Generator re-voltage overcurrent protection

灞波儿奔

Generator re-voltage overcurrent protection [Copy link]

Compound voltage overcurrent protection, it is a voltage compound element composed of a negative sequence voltage relay and a low voltage relay connected to the phase voltage. As long as one of the two relays is activated, the overcurrent relay will also be activated, and the whole device can be started.
　　The protection responds to phase-to-phase short circuit faults and serves as a backup protection for differential protection. It includes the following elements:
　　1. The voltage is taken from the YH on this side or the YH on each side of the transformer. Action criterion: the action value is less than the setting value of the low voltage element.
　　2. Negative sequence voltage element, the voltage is taken from this side or each side of the transformer, the action criterion: the action value is greater than the setting value of the negative sequence voltage element. 3.
　　Overcurrent element, the current is taken from the LH on this side, and the current of any phase is greater than the overcurrent setting value.
　　

　　Generator re-voltage overcurrent protection
　　Protection principle?
　　The protection reflects the generator voltage, negative sequence voltage and current. The current is preferably taken from the neutral point side, mainly as a backup protection for the generator phase short circuit. When the generator is self-excited, the overcurrent element should have a current memory function. See Figure 1.
　　

　　Generators are generally equipped with overload protection and overcurrent protection. Overload is used as overload protection under abnormal operation conditions of the generator, and it acts on signals or automatically reduces load. On the one hand, overcurrent is used as a near backup protection for the generator, and on the other hand, it is used as a far backup protection for adjacent components. It is required that the setting value of overcurrent protection should have the necessary sensitivity to the short-circuit fault of adjacent components.
　　Since the overcurrent caused by external short circuit of the generator and the overload current caused by abnormal operation of the generator are not much different in value, in order to distinguish between overload and overcurrent, overcurrent protection needs to be equipped with low voltage elements (symmetrical short circuit) and negative sequence voltage elements (asymmetrical short circuit) as locking elements (also called starting elements), forming the so-called composite voltage locking (starting) overcurrent protection. This protection requires the low voltage element or negative sequence voltage element to act at the same time as the overcurrent element before the export can act on the trip.
　　Composite voltage locking memory overcurrent protection is used as a backup protection for internal short circuit faults and out-of-zone short circuit faults of the generator. When the generator excitation adopts the self-parallel excitation system, since the excitation transformer is connected to the generator outlet, when an external short circuit fault occurs and the main protection fails to operate, the normal backup protection should operate. However, due to the short circuit fault, the generator outlet voltage decreases, causing the output of the excitation transformer to decrease, resulting in a decrease in the generator excitation current, and then the generator stator current decreases, which cannot maintain the short circuit current, so that the protection returns, causing the backup protection to also fail to operate. For this reason, a memory overcurrent function is designed in the composite voltage locking overcurrent protection, so that the protection device can remember the short circuit current at the beginning of the fault and ignore the subsequent current.