Advantages and Disadvantages of Minimum Oil C.Bs.

Advantages:

1.    It requires lesser quantity of oil.

2.    It requires smaller space.

3.    Maintenance is less.

4.    Fire risk is reduced.

5.    Cost per breaking capacity in MVA is less.

6.    Suitable for both manual and automatic operation.

Disadvantages:

1.    Smaller quantity of oil is used so the degree of carbonization is increased.

2.    It is difficult to remove gases from the space between contacts.

3.    Carbonization is more so oil deteriorates rapidly due to frequent operations.

4.    Difficulty in locating CT.

5.    Possibility of fire and explosion.

Maintenance of OCBs

The main inspection is:

a)    Checking of contacts.

b)    Checking of dielectric strength of oil.

Due to the switching ON-OFF on load and in case of tripping in faults, the contacts get burnt by arcing. The insulating oil also lost by some amount and loses it dielectric strength due to carbonization. This results in reducing rupturing capacity of the breaker.

Following is the maintenance schedule (every after 3 months/6 months).

1.    Inspect the condition of contacts, if burnt replace.

2.    Check dielectric str4ength of oil seeing the color of oil. If badly discolo9ured then take it out for filtration or replace it by new one.

A good oil withstands 30kV for 1 minute in a testing set of 4 mm  gap.

3.    Check the insulation for possible damage, clean the surface to remove carbon deposits with fabric cloth.

4.    Oil level to be checked, if gone down make up the level by fresh oil.

5.    Mechanisms shall be checked frequently.

Comparison of Bulk Oil C.B. with Air Blast C.Bs.

Sn	Bulk Oil C.B.	Sn	Air Blast C.B.
1	Decomposed products are inflammable and explosive. If arc is not extinguished explosion may occur.	1	No such danger exists because no products are formed.
2	Oil absorbs moisture and also get carbonized due to frequent operations, hence needs regular replacement.	2	Air compressor takes air from atmosphere hence no replacement i.e. arcing products are completely removed.
3	Not suitable for repeated operations because oil gets deteriorated.	3	Suitable for repeated operations.
4	Used for medium capacities.	4	Used for high capacities.
5	Reliability is less as compared to ABCB.	5	Reliable operation.
6	If leakage occurs it can be detected easily.	6	If leakage occurs operation is not satisfactory.
7	Better design, hence contact remain smooth.	7	Complex design and contacts get eroded.
8	For single C.B. huge oil storage is required.	8	A single compressor unit can be used for multi C.B. system.
9	Maintenance required is more.	9	Less maintenance.
10	Costly.	10	Less costly.
11	Transportation becomes difficult due to huge size and large weight.	11	No such problem.

SF₆ Circuit Breaker (Sulphur Hexa Fluoride C.B.)

Prior to study the constructional details let us know the SF6.

Properties of SF₆ gas:

1.    Electronegative: It is the ability of an atom to attract and hold electrons. Such gases have high dielectric strength SF₆ is electronegative, it forms negative ions. Negative ions are heavy and immobile so they do not flow easily. Hence, SF₆ has high dielectric strength.

2.    Due to SF₆ gas arc time constant is low. Because rate of rise of dielectric strength of SF6 is very high.

3.    At atmospheric pressure dielectric strength is 2.35 time more than air.

4.    It can be liquidified at moderate temperature and can be stored in a steel tank.

5.    Dielectric strength increases linearly with pressure

6.    SF₆ has great heat transfer ability so arc can be cooled quickly.

7.    Gas is inert. Therefore, contacts will not get eroded.

8.    Gas is non-inflammable.

9.    Colorless.

10.  Odorless

11.  Non-toxic.

12.  Thermally stable up to 500⁰C. The decomposed products (SF₂ and SF₄) recombine to form same volume of original gas.

13.  Density is 5 times more than air.

(A) Puffer Type SF₆ Circuit Breaker:

Principle: It makes use of puffer action. Puffer means short and quick blast of gas or air. For this circuit breaker the SF₆ gas and blast is produced by puffer action which is passed on to the arc and arc is extinguished.

Construction: The fixed and moving contacts are kept in insulating nozzle section. The whole assembly i.e. insulating nozzle, puffer action cylinder moves with the moving contact. A fixed piston is provided at the other end of moving contact. The chamber is filled up with SF6 gas.

               

Working: Under normal working conditions the contacts are closed. On occurrence of fault, the contacts are opened, movable contact moves towards right, the movable cylinder, insulating nozzle move simultaneously.

An arc is struck between fixed contact and moving contact. The travel of movable cylinder causes increase in pressure on SF₆ gas, the SF6 gas is now compressed i.e. it is under high pressure. The SF₆ travels to left towards the arc i.e. it puffs over the arc. This results in extinction of arc.

(B) Double Pressure Dead Tank Type SF₆ Circuit Breaker:

The gas from the compressor is taken into auxiliary H.P. reservoir through the inlet as shown in figure. It is then admitted into main SF₆ reservoir at 14 kg/cm² from which it travels axially to the arc extinction chamber where the arc is formed. The axial flow of this gas takes away the heat and arc is cooled and is extinguished. The arcing time becomes less as the dielectric strength is more. Finally, the gas reaches L.P. chamber from which it is pumped to the compressor unit from which it is taken to the auxiliary H.P. cylinder for the next operation.

The CB consists of:

1.    Gas system.

2.    Tank.

3.    Interrupted or arc extinction chamber.

1. Gas system: The gas follows closed path as indicated above and do not escape to the atmosphere. The activated alumina (AL₂O₃) filters the gas and absorbs the products which have failed to recombine. Further the moisture also is removed by alumina.

2. Tank: The tank is earthed.

3. Arc Extinction Chamber: The chamber is located axially in the tank and is supported by insulators. The arc extinction process takes place in this chamber by the axial flow of SF₆ gas.

The type is becoming absolute and puffer type is coming up as explained here after.

                                     

Advantages and Disadvantages of SF6 C.B.:

1.    It’s working is silent with good overload capacities. The failure from moisture can be avoided by using sealed constructions.

2.    It has ability to interrupt low and high fault current inductive and capacitive current with minimum fault clearing time.

3.    Some gas is re-circulated; hence the quantity of SF₆ gas required in the long run is small.

4.    Maintenance is a minimum.

5.    No over voltage problems and no contact replacement because the contact corrosion is very small (Because gas is inert).

6.    For the same rating size of SF₆ C.B. is small as compared to ABCB because dielectric strength of SF₆ is high. Hence, clearance is less and size is smaller.

7.    Excellent arc extinction capacity, the insulating physical and chemical properties are advantageous and the possibility of fire and explosion is very less.

i)        Special material is required for construction because of the sealing problems.

ii)       The entry of moisture in a gas cause the failure of C.B. but the latest design takes due care about it.

Comparison of SF₆ C.B. with Air Blast C.B.

Sn	Features	Air blast C.B.	SF6 C.B.
1	Compressed air	Required	Not required
2	Gas	Not required	SF6 gas required
3	Number of interrupters	6 to 12	2 to 4
4	Maintenance	More	Less
5	Pressure	30 bars	5 bars
6	Switching	Excellent	Excellent
7	Breaking time	2 cycles	2.5 cycles
8	Medium	Fresh every time	Same forever
9	Construction	Comp0lex	Compact/simple
10	Cost	Very high	Medium
11	Use	Used early days	Used now-a-days

Vacuum Circuit Breaker

Principle: When two contacts of this circuit breaker and separated in vacuum an arc is struck and hot spots are formed on the surface of contacts. These hot spots produce metal vapor and plasma. The amount of vapor in the plasma depends on how rapidly the vapor is emitted from contact surface which indeed depends on arc current. The current is of alternating nature, it passes through zero several time, so the rate of vapor emission also becomes zero, and the vapor already emitted gets condensed. During this process the dielectric strength builds up rapidly and re-striking of arc is prevented.

                                     

Construction: It consists of a enclosure made up of insulating material like glass. The vacuum retained inside the container is about 10-7 torr. The enclosure is supported by means of end flanges made up of non-magnetic material. The contacts are disc shaped. The contact tips are made of a copper-chromium or copper-bismuth alloy. The contact region is surrounded by vapor condensing shield. It does not allow the condensed vapor to pass on to the glass enclosure.

The metallic bellows are used which can permit the movement of moving contact without disturbing the vacuum. Metal ceramic seals are used a respective points to be sealed.

Working: Under normal working conditions the contacts are closed. On occurrence of fault, the moving contact moves upwards. An arc is produced between the contacts due to ionization of metal vapor. The arc gets extinguished. The arc extinction process is already explained in principle of working of vacuum C.B.

Advantage:

1.    Vacuum C.Bs. are compact in size.

2.    Reliable and longer life.

3.    Heavy fault current can be interrupted effectively.

4.    No gas is generated after arc extinction operation.

5.    Operation is not noisy.

6.    Arc energy is low

7.    Inertia is low.

8.    No risk of fire.

Disadvantages;

Vacuum has to be maintained at desired level always.

Application: Outdoor application where maintenance required si minimum. In the high voltage system from 22 kV to 66 kV power circuits. This breaker is also useful in rural area with limited rating 60 to 100 MVA.

L.T. CIRCUIT BREAKER

Air Break Circuit Breaker

The main contact carries current when C.B. is closed. When the fault occurs first the main contacts operate and then the arcing contacts. When the arcing contacts separate out the blow out coils which are provided across the arcing contacts are brought into circuit and the current will flow through the blow out coil. When the arcing contacts separate, the arc strikes across and will be placed in the magnetic field provided by the blow out coil. The arc being a path of current it becomes as if a current carrying conductor placed in a magnetic field. Hence, the arc experiences electromagnetic force. The arc experiences electromagnetic force. The arc heats the air around the contacts and hence due to thermal action, arc travels upward and will be directed along he arc runners. Finally, arc will reach the arc splitter plates and these plates increase the length of the arc therefore an resistance is increased, arc voltage drop is increased and arc is cooled down when the recovery voltage is less than the arc voltage the arc is extinguished and the C.B. opens the circuit.

                                               

These breakers are available in different ratings:

460 volts,         400 to 3500 Amp,       40 to 75 kA

3.3 kV,             400 – 3500 Amp.,       13.1 – 31.5 kA

6.6 kV,             400 – 2400 Amp.,       13.1 – 20 kA