Negative phase-sequence voltages on E&W transmission system

In the absence of phase transposition, overhead lines are a major source of unbalance in both currents and voltages on the transmission system. In the England and Wales (E&W) transmission system, the individual circuits are phased such that the average unbalance produced is minimized. The main negative phase-sequence (NPS) voltages on the transmission system arise at loop-in points for generation and load connection under an outage condition of one of the connection circuits and where there is a large NPS voltage induction from the parallel circuit. If not limited, excessive unbalance can cause system incidents and lead to damage of power system equipment.