Bulk oil circuit breakers (OCBs) are most popular in North America when used in HV applications. These types of circuit breakers are of the dead tank design and are regarded as old technology. For the past 30 + years, grid asset managers have been steadily replacing these with newer SF6 Dead Tank Circuit Breakers. OCBs have been more widely used at the distribution level but they demand a lot of maintenance on the main contacts and on the mechanism.
The current interruption takes place in a large volume, oil-filled tank. The arcing in oil creates gases (hydrocarbons). Through the use of a vented interrupter chamber, gas bubbles create pressure that force the arc to expand further into the vents, until it is able to extinguish itself at a zero crossing current. The interrupter chambers are particularly prone to moisture contamination. In addition to its use as the arc extinguishing media, the oil serves as the insulating media between current-carrying contacts and grounded parts of the breaker.
Bulk oil circuit breakers often use oil- or resin-impregnated-paper capacitive-graded bushings that are equipped with tap electrodes and have CTs mounted on the lower ground sleeve. In addition to recommended circuit breaker tests, these bushings should be isolated and tested. The integrated CTs may be tested as well. Power factor/dissipation factor/tan delta tests are particularly recommended for oil circuit breakers.
Minimum oil circuit breakers: Minimum oil circuit breakers operate very fast and are used in transmission networks and substations. These types of circuit breakers are of the live-tank design and require only a small amount of oil. The oil is utilized primarily as the interrupting media, unlike in a bulk oil circuit breaker where the oil is utilised as an insulating media as well. In a minimum oil circuit breaker, the interrupter is enclosed in an oil-filled chamber at live potential.
It is essential that the insulation properties of the oil are monitored, click here to see appropriate equipment