Transformer Zero-Seq Directional

Set a transformer zero-sequence (3I0) overcurrent with directional element: 3I0 from phase currents, pick-up riding through external/unbalance current, relay secondary current, directional operation zone and sensitivity (IEC 60255-151 / protection coordination practice).

Inputs

Phase A current Iₐ A

Phase B current I_b A

Phase C current I_c A

Max external / unbalance 3I0 A

Reliability factor K_rel —

Neutral CT ratio /1

3U0 angle °

3I0 angle °

Transformer rated capacity Sₙ MVA

Through-load apparent power Pᵣ MVA

Results

Note — 3I0 = I_a + I_b + I_c. The directional element operates when the measured 3I0 lags/leads 3U0 within ±margin of the sensitive angle φ_sen (commonly −90° for a 3U0/3I0 element; confirm against the relay wiring). The pick-up rides through the maximum external or unbalance 3I0 with K_rel 1.2–1.5, and the sensitivity K_sen = 3I0_min / I_set must be ≥ 1.5 for the in-zone fault. Verify the neutral CT ratio and the earthing-transformer capacity against the supplier data.