Transformers
Power transformers
Power transformers are rated in MVA and engineered for each project, for primary substations, generation connections and large industrial supplies.
- Rating
- in MVAset by the connection and the load or generation
- Voltage regulation
- commonly an on load tap changerset by the network voltage range
- Cooling
- ONAN, ONAF or forced oil stagesset by the load profile
- Engineering
- designed per projectno standard catalogue value applies
- Standard
- IEC 60076 seriesIEEE C57.12.00 on North American specifications
Power transformers sit above the distribution range. They connect transmission and subtransmission networks to distribution, connect generation to the grid and supply the largest industrial loads. Each is engineered to its project, so the specification carries more weight than for a distribution unit, and the documents exchanged before an order run longer.
Three questions settle feasibility early. The weight and dimensions of the main tank decide the route, and sometimes the site. An on load tap changer adds cost and maintenance, and lets the unit follow the network voltage. Factory acceptance tests are normally witnessed, so they go into the programme.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Rating | In MVA, with cooling stage ratings where applicable | The connection, the load or the generation |
| Voltage ratio | Transmission or subtransmission to distribution voltage | The networks on either side |
| Tap changer | On load or off circuit, with the tapping range | The voltage variation to be followed |
| Cooling | ONAN, ONAF, OFAF or equivalent stages | The load cycle and the hottest ambient on site |
| Impedance | Specified per project | The fault level and the parallel operation |
| Transport limits | Maximum shipping weight and dimensions | The route to site |
| Testing | Routine, type and special tests, and witnessing | The specification and the owner's standard |
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 60076-1 | General requirements, with the project specification adding what a catalogue unit would not need |
| IEC 60076-7 | Loading guide, used when the owner wants ratings for cyclic or emergency loading |
| Regulation (EU) No 548/2014, as amended | Above 3150 kVA the regulation sets a minimum Peak Efficiency Index rather than absolute loss ceilings |
| IEEE C57.12.00 | General requirements for liquid immersed power transformers on North American specifications |
Documents normally exchanged
| Document | What it evidences |
|---|---|
| Technical specification and data schedule | The owner's requirements, and the values the manufacturer declares against them |
| Outline and transport drawings | Dimensions, shipping weight and the arrangement on site |
| Test plan | Routine, type and special tests, and which are witnessed |
| Factory acceptance test report | Results of the tests carried out before dispatch |
| Transport and erection instructions | Handling, assembly and filling on site |
| Operating and maintenance manual | Operation, the tap changer and maintenance intervals |
What a power transformer request needs
- Rating in MVA and any cooling stage ratings
- Voltage ratio, vector group and impedance requirement
- Tap changer type and tapping range
- The standard and the owner's technical specification
- Site location and any known transport restriction
- Testing and witnessing requirements
Delivery and logistics
Power transformers normally travel as abnormal loads, often drained and with fittings removed, and are assembled, filled and tested on site. The route survey belongs in the project from the start, and the shipping weight of the main tank is one of the first values to fix.
Delivery dates follow manufacturer loading. Each quotation states the date for the unit offered.