Reasons for Demagnetization of Generators Set

The role of diesel generators in society is becoming increasingly important. In the use of diesel generator sets, the phenomenon of generator demagnetization may occur. So, how does the demagnetization of the generator occur? The following Guangxi diesel generator manufacturer will give you a brief explanation using Yuchai generator as an example: Generator demagnetization refers to the sudden or partial disappearance of the excitation current of the generator. The reasons for generator demagnetization include rotor winding failure, excitation machine failure, false tripping of automatic demagnetization switch, excitation system failure, failure or misoperation of automatic adjustment excitation device, etc.

When the generator completely loses excitation, the excitation current gradually decays to zero, and the induced electromotive force Ed of the generator will decrease with the decrease of excitation current, resulting in a decrease in the electromagnetic torque of the generator. Since the mechanical torque acting on the generator has not yet decreased, the generator will increase the rotor acceleration power angle under the action of the prime mover to maintain the balance between electromagnetic power and mechanical power.

When the power angle is less than 90 degrees and the generator has not yet lost step, it is in the synchronous oscillation stage,

When the power angle is equal to 90 °, the generator is in a critical out of step state,

When the power angle exceeds 90 °, the generator loses synchronization with the system. (i.e. entering a new balance).

At this time, the generator will absorb reactive power from the system to supply excitation current, establish an air gap magnetic field, induce electromotive force in the stator winding, and induce a frequency of Fg Fs (fg is the frequency corresponding to the generator speed) in the rotor circuit after the generator exceeds the synchronous speed. The single-phase current and electromotive force of the system frequency (Fs) generate asynchronous braking torque. When the asynchronous torque reaches a new balance with the torque of the prime mover, it enters a stable asynchronous operating state. After entering asynchronous operation, the active power output of the generator will decrease compared to before demagnetization.

Generator demagnetization will generate differential frequency currents in the damping system of the rotor, the surface of the rotor iron core, and the rotor winding, causing additional temperature rise and potentially endangering the safety of the rotor. Generator demagnetization will result in pulsed current, also known as differential current, in the stator winding, which will generate alternating mechanical torque and may affect the safety of the generator.

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