The secondary of CT has a large number of turns. The secondary winding is closed through an ammeter of range 0-5 A. Thus, a current transformer is like a short-circuited transformer having a limited current flowing through the secondary, the voltage across the primary winding being very small.
12. The secondary of CT is never left open-circuited because
Heat dissipation in the core will be very large
The core will be saturated and permanently magnetized rendering it useless
Dangerously high EMFs will be induced in the secondary
All of the above
Answer.4. All of the above
Explanation:
Most important precaution in use of a CT is that in no case should it be open-circuited (even accidentally)
As the primary current is independent of the secondary current, all of it acts as a magnetizing current when the secondary is opened
This results in deep saturation of the core which cannot be returned to the normal state and so the CT is no longer usable and the heat dissipation in the core will be very large
Again, due to the large flux in the core, the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT
This large voltage across the secondary terminals will be very dangerous and will lead to the insulation failure and there is a good chance that the person who is opening the CT secondary while the primary is energized will get a fatal shock
13. Turns ration for a C.T. is _________
n = Np ⁄ Ns
n = Ns ⁄ Np
n = 1 ⁄ Np
n = N
Answer.1. n = Np ⁄ Ns
Explanation:
The turns ratio for a C.T is defined as the ratio of the number of turns in the primary to the number of turns in the secondary. For the current transformer, the Turn ratio is given as
N = Number of turns in Primary winding/Number of turns in Secondary winding
N = Np ⁄ Ns
14. Which of the following is TRUE in the case of current transformers?
It helps in measuring high current using high range ammeter.
It helps in measuring high current using low range ammeter.
It helps in measuring high voltage using high range voltmeter.
It helps in measuring low voltage using high range ammeter
Answer.2. It helps in measuring high current using low range ammeter.
Explanation:
A CT is an instrument transformer in which the secondary current is substantially proportional to the primary current and differs in phase from it by ideally zero degrees.
Current transformers are used in electrical power systems for stepping down currents of the system for metering and protection purposes.
Actually, relays and meters used for protection and metering, are not designed for high currents.
High currents of the electrical power system cannot be directly fed to relays and meters. CT steps down rated system current to 1 Amp or 5 Amp.
15. The primary winding of a P.T. has _________
An intermediate number of turns
No turns at all
Larger number of turns
Few turns
Answer.3. Larger number of turns
Explanation:
The primary winding of the PT is connected across the high-voltage line whose voltage is to be measured so primary winding has a larger number of turns. It is connected in parallel with the load whose voltage is to be measured.
16. The burden across the secondary of an instrument transformer is specified as:
V2/V1
V2/I2
V2/I1
I2/I1
Answer.2. V2/I2
Explanation:
The nominal ratio of an instrument transformer does not remain constant as the load on the secondary charges. It changes because of the effect of secondary current and this causes errors in the measurement.
The specific loading at rated secondary winding voltage is specified such that the errors do not exceed the limits. Such a permissible load is called the burden of an instrument transformer.
The burden across the secondary of an instrument transformer is specified as V2/I2
17. The secondary winding of a P.T. has _________
An intermediate number of turns
No turns at all
Larger number of turns
Few turns
Answer.3. Larger number of turns
Explanation:
The Primary winding of PT consists of large numbers of turns while the secondary has less number of turns. The primary is connected across the high voltage line while the secondary is connected to the low range voltmeter coil.
The voltage being measured connect across the primary winding which has a large number of turns and is connected across the circuit. The secondary winding, which has a much smaller number of turns, is coupled magnetically through the magnetic circuit to the primary winding.
The turn ratio is adjusted so that the secondary voltage is 110V when full-rated primary voltage is applied to the primary.
18. The primary of a _______ should never be energized when its secondary is open-circuited.
Potential transformer
Current transformer
Autotransformer
Power transformer
Answer.2. Current transformer
Explanation:
Current transformers are always used with their secondary circuit closed through very low resistance loads such as ammeters, wattmeter current coils, or relay coils. In such a case, a current transformer should never have its secondary terminals open-circuited while the primary circuit is still energized.
The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.
The current transformer works on the principle of shorted secondary.
It means that the burden on the system Zb is equal to 0.
Thus, the current transformer produces a current in its secondary which is proportional to the current in its primary.
19. Turns ration for a Potential Transformer (PT) is _________
n = Np ⁄ Ns
n = Ns ⁄ Np
n = 1 ⁄ Np
n = Ns
Answer.1. n = Np ⁄ Ns
Explanation:
The turns ratio for a P.T is defined as the ratio of the number of turns in the primary to the number of turns in the secondary. For the current transformer the Turn ratio is given as
N = Number of turns in Primary winding/Number of turns in Secondary winding
N = Np ⁄ Ns
20. The burden value in Potential Transformer (PT) should be ______ to achieve low errors in PT.
Low
High
Medium
Infinite
Answer.2. High
Explanation:
The nominal ratio of an instrument transformer does not remain constant as the load on the secondary charges. It changes because of the effect of secondary current and this causes errors in the measurement.
The specific loading at rated secondary winding voltage is specified such that the errors do not exceed the limits. Such a permissible load is called the burden of an instrument transformer.
For low errors in PT, the burden value should be high.
The burden across the secondary of an instrument transformer is specified as V2/I2
