitor and arrester. There is only zero-sequence component in the third harmonic under the steady state condition. On the other hand, the third harmonic voltage and current caused by the sympathetic inrush is not completely symmetric. Consequently, there are positive-sequence, negative-sequence and zero-sequence components. The positive-sequence and negative-sequence components in the sympathetic inrush would flow through the delta-side of the transformer and into the capacitor and load. Meanwhile, if the reactance ratio of the parallel capacitor bank is 6%, there will be a series resonance point of the third harmonic, which is the reason for further amplification of the third harmonic. With theoretical analysis and simulation study, the above point of view is validated. Besides, a parallel capacitor bank with a mixed series reactor is designed. The dynamic simulation test shows that the parallel capacitor bank with a mixed series reactor is able to effectively suppress the third and the fifth harmonics, which is much better than the parallel capacitor bank with unified series reactance.