Circuit breaker ratings: Under fault conditions
the C.B. should perform the following three duties:
i)
It
must be capable of opening faulty circuit and breaking the faulty current.
ii)
It
must be capable of being closed on to a fault.
iii)
It
must be capable of carrying fault current for a short time while another, is
clearing the fault.
Corresponding
to above thing C.B. has three ratings:
1) Breaking capacity: It is the current
(r.m.s.) that a C.B. is capable of breaking at given recovery voltage and under
specified conditions (e.g. p.f., RRRV).
It
is a common practice to express the breaking capacity in MVA by taking into
account rated breaking current and rated service voltage.
Breaking
capacity = √3
x V x I x 10-6 MVA
2) Making Capacity: The peak value of current
(including D.C. component) during the first cycle of current wave after closure
of circuit breaker is known as making capacity.
The
C.B. may be closed in S.C. conditions. The electromagnetic forces are produced
which are proportional to square of the maximum instantaneous current of
closing. So making capacity is expressed in peak value instead of r.m.s. and
the maximum value of fault current is in the first cycle.
Making
capacity = 2.55 x Symmetrical breaking capacity
3) Short Time Rating: It is the period for
which C.B. is able to carry fault current while remaining closed. Sometimes
fault occurred in a system may be of temporary nature and it occurs only for 1
to 12 sec. and afterwards it is cleared automatically. They should not trip in
such conditions i.e. it should be able too carry this large current for a very
few time i.e. short time rating. If fault persists for longer time than
specified limit then should trip.
The
short time rating depends upon: i) Electromagnetic force, ii) Temperature rise.
4) Normal Current Rating: It is the r.m.s. value of current which the C.B. is capable of
carrying continuously at its rated frequency under specifie4d conditions.
√3 x
Recovery voltage
1. Peak re-striking voltage
= ------------------------------- x 2 = 2 Emax
√3
2.
Frequency of oscillations fn = 1/2t
Time up to 1st peak
Peak re-striking voltage
3. Average RRRV
= --------------------------------
T
1
4. Frequency of oscillations fn = --------------------
2π√LC
Difference between Isolator and
Circuit Breaker
|
Sn
|
Isolator
|
Sn
|
Circuit
Breaker
|
|
1
|
Operation is on no load.
|
1
|
Operated ON-load/on occurrence of fault.
|
|
2
|
No arcing during ON-OFF so no arc quenching provision
|
2
|
The current is interrupted and hence heavy arc is
produce. Arc quenching facility is provided
|
|
3
|
It’s operation is manual or manual plus mechanical or
pneumatic.
|
3
|
Manually operated in normal condition and automatic
tripping in fault condition.
|
|
4
|
No definite current capacity and specifications.
|
4
|
CBs. Are specified by breaking capacity, making
capacity, etc.
|
|
5
|
Visible operation in open-air (opening and closing of
contacts).
|
5
|
Operation in the oil or gas chamber (not visible).
|
|
6
|
Noise-less operation.
|
6
|
Big sound on operation.
|
|
7
|
Cheaper.
|
7
|
Costly.
|
|
8
|
No periodic maintenance required only contact cleaning.
|
8
|
Periodic maintenance is necessary.
|
|
9
|
Types are very few:
a)
Vertical breaking,
b)
Horizontal breaking,
c)
Pentagonal type.
|
9
|
Vide varies like, Air break CB, ABCB, MCB, MCCB, OCB,
SF6, DCB VCB etc.
|
|
10
|
For safety and mal-operation it is interlocked.
|
10
|
CB and isolators interlocked to avoid mal-operation.
|
|
11
|
Occupy less space.
|
11
|
Occupy more space.
|
|
12
|
No tripping circuit.
|
12
|
Requires tripping circuit for operation.
|
CIRCUIT BREAKERS
Concept of Circuit Breakers
Power
system consists of generating plants, transformers, transmission line,
distributors etc. Many times it is essential to switch off the various circuits
for maintenance work or due to occurrence of fault. In earlier days, i.e.
during 1890 switch fuse unit was used for this purpose. Switch fuse unit is
inserted in series with the circuit. On occurrence of fault the fuse blows out,
the switch is then opened and the fault finding is done and fault is cleared.
Then a new fuse is put, switch is closed and thus supply is restored. So a lot
of time is required to restore the supply.
Modern
power system operates with high voltage so heavy fault current is there, the
fuse cannot interrupt is effectively. So this switch fuse unit cannot be used
for such purpose.
The
use of switch fuse unit is for low voltage circuits where frequent operations
do not occur e.g. distribution transformer circuits, lighting circuits etc.
For
modern power system a more reliable switchgear is required because switch fuse
unit cannot effectively interrupt heavy fault current. So a more dependable
device is required and that is circuit breaker. A circuit breaker can make or
break a circuit either manually or automatically on occurrence of fault (short
circuit etc). If required it can be operated during normal working conditions
also for the purpose of maintenance of any device, circuit etc.
The
automatic operation of circuit breaker takes place with the help of relays and
CTs.
Operating Principle
A
protective relay detects abnormal conditions and sends a tripping signal to
circuit breaker. After receiving the trip command from relay, the circuit
isolates the faulty part of the power system. This is the principle of
operation for all types of circuit breakers.
A
circuit breaker has two contacts fixed contact and moving contact.
Under
normal operating conditions these contacts remain closed. When fault occurs on
any part of the system, the trip signal is received and moving contacts moves
away, thus circuit is interrupted. This results into formation of arc. This arc
is to be extinguished or quenched at the earliest.
These
contacts are placed in a closed chamber. The chamber is filled up with some
insulating medium (liquid or gas) which extinguishes the arc. The insulating
medium used for arc extinction is usually oil, air, vacuum, SF6 gas
(sulphur hexa floride).
Classification of Circuit Breaker: The basic classification of
circuit breaker is made on the basis of insulating medium used for extinction
of arc. The classification can be further made on the basis of operating
voltage of the system i.e. HT and LT.
Following
three pattern shows classification of CBs.
Circuit
breaker(C.Bs.)
C.Bs used for HT
CBs used for LT
Plain Break Arc control Air CB Miniature Molded Case Earth leakage
Oil CB oil CB CB(MCB) CB CB (ELCB)
Medium used for arc
extinguishing;
1.
Oil
circuit breaker – insulating oil – transformer oil.
2.
Air
blast circuit breakers – high pressure air-blast.
3.
SF6
circuit breakers – SF6 – gas
4.
Vacuum
circuit breaker – vacuum.
Properties of C.B. Contacts
1.
The
contact area should be well defined.
2.
Contacts
should have low contact resistance to avoid over heating.
3.
The
contact pressure should be adequate.
4.
The
contact materials should be of self cleaning type because the thin oxide films
are developed at the surface of contacts.
5.
The
tips of the contacts should be replaceable, so that entire contact assembly
need not be replaced.
6.
Contacts
grip is an important electromagnetic phenomenon and the grip should not be
loosen under the operation of the electromagnet forces.
7.
Contact
material should be able to carry load current without a temperature rise.
8.
Contact
material should carry S.C. current for a short duration without deterioration.
Contact Materials
The
materials, used for C.B. contact are generally copper, silver and various
alloys. These materials must have ionization energies between 12 to 16 volts.
Operating mechanism of
C.Bs.: The
different types of operating mechanisms are:
1.
Hydraulic
mechanism
2.
Pneumatic
operating mechanism
3.
Spring
operated (spring opened and spring closed) mechanism: used in miniature CBs.
4.
Solenoid
operated mechanism.
Expected Properties of
Operating Mechanisms of CBs
Next
to the arc extinguishing chamber the important element is the operating mechanism.
Due to this mechanism correct operation of CB is assured. The mechanism should
be such that:
i)
It must close the CB contacts within a few
tenths of a second which in the case of high surge currents.
ii)
Its
trip mechanism must e prepared to open the CB contacts within a few
milliseconds
iii)
Design
of mechanism must withstand to face heavy stresses, so I should be robust and
reliable and mechanically strong.
Mechanisms Available are of
the Following Types:
1.
Hand operated type: Suggested for
low capacity breakers only. Energy required for operation is supplied by human
muscles. The mechanism is very cheap in cost. But is not useful in modern power
system.
2.
Solenoid operated mechanism: This
mechanism is also very simple and not so costly. Electrical energy of an
electromagnet (solenoid) is transferred to mechanical energy necessary to close
the CB.
Drawbacks and limitations: Heavy power demand for
operation. Solenoid coil requires time to get in operation. It gives low
initial acceleration. The speed of operation is reduced. So is not suggested
for high power circuits.
1.
Motor operated mechanism: This mechanism also
suffers due to longer “make time”. In this mechanism, the necessary mechanical
power is supplied by D.C. or A.C. motor. The advantage is not power required
for operation of mechanism is comparatively lesser.4.
2.
Motor operated
spring-mechanism:
In this type, the spring is charged compressed by previous operation of
electric motor and thus energy is stored in it. Therefore, the closing
operation is affected by releasing the compressed spring. So making time is
very short and depends on elasticity of the spring. Motor required for the
mechanism is of small capacity. Shock due to contact closing of CB is small.
The mechanism is suitable for high power circuits.
5.
Pneumatically operated mechanism: In
this type, potential energy of compressed air is made use for operation of
mechanism. The speed of operation is much more. Quick reversal of breaker
moving parts for re-closer is possible because of low inertia of the operating
piston. Force exerted by compressed air is mostly uniform.
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