In order to ensure the safe operation and reliable power supply of the hv transformer testing, when the transformer has abnormal conditions, it can be discovered and handled in time to eliminate the fault in the bud to prevent the occurrence and expansion of accidents. Therefore, the transformer in operation must be inspected regularly and the operation records must be kept.

(1) Normal operation mode of the transformer

① Rated operating mode. Under the specified cooling conditions, the transformer can operate according to the nameplate specifications. The allowable temperature of the oil-immersed transformer during operation should be checked according to the upper oil temperature. The upper oil temperature should comply with the manufacturer’s regulations, but the maximum should not exceed 95°C. In order to prevent the transformer oil from deteriorating too quickly, the upper oil temperature should not often exceed 85°C.
The applied voltage of the transformer should generally not exceed 105% of the rated value. At this time, the secondary side of the transformer can carry the rated current. In some cases, after testing or with the manufacturer’s consent, the applied voltage can be 110% of the rated voltage.
② Allowable overload. The transformer can operate under normal overload or accident overload. Normal overload can be used frequently, and its allowable value is determined based on the transformer’s load curve, cooling conditions, and the load carried by the transformer before overload. Accident overload is only allowed to be used in accident situations (transformers that can still operate).
The allowable value of accident overload should comply with the manufacturer’s regulations; if there are no manufacturer’s regulations, for self-cooled oil-immersed transformers, they can be operated according to the requirements in the following table.

(2) Abnormal operation and emergency treatment of transformers

(a) Abnormal phenomena during operation. If any abnormal phenomena are found during the operation of the transformer (such as oil leakage, insufficient oil level in the oil pillow, abnormal heating, abnormal sound, etc.), try to eliminate them. If any of the following situations occurs, stop and repair immediately.
① The internal sound is loud, very uneven, and there is a bursting sound.

② Under normal cooling conditions, the temperature is abnormal and keeps rising.

③ Spray oil from the oil pillow or explosion-proof pipe.

④ Oil leakage causes the oil level to drop below the limit on the oil level indicator.

⑤ The oil color changes excessively and carbon appears in the oil.

⑥ The bushing has serious damage and discharge.

(b) Unacceptable overload, abnormal temperature rise and oil level. If the transformer overload exceeds the allowable value, the transformer load should be adjusted in time. When the transformer oil temperature rises beyond the allowable limit, the cause should be identified and measures should be taken to reduce it. Therefore, the following work must be carried out.

① Check the transformer load and cooling medium temperature and compare them with the expected temperature at this load and cooling temperature.

② Check the temperature gauge.

③ Check the transformer mechanical cooling device or the ventilation of the transformer room.

If it is found that the oil temperature is more than 10°C higher than usual under the same load and cooling temperature, or the oil temperature continues to rise while the load remains unchanged, and the cooling device, transformer room ventilation and thermometer are all normal, it may be an internal fault of the transformer (such as Fire in the iron core, short circuit between coil layers, etc.), stop and repair immediately.

If the oil in the transformer has solidified, the transformer is allowed to be put into operation with load, but attention must be paid to whether the upper oil temperature and oil circulation are normal. When it is found that the oil level of the transformer is significantly lower than what the oil temperature should be at that time, oil should be added immediately. If the oil level drops rapidly due to a large amount of oil leakage, it is forbidden to change the gas relay to only act on the signal, but measures must be taken quickly to plug the leakage and refuel immediately.

(c) Processing when the gas relay operates. When the gas relay signal is activated, the transformer should be inspected to find out the cause of the signal activation, whether it is due to air intrusion into the transformer, a drop in oil level, or a secondary circuit failure. If the fault cannot be detected outside the transformer, the nature of the gas accumulated in the relay needs to be identified. If the gas is colorless, odorless and non-flammable, it is air separated from the oil and the transformer can still continue to operate. If the gas is flammable, the transformer must be stopped and the cause of the operation must be carefully studied.
When checking whether the gas is flammable, special care must be taken not to place the fire close to the top of the relay, but 5 to 6 cm above it.
If the operation of the gas relay is not caused by air intrusion into the transformer, the flash point of the oil should be checked. If the flash point is lower than the past record by more than 5°C, it means there is a fault in the transformer.
If the transformer trips due to the action of the gas relay and is proved to be flammable gas, the transformer shall not be put into operation again without special inspection and passing the test.
There are generally two types of gas relay actions depending on the nature of the fault: one is a signal action without tripping; the other is both actions at the same time.
If the signal acts but does not trip, there are usually the following reasons.

① Air enters the transformer due to oil leakage, refueling or loose cooling system.

② The oil level slowly drops due to temperature drop or oil leakage.

③ A small amount of gas is generated due to transformer failure.

④ Caused by a through short circuit.

If the signal and the switch are actuated at the same time, or only the switch is actuated, it may be due to a serious fault inside the transformer, the oil level drops too quickly, or there is a fault in the secondary circuit of the protection device. In some cases, such as after repair, the air in the oil separates too quickly, which may also cause the switch to trip.

(d) Treatment of transformer oil leakage

There are two types of oil leakage: weld oil leakage and seal oil leakage. The treatment of weld oil leakage is repair welding. During welding, the body of the transformer should be lifted out and the oil should be drained. The cause of seal oil leakage should be found out. If the operation is poor (the sealing gasket is not placed correctly, the pressure is uneven, the pressure is insufficient, etc.), it should be repaired as appropriate. If the gasket is aged or damaged (such as the oil-resistant rubber is sticky, loses elasticity, cracks, etc.), the sealing material should be replaced.

(3) Inspection tour of oil-immersed transformers

Transformers in operation should be inspected and monitored regularly to detect abnormal phenomena or faults in time and avoid serious accidents.

The items that should be inspected and monitored generally include:
(1) Whether the bdv transformer has any abnormal sound, such as uneven sound or discharge sound.
(2) Whether the oil level is normal and whether there is any seepage or leakage.
(3) Whether the oil temperature is normal (the upper oil temperature should not exceed 85℃).
(4) Whether the bushing is clean and whether there are any cracks, damages or discharge.
(5) Whether the joint is hot.
(6) Whether the explosion-proof membrane of the explosion-proof pipe is intact.
(7) Whether the gas relay is leaking oil and whether it is full of oil.
(8) Whether the breather is unobstructed, whether the oil level of the oil seal breather is normal, and whether the silicone in the breather is saturated with moisture.
(9) Whether the cooling system is operating normally.
(10) Whether the shell grounding wire is intact.