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Analysis on the Causes of Main Insulation Damage of Hydrogenerator

August 12, 2020

Analysis on the Causes of Main Insulation Damage of Hydrogenerator
 

Currently, there are small hydropower generating units distributed all over the country, which have made great contributions to the local economic and social development. However, there are many problems that need to be further studied and resolved in safety production and technical management, among which the main insulation damage of generators accounts for a large proportion.

 

1. Poor design and manufacturing cause damage to the insulation of the turbine generator
1.1 In order to reduce cost and volume, the main ball margin is low when designing, and the number of main insulation wrappers in some parts is not enough to cause disconnection. To
1.2 The production process directly affects the quality of the generator.
Vibration of the iron core of hydro-turbine generator sets occurs from time to time, and it is often difficult to make a correct judgment during operation. For example, a hydroelectric generating unit put into operation in 1990 at a power station,
Soon after it was put into production, the operating personnel discovered that after the unit was boosted and connected to the grid, abnormal squeals were produced when the load reached a certain level. It is judged by the technicians that the iron core is vibrating. The main reason is that the chips are not stacked enough during manufacturing
Tight, causing the silicon steel sheet to vibrate during operation. If it is not handled in time, it may cause the silicon steel sheet to break due to long-term vibration fatigue, and finally cut the coil insulation to cause grounding short-circuit or phase-to-phase short-circuit failure. In severe cases, it may not be repaired on site.

It needs to be shipped back to the manufacturer for core compression and reinserting processing. After being compacted on site by the new technology, the DC leakage test and the DC withstand voltage test of 2 times the rated voltage are passed, and then 1.5 times the rated voltage

 

AC withstand voltage test. The current operation is relatively normal.
2 The influence of the operating environment on the main insulation damage of the generator
2.1 The operating environment temperature directly affects the life of the motor.
Most of the small hydro-generator units that have been put into operation are insulated with pitch mica. This insulation uses mica tape to continuously wrap the entire line and ends of the bar and then undergo vacuum dipping treatment to eliminate the disadvantages of end overlap. However, the asphalt has a low softening point and a lower limit of the heat resistance of the main insulation, generally 105K. The vacuum dipping process is complicated, and it is not easy to penetrate if it is not strictly controlled. There may be air bubbles inside, resulting in unstable coil quality. In addition, because some power stations have built very low powerhouses in order to increase the operating head, and in order to prevent the tail water from flooding the equipment through the windows in the flood season, most of the main floor walls are not equipped with ventilation and light windows. The operating environment temperature is very high. The insulation is aging. In particular, some coils are poorly manufactured, and the insulating material contains an air gap, which increases the insulation temperature difference, and the hottest temperature directly shortens the service life of the stator insulation.
2.2 Small hydropower stations are located in remote mountainous areas and are often at the end of the line in electrical wiring, and the voltage deviation is large.

Especially in the wet season, the bus voltage of 400V units sometimes rises to 470V, and the 6kV bus sometimes reaches 7kV. The grid-connected transformer of a small power station has only a three-stage tap changer, which is far from meeting the needs of actual operation and has to raise the generator outlet voltage.

 

3. Damage to generator main insulation caused by poor maintenance

When the vertical hydro-generator undergoes extensive maintenance, most of the rotors need to be lifted out, and the runner of the hydro-turbine also needs to be lifted out of the stator bore for repair welding. In the lifting, if you are not careful or the crane is not skilled, it will hit the stator coil. To

 

4. Measures for damage to the main insulation of the unit coil
(1) Prevent the iron core from loosening. Pay attention to observation when cleaning the stator core during overhaul. If red powder is found on the core, it indicates that
There is looseness. Use electric knife and other thin sheet tools to test the degree of looseness. Normally, the insertion depth of the iron core teeth is generally not more than 3mm. Pay attention to the tightness of the coil during operation. The upper and lower coils are in phase and electrical
When the flow direction is the same, the maximum force is pressed towards the bottom of Kai. If the coil is not firmly fixed in the slot, vibration will occur and cause the anti-corona layer of the coil surface to wear and damage. When the current direction of the out-of-phase coil on the same layer is opposite, the tangential alternating bending force will be generated.
The maximum moment will destroy the insulation. For loose coils, the slot wedges should be tightened in time, and oblique key slot wedges can be used when necessary. If the ends are loose, they can be tied up with a weftless glass ribbon, and then sprayed with epoxy paint to cure.
(2) Prevent electric corrosion. Hydrogenerator unit using epoxy powder mica as main insulation
The main problem exposed during operation is electric corrosion. Electrical corrosion is divided into internal corrosion and external corrosion. Internal corrosion is because there is an air gap between the main insulation and the anti-corona semiconductor branch, and the ground voltage is distributed between the main insulation and the air gap.
Top. Discharge the air gap freely; the external corrosion is due to the free discharge in the air gap between the anti-corona layer and the iron core. The internal corrosion first destroys the adhesive glue between the main insulation and the wire, so that the insulation peels off and the glue wire becomes loose. Generate electromagnetic vibration, glue line grinding

Broken, damage to the main insulation; external corrosion is first to wear the anti-corona layer, aggravate the corona discharge, causing damage to the surface of the coil.

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In order to prevent electrical corrosion, the following measures can be adopted:
(1) The degree of electrical corrosion is related to the voltage of the coil. Most of the corrosion occurs in the coil with a generator voltage greater than 4kV. After the motor has been running for a period of time, the coil center point and outlet end can be inverted to alleviate it.
(2) If the running motor is found to smell of ozone, it is often a precursor to electrical corrosion, and it can be checked with a partial discharge meter. Small hydropower systems generally do not have this condition, and preliminary observations can be made with the naked eye in a dark environment. If there is a blue glow at the junction of the coil slot and the coil end hoop, there is a corona phenomenon. Need to be overhauled.