A transformer where a single winding acts as both the primary and secondary, with the output voltage taken from a tapping point along the coil. Because it does not provide electrical isolation between the input and output, it is smaller, lighter, and cheaper than a two-winding transformer for the same rating. They are heavily used in power grids to step voltages between transmission levels, such as from 275 kV to 132 kV.
We're installing a 500 MVA autotransformer to tie the new transmission line into the existing substation bus.
Related terms
Transformer
A static electromagnetic device that transfers electrical energy between circuits at changed voltage levels.
Tapping
A connection point made along a transformer winding to allow for voltage adjustment.
OLTC
On-load tap changer: a transformer switch that changes the turns ratio under load to regulate voltage.