How Does an On-Load Tap-Changer (OLTC) Work?
Quick answer — An on-load tap-changer (OLTC) changes the transformer ratio without interrupting load by using a diverter switch (or vacuum interrupter) and a transition impedance to bridge between tap positions while current flows. A motor drive and control select the position, and a selector moves the connection between the winding tappings. OLTCs are essential for holding the secondary voltage within limits under varying load.
How does an on-load tap changer work?
The principle
To change taps under load, the current must never be interrupted and adjacent tap contacts must never be shorted by a circulating current; a transition resistor (or reactor) and a diverter bridge the gap.
Construction
The OLTC has a tap selector, a diverter switch (oil or vacuum) and a motor-drive mechanism, sealed in its own oil compartment. The control selects the position and stops at the correct tap.
Maintenance
OLTC contact wear and oil condition are a leading cause of transformer faults, so oil testing (acetylene) and periodic contact/maintenance are essential.
References & standards
- IEC 60214 (tap-changers)
- IEC 60076-1
Related
Related manufacturers
Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp
Related components
Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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