Objective The auxiliary power source in nuclear power plant operates at no-load normally, the protection cannot alarm when it occurs single phase open, but this open-phase condition will threaten the nuclear safety seriously, it should assemble special open-phase protection. Besides the near-end open-phase, the open-phase protection should consider the far-end open-phase and neutral ungrounded.

Method Based on the analysis of voltage and current behavior of the auxiliary transformer in nuclear power plant during the near-end open-phase and far-end open-phase failure, the different points of voltage, current, and zero-sequence impedance in normal operation and fault operation were identified to set open-phase protection. And the auxiliary power source of "Hualong No. 1" nuclear power unit was analyzed, the open-phase protection was recommended according to its characteristics.

Result After research and analysis, when the auxiliary transformer of nuclear power plant occurs single open-phase at near-end, the three-phase voltage is unchanged, the break phase current is zero and it has zero sequence current, the zero sequence impedance is increased. When it occurs single open-phase at far-end, the three-phase voltage is unchanged, the three-phase current is increased and in the same phase angle, also it has zero sequence current. when the neutral ungrounded transformer occurs single open-phase, the break phase voltage is drops, the break phase current is zero, and there is negative sequence current. Open-phase protection can use these electrical parameter changes to set protection, there are two main types, neutral injection method set protection with zero sequence impedance increase after open-phase, optical CT measurement method set protection with current and its phase change after open-phase.

Conclusion Both of the schemes have their advantages and disadvantages, and both can be used for the open-phase protection of auxiliary power in nuclear power plant. For the auxiliary power source of two lines and six transformers after the completion of six-unit nuclear power base of "Hualong No. 1", it is recommended to adopt two schemes: one set of neutral injection method for each transformer or one set of optical CT measurement method for each transformer. If it is an ungrounded neutral transformer, it is recommended to use optical CT measurement method with low voltage and negative sequence current. All the protection are used for alarm.