AC filter of DC transmission project is one of the main equipment of converter station. At present，such accidents as unbalanced current protection tripping due to bird damage between AC filter high-voltage

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capacitor joints and between capacitor tower layers occasionally occur. Since there is no monitoring value of overvoltage and overcurrent of each capacitor，the harm extent of capacitor performance is concerned. In this paper，the type H unbalanced current protection tripping of capacitor of AC filter of one converter station due to bird damage is taken as an example. The equivalent circuit of short-circuit impedance

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between layers due to bird damage flashover is set up，and the simulation software is used to analyze the variation of impedance of AC filter，variation of unbalanced current and variation of over voltage as well s over current parameters of capacitor in case of bird damage between different layers of high voltage

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capacitor bank of AC filter and different short circuit impedance

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，so as to assess the extent of the damage of the performance of capacitor. The conclusion shows that in case of flashover in the same layer due to bird damage，the overvoltage and overcurrent

of high voltage

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capacitor unit are increased with the decrease of short circuit impedance

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. In case of bird damage between the bracket which is furthest or nearest to the current transformer

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and the adjacent layer，the over voltage and over current of

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capacitor unit is the highest. When the short-circuit impedance

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is less than a certain value，the high-voltage

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capacitor can withstand the overvoltage exceeding the allowable limit，which shows that frequent bird damage has a certain harm to the performance of the capacitor.