What are Minimum oil circuit breakers?
The Circuit breaker which uses a minimum or small quantity of oil for arc quenching medium is called a minimum oil circuit breaker. that circuit breaker is also called a low-oil circuit breaker. the minimum oil circuit breaker oil is used only for arc extinction purposes only. In the bulk oil circuit breaker oil is used for both purposes for arc extinction purposes and insulates the current-carrying contacts from the earth. The minimum oil circuit breaker requires a small percentage of oil about 10 % of oil.Principle of Minimum oil circuit breaker
During normal operating conditions fixed and moving contacts that are immersed in insulating oil remain in closed condition.
When the fault occurs in the system moving contacts separate from fixed contacts through the breaker control mechanism and the arc is produced between the contacts. Due to the production arc between the fixed and moving contacts, heat energy is produced between the circuit breaker contacts which evaporate the oil between the contacts and produce the hydrogen gas at high pressure.
The oil between contacts is pushed away from the arc, and a hydrogen gas bubble surrounds the arc and portion of fixed and moving contacts. the hydrogen gas cools down the arc by its high heat conductivity and hydrogen gas produced the violet movement in oil and fresh oil enters between the contacts.
The arcing products between fixed and moving contacts are pushed away and the arc is extinguished and the fault current is interrupted.
Construction of minimum oil circuit breaker
Fig shows the cross-sectional view of the minimum oil circuit breaker. The minimum oil circuit breaker consists two-compartment separated from each other and both are filled with transformer insulating oil. The minimum oil circuit breaker consists upper chamber and a lower chamber. The upper chamber is called the circuit breaking chamber and the lower one is called the supporting chamber.
The two-chamber are separated by a partition of bakelite insulating material which is preventing the mixing of oil from one chamber to another chamber. The circuit-breaking chamber uses a small volume of oil which is just enough for arc extinction purposes. So the oil to be replaced is reduced because the oil used in the supporting chamber is not mixed with arc products.
The minimum oil circuit breaker consists following parts
1. Supporting Chamber
2. Circuit breaking chamber
3. Top Chamber
1. Supporting Chamber
The supporting chamber is made from porcelain and mounted on a metal chamber. The supporting chamber consists operating rod and insulating oil. The operating rod is used for opening and closing the fixed contacts.
The oil inside the supporting chamber is used only for insulating the current-carrying parts from the earth. The supporting chamber is separated from the circuit breaker chamber by the mica or backlit insulating material. due to this partition, the oil from the supporting chamber is mixed with the circuit breaking chamber.
2. Circuit Breaking Chamber
The circuit-breaking chamber consists of the following equipment
1. upper and lower Fixed contacts
2. moving contacts
3. Turbulator
The circuit-breaking chamber is also made from porcelain and mounted on a supporting chamber and filled with the insulating oil which is used for arc extinction purposes.
The moving contacts are hollow which is includes a cylinder which is moves up and down over a fixed piston. Fixed contacts are controlled by an operating rod through the mechanism which is available in the supporting chamber.
The upper fixed contacts consist of a turbulator which is used for arc controlling devices. the turbulator consists of axial and radial vents. The axial vents are used for interruption of low currents and radial vents are used for interruption of high currents.
3. Top chamber
The top chamber is made from metal and mounted on the circuit breaking chamber. the top chamber provides expansion space for insulating oil in the circuit-breaking compartment. The top chamber consists of the following equipment
1. Breather
2. Gas vent
1. Breather
It is used for the breathing process for oil. when atmospheric temperature increases, the volume of oil increases,s and when atmospheric temperature decreases, the volume of oil decreases.
2. Gas Vent
It is used for removing the gases produced in the circuit-breaking compartment.
Working of Minimum oil circuit breaker
During the normal operating conditions, fixed contacts are engaged with the moving contacts. when the fault occurs in the system, moving contacts are separated from fixed contacts through the control mechanism of the circuit breaker, and an arc is produced between the contacts. this arc produces high heat and oil surrounds the fixed and moving contacts are vaporized and produce gases under high pressure.
The oil between contacts is pushed away from the arc, and a hydrogen gas bubble surrounds the arc and portion of fixed and moving contacts. the hydrogen gas cools down the arc by its high heat conductivity and hydrogen gas produced the violet movement in oil and fresh oil enters between the contacts.
The arcing products between fixed and moving contacts are pushed away and the arc is extinguished and the fault current is interrupted.
Advantages of minimum oil circuit breaker
1. The minimum oil circuit breaker requires a small quantity of oil. In minimum oil circuit breaker oil is used only for arc quenching medium only and for insulating purposes other hard insulating materials are used.
2. The minimum oil circuit breaker required small space because the required oil quantity is small, so the breaker size is reduced as compared to the bulk oil circuit breaker.
3. In the minimum oil circuit breaker less maintenance is required.
4. In the minimum oil circuit breaker less risk of fire is reduced because less quantity of oil is used.
5. The weight of the minimum oil circuit breaker is reduced.
6. The oil has high dielectric strength which provides higher insulation after the arc is extinguished.
Disadvantages of minimum oil circuit breaker
1. In the minimum oil circuit breaker, there is difficulty in removing gases between the fixed and moving contacts.
2. In a minimum oil circuit breaker smaller quantity of oil is used so, the degree of carbonization increases.
3. Due to the high degree of carbonization, the dielectric strength of oil is reduced rapidly.
Applications of minimum Oil circuit breaker
1. The minimum oil circuit breaker is used for voltage range up to 33kv to 220Kv.
2. The minimum oil circuit breakers are operates very fast and are widely used for transmission lines and electrical substations.