A unit transformer – also called a generator step-up transformer – connects a generator’s relatively low output voltage directly to the higher voltage of the transmission or distribution grid. Found at every power plant, from large thermal stations to wind and hydro installations, it is the critical link that makes a generator’s output usable on the wider grid. Without a correctly rated unit transformer, even the most efficient generator could not deliver its power beyond the plant fence.
How is a unit transformer designed?
Unit transformers are typically connected directly to a single generator, without intervening switchgear on the low-voltage side, and are therefore designed to match that generator’s exact voltage, current, and short-circuit characteristics. Because they operate continuously at high loading, they require robust cooling systems and a conservative thermal design. Many unit transformers also include a tertiary winding to supply the plant’s own auxiliary systems.
Key Characteristics at a Glance
- Steps up generator voltage to transmission or distribution grid level
- Dedicated, direct connection to a single generator in most designs
- Continuous high loading requires robust thermal design
- Often includes a tertiary winding for station auxiliary power
- Protected by differential protection matched to the generator protection scheme
- Ratings scale with generator size, from a few MVA to over 1,000 MVA
- Critical single point of failure requiring high reliability and redundancy planning
Frequently Asked Questions
How does a unit transformer differ from a standard power transformer?
The core technology is similar, but a unit transformer is specified around one particular generator’s characteristics and operates under a very consistent, continuous loading profile, whereas a general power transformer must accommodate more variable network conditions.
Why do many unit transformers have a tertiary winding?
The tertiary winding supplies the power plant’s own auxiliary systems – pumps, fans, control systems – independently of the main grid connection, which is important for reliable plant startup and operation, including during grid disturbances.
What happens if a unit transformer fails?
Because it is often the only connection between a generator and the grid, a unit transformer failure typically takes the entire generating unit offline. This is why unit transformers receive particularly thorough protection, monitoring, and, in critical plants, spare-unit planning.
Are unit transformers used for renewable generation too?
Yes. Wind and solar parks use comparable step-up transformers to raise inverter or generator output to grid voltage, though at a smaller scale than in conventional power plants – the underlying principle of matching generation voltage to grid voltage remains the same.
Are you planning a generation project and need a correctly specified unit transformer? Our engineers will design a solution matched exactly to your generator – contact us.