Protection Against Overvoltages
Modification of Transients
When considering the protection of a power system against overvoltages, the transients may be modified or even eliminated before reaching the substations. If this is not possible, the lines and substation equipment may be protected, by various means, from flashover or insulation damage.
By the use of over-running earth (ground) wires, phase conductors may be shielded from direct lightning strokes and the effects of induced surges from indirect strokes lessened. The shielding is not complete, except perhaps for a phase conductor immediately below the earth wire. The effective amount of shielding is often described by an angle f, as shown in Figure 10.8; a value of 35° appears to agree with practical experience. Obviously, two earth wires horizontally separated provide much better shielding.
It has already been seen that the switching-in of resistance across circuit-breaker contacts reduces the high overvoltages produced on opening, especially on capacitive or low-current inductive circuits.
An aspect of vital importance, quite apart from the prevention of damage, is the maintenance of supply, especially as most flashovers on overhead lines cause no permanent damage and therefore a complete and lasting removal of the circuit from operation is not required. Transient faults may be removed by the use of autoreclosing circuit breakers. In distribution circuits and many transmission systems all three
Figure 10.8 Single earth wire protection; shielding angle f normally 35’ phases are operated together. Some countries use single phase auto-reclose to improve transient stability. Only the faulted phase is opened and re-closed while two phases remain connected.
It is uneconomic to attempt to modify or eliminate most overvoltages, and means are required to protect the various items of power systems. Surge divertors are connected in shunt across the equipment and divert the transient to earth; surge modifiers are connected to reduce the steepness of the wavefront- hence reduce its severity. A surge modifier may be produced by the connection of a shunt capacitor between the line and earth or an inductor in series with the line. Oscillatory effects may be reduced by the use of fast-acting voltage or current injections.