electrical-engineering-portal.com-the main components of an oil circuit breaker and how does it...

7/27/2019 Electrical-Engineering-portal.com-The Main Components of an Oil Circuit Breaker and How Does It Interrupt the Arc

1/6

This is a 230,000 volt circuit breaker. We are using it on a 138kV system in this station. Today Ihad to re- hang the leads f rom the bus. We took them down f or maintenance on the breaker.

Each of those tanks has about 2500 gallons of mineral oil. 2800PSI Hydraulic pressureoperates the breaker. Just a litt le beef ier than the little breaker in your panel at home (photo by

doug911 at Flickr)

http ://ele ctrical- eng ine ering -p orta l.co m/oil- circu it- bre ake r- ho w- do es- it- in terru pt- th e- arc Fe bru ary 5, 2013

The main components of an oil circuit breaker and how

does it interrupt the arc

Edvard

They are simple in construction. The major parts of a minimum oil circuit breaker excluding thepoles are the base frame,the drive which is constructed as a stored energy opening and closingmechanism (the operating mechanism). The opening spring of the stored energy mechanism ischarged automaticallyduring the closing action. The closing spring is charged either by meansof an electric motor (is built into the drive housing) or by means of a removable crank.

The pole constitute of: insulat ing cylinder, arc chamber, fixed, guiding and moving contacts. Italso has the gas expansion chamber, terminals, oil sump, oil draining and oil f illing plugs and the

oil level indicator.

Arc Interruption in Oil
http://electrical-engineering-portal.com/buchholz-relay-on-oil-transformer-with-conservatorhttp://electrical-engineering-portal.com/what-is-incident-energy-caused-by-arc-flashhttp://electrical-engineering-portal.com/buchholz-relay-on-oil-transformer-with-conservatorhttp://electrical-engineering-portal.com/oil-circuit-breaker-how-does-it-interrupt-the-arc


2/6

Figure 1a - Arc in oil

Figure 1b - Arc in SF6 gas

On separat ion of the moving contact f rom the f ixed contact in the arc chamber, the currentcont inues to f low through the vaporizing metallic current paths. The high temperature occurringunder such conditions, decomposes the oil (which boils at 658K), in the immediate vicinity anda gas bubble is formed (under high pressure).

It consists of (from outside inward): wet oil vapour, superheated oil vapour, hydrocarbons(C2H2 at around 4000K), the arc (approximate temperature 7000K) as shown in Figure 1.

Explanation

CF4 Carbon Tet raf luoride

CuF2 Copper Difluoride

HF Hydrogen Fluoride

H2O Water

SF4 Sulphur Tet rafluoride

SF6 Sulphur Hexaf luoride

SO2 Sulphur Dioxide

SO2F2- Sulphuryl Fluoride

SOF2 Thionyl Fluoride

WF6 Tungsten Hexafluoride

WO3 Tungsten Trioxide

As can be seen, the arc runs in a mixture of hydrogen(in both molecular and atomic states), carbon andcopper vapour. The thermal conductivity is high due tothe dissociation of hydrogen molecules into atoms. Thethermal energy generated in the arc is primarilydissipated outward through the surrounding gasenvelope to the oil.

Also, the gas in the arc chamber escapes to t he gasexpansion chamber, so that a t ype of heat dissipationby convection is created, thus the rate at which heat isdissipat ing is increasing. Near current zero, the thermalpower generated by t he current (in the arc) approacheszero.

If the heat dissipation out wards is suff icient ly large, thetemperature in the arc zone can be reduced in such amanner that the arc would lose conductivity and extinguish.

An arc in hydrogen has a short thermal t ime constant , sothat the conditions are favourable for quenching. There aretwo ot her situations that may occur under certainconditions: thermal Restriking of Arc, reignition.

Thermal restriking is when the post -arc current rises again and passes into the next half cycle
http://electrical-engineering-portal.com/download-center/books-and-guides/electrical-engineering/understanding-arc-flash-hazardshttp://electrical-engineering-portal.com/consequences-of-internal-arc-for-personal-safety-and-mv-electrical-equipment


3/6

of SCC, as the arc plasma heats up due to t he insuff iciency of heat dissipation to makeconductance of the arc zone equal to zero. Reignit ion happens when therestriking voltage ofthe system causes a renewed formation o f the arc, (after completion of the first interruption)and cont inuat ion of f low of current. The arcing chamber designs are either of the axial or radialventing type. Of ten, a combination of bot h are used in the design of minimum oil, MV CBs.

The axial vent ing process generates high gas pressures and has high dielectric strength. This isused mainly for interruption of low currents. The radial vent ing is used for high current

interruptions, as the gas pressures developed are low and the dielectric strength is low.

The higher the current to be interrupted, the larger the gas pressure developed.


4/6


5/6

Resource: Vepi TheElectrical PowerSystems Division; TheSwitchgear & circuitbreakers section


6/6

Low oil circuit breaker components

electrical-engineering-portal.com-the main components of an oil circuit breaker and how does it...

Documents