Oil circuit brakers (OCB)

Circuit breakers

Electrical circuit breaker is a switching device which can be operated both manually and automatically for controlling and protection of any electrical power system. As the modern power system deals with huge currents, the spacial attention should be given during designing of circuit breaker to safe interruption of arc produced during the opening/closing operation of circuit breaker.

According to their arc quenching (rapid cooling) media the circuit breaker can be divided as:

1- Air circuit breaker

2- Oil circuit breaker

3-Vacuum circuit breaker

4-) SF6 circuit breaker

According to their services the circuit breaker can be divided as:

1-Outdoor circuit breaker

2-Indoor circuit breaker

According to the operating mechanism of circuit breaker they can be divided as:

1-Spring operated circuit breaker

2- Pneumatic circuit breaker

3- Hydraulic circuit breaker

According to the voltage level of installation types of circuit breaker are referred as:

1- High voltage circuit breaker (> 72 kV)

2- Medium voltage circuit breaker (1-72 kV)

3- Low voltage circuit breaker (< 1 kV)

Circuit breaker ratings according to the IEC 62271-100, IEC 62271-1 and IEC 60947-2 are explained on this page "Circuit Breaker Ratings".

Short summary for breakers:

Plain-break air breakers are used in low voltage and medium voltage up to 15 kV

For low and medium voltages fuses can be also used, but the main disadvantage is that they must be replaced after fault clearing

In medium voltage systems minimum oil, SF6 and vacuum breakers are also being used

For high voltages minimum oil, SF6 and blast-air breakers are used, but always with multiple interrupters in series

The maximum voltage per interrupter is 100 kV for air-blast and SF6 breakers, 170 kV for minimum oil breakers

Oil circuit brakers (OCB)

Mineral oil has better insulating property than air. The oil is used to insulate between the phases and between the phases and the ground, and to extinguish the arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble of hydrogen that surrounds the arc. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization and extinction of the arc. Disadvantage of the oil circuit breakers is the flammability of the oil, and the maintenance necessary (i.e. changing and purifying the oil). The oil circuit breaker is the one of the oldest type of circuit breakers.

Bulk oil circuit breakers (BOCB)

Bulk oil circuit breaker (or BOCB) is a such type of the circuit breakers where oil is used as arc quenching media as well as insulating media between current carrying contacts and earthed parts of the breaker. The oil used here is same as transformer insulating oil. These types of breakers are designed in all voltage ranges from 1 kV up to 330 kV.

Modern arc-controlled oil breakers have an arc control device surrounding the breaker contacts to improve extinction of arc. In cross blast interrupters, the arc is drawn in front of several lateral vents. The gas formed by the arc causes high pressure inside the arc control device. The arc is forced to bow into the lateral vents in the pot, which increases the length of the arc and shortens the interruption time. The axial blast interrupters use similar principle. Oil breakers are design for both three-phase and single-phase circuit brakers.

At voltages higher than 115 kV, separate tanks for each phase are used. The practical limit for the bulk oil breakers is 275 kV

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Minimum oil circuit brakers (MOCB)

In minimum oil breakers the oil is used only for extinguishing of the arc. The arc control devices are the same as for the bulk-oil breakers. However, unlike bulk oil circuit breakers, these designs place the interrupting units in insulating chambers at live potential. To improve breaker performance, oil is injected into the arc. The interrupter containers of the minimum oil breakers are made of insulating material and are insulated from the ground. This is usually referred to as live tank construction. For high voltages (above 132 kV), the interrupters are arranged in series. It is essential to ensure that each interrupter carries its share of the duty. Care must be taken that all breaks occur simultaneously, and that the restriking voltage is divided equally across the breaks during the interrupting process. The features of designing MOCB is to reduce requirement of oil, and hence these breaker are called minimum oil circuit breaker. These designs are available in voltages ranging from 1 kV to 765 kV using the multi-break technique.