Auto-Transformers
An auto-transformer shares a single winding between primary and secondary instead of using two fully isolated windings, which makes it smaller, lighter and lower-loss than a two-winding transformer of the same rating for a modest voltage change. NLS Power sources and supplies auto-transformers built to IEC 60076-1 for voltage step and interconnection duty on Saudi transmission and industrial projects.
An auto-transformer uses a single winding tapped at the point that defines the secondary voltage, rather than two electrically separate windings coupled only by the magnetic core the way a conventional two-winding transformer works. Because most of the winding is shared between primary and secondary, an auto-transformer is smaller, lighter and runs lower losses than an equivalent two-winding unit, and the saving gets larger the closer the primary and secondary voltages are to each other. That's why auto-transformers are the standard choice for voltage ratios like 132/33kV or 33/11kV at transmission and sub-transmission level, where the ratio is modest, but rarely used for a large step-down like 33kV to 400V, where a two-winding unit is the more sensible design.
The tradeoff is electrical isolation. Because the windings are physically connected rather than only magnetically coupled, a fault on the secondary side has a more direct path back to the primary than it would through a two-winding transformer, which shapes how the protection scheme, earthing arrangement and insulation coordination around an auto-transformer get designed. IEC 60076-1 covers the general requirements that apply, and the same vector group and cooling class considerations that apply to conventional transformers, Dyn11 or Yy0 depending on application, ONAN through ONAF cooling, still apply here.
On the Saudi grid, auto-transformers show up mainly at SEC transmission and sub-transmission substations interconnecting adjacent voltage tiers, and on some industrial sites doing a moderate voltage step ahead of process equipment. A variable-ratio version, using a moving tap or variac-style winding, also serves voltage regulation duty where a site needs to trim supply voltage rather than step it.
NLS Power sources and supplies auto-transformers sized to the voltage ratio, rating and cooling class a transmission interconnection or industrial voltage-step application requires.
Specifications
- ✓Winding: single shared winding between primary and secondary, per IEC 60076-1
- ✓Efficiency: lower losses and smaller frame than an equivalent two-winding transformer at a modest voltage ratio
- ✓Typical ratio: best suited to close primary/secondary ratios, e.g. 132/33kV or 33/11kV interconnection
- ✓Isolation: no full electrical isolation between primary and secondary, factored into protection and earthing design
- ✓Cooling classes: ONAN through ONAF depending on rating, same as conventional oil-immersed units
- ✓Typical use: transmission and sub-transmission voltage interconnection, industrial voltage-step and regulation duty
Frequently Asked Questions
What's the difference between an auto-transformer and a regular two-winding transformer?+
An auto-transformer shares a single winding between primary and secondary rather than using two fully separate windings, which makes it smaller and lower-loss for a modest voltage change, but it doesn't give the same full electrical isolation a two-winding transformer provides.
When does it make sense to use an auto-transformer instead of a two-winding unit?+
When the primary and secondary voltages are relatively close, like 132/33kV or 33/11kV interconnection duty. The size and loss advantage of an auto-transformer shrinks as the voltage ratio gets larger, which is why a big step-down like 33kV to 400V still uses a conventional two-winding transformer.
Does an auto-transformer need different protection than a two-winding transformer?+
Yes. Because the windings are electrically connected rather than only magnetically coupled, a fault has a more direct path between primary and secondary, so the protection scheme and earthing arrangement are designed specifically around that reduced isolation rather than reused from a standard two-winding design.