Page 89 - Practical Power System and Protective Relays Commissioning

Circuit Breakers Theory Testing and Commissioning Chapter | 7 87

The extinguishing of the arc is done at a current of zero and a
dielectric is injected between the contacts faster than the recovery voltage
can build up. This process is the same regardless of the dielectric medium
used.
Fig. 7.1 shows only the voltage waveform from a normal operating condi-
tion, through a short circuit, and then back to the recovery voltage. Fig. 7.1A
shows the system operating under normal conditions. At the beginning
Fig. 7.1B shows a short circuit has occurred, which will cause the voltage to
theoretically go to zero during the prearcing stage. As the contacts begin to
separate, the short circuit current that was following just prior to the contacts
separating continues to follow and establish an arc. The arc interruption will
depend on the type of insulating media (air, oil, vacuum, SF 6 ). During the
arc stage (Fig. 7.1B) shows the arc is extinguished at each current zero, due
to ionization of the insulating media gap the arc will once again begin to
form due to the system voltage across the contact gap. This condition of
current flow again causes the restrike voltage peaks to occur. Then, as shown
in Fig. 7.1C, after the contact has separated the short circuit should be extin-
guished and the recovery voltage appears.
Fig. 7.2 shows the break downs in the current zero window of the restrike
condition.
Fig. 7.3 shows the recovery condition in the current zero window.

FIGURE 7.2 Break downs in the current zero window of the restrike condition.