Resources
Transformer specification glossary
Plain definitions of the terms that come up most often when specifying or buying a large power transformer. Each term has its own link you can share.
Terms A to Z
- Autotransformer
- A transformer in which part of the winding is shared between the high-voltage and low-voltage circuits. Autotransformers are smaller and lighter than a two-winding unit of the same rating, and are common for transmission interties where the voltage ratio is modest, such as 345 kV to 138 kV.
- BIL (basic impulse insulation level)
- The peak voltage of a standard lightning impulse that a winding or bushing is designed to withstand. Each winding has its own BIL. A unit with a lower BIL than your system requires generally cannot be used without changes to protection.
- DETC (de-energized tap changer)
- A tap changer that can only be operated with the transformer switched off. Used where the voltage ratio rarely needs to change. Sometimes called a no-load tap changer.
- DGA (dissolved gas analysis)
- A laboratory test of the insulating oil that measures gases produced by faults such as overheating or arcing. A DGA history is one of the most useful indicators of a transformer's internal condition.
- FAT (factory acceptance test)
- The set of tests carried out at the factory before a transformer ships, to the applicable standard such as IEEE C57.12.90 or IEC 60076. Buyers often witness the FAT. The FAT report is a key document when buying any existing unit.
- GSU (generator step-up transformer)
- A transformer that raises a power plant's output voltage to transmission voltage. See GSU transformers.
- Impedance
- Expressed as a percentage, it describes how much the transformer limits current under a short circuit and how voltage changes with load. Impedance is usually set by system studies, and is one of the most common reasons an existing unit does not fit.
- MVA rating
- The apparent power a transformer can carry continuously. It is usually stated for each cooling stage, for example 60/80/100 MVA for ONAN/ONAF/ONAF, at a stated temperature rise.
- Natural ester fluid
- An insulating fluid made from vegetable oils, used in place of mineral oil. It has a much higher fire point and is biodegradable, which makes it attractive near buildings or in sensitive locations.
- ODAF (oil directed, air forced)
- A cooling class in which pumps direct oil through the windings and fans blow air over the radiators. It allows a higher rating from the same size of unit.
- OLTC (on-load tap changer)
- A tap changer that can change the voltage ratio while the transformer is energized and carrying load. Used where voltage must be regulated during operation. OLTCs need regular maintenance.
- ONAF (oil natural, air forced)
- A cooling class in which oil circulates naturally and fans blow air over the radiators. Often the second or third stage of a transformer's rating.
- ONAN (oil natural, air natural)
- The base cooling class, with natural oil circulation and natural air flow over the radiators. The ONAN rating is the transformer's rating with no fans or pumps running.
- Phase-shifting transformer
- A specialized transformer that controls the flow of power on a transmission line by shifting the phase angle between its input and output. Each one is designed for its specific location.
- SFRA (sweep frequency response analysis)
- A test that compares the frequency response of the windings against a baseline. Changes can reveal winding movement or core damage, especially after transport.
- Temperature rise
- The allowed increase in winding or oil temperature above ambient at rated load, for example 65 °C. Units with the same MVA rating but different temperature rise are not directly interchangeable.
- Vector group
- A code such as YNd1 or Dyn11 that describes how the windings are connected and the phase shift between them. Transformers running in parallel must have compatible vector groups.
Using these terms in a search
Our specification matching section shows how these parameters are used to decide whether a unit fits. For equipment-specific notes, see power transformers and GSU transformers.
Sources
FAQ
Common questions
Which specification terms matter most for a search?
MVA rating, voltages, impedance, BIL, vector group and cooling class decide whether an existing unit can fit your project.
What is the difference between OLTC and DETC?
An on-load tap changer adjusts voltage while the transformer is energized. A de-energized tap changer can only be adjusted with the unit out of service.
Where do these definitions come from?
They follow the IEEE/ANSI C57 and IEC 60076 standards listed in the sources on this page.
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