Why Regular Transformer Maintenance Matters: 5 Serious Consequences of Neglecting It

I. Allowable Temperature

When a transformer is in operation, its windings and iron core generate copper loss and iron loss. These losses are converted into heat energy, causing the temperature of the transformer's iron core and windings to rise. If the temperature exceeds the allowable value for a long time, the insulation will gradually lose its mechanical elasticity and age.

The temperature of each part of the transformer during operation is different: the winding temperature is the highest, followed by the iron core temperature, and the temperature of the insulating oil is lower than that of the windings and iron core.

The oil temperature in the upper part of the transformer is higher than that in the lower part. The allowable temperature of the transformer during operation is checked by the upper oil temperature. For transformers with Class A insulation, when the maximum ambient air temperature is 40°C during normal operation, the maximum operating temperature of the transformer windings is 105°C.

Since the temperature of the windings is 10°C higher than that of the oil, to prevent the deterioration of the oil quality, it is stipulated that the maximum upper oil temperature of the transformer shall not exceed 95°C. Under normal circumstances, to prevent the accelerated oxidation of the insulating oil, the upper oil temperature shall not exceed 85°C.

For transformers with forced oil circulation water cooling and air cooling, the upper oil temperature should not often exceed 75°C (the maximum allowable upper oil temperature for such transformers is 80°C).

II. Allowable Temperature Rise

Monitoring only the upper oil temperature of the transformer during operation cannot ensure the safe operation of the transformer; it is also necessary to monitor the temperature difference between the upper oil temperature and the cooling air, i.e., the temperature rise. The temperature rise of the transformer refers to the difference between the transformer temperature and the ambient air temperature.

For transformers with Class A insulation, when the maximum ambient temperature is 40°C, the national standard stipulates that the temperature rise of the windings is 65°C, and the allowable temperature rise of the upper oil temperature is 55°C.

As long as the temperature rise of the transformer does not exceed the specified value, the transformer can operate safely within the specified service life under rated load (a transformer can operate continuously with rated load for 20 years under normal operation).

III. Reasonable Capacity

During normal operation, the electrical load borne by the transformer should be approximately 75-90% of the transformer's rated capacity.

IV. Reasonable Current Range

The maximum unbalanced current of the transformer's low-voltage side shall not exceed 25% of the rated value; the allowable variation range of the transformer's power supply voltage is ±5% of the rated voltage. If this range is exceeded, a tap changer should be used for adjustment to bring the voltage into the specified range.

(Adjustment should be carried out with the power cut off.) Usually, the voltage is adjusted by changing the position of the tap on the primary winding. The device used to connect and switch the position of the tap is called a tap changer, which adjusts the transformation ratio by changing the number of turns of the transformer's high-voltage winding.

Low voltage has no impact on the transformer itself, but only reduces its output slightly; however, it has an impact on electrical equipment. High voltage increases the magnetic flux, causes saturation of the iron core, increases iron core loss, and raises the transformer temperature.

V. Overload

Overload is divided into two cases: normal overload and emergency overload. Normal overload occurs when the user's electricity consumption increases under normal power supply conditions. It will cause the transformer temperature to rise, leading to accelerated aging of the transformer insulation and reduced service life. Therefore, overload operation is generally not allowed.

Under special circumstances, the transformer can operate with overload for a short time, but the overload shall not exceed 30% of the rated load in winter and 15% of the rated load in summer. In addition, the overload capacity of the transformer should be determined according to the transformer's temperature rise and the manufacturer's specifications.

VI. Transformer Maintenance

Transformer faults are divided into open circuit and short circuit. Open circuit can be easily detected with a multimeter, while short circuit faults cannot be detected with a multimeter.

1. Inspection of Power Transformer Short Circuit

(1) Disconnect all loads of the transformer, turn on the power supply, and check the no-load temperature rise of the transformer. If the temperature rise is relatively high (too hot to touch), it indicates that there must be an internal partial short circuit. If the temperature rise is normal 15-30 minutes after the power is turned on, the transformer is normal.

(2) Connect a 1000W light bulb in series in the transformer's power circuit. When the power is turned on, if the bulb only glows dimly, the transformer is normal; if the bulb is very bright or relatively bright, it indicates that there is an internal partial short circuit in the transformer.

2. Transformer Open Circuit

One type of open circuit is the disconnection of the internal winding, but the disconnection of the lead wire is the most common. Careful inspection should be carried out, and the broken part should be re-soldered. If there is an internal disconnection or signs of burning are visible on the outside, the transformer can only be replaced with a new one or have its windings rewound.