The study on the effects of the transformer Δ-side unbalanced third harmonic on the parallel capacitor bank caused by the sympathetic inrush current and the countermearsures

Under steady state condition, the third harmonic is thought to have the same features with zero-sequence component, which would not couple to the Δ-side device of the transformer. However, the field test result shows that the unbalanced third harmonic caused by the sympathetic inrush can flow into the Δ-side of the transformer. Hence, the third harmonic would form a loop with the capacitor at the low voltage side and may damage the capacitor and arrester. There is only zero-sequence component in the third harmonic under steady state condition. On the other hand, the third harmonic voltage and current caused by the sympathetic inrush are not 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 Δ-side of the transformer and flow into capacitor and load. Meanwhile, if the series reactance ratio of the parallel capacitor bank is 6%, there would be a series resonance point of the third harmonic, which is the reason of the amplification of the third harmonic. With theoretical analysis and simulation study, the above point of view is validated. Besides, a parallel capacitor bank with mixed series reactance is designed. The dynamic simulation test demonstrates that the parallel capacitor bank with mixed series reactance ratios can effectively suppress third harmonic and fifth harmonic. Hence, the parallel capacitor bank with mixed series reactance ratios is much better than the parallel capacitor bank with unified series reactance ratio.