A VT, voltage transformer, supplies a scaled replica of primary voltage, typically 110 V or 120 V line to line, for metering, protection and synchronising. On transmission systems capacitor voltage transformers (CVTs) are used, which can also couple power line carrier signals. Unlike a CT, a VT secondary must never be short-circuited, and its fuses or miniature breakers matter: when one fails, a distance relay can see a false fault.
VT fuse blew on the line panel and the distance relay went into fuse-fail block, replace it before we lose the line.
Also heard as
- voltage transformer
- PT
- potential transformer
Do not mix it up with
- CT: A CT secondary must never be opened; a VT secondary must never be shorted.
Related terms
CT
Current transformer: an instrument transformer that scales large primary currents down to a standard secondary.
Distance protection
Line protection that measures impedance to a fault and trips fastest for faults close to the relay.
Burden
Load connected to the secondary of an instrument transformer, stated in VA or ohms.