IEEE Std C37.91-2021
Protecting Power Transformers
5. Types of Failures in Transformers
Types of Transformer Failure
- Expansion and Contraction due to Thermal Cycling:
Transformers generate heat when they operate. This heat can cause the materials in the transformer to expand. When the transformer cools down (such as when it's switched off), these materials contract. This repeated expansion and contraction (thermal cycling) can eventually cause wear and damage.
- Vibration:
Transformers can generate vibrations during operation. Over time, these vibrations can potentially cause physical damage to the transformer's components.
- Local Heating due to Magnetic Flux:
Transformers operate by magnetic induction, which can cause localized heating, especially if the magnetic flux density is high. If not managed correctly, this heat can degrade the transformer's insulation and other components.
- Forces due to the Flow of Through-Fault Currents:
Fault currents (abnormally high currents due to faults like short circuits) can generate strong forces in the transformer. These forces can potentially cause mechanical damage.
- Excessive Heating due to Overloading or Inadequate Cooling:
Transformers are rated for a specific maximum load. If this load is exceeded, the transformer can overheat. Additionally, if the cooling system of the transformer isn't working properly, even normal operation can lead to overheating.
These failures can lead to various problems in transformers, including
Insulation failure
Mechanical damage
Overall degradation of transformer performance and life
Therefore, it's crucial to have appropriate measures in place to monitor these stresses and maintain the transformer's operation within its design specifications.
The key preventive and detection mechanisms mentioned include:
- Temperature Monitoring Devices:
These are used to track the temperature of the transformer's windings and/or insulating liquid. If temperatures exceed safe levels, an alarm is triggered, prompting investigation by maintenance staff. Operators may then reduce the transformer's load to prevent a trip (an automatic shutdown due to a fault condition).
- Gas Detection Relays:
These devices detect gases produced within the transformer's insulating liquid. Different gases can indicate specific types of issues. For instance, the presence of acetylene can suggest electrical arcing, while other gases may signify partial discharge or thermal degradation of the cellulose insulation. Depending on the utility's practice, gas detection relays may trip the transformer or generate an alarm.
- Sudden-Pressure Relays (Below Insulating Liquid Level):
These relays respond to pressure waves in the insulating liquid caused by internal arcing - a serious fault condition.
- Sudden-Pressure Relays (Above Insulating Liquid Level):
Located above the insulating liquid level, these relays respond to sudden changes in gas pressure, which can also be indicative of arcing within the insulating liquid.
- Liquid Level Detectors:
These devices monitor the level of insulating liquid within the transformer. They can generate an alarm for minor level reductions and initiate a trip for significant decreases, which can prevent catastrophic failures.
- Online Monitoring Devices:
These devices track the health of the transformer's bushings, the current transformers (CTs) installed in those bushings, and the surge arresters installed on the transformers. They can trigger an alarm indicating the urgent need for repair to avoid severe damage.
These various monitoring and protection methods play a crucial role in maintaining transformer reliability and longevity.
Image source: Tamil Nadu Electric Utility (TNEU), India
Image source: OilRegeneration.com