Representation of Plant in the Phase-Sequence Networks
The Synchronous Machine (see Table 3.1)
The positive-sequence impedance Zi is the normal transient or subtransient value. Negative-sequence currents set up a magnetic field rotating in the opposite direction to that of the positive-sequence currents and which rotates round the rotor surface at twice the synchronous speed; hence the effective impedance (Z2) is different from Z1. The zero- sequence impedance Zo depends upon the nature of the connection between the star point of the windings and the earth and the single-phase impedance of the stator windings in parallel. Resistors or reactors are frequently connected between the star point and earth for reasons usually connected with protective gear and the limitation of overvoltages. Normally, the only voltage sources appearing in the networks are in the positive-sequence one, as the generators only generate positive-sequence e.m.f.s.
Lines and Cables
The positive- and negative-sequence impedances are the normal balanced values. The zero-sequence impedance depends upon the nature of the return path through the earth if no fourth wire is provided. It is also modified by the presence of an earth wire on the towers that protect overhead lines against lightning strikes. In the absence of detailed information the following rough guide to the value of Zo may be used. For a single-circuit line Zo/Z1 = 3.5 with no earth wire and Zo/Z1 = 2 with one earth wire. For a double-circuit line, Zo/Z1 = 5.5. For underground cables, Zo/Z1 = 1 — 1.25 for single core and 3-5 for three-core cables.