Power Systems Lab · 18 modules · all computed in your browser

Per-unit & vector groups

Bases at 115/230/500 kV, rebasing, Dyn1/Dyn11 shifts.

Per-unit calculator

Base kV (L-L)Base MVA (3φ)

Z_base

529.00 Ω

I_base

251.0 A

Line impedance Ω

Z in pu

0.0500 pu

Z_pu = Z_Ω / Z_base

Xt puon MVAat kV

Xt on system base

0.0400 pu

Nameplate impedance is on the transformer’s own MVA. Rebase before adding it to the system.

Transformer vector groups · IEC clock notation

Clock number

1 → 30°

LV vs HV (pos. seq.)

lags 30°

Zero sequence: Blocked HV↔LV; delta circulates I0, LV neutral provides ground source

Negative-sequence quantities shift by the opposite angle. That is why an LV fault looks different on HV-side relays.

Learn it

Per-unit expresses every quantity as a fraction of a chosen base, so a 500 kV line and a 13.8 kV feeder can be added in one network. Transformers effectively disappear: with bases related by the turns ratio, a transformer becomes just its leakage impedance.

Vector groups describe how the winding connections rotate the LV phasors relative to HV. Differential relays must undo that rotation, and remove zero-sequence current, before comparing the two sides.

Quiz · 0/3

1. Z_base for 500 kV on a 100 MVA base is…
2. In Dyn11 the LV positive-sequence voltage…
3. Why does a transformer differential remove zero-sequence current from the wye side?

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