Minimum Oil Circuit Breaker
These types of circuit breaker also utilize oil (transformer oil) as an interrupting medium. Unlike bulk oil circuit breakers, these designs place the interrupting units in insulating chambers at the live potential. This feature of the design of MOCBs reduces the requirement of oil and these breakers are therefore known as minimum oil circuit breakers. These I designs are available in voltages ranging from 1000 V to 765 kV using the multi-break technique. A typical view of 36 kV MOCB indicating main parts is shown in figure 4.24.
This Type of breaker is Widely used in transmission and distribution network. In an oil circuit breaker, they are drawn across the contacts are contained inside the interrupting pot and thus the hydrogen bubble, formed by the vaporized Oil (gas) is also contained inside the chamber. As the contacts continue to move and When the moving contact rod separates itself from the orifice at the bottom of the chamber, an exit similar to a nozzle becomes available for exhausting the hydrogen that is trapped inside the interrupting chamber see the Figure 4.24(b).
It is well-known that minimum oil circuit breakers are sensitive to high TRV peak and prone to restrikes during switching capacitor banks. However, this problem has been effectively countered by pressurizing the interrupter heads with dry nitrogen.
These types of breakers were found to be more suitable for applications in countries with very low ambient temperatures like Canada, Russia, and Finland, where the gas blast circuit breakers become more sensitive to icing and liquification problems.
Due to their cross blast designs, minimum oil HV circuit breakers suffer from a few major inherent problems such as sensitivity to high peak voltage and the effects of pre-arcing. Usually, the out of phase switching test is the most difficult, test on MOCBs, which generally dictates the number of interrupters to be used in series for a particular voltage class. The energy released due to pre-arcing is very high and high strength insulating materials are required to enable cross jet pots and jet plates to “withstand high-pressure surges. The number of breaks per pole also depends upon the ability‘of the arc interrupting chamber to withstand the energy released due to PRC-arcing, which is a function of current in the arc and voltage per break.
Presently this technology is considered outdated due to the advent of VCB technology medium voltage and SF6 technology m the high voltage range.