Technical Article

Examining the Causes of Transformer Insulation Failure

January 04, 2024 by Ahmed Sheikh

Learn the reasons behind insulation-related transformer failures.

Transformers have few, if any, moving parts and are very reliable. Because they are so reliable, we tend to ignore them until something catastrophic happens and we have no power. Replacing small transformers that work with standard voltages can be accomplished quickly as suppliers stock the transformers. Large transformers above 500 KVA may not be stocked and require more time to remove and replace. 


Image used courtesy of Adobe Stock


The causes of failure fall into two categories: mechanical failure and insulation failure. Here, we will focus on insulation failure. 


Causes of Insulation Failure

The causes of insulation failure may be any of the following:

  • Transformer overloading and temperature rise
  • Voltage transients
  • Excessive heat
  • Contamination or decay of insulating liquids 
  • Dirt


Transformer Overloading and Temperature Rise

Increasing the load on a transformer causes an increase in the rise in the temperature of the windings. As long as the temperature rise does not cause the winding temperature to exceed its rating, the winding insulation will not be degraded. As soon as the insulation begins to break down, either because of heat or some other factor. The normal life expectancy for a transformer is about 20 years, based on winding temperatures that do not exceed the winding temperature rating. When the transformer is not fully loaded, the winding temperature rise will be less than the full-load rated value; Table 1 shows how the temperature rise changes with load. If the winding temperature is less than its rated value, it should be permissible to increase the load above the rated full-load amperage for a short period because the additional heat created by the higher load will take some time to raise the winding temperature to the maximum value.


Table 1. Load and Temperature Rise

Percentage of Transformer Load





Percentage of Temperature Rise







Transformers are rarely operated at full load, and the power they must supply constantly changes. Short periods of overload will not shorten transformer life as long as they are preceded by and followed by periods of under-loading. The excess heat caused by overloading will be dissipated in periods of less than full load. Transformers may also be loaded more heavily if the ambient temperature is much lower than the 40ºC standard. Liquid-cooled transformers may be overloaded by as much as 200% for one hour without adverse effects, and dry transformers by as much as 125%.


Effects of Voltage Spikes

When lightning strikes a utility power line, very high voltage spikes result. These spikes are known as voltage transients and may also be caused by the utility switching to connect or disconnect parts of the distribution system. Although the voltages last short periods, they can reach above 100 KV. Transformers may have surge arrestors to limit the spikes. They are manufactured with a basic impulse level (BIL) rating that will allow them to withstand specific transient voltage spikes; for example, a 600-volt transformer will have a BIL rating of 10 KV. It is difficult to predict the value of these voltage spikes, and they may be larger than the BIL rating of the transformer. The high voltages will weaken the insulation and cause arcing and instantaneous failure.


Effects of Excess Heat

Heat causes winding insulation to become brittle and crack. As the transformer materials expand and contract with changing temperatures and loads, the cracks become larger, and arcing may occur. Heat may also cause the general degradation of the insulating material, making it less effective and shortening its life. Heat also increases the resistance of conductors, resulting in a higher voltage drop on the transformer secondary. The load will have to draw more current to provide the same power at reduced voltage, which will cause even more heat in the windings. Excessive heat may be caused by too much load, failure of the transformer cooling system, improper location of the transformer, or high ambient temperatures, and is a slow but sure way to destroy a transformer.


Effects of Impurities in Transformer Cooling Liquids

The windings of liquid-cooled transformers are in complete contact with the cooling medium and must have a high insulating value. Any impurities in the liquid will decrease its effectiveness. If the impurities are too high, the liquid can provide a path for current flow between parts of the transformer windings, especially where the winding insulation is cracked or weak. Once the current begins to flow, the liquid will further break down, and a short circuit may result. In its normal lifespan, the transformer cooling liquid will interact with the windings of the transformer, its chemical composition will change, and it will break down and be unable to insulate. Heat will speed up the process, as will moisture, resulting in transformer failure.


Effects of Dirt

Dirt, especially on air-cooled transformers, will prevent heat transfer, causing windings to become hot. The dirt may also provide a conductive path for arcs in high-voltage windings, causing insulation breakdown and failure. If the dirt is mixed with oil or other types of hydrocarbons, insulation breakdown may occur on the winding insulation covered by the dirt and oil mixture. Liquid-cooled transformers may be affected by dirt buildup at connection points where the dirt will provide a conductive path that can lead to arcing.


Takeaways of Insulation-Related Transformer Failures

Exploring the causes of transformer failures proves essential for power systems applications. Insights into insulation failures, such as overloading, temperature rise, voltage spikes, excess heat, and contamination of insulating liquids, enable engineers to optimize transformer performance. This knowledge proves crucial for refining maintenance strategies, enhancing system reliability, and ensuring the longevity of critical components in electrical power networks.