Circuit-breaker ( 2 )
HIGH VOLTAGE CIRCUIT BREAKERS :
ii) SF6 ( sulphur hexa flurried ) :
In SF6 circuit breakers, sulphur hexafluoride (SF6) gas is used as the arc quenching medium. The SF6 is an electro-negative gas and has a strong tendency to absorb free electron .The contacts of the breaker are opened in a high-pressure flow of sf6 gas and arc is struck between them. The conducting free electrons in the arc are rapidly capture by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc the SF6 circuit breakers have been found to be very effective for high power and high voltage service .
iii) OCB ( oil circuit breaker ):
Oil circuit breaker is such type of circuit breaker which used oil as a dielectric or insulating medium for arc extinction. In oil circuit breaker acure in the system the contacts of the breaker are made to separate within an insulating oil. When the fault the contacts of the circuit breaker are open under the insulating oil and an arc is developed between them and the heat of the arc is evaporated in the surrounding oil. The oil circuit breaker is divided into two categories
1- bulk oil circuit breaker
2- minimum oil circuit breaker
ADVANTAGE and DISADVANTAGE of OIL CIRCUIT BREAKERS :
ADVANTAGE :
--Transformer oil is good insulating materials which insulates both contracts
--Hydrogen gas has high diffusion rate
that's why, deionized the ionized particles
DISADVANTAGE :
-- oil remains inflammable
risk of fire hazards
-- Due to mixture of air and hydrogen gas
can be explosion
-- When ionized particles started to deionized
carbon particles mixed in to oil
iv) ACBC ( air blast circuit breaker ):
In this type of circuit breaker, shown in Fig , compressed air is used as the medium for closing or opening the contacts. As such there will be a high pitched blasting noise when the ABCB closes or opens. The type shown is a 110KV 3-phase breaker having all the 3-phases mounted on one air reservoir and has a single closing mechanism.
.jpg "ELCB")
Over the years, it has been established that current operated ELCBs are much more reliable in operation, easier to install and maintain. Besides, there is no dangerous ageing of the protective system components involved, as in the voltage operated ELCBs, where the earth electrode resistance changes with time and hence the earth loop impedance does not remain constant over a period. This leads to dangerous touch voltages on the metal enclosures without being sensed by the voltage operated ELCBs, whereas current operated devices are safer as they operate on the principle of the vector sum of the line currents and the neutral current. Any current even in milliamperes which is not returning to the source through the neutral is assumed to be flowing through the earth or through any insulating body. This differential current is immediately sensed by the current operated ELCB which switches off the electricity supply, protecting the people from dangerous electrical shocks and the insulations from failing and inviting dangerous fires.
They also provide a high degree of protection against earth faults and fires. Fig 9 shows the circuit diagram of a residual -current earth leakage circuit breaker. The essential part of the ELCBs is a toroid type core transformer with two opposed windings called primary. One is connected in series with the line and the other in series with the neutral As far as there is no leakage current, the line current is equal to the neutral current and the magnetic flux produced by the two primary windings oppose and cancel each other. Thus the secondary winding which is connected to the trip (operating) coil does not induce any voltage.
SOME IMPORTANT QUESTIONS ANSWER FOR comparative EXAMS :
1.Arc in a circuit breaker behaves as-
Ans = A resistance decreasing with voltage rise across the arc
2. The main function of fuse is to-
ans= prevent excessive current
3. Directional relays are based on the flow of
ans= power
4. The voltage appearing across the contacts after opening of the circuit breaker is called
Ans= recovery voltage
5. Wave trap is used to trap waves of
ans= Higher frequencies
6. The reflection coefficient of voltage at the open circuited end of a transmission line is
Ans= 1
7. To reduce short circuit fault currents __ are used
Ans= reactor
8. For which of the following protection from negative sequence currents is provided-
a) transformer
b) generators
c) motors
d) transmission line ANS=B
9. Which relay is preferred for phase fault on short transmission line
a) Induction type
b) reactance
c) impedance
d)none Ans=B
10. Short circuit currents are due to which fault-
a) Single phase to ground
b) phase to phase
c) double phase to ground
d)three phase
e) all Ans=E
11. What will be the reflection coefficient of the wave if surge impedance) line is equal to load impedance?
Ans= zero
12. Which device will receive voltage surge first travelling on transmission line?
Ans=lighting arrestor
13. Arc in a circuit breaker is interrupted at-
Ans= current zero
14. Overfluxing protection is recommended for-
Ans= Generator transformer of the power plant
15. Admittance relay is -
a) Impedance
b) directional
c)non directional
d)non Ans=B
16. The material used for fuse must have-
Ans = Low melting point and high specific resistance
17. A fuse is connected in-
Ans= Series with circuit
18. The inductive interference between power and communication line can be minimised by-
Ans= Transposition of both lines
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