Page 90 - Practical Power System and Protective Relays Commissioning

88 Practical Power System and Protective Relays Commissioning

FIGURE 7.3 The recovery condition in the current zero window.

7.3 CIRCUIT BREAKER TYPES BASED ON INSULATING
MEDIUM

7.3.1 Air Circuit Breaker
An air CB employs air as the interrupting insulation medium. The air CB
poles consist of a set of main contacts and a set of arcing contacts. The arc-
ing contacts open after the main contacts and the arc is drawn on them.
A puff of air from a puffer is used to aid in blowing the arc deeper into the
throat of the arc chute.
A blast of air trained on the arc from a reservoir of compressed air
enables a heavier arc to be extinguished in an air blast CB.
Refer to Fig. 7.4 for an air CB contact assembly.

7.3.2 Oil Circuit Breaker
Oil CB have their contacts immersed in insulating oil, during Oil Circuit breaker
opening the insulating medium- Oil- will cools the arc, the heat and chemical
reaction create hot gases which expand rapidly, blowing the arc out. There are
two types of oil CB: the original type was the bulk oil CB and the current one
is the minimum oil CB. Refer to Fig. 7.5 for a schematic of a bulk oil CB.
Fig. 7.6 illustrates a schematic of a minimum oil CB.

7.3.3 Vacuum Circuit Breaker

In this breaker the isolating medium is a vacuum in a chamber in which the
air has been evacuated as illustrated in Fig. 7.7.