Unit - 3
Circuit Breakers
Classification of C.B.:-
The classification of C.B. Has been made on the basis of insulating medium employed in the C.B. To extinguish the arc. Depending on the arc quenching medium employed, the following are important types of C.B.
i) Oil circuit breaker
Ii) Fir blost circuit breaker
Iii) Saiphar Hexafluoride(SF6) C.B.
Iv) Vaccume C.B.
Rating Of Circuit Breakers
A C.B. Has to perform the following major duties under S.C. Conditions.
i) To open the contacts to clear the result
Ii) To close the contacts after fault
Iii) To carry fault current for a short time.
[While another C.B. Is clearing the fault]
Therefore in addition to the rated vtg current and frequenting C.B. Have the following imp ratings.
i) Breaking capacity
Ii) Making Capacity
Iii) Short – time capacity
i) Breaking Capacity :-
The Breaking Capacity of a C.B. Is of two types
a) Symmetrical Breaking Capacity
b) Asymmetrical Breaking Capacity
a) Symmetrical Breaking Capacity:-
It is the rms value of the ac component of the fault current that the C.B. Is capable of breaking under specified conditions of reveres vtg.
b) Asymmetrical breaking capacity:-
It is the rms value of the total current comprising of both AC & DC components of the fault current that the C.B. Can break under specified conditions of recovery vtg.
1) Above fig shows a S.C. Current wave. The S.C. Current contains dc components which dies out gradaaug.
2) In the beginning, the S.C. Current is asymmetrical due to the dc component. When dc dies out completing, the short current becomes symmetrical.
3) The line x-x indicates the instant of contact separation AB is the peak value of the ac component of the current at this instant.
4) Therefore the symmetrical breaking current which is the rms value of the ac component of the current at the instant of contact separation is equal to AB/√2
5) The section BC is the ac component of the sc current at this instant.
6) Therefore asymmetrical braking current is given by
Iasgm = (AB/√2 )2 + (BC)2
7) The breaking capacity of a C.B. Is generating expressed in MVA.
8) For a 3 – Ø C.B. It is given by
Breaking Cap = √3 * rating vtg. In kv * rated current in KA
- Making Capacity:-
- The possibility of C.B. To be closed on SC is also considered. The rated making current is defined as the peak value of the current (including dc component) in the first cycle at which a C. B. Can be closed on to a SC.
- Making current =√2 * 1.8 * symmatricals breaking current
- The multiplication of √2 is to peak value and again by 1.8 to take D.C. Component into account.
- Making Capacity = √2 *1.8 * Symmetrical breaking capacity
- The possibility of C.B. To be closed on SC is also considered. The rated making current is defined as the peak value of the current (including dc component) in the first cycle at which a C. B. Can be closed on to a SC.
- Rated interrupting duties
- Rated Operating Sequence
- Short time current rating:-
- The short time current rating is based on thermal and mechanical limitations. The C.B. Must be capable of carrying SC current for a short period. The rated short time current is the rms value (total current both AC and DC compo) of the current that the C.B. Can carry safely for a specified short period.
- Rated vtg current and frequency :-
- In a power system, the voltage level all point is vwt same. It varies depending upon system operating condition due to this reasons, manufacturing level specified rated max. Voltage at which the open of C.B. Is guaranteed.
The rated current is the rms value of current that C.B. Can carry continuously without any temp. Rise in excess of its specified limits
The rated freq. Is also mentioned i.e. manufacturing. It is the freq. At which C.B. Has been designed to operate.
Standard freq. Is 50 H2 It is C.B. To used of freq. Other than its rated freq its effect should be tken in to consideration .
Air – Blast Circuit Breaker:
1) In air blast C.B. Compressed air at a pressure of 20-30kg/cm2 is employed as an arc quenching medium.
2) Air blast C.B. Are suitable for repeating vtg of 132kr and above. They have also been used in 11-33kv rang for certain applications.
3) At present SF6 C.B. Are proffered for 132kv and above VCB preferred for 11-33kv. So ABCB’s are becoming absolute.
Advantages
1) Cheapness and free availability of the interrupting medium.
2) High speed operation
3) Elimination of fire hazard
4) Less maintains
5) Suitability for frequent operation
6) Short and consistent arching times and therefore less burning of contacts.
Dis-Advantages:-
1) An air compressor plant has to be installed and maintaina
2) Upon arc interruption the air blast C.B. Produces a high – level noise when air is discharged to the open atmosphere. In residential arcas silencers need to be provided to reduce the noice level to an acceptable level.
An A.B.C.B may be either of the following two types
i) Cross blast C.B.
Ii) Axial Blast C.B.
i) Cross Blast C.B. :-
a) In cross – blast C.B. a high pressure blast of air is directed perpendicularly to the are for it’s interruption.
b) The arc is forced into a suitable chute.
c) Sufficient lengthing of the arc is obtained resuming in a introducing of appreciable.
d) Therefore resistance swiching is not comm. In this type of C.B.
e) Cross hast C.B. Arc suitable for interrupting high current ( Up To lookA)
Ii) Axial Blast C.B. :-
a) In an axial blast C.B. a high – pressure blast of air is directed longitudinally i.e in line with the arc.
b) Fig. (a) shows a single blast type and (b) ahows double blast type or adial blast type.
c) Axial blast C.B. Are auitable for EHV and super high vtg application. This is because interrupting chambers can be fully enclosed in porcelain tubes.
d) Resistance switching is employed to reduce the transient over voltage.
a)single blast type
b)double blast type or radial blast type
Air Break Circuit Breaker:-
1) Oil circuit breaker are not suitable for heavy current interruption at low voltage due to carbonization of oil, where as air –break circuit breakers are quite suitable for high current interruption at low voltage.
2) In this type of C.B. Air at atmosphere pressure is used as an arc exiting wishing medium.
3) It employs two pairs of contact main contents and arcing contacts.
4) The main contacts carry when the breaker is in closed position
5) When the contacts are opened, the main contact first, the arcing contacts stin remains closed.
6) Therefore, the current is shifted from main contact to the arcing contacts.
7) The arcing contacts separate later on and the arc is drawn between them
8) In air break C.B. The principle of high resistance is emptied for arc interruption
9) The arc resistance is increased by lengthing splitting and cooling the arc.
10) The arc length is rapidly increased employing arc rankness and arc chutes.
11) The arc moves upwards by electromagnetic effects
12) It moves along the arc runner and then it is forced into a chute. It is split by arc splatters.
13) A blow out coil employed to provide magnetic field to speed up arc movement and to direct the arc into arc splitters
14) AC air break C.B. Are available in the voltage range 400v to 12kv. They are widely used in low and medium vtg system.
15) Air break C. B. Are also used in DC circuit up to 12kv.
Physical Properties of SE6 gas :-
1) Colorless
2) Odorless
3) Non- toxic Pure SE6 gas is not harmful to health
4) Non inflammable
Advantages:
1) The gas is non –inflammable hence there is no danger of fire or explosion
2) Same gas is recalculated in the ckt. Have requirement of SE6 gas is small e.g 145kv SE6 C.B. Requires only about 30kg of SE6 for dirst filling No replacement is required for five years.
3) Ample overload margine. For the some size of conductors the current carrinability of SE6 C.B. Is about 1.5 times that of A.B.C.B. Because of superior heat transferability of SE6 gas.
4) The breaker is silent and does not make sound like ABCB during operation.
5) The sealed construction avoids the contamination by moisture dust and moisture dust and sand etc. No costely air compressed system like ABCB.
6) The maintenance required is minimum. The breaker may need maintenance once in four to ten years.
Dis – Advantages
1) Imperfect joints lead to leakage of gas.
2) Sf6 gas is poisonous and should not be let-out
3) Mechanism of higher energy level is necessary for puffer type SF6 C.B. Lower speeds due to friction misalignment can cause failure of breaker.
4) The internal parts should be cleaned thoroughly during periodic maintenance under clean dry environment.
5) Special facilities are needed for transporting gas and maintaining the quality of gas.
6) The deterioration of quality of the gas affects the reliability of the SF6 C.B.
Types of SF6 C.B.:-
1) Double –pressure SF6 C.B.
2) Puffer type SF6 C.B.
1) Double pressure SF6 C.B.
i) This is the early design of SF6 C.B.
Ii) It’s operating principle is similar to that of A.B.C.B.
Iii) In this type of C.B. The gas from a high – pressure compartment is released to the low pressure compartment to extinguish the arc.
Iv) Because of it’s complicated design and construction and it’s need for various auxiliaries such as gas compressor, filters and control devices, this type of C.B. Have become obsolete.
2) Puffer type SF6 C.B.
a) Closed Position
b)Open Position
i) This type of C.B. Are also sometimes called single pressure or impulse type SF6 C.B.
Ii) In this type of C.B. Gas is compressed by a moving cylinder system and is relased through nozzle to quench the arc.
Iii) This type is available in the vtg range 3.6kv to 765kv.
Iv) Fig/(a) shows a puffer type breaker in closed position.
v) The moving cylinder and ,moving contacts coupled together.
Vi) When the contact seprate and the moving cylinder moves, the trapped gas is compressed.
Vii) The gas is released through a nozzle and flows axially to quench the arc as shown in fig(b).
● Vaccume Circuit Breakers:-
Advantages:-
1) VCB is self contained does not need fiiled of gas or oil. They do not need auxillary air system. Oil handling system etc. No need periodic refilling
2) No emission of gases, pollution free.
3) Breaker forms a unit which can be installed at any required orientation. Breaker unit is compart and self contains.
4) Non explosive and silent operation
5) Large number of operations sutable for repeated operating duty, long life.
Dis-Advantages:-
1) The VCB is more expensive than the interrupting devices in other types of C.B.
2) In the event of loss of vacuum due to transient damage or failure, the breaker is useless; It cannot be repaired at site.
3) For interruption of low magnetizing currents in certain range, additional surge suppressors are required in parallel with each phase of VCB.
1) The dielectric strength and arc interrupting ability of high vacuume is superior to those of oil air and SF6 gas.
2) Its enclosure is made of insulating material such as glass porcelain or glass fiber.
3) The vapour condensing shield is made of synthetic resin.
4) This shield is provided to present the metal vapour reacting the insulating envelope.
5) As the breaker has the sealed construction a stainless metallic below is used to allow the Moment of the lower contact.
6) One of its end is welled to the moving contact. Its other end is welded to the lower end flange.
7) When contacts are separated in high vacuumed an arc is drawn between them.
8) Arc does not takes place on the entire surface of the contacts hut only on a few spots.
9) The contact surface is not perfecting smooth . It has certain micro projections.
10) At the time of contact sepration, these projection forms, the last point of separation.
11) The current flows through these point of separation resulting in the formation of a few hot spots.
12) These not spot emits electrons and acts as cathode spots and vapour emission forms.
13) The vapor emission depends on the arc current. In A.C., when the current decreases, vapor emission decreases.
14) Near current zero the rate of vapor emission tends to zero.
15) I mediating after current zero the remaining vapor condenses and the dielectric strength increases rapidly
16) At current zero cathode spots extinguish within 10-8 sec
17) The rate of dielectric recovery is many times of dielectric recovery is many times higher than other types of C.B. [20kv/us].
● Selection Of C.B.
| Rated Voltage | Choise of C.B. | Remark |
1) | Below 1kv | Air – Braek C.B. |
|
2) | 3.3kv – 33kv | VCB SF6 MOCB | Vaccume preffered |
3) | 1.32kv – 220kv | SF6 C.B., air – blast C.B. M. OCB | SF6 preferred |
4) | 400kv – 760kv | SF6 C.B. Air Blast C.B. | SF6 is Preferred |
1) Many facts on overheads transmission lines are transient in nature.
2) Statically evidence shows that about 90% of faults are used by lighting , birds, vines, tree brachesek.
3) These conditions result in arcing faults and the arc in the fault can be extinguished by dc- energizing the line by simultaneous opening of C.B. On both ends of the line or one end of line.
4) Since the cause of transient faults mentioned above disappear after a short time the C.B. Can be reclose as soon as the arc in fault has been extinguished and the path has regained its dielectric strength.
5) Reclosing of lines restores the supply continuity of service is the major advantage of auto – enclosure.
6) If the fault is transient one the normal condition is restored by auto re closure.
7) High speed tripping and high speed reclosing improves the stability of power system.
8) Hence the C.B. And relaying on EHV lines are provided with auto reclosing features.
Reference Books
- [R1] Badri Ram, D. N. Vishwakarma, “Power System Protection and Switchgear”, Tata McGraw Hill Publishing Co. Ltd.
- [R2] J Lewis Blackburn , “Protective Relaying- Principles and Applications”, Dekker Publications.
- [R3] Prof. Dr S.A. Soman, IIT Mumbai, A Web course on “Digital Protection of power System”
- [R4] A.G. Phadke, J.S. Thorp ,Computer relaying for Power System , Research Studies Press LTD, England.(John Willy and Sons Inc New York)
- [R5] Mason C.R., “Art and Science of Protective Relaying”, Wiley Eastern Limited.
- [R6] Arun Ingole, “Switchgear and Protection”, Pearson.
