There are many forms of generators, but their working principles are based on the law of electromagnetic induction and the law of electromagnetic force. Therefore, the general principle of its construction is to use appropriate magnetic and conductive materials to form magnetic circuits and circuits that conduct electromagnetic induction with each other to generate electromagnetic power and achieve the purpose of energy conversion.
During the normal operation of the generator, the excitation suddenly disappears in whole or in part, which is called the generator loss of excitation. The main reasons for loss of excitation include: rectifier cabinet failure, automatic adjustment excitation device failure, operator misoperation, excitation circuit disconnection, de-excitation switch malfunction, and rotor winding failure.
After the generator loss of field failure occurs, the harm to the power system is manifested in:
(1) After low excitation or demagnetization, the generator will transition to the asynchronous operation state, absorb reactive power from the system, and cause the voltage of the power system to drop. If the reactive power reserve of the system is insufficient, the system may collapse due to voltage collapse;
(2) The active power of the loss-of-excitation generator changes, and the system voltage drops, the system may oscillate, and a large amount of load shedding occurs.
After the generator loss of field fault occurs, the harm to the generator itself is mainly manifested in:
(1) If there is a loss of magnetism under heavy load, the stator current will increase, resulting in overheating of the stator winding;
(2) The difference frequency current appears in the rotor circuit, which produces additional loss of the rotor. If it exceeds the allowable value, the rotor will be overheated.
The generator manufacturer JUNVOCH reminds: After the generator loses its magnetic field, the active power should be reduced to the specified value within the specified time. If the voltage of the factory power is too low, the factory load should be poured to the backup power belt, and then quickly find the cause of the loss of magnetic field and fix it. Eliminate and restore synchronization; for units that have not been subjected to the demagnetization test or have been tested and demonstrated to be unsuitable for non-excitation operation, the unit should be cut off by the demagnetization protection or manually disconnected to stop.
