Minimum oil circuit breaker

A minimum oil circuit breaker (MOCB) is an oil circuit breaker in which oil is used only for arc extinction, while insulation between live parts and earth is provided by solid insulation (porcelain or organic material). It is used from 3.3 kV to about 132 kV.

Why it is called "minimum oil"

In a bulk oil CB a large earthed steel tank is filled with oil, which serves both for arc quenching and for insulating live parts from the earthed tank, so a lot of oil is needed. In the MOCB the interrupter is housed in a small insulated chamber, which is itself kept live, so only the small quantity of oil needed for arc quenching is used (about 10% of a bulk oil CB of the same rating). Hence the name "minimum" or "small" oil circuit breaker.

Construction

• Supporting chamber: porcelain, filled with oil, physically separated from the interrupting chamber; it carries the breaker and gives insulation to earth.
• Circuit breaking chamber: a porcelain/insulating tube holding the fixed contact, a moving contact (rod), and a turbulator (side-vent or axial-vent arc control device).
• Top chamber: a metal chamber with an oil separator to cool and separate gases from oil; a vent releases gas.
• Operating mechanism: spring or solenoid, connected to the moving contact through an insulating rod.

          ┌──────────────┐  Top chamber
          │ gas separator│──► vent
          ├──────────────┤
          │ fixed contact│
          │   ┌──┴──┐    │  Breaking chamber
          │   │turbu│    │  (oil, porcelain)
          │   │lator│    │
          │   └──┬──┘    │
          │ moving contact│
          ├──────┼───────┤
          │ support      │  Supporting chamber
          │ chamber (oil)│  (porcelain)
          └──────┼───────┘
            operating rod

Principle of operation

1. In the closed position the moving contact rod is held engaged in the fixed contact.
2. On a trip signal the mechanism pulls the moving contact down, and an arc is struck between the contacts in oil.
3. The arc heat decomposes oil into gas (mainly hydrogen, about 70%) forming a gas bubble at high pressure.
4. The turbulator directs this gas and oil flow through side vents (cross blast) or axially across the arc, which cools it, lengthens it and deionises the gap.
5. Hydrogen has high thermal conductivity, so the arc loses heat fast; at current zero the arc is extinguished and fresh oil fills the gap.
6. Gases rise to the top chamber where oil is separated and the gas is vented.

Advantages

• Very small oil requirement, less fire risk, lighter and smaller.
• Lower cost and less space than bulk oil CB.
• Suitable for indoor and outdoor substations.

Disadvantages

• Small oil volume gets carbonised quickly, so frequent oil replacement is needed.
• Not suitable for frequent operation or high-speed reclosing.
• Danger of explosion if oil level falls.
• Difficult to remove gases from the small space.