Consider the schematic presentation of a section of a power system, shown in Fig. 3.10.
The gap shown typically represents the shortest distance from the high voltage wire to
ground – often across an insulator. Due to an overvoltage the air in the gap breaks down
(flashes over) due to the increased electric field strength. The mechanism of flashover
depends on the nature of the gap as indicated in Table 3.1. The wave shape of the
overvoltage also affects the value of the breakdown voltage. Typically the flashover voltages
of short duration impulses are higher than for AC and DC.
Fig. 3.10: Section of a power system with gap
Often the flashover of the gap is initiated by an external overvoltage, such as a
lightning surge. Once the gap has flashed over an arc is formed (provided that the
impedance Z is not too high), maintained by the system voltage (AC or DC) and a large fault current flows that has to be cleared by the circuit breaker (CB in Fig. 3.10). If the impedance is high, it may not be possible for a stable arc to form; in such cases intermittent or repetitive sparking may occur
The transition to an arc can be explained with reference to Fig. 3.11, showing the DC
voltage/current characteristic a gas-filled gap.
Fig. 3.11: The V-I curve, pertaining to corona and arc transition.
Table 3.1: Occurrence of the different flashover mechanisms
Although this illustration is strictly only valid for DC discharges in gas discharge tubes,
similar phenomena occur in the case of AC air flashover. Initially, electron avalanches lead to
Townsend discharges, until the sparking potential VC is reached. At this point, glow
corona sets in and the glow discharge changes into an arc. Fig 3.11 shows an important
feature of arcs: the non-linear "negative resistance" characteristic, i.e. the arc voltage
decreases with increase in current.
As explained in chapter 1, arcs also form when the contacts of a circuit breaker move
apart. The circuit breaker’s main function is to interrupt the arc and current.
Fig. 3.12: Circuit for interruption of an arc.
Consider the problem of interrupting a DC current by a circuit breaker, as shown
in Figure 3.12. In Figure 3.13 the nonlinear arc characteristics of arcs of increasing
length are shown, together with the load line, representing the voltage source, E,
and the load resistance, R. The stable operating points are also shown for each
length of arc. It is clear that if the arc length exceeds a certain critical length, no stable operating point exists. In a DC circuit breaker the arc is elongated by: moving the circuit
breaker contacts apart, allowing the arc to move to the region where the contacts are further
apart (arcing horns). This movement is facilitated by magnetic blow-out coils or by the
thermal effect. The circuit usually also contains some inductance that complicates the
current interruption process.