What is a generator step up transformer?
A generator step up (GSU) transformer is a power transformer used in power plants to raise the voltage from generator level, typically 11 to 25 kV, to transmission level (110 kV, 220 kV, 500 kV or higher) so power can be transmitted to the grid efficiently. It sits between the generator and the plant’s outgoing connection, and it carries the full output of the plant whenever the unit is running.
Key characteristics of GSU transformers
High efficiency design
GSU transformers run continuously at full load, so they are designed for the highest efficiency the design can carry. Typical efficiency exceeds 99.5% at rated load.
Low impedance
Lower impedance gives better voltage regulation and higher short-circuit strength. Typical values range from 10% to 18%.
Impedance is the parameter where the plant and the grid disagree most often. A lower value gives tighter voltage regulation and better transient behaviour, which the generator side prefers, but it raises short-circuit current, which the switchgear and the grid side must withstand. The specified value is normally set by a system study rather than chosen by either party alone.
Special requirements
- Low no-load loss, since the unit is always energised
- High short-circuit withstand capability
- Low noise level, for the plant environment
- On-load tap changer (OLTC) for voltage regulation
- Protection and monitoring systems
Construction features
- Grain-oriented silicon steel core with step-lap joints for low no-load loss
- Disc or interleaved windings for short-circuit strength
- Reinforced clamping structure to resist short-circuit forces
- Tank with radiators or separate coolers depending on cooling class
- Bushings rated for the highest system voltage at the connection
Typical GSU transformer ratings
| Power Plant Type | Typical Capacity | Voltage Ratio |
|---|---|---|
| Hydro Power Plant | 10 to 300 MVA | 10.5 kV / 110 to 220 kV |
| Thermal Power Plant (Coal/Gas) | 50 to 1000 MVA | 15.75 kV / 220 to 500 kV |
| Solar Farm | 5 to 100 MVA | 0.4 kV / 35 to 110 kV |
| Wind Farm | 10 to 200 MVA | 0.69 kV / 35 to 220 kV |
Nuclear plants use 500 MVA and above at 500 kV and higher. The table gives typical ranges for planning; the actual rating follows the generator output plus the margin the specification allows.
GSU transformer vs distribution transformer
GSU transformers differ from distribution transformers in several ways:
- Voltage level: GSU 110 kV and above; distribution 10 to 35 kV
- Power rating: GSU tens to hundreds of MVA; distribution up to a few MVA
- Duty: GSU runs continuously at full load; distribution follows the daily load curve
- Tap changer: GSU normally has an OLTC; distribution normally uses off-circuit taps
- Loss priority: GSU is dominated by load loss at full load; distribution units spend much of their life lightly loaded
- Protection: GSU carries differential protection and detailed monitoring; distribution is protected more simply
Cooling methods for GSU transformers
Because a GSU runs at full load continuously, its cooling class is normally higher than a distribution unit of similar size:
- ONAF for smaller hydro and renewable plants
- OFAF for large thermal plants
- OFWF or ODAF where space or ambient conditions demand it
Most large GSUs are specified with multi-stage cooling, so the unit has a lower rating with auxiliaries off and a higher rating with them running.
Selection factors
1. Power rating
The rating follows the generator capacity, usually sized at 105 to 110% of the generator rating to allow for overload capability.
2. Voltage ratio
Set the generator-side and grid-side voltages from the generator rated voltage, the grid interconnection voltage and the voltage regulation requirement.
3. Impedance
Specified from the system study, typically between 10% and 18%.
4. Tap range
The OLTC range follows the expected variation in generator output voltage and in grid voltage during the plant’s operating regime.
5. Short-circuit withstand
A GSU sits close to a large generator, so it sees significant fault current. Fault current limited by design, not by hope.
Plant-specific considerations
Thermal power plants
Large GSU transformers, 200 to 1000 MVA, usually with OFAF cooling.
Hydro power plants
Rugged designs that tolerate frequent load changes and remote locations.
Solar and wind plants
Units built for renewable duty, handling harmonics from inverters and fluctuating load, and often working at higher ambient temperatures.
Replacing or Retrofitting a GSU Transformer
Most GSU enquiries are replacements rather than greenfield. The existing unit is at end of life, has failed, is no longer supported, or the plant has been uprated and the original rating no longer matches the generator. Replacement is more constrained than new build because the new unit has to fit what is already there.
What Has to Match
Start with the rating plate: capacity, voltage ratio, vector group, impedance, cooling class and tap range. Then check the physical constraints, which are usually the deciding factor — foundation and rail dimensions, busbar or cable termination positions and phases, bushing locations, and the available clearance for the conservator and cooling equipment. Where the original drawings exist they settle most of this; where they do not, we work from the rating plate and site measurements.
When the Replacement Does Not Have to Be Identical
A like-for-like replacement is the safe default, but it is not always the right answer. If the plant output has changed, the impedance requirement may have changed with it, because impedance is set partly by the fault level the unit has to survive. If the original unit was specified conservatively, a review may show a smaller or more efficient rating is adequate. If the site has added generation or changed its grid connection, the tap range may need to be wider than the original. These are worth checking before ordering an exact copy.
Planning the Outage
On a single-unit plant the GSU replacement sits on the critical path, so the sequence matters more than the equipment. Decommissioning and oil handling, foundation work, positioning, termination, drying and oil filling, then the test and commissioning programme all have to be sequenced against the outage window. We plan the test slots with the manufacturing slot so inspection does not sit behind production, and we can deliver the unit to a pre-agreed programme where the outage date is fixed.
Related: step-up transformers, high-voltage transformers, and the installation guide for site sequencing.
Testing and commissioning
GSU transformers are subject to full routine tests on every unit — winding resistance, ratio, no-load loss and current, load loss and impedance, and dielectric tests — plus type tests on the design. Our page on testing standards sets out the tests and the standards they follow, and our installation guide covers the commissioning sequence from site preparation to first energisation.
Key takeaways
- A GSU carries the plant’s entire output, so reliability and efficiency dominate every other consideration.
- Impedance is set by the system study and affects both regulation and short-circuit duty.
- GSU units run at full load continuously, which is why low-load-loss and high-efficiency designs matter.
- An OLTC is standard on GSU units.
- The cooling class is normally above that of a distribution unit of the same size.
Learn more about our power transformers and generator transformers, or read our guide on selecting a transformer for a power plant.
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.
Contact us to discuss your project, or browse the
power transformer range.
Frequently Asked Questions
Can you replace a GSU transformer with the same footprint?
Usually yes. Send the rating plate and the foundation, termination and clearance dimensions, and we will confirm whether a matching unit can be built to the existing arrangement or whether the layout needs adaptation.
Does the replacement have to be identical to the original?
Not always. If the plant output, fault level or grid connection has changed, the rating, impedance or tap range may need to change with it. An exact copy is the safe default but worth reviewing against current conditions.
How long does a GSU replacement take?
Manufacture is 30 days from drawing approval for standard ratings. The outage window is set by the site sequence: decommissioning, positioning, termination, oil filling and the test programme.
Can you work without the original drawings?
Yes. Where drawings are not available we work from the rating plate plus measurements taken on site, and confirm every dimension on the drawing before manufacture.