The main components of an oil circuit breaker and how it interrupts the arc

This is a 230,000-volt circuit breaker. We are using it on a 138 kV system at this station. Today I had to hang up the bus cables. We took them apart for the maintenance of the circuit breaker. Each of these tanks holds approximately 2,500 gallons of mineral oil. 2800PSI Hydraulic pressure activates the circuit breaker. Just a little beefier than the little puzzle on your home-panel

They are simple in construction. The main parts of an oil circuit-breaker excluding the poles are the base frame, the drive which is constructed as a stored energy opening and closing mechanism (the operating mechanism). The opening spring of the stored energy mechanism is charged automatically during the closing action. The closing spring is loaded either by means of an electric motor (integrated into the drive unit) or by means of a removable crank.
The pole consists of an insulating cylinder, arc chamber, fixed, guiding, and mobile contacts. It also includes the gas expansion chamber, terminals, oil pan, oil drain and fills plugs, and oil level indicator.

oil circuit breaker
main components of an oil circuit breaker

Interruption of the arc in petroleum
As the moving contact separates from the fixed contact in the arc chamber, current continues to flow through the metal current paths which vaporize. The high temperature that occurs under such conditions decomposes the oil (which boils at 658 ° K) in the immediate vicinity and a gas bubble forms (under high pressure).
It consists of (from the outside to the inside): wet oil vapor, superheated oil vapor, hydrocarbons (C 2 H 2 around 4000 ° K), the arc (approximate temperature 7000° K)
CF 4 – carbon tetrafluoride
C you F 2 – Copper Difluoride
HF – Hydrogen fluoride
H 2 O – Water
SF 4 – Sulfur tetrafluoride
SF 6 – sulfur hexafluoride
THEN 2 – Sulfur dioxide
THEN 2 F 2 – Sulfuryl fluoride
SOF 2 – Thionyl fluoride
WF 6 – tungsten hexafluoride
WO 3 – Tungsten trioxide

As can be seen, the arc spins in a mixture of hydrogen (in molecular and atomic states), carbon and copper vapor. Thermal conductivity is high due to the dissociation of hydrogen molecules into atoms. The thermal energy generated in the arc is mainly dissipated outward through the surrounding gas jacket to the oil.
In addition, the gas in the arc chamber escapes to the gas expansion chamber, so that a type of convection heat dissipation is created, so that the rate of heat dissipation increases. Near the current zero, the thermal power generated by the current (in the arc) is close to zero.
If the heat dissipation to the outside is large enough, the temperature in the arc area can be reduced so that the arc loses its conductivity and is extinguished. An arc under hydrogen has a short thermal time constant, so conditions are favorable for quenching. Two other situations can occur under certain conditions: Thermal arc restart, reignition.
Thermal recovery occurs when the post-arcing current rises again and passes into the next half-cycle of the SCC, as the plasma of the arc heats up due to insufficient heat dissipation to make the conductance of the zone of the arc equal to zero. Reignition occurs when the braking voltage of the system causes the arc to form again (after the first interruption) and the current flow to continue. The design of the interrupting chambers is of the axial or radial ventilation type. Often a combination of the two is used in the minimal oil design, CB MVs.
The process of axial ventilation generates a lot of gas pressures and has high dielectric strength. This is mainly used for interrupting low currents. Radial ventilation is used for high power outages because the developed gas pressures are low and the dielectric strength is low.
The higher the current to be interrupted, the greater the gas pressure developed.

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