Ferro-resonance over-voltage and power frequency over-voltage on the neutrals of 220kV power transformer and corresponding protection strategy

In order to limit short-circuit current and satisfy the need of relay setting, only part of the 220kV power transformer is grounded, so on the neutrals of ungrounded transformers will appear the over-voltage. Currently the value of over-voltage on transformer neutral point and the corresponding protection strategy is based on theoretical formula, this article use the PSCAD/EMTDC software to calculate the over-voltage on the neutral point of 4 ungrounded power transformers in Chongqing 220kV power grid, the result shows that the power frequency transient over-voltage on the neutrals may reach 178kV, If the single-phase grounding fault occurs in ungrounded power system, the power frequency over-voltage on the neutrals will be more serious and rise to 138.6kV. Non-full phase operation may cause serious ferro-resonance over-voltage on the neutrals of no-load transformer, which may last for seconds and may rise to 723.7kV, causing serious threat to the transformer neutrals and line-side equipment. The article also studies the gap parameter which should be taken on the neutrals of 220kV transformers at the end.