How to Select a Power Transformer for a Power Plant

What is a GSU transformer?

A generator step-up (GSU) transformer raises the voltage from the generator output, typically 11 to 25 kV, to transmission level (110 kV, 220 kV, 500 kV and so on) so power can be transmitted to the grid efficiently. It carries the entire output of the plant, and unlike a distribution transformer it runs at or near full load for as long as the plant is generating.

Learn more about generator transformers.

Key selection factors

1. Power rating

The transformer rating follows the generator capacity, usually sized at 105 to 110% of the generator rating to allow for overload capability. The margin also covers the case where the generator operates above its nominal rating at low ambient temperature.

2. Voltage ratio

Set the generator-side and grid-side voltages from:

Generator rated voltage
Grid interconnection voltage
Voltage regulation requirements

3. Impedance

Lower impedance gives better voltage regulation but higher short-circuit currents. Typical values fall between 10% and 18%. The final figure normally comes from the interconnection study, because it affects the fault level the grid has to tolerate as well as the generator’s own stability.

4. Cooling method

For power plants:

Small to medium plants: ONAN or ONAF
Large plants: OFAF or OFWF

5. On-load tap changer (OLTC)

An OLTC regulates voltage while the transformer is energised, and grid interconnection normally requires one. The tap range has to cover grid voltage variation plus the change in generator terminal voltage between operating points, for example between full load and part load.

6. Short-circuit withstand

A GSU connects directly to a generator with a low subtransient reactance, so fault current is high. The winding clamping structure and the supporting insulation have to be designed for it, and the short-circuit test result is the evidence.

7. Losses

Two separate considerations apply. No-load loss is charged continuously whenever the unit is energised, including during periods when the plant is not generating. Load loss is charged in proportion to the square of output. Because load factors are high, load loss usually dominates the capitalised cost — but the balance flips for peaking plants, which spend long periods idle.

Plant-specific considerations

Thermal power plants

Large GSU transformers, 200 to 1000 MVA, usually with OFAF cooling. Unit sizes follow the turbine rating, so a single-line plant uses one very large unit.

Hydro power plants

Rugged designs that tolerate frequent load changes and remote locations. Hydro units also see more start-stop cycling, which works the OLTC harder than base-load thermal duty.

Renewable energy plants

Units built for solar and wind duty, handling harmonics and fluctuating loads. Inverter-fed generation adds harmonic content, so a harmonic loss allowance belongs in the specification.

Nuclear plants

Very large units at the highest voltage classes, with redundancy and seismic requirements that dominate the design.

Testing and documentation

Every GSU is subject to routine tests before despatch: winding resistance, ratio, no-load loss and current, load loss and impedance, and dielectric tests, plus oil tests where applicable. Type tests — temperature rise, lightning impulse and short-circuit — are performed on the design. Our testing standards page lists the full schedule, and our installation guide covers commissioning from site preparation to first energisation.

Common specification mistakes

Leaving impedance to the supplier, so each bid limits fault current differently.
Specifying a tap range that covers grid variation but forgets generator voltage change.
Buying on price without capitalising losses in a continuously loaded unit.
Omitting a harmonic loss allowance where the plant is inverter-fed.
Not defining the monitoring scope at order stage.

Conclusion

GSU specification sits between two disciplines, generation and transformer design, and the details are worth settling early: rating, voltage ratio, impedance, cooling and tap changer are fixed by the system, not by preference. Our guide to GSU transformers covers the design in more depth.

Talk to our engineering team

Hebei Youge Electrical Equipment Co., Ltd. designs and builds power transformers to
IEC 60076 and IEEE C57, with routine tests on every unit and a test report supplied with
the equipment. Send us your rating, voltage class and site conditions and we will come back
with a technical proposal.

Ratings by plant type

Plant type	Typical GSU rating	Typical voltage ratio	Cooling
Hydro	10 to 300 MVA	10.5 kV / 110 to 220 kV	ONAF or OFAF
Thermal (coal/gas)	50 to 1000 MVA	15.75 kV / 220 to 500 kV	OFAF
Solar	5 to 100 MVA	0.4 kV / 35 to 110 kV	ONAN or ONAF
Wind	10 to 200 MVA	0.69 kV / 35 to 220 kV	ONAN or ONAF

The table gives planning ranges. The actual rating follows the generator output plus the margin the specification allows, and the cooling class follows the ambient conditions and the intended loading.

Protection and monitoring typically provided

Differential protection covering the transformer and the connections to the generator and the switchyard
Overcurrent and earth fault protection, with settings coordinated against the generator protection
Restricted earth fault protection for the generator-side winding
Buchholz relay and pressure relief on oil-immersed units
Oil and winding temperature indication with alarm and trip contacts
Dissolved gas monitoring, where specified for critical units
Tap changer position indication and control
On-load tap changer monitoring, since OLTCs account for a large share of transformer failures

Working with the generator and the grid interface

A GSU sits between two systems with different priorities. The generator side wants low impedance to keep terminal voltage steady under changing output; the grid side wants enough impedance to limit fault current to what the switchgear can interrupt. The interconnection study normally settles the value, and both parties have to accept it.

The same tension applies to the tap changer. The generator’s terminal voltage changes with output, and the grid voltage changes with system conditions. The OLTC range has to cover both, and the control scheme has to decide which side it is regulating — normally the grid-side voltage, with the tap changer responding automatically.

Delivering a GSU project

Establish the generator data, including subtransient reactance and rated voltage
Confirm the grid interconnection voltage, fault level and protection requirements
Run the system study to fix impedance and tap range
Fix the loss specification and capitalise it in the evaluation
Agree the test schedule, including short-circuit type-test evidence
Agree the documentation, protection and monitoring scope
Plan transport, since a large GSU is often the heaviest single item on site

How to Select a Power Transformer for a Power Plant: GSU Guide