Causes, Hazards, and Treatment of Generator Rotor Grounding

The grounding of the rotor of the generator has one-point grounding and two-point grounding of the rotor. In addition, there will be short circuit faults between rotor layers and turns. Like stator grounding, rotor grounding can be divided into instantaneous grounding, intermittent grounding, permanent grounding, internal grounding and external grounding, metallic grounding, and resistive grounding.

During the long-term operation of the generator, due to many reasons such as moisture inside the rotor, leakage of the cooling medium, insulation aging, and mechanical vibration, the insulation level of the rotor to the ground is likely to decrease and cause a ground fault of the rotor. When the rotor has a ground fault at one point, although it will not cause direct harm to the generator itself, if two points of ground fault occur successively, it will seriously threaten the safety of the generator.

1. The reason for the rotor grounding

When the staff is working on the excitation circuit, the rotor is grounded due to accidental touch or other reasons; the insulation of the rotor slip ring, slot and notch, end, lead wire, and other parts is damaged; the insulation is aging due to long-term operation, and the Part of the inter-turn insulation gasket of the rotor is displaced, which partially blocks the rotor ventilation holes, causing local overheating and aging of the rotor winding insulation, causing the rotor to be grounded; rats and other small animals enter the excitation circuit, and the insulation water pipe of the stator inlet and outlet branches breaks and leaks, causing the excitation circuit dirty, etc. cause the rotor to ground.

2. The hazard of one-point grounding of the rotor

The one-point ground fault of the generator rotor is one of the common fault forms. When a one-point ground fault occurs, an electrical circuit has not been formed between the excitation winding and the ground, and the excitation voltage of the rotor and the rotor current flowing through the rotor are slightly affected, so it does not affect If the generator causes harm, the load can be shifted at this time, and the fault can be checked after a smooth shutdown.

3. The hazards of two-point grounding of the rotor

(1). Destroy the symmetry of the air-gap magnetic field of the generator, distort the air-gap magnetic field, lose the balance of the air-gap magnetic flux, cause the generator to vibrate violently, damage the motor and reduce the reactive power output. The two-point grounding of the excitation circuit of the turbogenerator can also cause the magnetization of the shafting and the turbine, with serious consequences. If the lateral difference protection is installed, it will also cause its malfunction. Therefore, after the one-point grounding protection of the rotor is activated, a short time delay should be added to the lateral difference protection to prevent malfunction.

(2). Two-point grounding causes the non-short-circuit winding current to increase. If the short-circuit current flowing through the rotor body is large (usually 1500 A is the limit), the thermal effect will burn the rotor and cause the rotor to deform slowly, resulting in increased eccentricity. , intensifies the vibration. In addition, other excitation devices may be damaged, resulting in loss of excitation failure, and endangering the safety of generators and systems. In order to ensure the safe operation of the generator, when the rotor winding of the generator is grounded at one point, a signal should be sent out, and the operator should deal with it immediately; if two points are grounded, the operation of the generator should be stopped immediately. Therefore, it is very necessary to install rotor one-point and two-point grounding protection for the generator.

4. The phenomenon and treatment of one-point grounding of the rotor

When the rotor of the generator is grounded at one point, the central signal alarm bell will sound, the “Generator Rotor One Point Grounded” light plate will light up, and the meter will indicate that there is no abnormality. When the rotor circuit is grounded at one point, no current loop is formed because one point is grounded, no current passes through the fault point, and the excitation system remains in a normal state, so it does not affect the normal operation of the unit. At this time, check whether the protection signal of “one point of rotor grounding” can be reset. If it can be reset, it is instantaneous grounding; if it cannot be reset, check whether the one-point grounding protection of the rotor is normal. If it is normal, use the rotor voltmeter to measure the positive and negative pole-to-ground voltages through the switch to identify whether grounding has occurred. If it is found that the voltage between one pole and the ground drops to zero, and the voltage between the other pole and the ground rises to full voltage (the voltage value between the positive and negative poles), it means that a little grounding has indeed occurred.

Operators should follow the steps below:

(1). Check whether the excitation circuit is working, if it is caused by the staff, it should be corrected;

(2). Check each part of the excitation circuit for obvious damage or grounding due to dirt. If the grounding is due to dirt, it should be purged;

(3). Carry out a detailed visual inspection of the relevant circuits, and if necessary, stop the rectifier cabinets, in turn, to determine whether it is caused by the grounding of the DC circuit of the rectifier cabinets;

(4). Check to distinguish whether the grounding is in the excitation circuit or in the measurement protection circuit;

(5). If the grounding of the rotor is a one-point stable metallic grounding, and the fault point cannot be identified, in addition to strengthening the monitoring of the operation of the unit, after obtaining the consent of the dispatcher, the two-point grounding of the rotor will be used for tripping, and an application for shutdown as soon as possible;

(6). When the rotor is running with one-point grounding, if the unit is under-excited or out of step again, it can generally be considered that the rotor has developed into two-point grounding. At this time, the rotor two-point grounding protection action trips, otherwise it should be shut down immediately.

5. The phenomenon and treatment of two-point grounding of the rotor

When the rotor is grounded at two points, the indications of the rotor ammeter will increase sharply, the indications of the rotor and stator voltmeters will decrease, the indications of the reactive power meter will decrease significantly, the power factor will increase, and even the phase will advance. Noisy, the unit vibrates greatly. In severe cases, generator out of step or loss of excitation protection action tripping may occur. When the two points are grounded, the rotor current increases a lot, causing the excitation circuit equipment to overheat or even be damaged. If one of the grounds occurs inside the rotor winding, part of the rotor winding will also overheat. In addition, the two-point grounding of the rotor destroys the symmetry of the magnetic field, so the unit generates strong vibrations, especially when two points are grounded, in addition to harsh screams, the gap between the bearings at both ends of the generator may also spray out black smoke with flame. For this reason, when the generator rotor is grounded at two points, it should be shut down immediately. If the “rotor one point presses the ground” signal is not sent, and the vibration of the unit exceeds the allowable value or the rotor current increases significantly due to the short circuit between the rotor layers, the load should be reduced immediately to reduce the vibration and rotor current to the allowable range. If it is invalid after processing, apply for shutdown according to the specific situation.

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