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. This work is supported by National Natural Science Foundation of China (No. 50977012

No. 51377022) and Jiangsu Provincial Natural Science Foundation of China (No. BK20140113).