Large Power Transformers – Operation, Protection and Maintenance

A Large Power Transformer (LPT) is a critical component of the bulk electric power generation and transmission grid, as well as a multitude of industrial applications. Because LPTs are very expensive and tailored to customers’ specifications, they are usually neither interchangeable nor produced with extensive inventories. In a transformer failure, the repair or replacement work can range from 12 to 24 months and pose significant business interruption risks to the operator.

Risk considerations

  • Machinery breakdown: A transformer’s life expectancy is about 40 years. Beyond this age, failure rates begin to increase. A lack of maintenance and/or consistent overloading will further reduce life expectancy. Good judgement is required when determining the “technical age” of each transformer.
  • Business interruption: Repair or replacement of a power transformer will require 12-24 months. Even if a spare unit is available, replacement of the affected unit can take between a few weeks to a month, depending on various factors.
  • Fire: Failure of oil-filled LPT’s may lead to an oil tank rupture and the release of large quantities of insulating oil with a subsequent oil-mist explosion. The result is typically a long-lasting pool-type fire that damages the entire equipment and exposes adjacent equipment, structures, or buildings.

Protect an oil-filled Large Power Transformer (LPT)

  • Whenever possible oil-filled transformers should be separated from adjacent equipment, including other transformers, structures, or buildings through adequate distance separation or properly built fire barriers.
  • When distance separation or barriers cannot be provided, oil-filed transformers should be fitted with fixed water spray protection, primarily to protect surrounding equipment, buildings, or structures.
  • Oil-filled transformers should be provided with oil containment basins to prevent any spilt oil from reaching other equipment, structures, or buildings.

Maintain a Large Power Transformer (LPT)

Regular maintenance and testing are vital to ensure reliable operation and a normal life span of a transformer.

  • Oil analysis, including regular oil screening as well as water content and gas-in-oil analysis should be completed at least annually. Furan analysis is also desirable to monitor ageing transformers. Testing should be performed more frequently if the unit's operating history, condition, and criticality warrant it.
  • Testing for corrosive Sulphur should be conducted in transformers built after the year 2000 or those that have recently had their oil processed or replaced, and the replacement oil has not been tested for corrosive Sulphur. Transformers that meet the following conditions should have corrosive Sulphur testing performed. These conditions put a transformer at the highest risk of failure due to corrosive Sulphur contamination.
    • The transformer fluid is a mineral oil
    • Mineral oil is an uninhibited type
    • The oil preservation system is sealed (not free breathing)
    • The transformer has a high sustained load factor
    • The transformer is operating in a hot climate
    • The transformer windings are bare copper
  • Every 5 years, perform the following electrical tests:
    • Relay testing and calibration
    • Insulation resistance testing – measures the winding to winding and winding to ground insulation resistance
    • Power factor testing – measures the power factor and capacitance of the transformer and high voltage capacitive bushings
    • Buchholz testing. The Buchholz relay can be defined as a device which is utilised to protect against certain dangers in electrical transformers of ratings over 500kVA.

Download the full loss prevention guide for Large Power Transformers.