Fault diagnosis and troubleshooting of synchronous generators

1. Generator overheating

(1) The generator is not operating in accordance with the specified technical conditions, such as the stator voltage is too high, the iron loss increases; the load current is too large, the stator winding copper loss increases; the frequency is too low, the cooling fan speed slows down, affecting the heat dissipation of the generator; the power factor is too low, the rotor excitation current increases, causing the rotor to heat up. Check whether the indication of the monitoring instrument is normal. If not, make necessary adjustments and treatments to make the generator operate in accordance with the specified technical conditions.

(2) The three-phase load current of the generator is unbalanced, and the overloaded phase winding will overheat. If the difference between the three-phase currents exceeds 10% of the rated current, it is a serious three-phase current imbalance. The unbalanced three-phase current will produce a negative sequence magnetic field, thereby increasing losses and causing heating of components such as the pole winding and the hoop. The three-phase load should be adjusted to keep the current of each phase as balanced as possible.

(3) The air duct is blocked by dust, resulting in poor ventilation and making it difficult for the generator to dissipate heat. Dust and grease should be removed from the air duct to ensure that the air duct is unobstructed.

(4) The air inlet temperature is too high or the water inlet temperature is too high, and the cooler is clogged. The air inlet or water inlet temperature should be lowered to clear the blockage in the cooler. Before the fault is eliminated, the generator load should be limited to reduce the generator temperature.

(5) If the bearing is filled with too much or too little grease, grease should be added according to regulations, usually 1/2~1/3 of the bearing chamber (the upper limit is taken for low speed and the lower limit is taken for high speed), and it is advisable not to exceed 70% of the bearing chamber.

(6) Bearing wear. If the wear is not serious, the bearing will be partially overheated; if the wear is serious, the stator and rotor may rub against each other, causing partial overheating of the stator and rotor. Check whether the bearing has any noise. If the stator and rotor are rubbing against each other, stop the machine immediately for inspection or replace the bearing.

(7) Damage to the insulation of the stator core causes short circuits between the plates, which increases the eddy current loss of the local core and causes heat. In severe cases, the stator winding is damaged. The machine should be shut down immediately for maintenance.

(8) The parallel wire of the stator winding is broken, causing the current in other wires to increase and heat up. The machine should be shut down immediately for maintenance.

2. There is abnormal voltage between the neutral line of the generator and the ground

(1) Under normal circumstances, due to the influence of high-order harmonics or the manufacturing process, the air gap under each magnetic pole is uneven, the magnetic potential is unequal, and very low voltage appears. If the voltage is between one and several volts, there will be no danger and no treatment is required.

(2) The generator winding is short-circuited or poorly insulated from the ground, causing the performance of the electrical equipment and the generator to deteriorate and become easily overheated. This should be repaired in a timely manner to prevent the accident from expanding.

(3) When the neutral line is unloaded, there is no voltage to the ground, but when there is load, there is voltage. This is caused by the imbalance of the three phases. The three-phase load should be adjusted to make it basically balanced.

3. The generator current is too large

(1) The load is too heavy and should be reduced.

(2) If a phase-to-phase short circuit or ground fault occurs in a transmission line, the line should be inspected and repaired, and normal operation can be restored after the fault is eliminated.

4. The generator terminal voltage is too high

(1) The voltage of the generator connected in parallel with the power grid is too high. The voltage of the generator connected in parallel should be reduced.

(2) Failure of the excitation device causes overexcitation, and the excitation device should be repaired in time.

5. Insufficient power

Due to insufficient compound excitation compensation of the voltage source of the excitation device, the excitation current required for the armature reaction cannot be provided, causing the generator terminal voltage to be lower than the grid voltage and unable to deliver rated reactive power. The following measures should be taken:

(1) Connect a three-phase voltage regulator between the generator and the excitation reactor to increase the generator terminal voltage and gradually increase the magnetic potential of the excitation device.

(2) Change the phase between the excitation device voltage magnetic flux potential and the generator terminal voltage to increase the synthetic total magnetic flux potential. A resistor of several thousand ohms and 10W can be connected in parallel at both ends of each phase winding of the reactor.

(3) Reduce the resistance of the rheostat to increase the excitation current of the generator.

6. Stator winding insulation breakdown and short circuit

(1) The stator winding is damp. For generators that have been out of service for a long time or have been overhauled for a long time, the insulation resistance should be measured before they are put into operation. Those that fail to meet the requirements are not allowed to be put into operation. Generators that are damp should be dried.

(2) Winding defects or improper maintenance process cause winding insulation breakdown or mechanical damage. The insulation material should be selected according to the specified insulation grade, and the winding installation, varnish dipping and drying should be carried out strictly according to the process requirements.

(3) Winding overheating. Overheating of the insulation will reduce the insulation performance, and sometimes it will quickly cause insulation breakdown at high temperatures. Daily inspections should be strengthened to prevent overheating of various parts of the generator and damage to the winding insulation.

(4) Insulation aging. Generally, after a generator has been in operation for more than 15 to 20 years, its winding insulation will age, its electrical performance will change, and even insulation breakdown will occur. The generator should be properly inspected and tested for preventive reasons. If the insulation is found to be unqualified, the defective winding insulation should be replaced in time or the winding should be replaced to extend the service life of the generator.

(5) Metal foreign objects may enter the generator. After inspecting the generator, do not drop metal objects, parts or tools into the stator chamber; tighten the rotor binding wire and tighten the end parts to prevent them from loosening due to centrifugal force.

(6) Excessive voltage breakdown: 1) The line is struck by lightning, but the lightning protection is not perfect. The lightning protection facilities should be improved. 2) Misoperation, such as raising the generator voltage too high when it is unloaded. The generator voltage should be increased strictly according to the operating procedures to prevent misoperation. 3) Internal overvoltage of the generator, including operating overvoltage, arc grounding overvoltage and resonance overvoltage, etc. The winding insulation preventive test should be strengthened to promptly discover and eliminate the defects in the stator winding insulation.

7. Stator core is loose

Due to improper manufacturing and assembly, the core is not tightened well. If the entire core is loose, for small generators, two iron plates smaller than the inner diameter of the stator winding end can be used, stud bolts can be put in, and the core can be tightened. After the original shape is restored, the original clamping bolts of the core can be tightened. If the core is partially loose, first apply silicon steel sheet paint between the loose pieces, and then drive hard insulating material into the loose part.

8. Short circuit between iron chips

(1) The core laminations are loose, and when the generator is running, the core vibrates and damages the insulation; the insulation of individual parts of the core laminations is damaged or the core is locally overheated, causing the insulation to age. In this case, the method in the original plan should be used for treatment.

(2) The edge of the iron sheet has burrs or is mechanically damaged during maintenance. Use a fine file to remove the burrs, trim the damaged area, clean the surface, and then apply a layer of silicon steel sheet paint.

(3) If there is solder or copper particles short-circuiting the iron core, the metal welding points should be scraped or chiseled off and the surface should be prepared.

(4) Arc short circuit in winding may also cause short circuit in core. The burned part should be removed with a chisel and the surface should be treated properly.

9. The generator loses residual magnetism and cannot generate electricity when starting.

(1) Residual magnetism is often lost after shutdown because the material used for the exciter pole is close to soft steel and has less residual magnetism. When there is no current in the excitation winding after shutdown, the magnetic field disappears. A battery should be prepared and magnetized before generating electricity.

(2) When the magnetic poles of the generator lose their magnetism, a DC current greater than the rated current should be passed through the winding (for a very short time) to magnetize it, so that sufficient residual magnetism can be restored.