Transformer Operation and Preventive Maintenance Procedures: A Complete Guide

In the power system, transformers hold significant importance. They enable safe transmission and flexible distribution of electrical energy. Due to factors such as transformer design, manufacturing, installation, and operation and improper maintenance, there are numerous accidents caused by transformers. Therefore, it is necessary to strengthen the maintenance of transformers, enhance the ability to analyze and handle common failures, and improve the accident prevention capabilities.

1. Normal Maintenance of Transformers

During the normal operation of transformers, the body should be intact without any defects. All electrical performances should comply with regulations. The rated parameters should meet the operation requirements. The cooling system should operate normally.

1.1 Oil filled transformer maintenance checklist before operation

(1) Check the grounding wire and confirm it has been removed.

(2) Measure the insulation resistance of each winding and ensure all are qualified.

(3) Close the neutral point grounding switch on the high-voltage side of the transformer.

(4) Make sure the primary and secondary circuits are normal, and the wiring is not loose or dislodged.

(5) The transformer shell is well grounded, the appearance is clean, there is no oil leakage or oil seepage, and the oil temperature and oil level are normal.

(6) The valves on the upper and lower parts of the cooler, the valves of the oil reservoir, the valves of the transformer body, and the valves of the oil purifier should all be opened, and there should be no gas in the gas relay.

(7) There are no foreign objects in the control box of the cooler, and the positions of all operation switches comply with the operation requirements.

(8) Accessories such as the oil level gauge, thermometer, desiccant, etc. are normal, and the silica gel remains unchanged in color

(9) The voltage tap changer is in the operating position.

(10) The current transformer wiring is in good condition, and the current transformer without load connection has been short-circuited.

1.2 Oil filled transformer maintenance checklist in operation

(1) The operating transformer is based on the condition that, the upper oil temperature does not exceed the allowable value.

(2) Check the oil quality. It should be transparent with a slight yellow tint. The oil level should comply with the standard line corresponding to the surrounding temperature. If the oil level is too low, check whether the transformer is leaking oil, etc. If the oil level is too high, check the operation of the cooling device and whether there is any internal failure.

(3) During normal operation, usually there is a uniform humming electromagnetic sound. If the sound changes, carefully inspect.

(4) Check if the bushing is clean, whether there are cracks and discharge marks. The working and standby power supplies and oil pumps should meet the operation requirements, etc.

(5) Special weather inspections: During strong winds, check if the leads are swinging violently. At the top cover of the transformer and the lead-out points of the bushing, there should be no foreign objects. On snowy days, after the contacts fall with the snow, they should not immediately melt or have discharge phenomena. On foggy days, check if there are any spark discharge phenomena, etc.

Regularly carry out cleaning and rust removal work

Remove dust from the cooler promptly to ensure proper air circulation. In areas with a large amount of accumulated dust, the cooler should be cleaned at least once every six months. Remove rust promptly to prevent the iron core from rusting. The iron core of a transformer is prone to rusting due to exposure to the air. Severe rusting of the entire iron core can directly threaten the lifespan of the transformer, increase its losses, and reduce its efficiency. Therefore, regular rust removal and prevention are necessary to avoid iron core rusting.

Common Transformer Failures and Preventive Maintenance

During the operation of a transformer, failures are inevitable. By conducting inspections to identify problems, and then correctly analyzing and handling them, as well as implementing preventive measures, safe operation can be achieved.

1. Common Transformer Failures

（1）Transformer winding failure

The main faults of transformer windings are inter-turn short circuits and short circuits to the casing.

Inter-turn short circuits mainly occur due to insulation aging, or when the transformer is overloaded or there is a transient short circuit, the insulation is mechanically damaged. When the oil level inside the transformer drops and the windings are exposed above the oil surface, inter-turn short circuits can also occur.

Additionally, during a transitory short circuit, due to the effect of excessive current, the windings deform and the insulation is mechanically damaged, resulting in inter-turn short circuits.

When inter-turn short circuits occur, the gas protection device acts, and in severe cases, the differential protection device will also act. The cause of short circuits to the casing is also due to insulation aging or the oil getting damp, the oil level dropping, or due to lightning and operational overvoltage.

Besides, during a transitory short circuit, due to excessive current causing the windings to deform, a short circuit to the casing can also occur. During short circuits to the casing, the gas protection device and the grounding protection device usually act.

（2）Core failure of transformer

The majority of core faults are caused by insulation damage to the through bolts of the core column or the clamping bolts of the iron wheel. The consequences may result in the through bolts and the core laminations forming a two-point connection, causing circulating current and resulting in local overheating. Even it may cause local melting of the core. It may also lead to local short circuits in the core laminations, generating eddy currents that cause overheating and damage to the insulation layer between the laminations, increasing the no-load loss of the transformer and deteriorating the insulation oil.

（3）Transformer bushing failure

Oil leakage from the bushing is the most common type of bushing failure. Severe bushing failures are manifested as breakdown between the bushing and the housing or flashover between phases. The cause of breakdown between the bushing and the housing often occurs due to cracks in the bushing caused by carelessness during manufacturing, installation or maintenance, or due to dirt on the surface of the bushing.

Inter-phase flashover of the bushing is rare because there is sufficient distance between phases. When the bushing is damaged, the differential protection device and the overcurrent protection device usually act.

(4) Transformer tap changer and casing faults

Most of the faults of the tap changer are caused by the burnout of the contact surfaces of the switch. The reasons are insufficient contact surfaces or insufficient contact pressure. When a short circuit occurs here, it is burned out due to the thermal effect of the excessive current. When a fault occurs in the tap changer, the gas protection device usually acts. The common faults that often occur in the casing and radiators of the transformer are oil leakage. It is easy for operation personnel to detect and find them.

(5) Abnormal transformer temperature rise and oil level

Under normal circumstances, the oil level of a transformer changes along with the variation of oil temperature, causing the oil level to rise or fall. Factors affecting the change in oil temperature include changes in load, variations in ambient temperature, internal faults, and the operation status of the cooling device.

When the oil level is too low, the gas protection will be triggered. In severe cases of low oil level, the iron core will be exposed to the air, which is prone to moisture and may cause wire overheating, insulation breakdown, and accidents. Operators should promptly identify the cause and take corresponding measures, inspect the load, cooling device, and oil leakage situation, notify maintenance, etc.

2. Preventive Maintenance of Transformer

Taking preventive measures can help avoid the occurrence of the above accidents as much as possible. The key points are as follows:

(1) Before putting the transformer into operation, all cooling equipment should be tested one by one. Any problems found should be dealt with promptly, and the standby power supply should be put into use.

(2) If the explosion-proof membrane has cracks, the cause should be investigated. Ensure that the breathing device is unobstructed, and replace the desiccant in time if it becomes ineffective.

(3) When there is gas in the gas relay or gas collection chamber, sample and test the gas to determine its composition.

(4) Before starting the forced oil circulation transformer after the oil system maintenance is completed, start all oil pumps to circulate and allow the residual gas to escape.

(5) The oil level of the transformer should not be too high or too low, and the temperature rise should be within the specified range.

(6) Regularly take oil samples for testing. For unqualified ones, pay attention to tracking monitoring and inspection.

(7) The transformer is not allowed to operate without protection. If the protection is withdrawn, go through the protection withdrawal procedures and obtain approval from the competent leader.

(8) When there is an abnormal internal sound of the transformer, the oil temperature keeps rising, the explosion-proof tube sprays oil, the body leaks oil, the bushing discharges and short-circuits, or catches fire, immediately stop the operation of the transformer.

Summary

In conclusion, transformers are one of the important equipment in the power grid. Although they are equipped with multiple protections such as lightning arresters, differential protection, and grounding protection, the accident rate is still very high, and serious accidents and significant losses occur from time to time, due to their complex internal structure, uneven electric and thermal fields, etc.

Therefore, doing a good job in the operation and maintenance of transformers, as well as the analysis and handling of faults and accident prevention, is very important in ensuring the safe and reliable operation of the power system.