Before entering the minimum oil circuit breaker (MOCB), let us recall our knowledge about the older version of the oil circuit breaker and which is the bulk oil circuit breaker. In a bulk oil circuit breaker,(BOCB) we use oil as the insulating medium. Also, we use oil as the arc-quenching medium. So, to serve both arc quenching and insulation, a BOCB needs a large quantity of oil. A bulk oil circuit breaker with a 31.5-kiloampere short-circuit breaking capacity requires more than 50 kiloliters of oil. Again, during an arc, the oil decomposes. The oil decomposes to produce hydrogen gas, and this decomposition also produces carbon particles.
Development of Minimum Oil Circuit Breaker (MOCB)
Repetitive short-circuit operations degrade the quality of the oil. This happens due to carbonization during arcing. As a result, after a certain specified number of operations with faults, we need to replace the oil. This replacement consumes a large quantity of new, fresh oil. Obviously, this is not economical. Also, a bulk oil circuit breaker occupies a large space. The erection of a bulk oil circuit breaker is also challenging because of its huge size and weight. To minimize these difficulties, engineers developed the minimum oil circuit breaker. Here, arc quenching happens only inside the oil-filled enclosure.
Construction of Minimum Oil Circuit Breaker (MOCB)
Here, in a minimum oil circuit breaker, a chamber contains the entire current-interrupting mechanism. This mechanism stays live. An insulated column supports this arc-quenching, current-interrupting chamber. This circuit breaker consists of two parts.
The upper part contains the arc-quenching, current-interruption chamber. Whereas the lower part serves as the supporting insulator. Although there is another compartment attached above the arc-quenching chamber. We refer to it as the top chamber. This top chamber provides the space above the oil level. The space is to accommodate the gases formed during the arcing phenomenon. This top chamber also contains an oil level indicator.
Just below the top chamber, the top terminal of the circuit breaker sits. That terminal connects to the external circuit. At the bottom of the arc chamber, there is another terminal. That bottom terminal connects the other part of the circuit.
Pole of a Circuit Breaker
So, there are three chambers. One is the top chamber, which provides the required vacant space above the oil for accumulating decomposed gas during the arc. The next chamber contains the current-interruption mechanism, and the third chamber is normally a hollow cylinder of porcelain insulator. Also, we fill that chamber with oil. Although this oil does not have any direct connection with the oil of the arcing chamber. We refer to the entire assembly as the pole of the circuit breaker.
In a three-phase circuit breaker, there are three such poles. Among them, one is for the red phase, one is for the yellow phase, and the third one is for the blue phase. Then a base holds these three poles. There is a tie rod and lever assembly to facilitate the movement of the moving contact during the operations of the circuit breaker.
Mechanism Box
Below this, there is the mechanism box. This box contains the spring-spring mechanism, spring-charging motor, and other lever-gear assembly to operate the circuit breaker. It also contains the DC control circuit of the CB, auxiliary circuit, anti-pumping relay, DC failure relay, and auxiliary AC circuit for door lights and heaters, etc. This is the basic structure of a minimum oil circuit breaker.
Working Principle of Minimum Oil Circuit Breaker (MOCB)
During the opening operation, the moving contact moves downward. So, arcs form between the contacts. Obviously, the contacts are opening inside the oil. The intense heat of a high-temperature arc vaporizes and decomposes the surrounding oil. Since oil is a hydrocarbon, decomposition produces hydrogen. As a result, a bubble of vaporized gas and hydrogen forms around the arc. As the arc continues, the bubble grows.
The increasing hydrogen gas serves two purposes. First, it raises the internal gas pressure. Second, it forces the hydrogen to exhaust through the upper vent. This creates a flow of hydrogen gas. Hydrogen has a very high cooling effect and excellent deionisation capability. Due to the combined cooling and deionizing action of the hydrogen flowing along the arc axis, hydrogen quenches the arc efficiently. Since the gas flows along the arc axis, we call this type of MOCB the axial vent MOCB (Minimum Oil Circuit Breaker).
After arc quenching, the gas escapes through the vents. At the zero crossing of the sinusoidal current, the arc naturally extinguishes. With no arc, no further heat is produced. So the oil no longer generates gas. The remaining gas from the previous current cycle quickly passes through the upper vents. As the moving contacts travel further downward after arc extinction, the lower oil vent opens. Therefore, this provides an open path for the oil to flow upward. As a result, a fresh, cold oil flows in to take its place. The fresh oil in the gap between the contacts restores dielectric strength. The high dielectric strength of this fresh oil prevents arc restriking, ensuring the current is fully interrupted.
Cross Venting (Radial Venting) Minimum Oil Circuit Breaker
In the previous discussion, we studied the axial venting minimum oil circuit breaker (MOCB). In that type, hydrogen gas flows axially, that is, along the axis of the arc.
Apart from this, another design of MOCB exists — the cross venting or radial venting MOCB.
Construction of Cross Venting (Radial Venting) Minimum Oil Circuit Breaker
The construction of a cross venting MOCB is like that of an axial venting breaker. The main difference lies in the working principle and the direction of venting.
The interrupter housing forms the arc interruption chamber. Specially designed paths allow oil to enter the contact gap radially. Other channels provide radial outlets for the gas that is generated during arc quenching.
Working Principle Cross Venting MOCB
When the breaker operates, the moving contact travels backward. As soon as the contacts separate, an arc strikes between the moving and fixed contacts. As the moving contact goes further away, the arc length increases. A longer arc produces more hydrogen gas, which leads to the formation of gas bubbles inside the chamber. As the moving contact continues to open, the vent openings are gradually uncovered. These vents allow the hot gases and arc plasma to exhaust safely.
While the gases escape, fresh oil flows in to fill the vacant space between the contacts. This oil not only occupies the gap but also pushes the hot gases radially outward through the venting paths. Eventually, the gas bubbles rise upward and accumulate in the free space at the top of the chamber.
Advantages of MOCB
The advantages of the minimum oil circuit breaker are
- It requires a very small quantity of oil. It requires around 10% of the oil compared to that of the bulk oil circuit breaker.
- As a result, the weight and size of the circuit breaker become much smaller than that of the bulk oil circuit breaker.
- Due to the lesser quantity of oil, these circuit breakers suffer less from fire hazards.
Disadvantages of MOCB
The carbonization also occurs here due to the decomposition of the oil during an arc. Therefore, we must replace the oil after certain faulty operations in a minimum oil circuit breaker. Although this replacement is much easier than that of the bulk oil circuit breaker. Because the quantity of oil is much less here. But it still suffers from fire hazards. However, the chance of fire hazard is less than that of the bulk oil circuit breaker.
If the circuit breaker fails to extinguish the arc properly, it generates excessive gas. This high gas pressure can burst a single pole or multiple poles. It can also create fire hazards.
Engineers developed vacuum circuit breakers and SF6 circuit breakers later. These modern breakers replaced both bulk oil and minimum oil circuit breakers. Today, bulk oil and minimum oil circuit breakers are obsolete.