1. Instrument transformers are designed to transform _____
Current only
Voltage Only
Both current and voltage
Low value of current and voltage
Answer.3. Both current and voltage
Explanation:
Instrument transformers are designed to transform voltage or current from the high values in the transmission and distribution systems to the low values that can be utilized by low voltage metering devices.
Instrument transformers isolate measurement, protection, and control circuitry from the high currents or voltages present on the circuits being measured or controlled.
2. If the secondary winding of a current transformer is opened when the primary winding is carrying current, then the following event occurs:
The transformer will burn immediately
High induced EMF is produced in the secondary winding
Flux density in the core gets weakened
High current is produced in the secondary winding
Answer.2. High induced EMF is produced in the secondary winding
Explanation:
The most important precaution in the 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.
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.
3. The current transformer is used with ______
A.C. ammeter
D.C. ammeter
A.C. voltmeter
D.C. voltmeter
Answer.1. A.C. ammeter
Explanation:
The primary winding of a current transformer is connected in series with the line carrying the main current. The secondary winding of the CT, where the current is many times stepped down, is directly connected across an ammeter, for measurement of current.
Current transformers are used to extend the range of ammeters, and the current coils of wattmeters and energy meters.
4. The ratio error in a current transformer is attributed to:
Leakage flux
magnetizing component of no-load current
power factor of the primary
The energy component of excitation current
Answer.2. magnetizing component of no-load current
Explanation:
Ratio error in current transformer:
In the current transformer, the primary current Ip should be exactly equal to the secondary current multiplied by the turns ratio, i.e. KTIs.
But there is a difference between primary current Ip should be exactly equal to the secondary current multiplied by the turns ratio.
This difference is contributed by the core excitation or magnetizing component of no-load current.
The error in the current transformer introduced due to this difference is called current error or ratio error.
5. The potential transformer is used with ______
A.C. ammeter
D.C. ammeter
A.C. voltmeter
D.C. voltmeter
Answer.3. A.C. voltmeter
Explanation:
Measurement of voltage, power, etc., of high voltage lines, requires the high level of voltage to be stepped down before being applied to the measuring instrument. Potential transformers are used to extend the range of voltmeters, and the pressure coils of wattmeters and energy meters.
6. In the current transformer, the number of primary turns is always:
less than the number of secondary turns
zero
greater than the number of secondary turns
equals the number of secondary turns
Answer.1. less than the number of secondary turns
Explanation:
A current transformer needs to step down the high currents i.e. the primary current is very high. So, the number of primary turns is always less than the number of secondary turns.
7. Current transformer and potential transformer are used for Measuring _________
Low current and voltages
Very low current and voltages
High current and voltages
Intermediate currents and voltages
Answer.3. High current and voltages
Explanation:
CT and PT both are measuring devices used to measure currents and voltages. They are used where large quantities of currents and voltages are used. The role of CT and PT is to reduce high current and high voltage to a parameter.
8. In which of the following transformers, the secondary winding is always kept closed?
Current transformer
Potential transformer
Power transformer
Distribution transformer
Answer.1. Current transformer
Explanation:
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.
9. The primary winding of a C.T. has _________
A larger number of turns
No turns at all
An intermediate number of turns
Few turns
Answer.4. Few turns
Explanation:
The current transformer (c.t.) normally has very few turns on its primary winding so that it does not affect the circuit to be measured, with the actual meter connected across the secondary winding.
10. Instrument transformers are
Used to extend the range of the AC measuring instruments only
Used to isolate the measuring instruments from the high voltage only
Used to extend the range and isolate the measuring instruments
Not used at generating stations and transformer stations
Answer.3. Used to extend the range and isolate the measuring instruments
Explanation:
In high voltage A.C. circuits, the measurement cannot be done by using the method of extension of ranges of low range meters by providing suitable shunts.
In such conditions, specially constructed accurate ratio transformers are used. These transformers are used to isolate the instruments from high current and high voltage A.C. circuits.
These are generally classified as
(i) Current transformer – large alternating currents can be measured
(ii) Potential transformer – High voltages can be measured
