How is the inspection cycle for transformers stipulated

1. During normal operation, conduct a patrol inspection every 4 hours.

2. Conduct a night inspection once a week, and perform a lights-out inspection during peak load times to check for discharges.

3. After power transmission or fault, an inspection should be conducted immediately.

4. In severe weather conditions such as strong winds, heavy snow, dense fog, and thunderstorms, which may pose a threat to safe operation, inspections should be conducted hourly or at any time.

5. During overload operation, it is necessary to check and record at any time.

6. When a new system is put into operation, it should be checked once every hour within the first 4 hours.

What are the inspection contents of transformer patrol during operation

1. Check whether the internal sound is normal. A transformer operating normally emits a uniform “buzzing” sound.

2. Check whether the casing is clean and free from damage, cracks, and discharge marks.

3. Check for any signs of oil leakage or seepage, and ensure that the oil level and color are normal. New transformer oil is light yellow, and it turns light red after operation.

4. Check whether the terminal block is loose and overheated.

5. Check whether the thermometer indicates normally, and ensure that the upper oil temperature does not exceed 85℃.

6. The glass of the gas relay should be transparent, filled with oil, and the lead wire should be free from corrosion.

7. The safety glass is not damaged.

8. Whether the shell is properly grounded.

9. The temperature of each radiator and iron shell should be uniform.

What phenomena would occur if a transformer experiences interlayer and interturn short circuits in its windings

When a transformer in operation experiences a short circuit between winding layers or turns, the following phenomena may occur:

1. Primary current increases.

2. The transformer sometimes emits a “guddu” sound, and the oil painting increases.

3. The high-voltage fuse is blown.

4. The secondary voltage is unstable, fluctuating between high and low.

5. The oil conservator is emitting smoke.

6. After the power outage, the three-phase resistance measured using an electric bridge was unbalanced.

How to judge the operation status of a transformer based on the sound it emits during operation

Place one end of a wooden stick or screwdriver on the transformer oil tank, and hold the other end close to your ear to listen carefully to the transformer sound. If it is a continuous “buzzing” sound that is louder than usual, you should check the voltage, current, and oil temperature. If there is no abnormality, it is mostly caused by a loose iron core.

When you hear a “squeaking” sound, check the surface of the bushing for flashover. When you hear a “cracking” sound, it indicates internal insulation breakdown.

Why is it necessary to conduct a surge closing test before a transformer is put into operation

The removal of an empty transformer operating in the power grid can generate switching overvoltages. When the system neutral point is not grounded or is grounded through an arc suppression coil, the amplitude of the overvoltage can reach 4 to 4.5 times the rated phase voltage; when the neutral point is directly grounded, it can also reach 3 times the rated phase voltage.

Therefore, in order to check whether the transformer insulation strength can withstand the rated voltage or the switching overvoltage occurring during operation, several impact closing tests need to be conducted when the transformer is put into operation.

At this time, when an empty transformer is put into operation in the power grid, it will generate an excitation inrush current, whose value can generally reach 6 to 8 times the rated current.

The excitation inrush current decreases to 0.25 to 0.5 times the rated current value after 0.5 to 1 second, but the total decay time is relatively long, which can reach tens of seconds for large-capacity transformers.

Due to the significant electrodynamic force generated by the excitation inrush current, the impact closing test is also conducted to assess the mechanical strength of the transformer and the reliability of the relay protection action.

The number of impact tests is 5 times for new products upon launch and 3 times after major maintenance. Only after passing the impact test can the product be operated with load.

What additional inspections should be conducted on newly installed or overhauled transformers before they are put into operation, besides the normal patrol inspection content

1. The indicator position of the tap changer pointer should be accurate. After major maintenance or position change, the contact resistance of the tap changer should be tested and qualified.

2. The oil valve of the gas relay should be fully opened.

3. The vent hole of the safety device should be open.

4. All radiator throttle valves should be fully opened.

5. The cooling device is operating well (including air cooling, water cooling, forced air cooling, etc.).

6. All temporary working grounding wires should be removed.

Why should the capacity of an oil-immersed air-cooled transformer be reduced after the fan is turned off

Installing a fan on an oil-immersed transformer can improve the cooling efficiency of the oil tank and the surface of the radiator. Generally, the heat dissipation rate of the oil tank can be increased by 50-60%, and its capacity is also increased by more than 30% compared to a self-cooled transformer.

Generally, when the fan of an oil-immersed air-cooled transformer is turned off, it can only operate at 70% of its rated load.

Otherwise, the temperature rise of the transformer will exceed the allowable value due to the reduced heat dissipation rate.

However, when the upper oil temperature does not exceed 55℃, it is possible to operate with rated load without the fan, as the *hot spot temperature of the coil will not exceed 85~95℃.