Types of Oil Circuit Breakers and Arc Extinguishing Principle

Oil circuit breakers are the oldest type of circuit breakers and have been in use for the longest time. Based on their insulation structure, they can be divided into two types: bulk oil circuit breakers and minimum oil circuit breakers. Currently, bulk oil circuit breakers are only used in small quantities at the 35kV voltage level, while minimum oil circuit breakers are still used in limited quantities at the 110kV and 220kV levels. Since oil circuit breakers still represent a significant portion of the power grid, this section focuses on minimum oil circuit breakers.

In an oil circuit breaker, when the current is interrupted, an electric arc forms between the moving and stationary contacts. The high temperature of the arc causes the insulating oil to rapidly vaporize into oil vapor and decompose into other gases. The generated gases, due to the surrounding oil’s inertia, form mixed gas bubbles around the arc. The oil vapor constitutes about 40% of the mixture, while other decomposed gases make up the remaining 60%, with 70%-80% of these gases being hydrogen, which has strong cooling and diffusion properties.

On one hand, the generated gases are sealed within the arc extinguishing chamber, which increases the pressure inside the chamber. This leads to an increase in the concentration of free particles in the arc, enhancing the recombination and deionization processes. On the other hand, the temperature of the gas bubbles' outer layer is lower than the temperature of the arc column inside the bubbles. The temperature and pressure difference between the inside and outside of the bubbles causes violent disturbances, which further enhances the cooling of the arc column. Meanwhile, the arc is in a high-pressure, strongly cooled, enclosed bubble, and as the contact gap increases, the arc is stretched. When the arc current crosses zero, the dielectric strength between the contacts rapidly recovers, extinguishing the arc.

Thus, the oil circuit breaker extinguishes the arc using the energy from the arc itself, that is, the oil in the breaker decomposes and vaporizes under high temperature, and the resulting gas forms high pressure in the special arc extinguishing chamber to blow out the arc. Therefore, the oil circuit breaker is a self-extinguishing type of circuit breaker. To improve the speed of dielectric recovery and shorten the arcing time, various types of arc extinguishing devices can be used, such as transverse blow arc extinguishing chambers, where the gas mixture's spraying direction is perpendicular to the arc's direction, longitudinal blow arc extinguishing chambers, where the gas spraying direction is aligned with the arc's direction, and combined transverse and longitudinal blow arc extinguishing chambers.

Usage of Oil Circuit Breakers

1.Inspection and Maintenance
During engineering handover and acceptance, the following checks should be made:

• Ensure the circuit breaker is securely fixed, with a clean and intact exterior.

• Verify electrical connections are reliable and have good contact.

• No oil leakage should be present, and the oil level and color should be normal.

• The interlocking between the breaker and the operating mechanism should be functional, with no blockages.

• The on/off indication should be correct.

• During commissioning, the auxiliary switch should operate accurately and reliably, with no arcing damage on the contacts.

• The porcelain insulators should be intact, clean, and free of cracks.

• Ensure the paint is intact, phase color markers are correct, and grounding is good.

During normal operation, inspections should include:

• Checking if the oil color and level are normal, and ensuring there are no leaks at oil filling points.

• Ensuring the porcelain bushings are clean and free of damage or discharge marks.

• Checking that all connections have good contact without overheating or loosening.

• Verifying the condition of insulation rods and their insulators.

• Ensuring correct operation of the on/off mechanisms and interlocks.

• Making sure the actuator box cover is tightly closed.

• Checking that the closing and opening coils have no burning smell, and that secondary wiring is free of moisture or corrosion.

2.Overhaul

Major Overhaul: Generally carried out every 3 to 4 years. A newly installed breaker should undergo its first major overhaul one year after being put into service. Major overhaul tasks include:

• External inspection and pre-test before disassembly.

• Draining the oil.

• Disassembling and inspecting the conductive system and arc-extinguishing unit.

• Disassembling and inspecting the insulation support system (e.g., insulators).

• Disassembling and inspecting the switching and transmission mechanisms.

• Inspecting the base.

• Replacing seals and gaskets.

• Servicing the operating mechanism.

• Reassembly and adjustment tests, including mechanical, electrical, and insulation tests.

• Rust removal, painting, and oil treatment (replacing or refilling oil).

• Cleaning the site and performing acceptance tests.

Minor Maintenance: Includes tasks such as:

• Checking and cleaning the external parts of the breaker and handling any oil leaks.

• Addressing any operational defects discovered during service.

• Inspecting external transmission mechanisms, springs, and bolts, and ensuring all parts are secure.

• Cleaning and lubricating the operating mechanism.

• Conducting preventive tests.

3.Emergency Maintenance

If defects arise that threaten the safe operation of the breaker (e.g., excessive loop resistance, severe overheating at contact points, high oil loss factor for bulk oil circuit breakers, or excessive DC leakage current for low-oil breakers, or severe oil leakage), or if the breaker reaches a specified number of operating cycles (200 operations or more, or after a specified number of fault trips), emergency maintenance should be performed.

This explanation provides a thorough understanding of oil circuit breakers, their types, working principle, maintenance, and inspection requirements. These practices ensure the breaker operates efficiently and safely in the electrical system.