Transformer Life Halved with Every 8°C Increase? Understanding Thermal Aging Mechanisms

The length of time a transformer can operate normally under rated voltage and rated load is called the service life of the transformer. Materials used in transformer manufacturing fall into two main categories: metallic materials and insulating materials. Metallic materials generally can withstand relatively high temperatures without damage, but insulating materials will rapidly age and degrade when the temperature exceeds a certain value. Therefore, temperature is one of the main factors affecting the service life of a transformer. In a certain sense, the life of a transformer can be said to be the life of its insulating materials.

Lowering temperature extends transformer service life

The length of time a transformer can operate normally under rated voltage and rated load is called the service life of the transformer. Materials used in transformer manufacturing fall into two main categories: metallic materials and insulating materials. Metallic materials generally can withstand relatively high temperatures without damage, but insulating materials will rapidly age and degrade when the temperature exceeds a certain value. Therefore, temperature is one of the main factors affecting the service life of a transformer. In a certain sense, the life of a transformer can be said to be the life of its insulating materials.

The gradual loss of original mechanical and insulating properties of insulating materials under prolonged exposure to electric fields and high temperatures is called aging. The rate of aging mainly depends on the following factors:

• Temperature of the insulation.

• Moisture content of the insulating material.

• For oil-immersed transformers, the amount of oxygen dissolved in the oil must also be considered.

These three factors determine the service life of a transformer. Practice and research show that if the winding can continuously maintain a temperature of 95°C, the transformer can be guaranteed a service life of 20 years. Based on the relationship between temperature and life, the "8°C rule" can be derived: taking the life at this temperature as the base, for every 8°C increase in winding temperature, the transformer's service life is halved.

Most power transformers in China use oil-paper insulation, i.e., Class A insulation. For Class A insulated transformers, under normal operation, when the ambient air temperature is 40°C, the maximum operating temperature of the windings is 105°C.

According to relevant data and practice:

• When the transformer's insulation operating temperature is 95°C, its service life is 20 years.

• When the transformer's insulation operating temperature is 105°C, its service life is 7 years.

• When the transformer's insulation operating temperature is 120°C, its service life is 2 years.

The internal insulation temperature of a transformer, under essentially constant voltage, mainly depends on the magnitude of the load current: higher load current leads to higher insulation temperature, while lower load current results in lower insulation temperature.

When a transformer is overloaded or operates at rated load during summer, its internal insulation runs at high temperatures, accelerating life loss. When the transformer operates under light load or at rated load during winter, its internal insulation runs at lower temperatures, slowing down life loss. Therefore, to fully utilize the transformer's load capacity throughout the year without affecting its normal service life, the monthly load can be appropriately adjusted.

High voltage accelerates transformer aging

For example, regulations stipulate that the operating voltage of a transformer must not exceed 5% of its rated voltage. Excessively high voltage increases the magnetizing current in the transformer core, may cause core saturation, generate harmonic flux, further increase core losses, and lead to core overheating. Excessively high voltage also accelerates transformer aging, shortening its service life; therefore, the operating voltage of a transformer must not be too high.

When insulating material ages to a certain extent, under the influence of operational vibration and electromagnetic forces, the insulation may crack, making electrical breakdown faults more likely and reducing the transformer's service life.

Adjusting transformer load to achieve ideal service life

The internal insulation temperature of a transformer, under essentially constant voltage, mainly depends on the magnitude of the load current: higher load current leads to higher insulation temperature, while lower load current results in lower insulation temperature.

When a transformer is overloaded or operates at rated load during summer, its internal insulation runs at high temperatures, accelerating life loss. When the transformer operates under light load or at rated load during winter, its internal insulation runs at lower temperatures, slowing down life loss. Therefore, to fully utilize the transformer's load capacity throughout the year without affecting its normal service life, the monthly load can be appropriately adjusted.

Proper maintenance helps maximize transformer service life
It is well known that once a transformer fails, not only are repair costs and downtime expenses substantial, but rewinding a coil or rebuilding a large power transformer can take 6 to 12 months. Therefore, a proper maintenance program will help the transformer achieve maximum service life.

Three key points of a good maintenance program

Installation and operation

A. Ensure the load remains within the design limits of the transformer. For oil-cooled transformers, carefully monitor the top-oil temperature.
B. The transformer's installation location should be suitable for its design and construction standards. If installed outdoors, ensure the transformer is suitable for outdoor operation.
C. Protect the transformer from lightning strikes and external damage.

Oil testing

The dielectric strength of transformer oil decreases sharply as moisture content increases. As little as 0.01% water content can reduce its dielectric strength by nearly half. Except for small distribution transformers, oil samples from all transformers should be regularly subjected to breakdown tests to properly detect moisture and remove it through filtration.

Fault gas analysis in oil should be performed. Using an online monitoring device for eight fault gases in transformer oil, continuously measure the concentration of gases dissolved in the oil as faults develop. By analyzing the types and concentrations of these gases, the type of fault can be determined. Physical property tests of the oil should be conducted annually to verify its insulating performance, including tests for dielectric breakdown strength, acidity, interfacial tension, etc.