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A transformer is only as strong as its weakest link

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Transformer rating

Capacity (or rating) of a transformer is limited by the temperature that the insulation can tolerate. Ratings can be increased by reducing core and copper losses, by increasing the rate of heat dissipation (better cooling), or by improving transformer insulation so it will withstand higher temperatures.

A transformer is only as strong as its weakest link
A transformer is only as strong as its weakest link

A physically larger transformer can dissipate more heat, due to the increased area and increased volume of oil.

A transformer is only as strong as its weakest link, and the weakest link is the paper insulation, which begins to degrade around 100 °C. This means that a transformer must be operated with the “hottest spot” cooler than this degradation temperature, or service life is greatly reduced.

Presspahn - A proven surface insulation material, resistant to high voltage and a high capacity for impregnation with transformer oil (by UKi - one of Europe’s largest suppliers of electrical insulation materials)
Presspahn – A proven surface insulation material, resistant to high voltage and a high capacity for impregnation with transformer oil (by UKi – one of Europe’s largest suppliers of electrical insulation materials)

Reclamation typically orders transformers larger than required, which aids in heat removal and increases transformer life.

Ratings of transformers are obtained by simply multiplying the current times the voltage. Small transformers are rated in “VA,” volts times amperes.

As size increases, 1 kilovoltampere (kVA) means 1,000 voltamperes, 1 megavoltampere (MVA) means 1 million voltamperes. Large generator step-up (GSUs) may be rated in hundreds of MVAs.

A GSU transformer can cost well over a million dollars and take 18 months to 2 years or longer to obtain.

Each one is designed for a specific application. If one fails, this may mean a unit or whole plant could be down for as long 2 years, costing multiple millions of dollars in lost generation, in addition to the replacement cost of the transformer itself.

This is one reason that proper maintenance is critical.

Transformer paper insulation damaged due to poor maintenance
This transformer used to be strong, but… (on photo: Transformer paper insulation damaged due to poor maintenance; credit: gcequipment.com)

Reference: Transformers: Basics, Maintenance, and Diagnostics – U.S. Department of the Interior Bureau of Reclamation

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on