Sizes a voltage transformer (VT): rated voltage factor by system earthing, turns ratio, and itemized secondary burden checked against standard rated outputs and the accuracy-guarantee range \u2014 plus advanced protection accuracy class limits, thermal limiting output, and open-delta residual voltage estimation.
Standards followed
IEC 61869-3 (inductive voltage transformers)
What it can do
- Rated voltage factor (Vf) selection by connection type and network earthing system.
- Turns ratio and rated primary/secondary voltage.
- Itemized secondary burden checked against standard rated VA sizes (10-500VA) and the 25-100%-of-rated-output accuracy-guarantee range.
- Secondary lead voltage drop to a remote instrument or relay panel.
- Advanced: protection accuracy class (3P/6P) limits, rated thermal limiting output check, and open-delta residual voltage estimate.
Example problem
An 11kV system, phase-to-earth VT, 110V rated secondary, connected to an 8VA burden.
- Rated primary voltage Up = 11kV / √3 = 6.351kV.
- Rated secondary voltage Us = 110V / √3 = 63.51V (phase-to-earth convention).
- Turns ratio = Up / Us = 100.0 (independent of the √3 division, since it cancels between line-to-line and phase-to-earth VTs on the same system).
- 8VA connected burden rounds up to the nearest standard 10VA rated output → 80% loaded, within the 25-100% accuracy-guaranteed range.
Up = 6.351kV, Us = 63.51V, ratio = 100.0, 10VA recommended standard size at 80% loading.
Want the full step-by-step version?
Two simplifications: the secondary lead voltage-drop calculation ignores burden power factor, and the open-delta residual voltage figure (3\u00d7 the residual-winding voltage, taken as Us/\u221a3 per winding on effectively earthed systems and Us/3 on non-effectively earthed systems) is a theoretical estimate \u2014 actual broken-delta winding ratios are manufacturer-specific.