Three-phase imbalance: neutral current, or a floating neutral
Balanced, the three line currents cancel and a neutral carries nothing. Unbalance them and the imbalance has to go somewhere — with a neutral (4-wire) it flows as neutral current; without one (3-wire) the currents are forced to sum to zero and it reappears as a shift in the neutral-point voltage. Switch the system and watch it move between the current view and the voltage view.
Current view. Left: phasor diagram of the three line currents from a common origin, with the neutral current as their vector sum. Right: the three phase currents over time plus the neutral as a fourth trace.
Voltage view. Left: the balanced source phase voltages form a fixed triangle with the source neutral at its centre; the load neutral point N is displaced by the neutral-point voltage, and the vectors from N to each line are the unbalanced load phase voltages. Right: bar chart of each phase-voltage magnitude against the 1.0 per-unit nominal line.
4-wire (wye with neutral): the neutral carries the vector sum of the three line currents, and the load phase voltages stay balanced on the source triangle. A dropped phase leaves a full 1.00 pu in the neutral — run “Drop L3” to see it.
3-wire (no neutral): with no return path the line currents must sum to zero, so the current phasors close and there is no neutral current. The imbalance instead slides the load neutral point N off the source neutral by Vₙ (Millman’s theorem) — the vectors from N to each line are the unbalanced phase voltages, and a lightly loaded or open phase swings to an overvoltage above 1.0 pu. Run “Drop L3”: a 1.00 pu neutral current (with a neutral) instead becomes a 0.50 pu neutral shift and a 1.50 pu terminal on the open phase.