Second, the principle of generator differential protection
1. Logical differential action of generator longitudinal differential protection
Since the generator neutral point is not directly grounded, when an interphase short circuit fault occurs inside the generator. There will be two-phase or three-phase differential relays operating simultaneously. According to this feature. Corresponding considerations can be made when protecting logic design. When a two-phase or three-phase differential relay operates. It can be judged that a short circuit fault occurs inside the generator: only one phase differential relay operates. Then it is judged that the TA is disconnected. In order to deal with a short-circuit fault caused by grounding in the area and grounding outside the area. When a phase differential relay operates and has a negative sequence voltage at the same time, it is also determined to be an internal short circuit fault of the generator. This action logic is characterized by a single-phase TA disconnection that does not operate. Therefore, the dedicated TA disconnection locking link can be omitted and the protection is safe and reliable.
2. The principle of incomplete longitudinal differential protection of generator
The conventional differential differential protection introduces all phase currents i and i of the stator end and the neutral point of the generator. The two-phase currents are still equal when the stator windings are short-circuited in phase. Protection will not work. In general, large-scale generators have two or more parallel branches per phase stator winding. If only the neutral point side branch current i of the generator is introduced to form the differential differential protection, the appropriate TA ratio can be selected to ensure normal operation and When there is no differential current in the fault outside the zone, a differential current will be formed when the generator phase and the inter-turn short circuit occur, and when it is greater than the setting value. The fault can be removed. This differential differential protection is called incomplete differential differential protection.