主设备保护若干问题的商榷

主设备差动保护一般选用P级互感器，如何保证在外部短路或其他暂态过程中不误动是普遍关注的问题。文中讨论了单元件（零序电流型）横差保护的定值整定，在整定计算导则中推荐为0.05I_gn，二滩水电厂在一次完全失磁故障中误动，建议定值增大为10%。文中还讨论了失磁保护系统侧判据与机组侧判据组成“与门”出口问题、大型发电机中性点经接地变压器接地时的阻值问题、转子两点接地保护的撤消、变压器相间差动保护的空载合闸问题，以及零序差动保护的推荐、后备阻抗保护的必要性、意外误合闸保护的必要性等问题。并给出了具体的建议。

DISCUSSION ON SOME PROBLEMS EXISTING IN THE MAIN EQUIPMENT PROTECTION

It is widely concerned how to avoid maloperation caused by the P class current transformer in external faults and other transient processes. which is generally applied in the main equipment protection.Taking0.05I gn as the setting of the single element transverse differential protection, which is recommended in the guide rule, the maloperation has happened during a complete loss-of-field fault in the Ertan Plant, so an increase of 10% on the setting is suggested.This paper also expands following subjects: the problem of logic product gate output which consists of loss-of-field protection criterions both in the system and the unit, selection of resistance value when the neutral point of large generator grounds through grounding transformer; cancellation of rotor double ground protection; problems resulted from switching on without load for the phase- to-phase differential protection of transformer and the recommendation of zero-sequence differential protection; necessities of backup impedance protection and the accidental false switch on protection; etc.Some advice are put forward.