The per-unit system divides every quantity by a base, typically a system MVA base and the nominal voltage at each level, so a transformer impedance becomes 0.08 pu instead of an ohmic value that changes depending on which winding it is referred to. Ideal transformers drop out of the network model, equipment of very different sizes ends up with comparable values, and errors stand out. Fault and load-flow studies are worked almost entirely in per unit.
Put everything on a 100 MVA base first, then the transformer comes out at 0.12 per unit and the fault calc takes five minutes.
Voltage in pu is the quick sanity check of every control room: 1.05 pu at a busbar is fine, 0.9 pu means the phone is about to ring.
Also heard as
- per unit
- pu
- p.u.
Related terms
Fault current
Abnormally high current that flows when insulation fails and a short circuit or earth fault forms.
X/R ratio
Ratio of reactance to resistance in the fault path, which sets how large and slow the DC offset in fault current is.
Swell
Short-lived rise in RMS voltage above normal, often seen on the healthy phases during an earth fault.