11. The coil of the current transformer is wound _____
Together
Not wound at all
Either separately or together
Separately
Answer.4. Separately
Explanation:
The windings should be close together in order to reduce the secondary leakage reactance as this increases the ratio error. The primary winding of CT is wound completely separately and then tapped with suitable insulating material and assembled with the secondary on the core.
12. The total impedance of the secondary winding, leads, and burden of a 5 A CT is 0.01 Ω. If the fault current is 20 times the rated primary current of the CT, the VA output of the CT is
100 VA
150 VA
200 VA
50 VA
Answer.1. 100 VA
Explanation:
Given that, impedance of secondary winding = 0.01 Ω
Rated current = 5 A
fault current = 20 times of rated current
= 20 × 5 = 100 A
VA output = I2z = 1002 × 0.01 = 100 VA
13. The windings of low voltage current transformer are ________
Tied together
Shorted
Wound over one another
Grounded
Answer.3. Wound over one another
Explanation:
The primary and secondary windings of low voltage current transformer may be on top of each other or side by side, on the same limb, or on different ones, however, properly insulated from one another. The core is insulated from the windings with the help of circumferential wraps of insulating tape or pressboard such as elephantine.
14. Burden of current transformer is usually expressed in
Amperes
Volts
Volt amperes
Kilowatt
Answer.3. Volt amperes
Explanation:
Load on Current Transformer is called as a burden because Current Transformer secondary winding can’t be kept open, high voltage can do the insulation failure.
So it is necessary to load the Current Transformer’s secondary winding continuously and ultimately it acts as a burden on the Current Transformer.
The burden of the Current Transformer can be specified as Volt-Ampere absorbed at a certain Power Factor.
15. At _____ the current transformers are enclosed in a tank.
Low voltage
High voltage
Both 1 and 2
None of the above
Answer.1. High voltage
Explanation:
For higher voltages, current transformers have oil-impregnated paper insulation and are housed in oil-filled metal tanks equipped with porcelain or composite bushings. Instrument transformers for direct use in GIS are metal-enclosed and have SF6-impregnated foil insulation. The current transformers are enclosed in a tank at high voltages of the order of more than 7 kV.
16. Permendur used in current transformer has __________
Medium saturation point of permeability
Low saturation point of permeability
No saturation point of permeability
High saturation point of permeability
Answer.2. High saturation point of permeability
Explanation:
Permendur is used for small specialized transformers and in magnetostrictive transducers and delay lines. Permendur 49 is an alloy with nominal composition 49% cobalt, 49% iron and 2% vanadium which combines high saturation with high permeability.
17. Turns compensation in current transformer is used to ________
Compensate the turns
Equalize the turns on the windings
Protect the equipment
Transformation ratio equal to the nominal ratio
Answer.2. Transformation ratio equal to the nominal ratio
Explanation:
The actual ratio of transformation varies with operating conditions and the error in secondary voltage is defined as
The ratio error can be compensated by using one or two turns less than that number which would make the turn ratio equal to the nominal ratio. It can be reduced by secondary turns compensation i.e. slightly decreasing the secondary turns.
If the actual transformation ratio, R is made equal to the nominal ratio then the ratio error will be zero.
18. Mumetal is used to make a core of the current transformer that has _______
Low permeability, high loss
High permeability, medium loss
Medium permeability, high loss
High permeability, low loss
Answer.3. Medium permeability, high loss
Explanation:
The core of the current transformer is built up with lamination of silicon steel. For a high degree of accuracy, Permalloy or Mumetal is used for making cores.
Mu-metal is a Nickel-Iron soft ferromagnetic alloy with approximately 77% Nickel, 16% Iron, 5% Copper, and 2% Chromium or Molybdenum.
It is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields.
Magnetic properties:
Low coercivity so that it saturates at low magnetic fields.
Very high magnetic permeability (Relative permeability is around 80,000 – 1,00,000)
It has low hysteresis losses when used in AC magnetic circuits.
19. What is the effect of reducing the secondary turns in a C.T. by 1%?
The transformation ratio reduces by the same value
No effect
Nominal Ratio increases by the same value
Secondary current increases by the same value
Answer.1. The transformation ratio reduces by the same value
Explanation:
The actual ratio of transformation varies with operating conditions and the error in secondary voltage is defined as
It can be shown that ratio error can be minimized by reducing the turn ratio by 1%. This can be done by reducing 1 or 2 turns of the secondary winding.
It can be reduced by secondary turns compensation i.e. slightly decreasing the secondary turns. As the number of secondary turns is reduced by a factor of 1%, the transformation ratio (R) is reduced by approximately the same percentage.
20. In a 1000/5 A C.T., the number of secondary turns would be ________
200
199 or 198
5
1000
Answer.2. 199 or 198
Explanation:
Compensation for ratio error
It can be shown that ratio error can be minimized by reducing the turn ratio by 1%.
It can be reduced by secondary turns compensation i.e. slightly decreasing the secondary turns.
This can be done by reducing 1 or 2 turns of the secondary winding. For example, 1000/5 C.T. with bar primary requires 200 turns on the secondary side.
Instead of employing secondary turns equal to 200, it is made equal to 199 in which case R = n+ Ie/Is, = 199 + Ie/Is, and is approaching nominal ratio 1000/5.
The phase angle error is very little affected by a change of one turn or two on the secondary side. Thus to reduce ratio error it is considered as 198 or 199 in order to keep R and Kn equal.
21. The secondary winding of which of the following listed transformers is always kept closed?
Step-up transformer
Step-down transformer
Potential transformer
Current transformer
Answer.4. Current transformer
Explanation:
The secondary side of the current transformer should not be kept open.
Either it should be shorted or must be connected in series with a low resistance coil such as current coils of wattmeter, a coil of ammeter etc.
If it is left open, then-current through secondary becomes zero hence the ampere-turns produced by secondary which generally oppose primary ampere-turns becomes zero.
As there is no counter magnetomotive force (MMF) to unopposed primary MMF (ampere-turns), it produces high flux in the core. This produces excessive core losses, heating the core beyond limits.
22. The wound-type current transformer has
Primary winding having one full turn wound on core
Primary winding having more than one full turn wound on core
Secondary winding having one full turn wound on core
Secondary winding having more than one full turn wound on core
Answer.4. Current transformer
Explanation:
Wound-type Current Transformer: The transformer’s primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn ratio of the transformer.
These are used at very low current ratios such as summing applications. Because of the higher values of primary ampere-turns, high accuracy can be achieved by these CTs.
In wound type construction the primary is wound for more than one full turn, on the core. In a low voltage wound-type current transformer, the secondary winding is wound on a bakelite former.
The heavy primary winding is directly wound on the top of the secondary winding with suitable insulation in between the two.
23. Current transformers are used to _______
Provide to measure voltages
Measure high value of currents
Short-circuit the unwanted instruments
Measure low value of currents
Answer.2. Measure high value of currents
Explanation:
The current transformer (CT) is used for the measurement of the high value of electric currents.
When the current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.
24. The bar-type current transformer has
Primary winding consists of a rod of suitable size and material
Primary winding consists of a bar of suitable size and material
Secondary winding consists of a rod of suitable size and material
Secondary winding consists of a bar of suitable size and material
Answer.2. Primary winding consists of a bar of suitable size and material
Explanation:
In bar-type current transformer construction, the primary winding is a bar of suitable size. It passes through the center of the hollow metalcore. The bar may be of rectangular or circular cross-section. The secondary winding is wound on this core.
25. The secondary of a current transformer is always kept short-circuited while operating because it ________
Avoids core saturation and high voltage induction
Is safe for human beings
Protects the primary circuit
keep the temperature within limits
Answer.1. Avoids core saturation and high voltage induction
Explanation:
The secondary side of the current transformer should not be kept open.
Either it should be shorted or must be connected in series with a low resistance coil such as current coils of wattmeter, a coil of ammeter, etc.
If it is left open, then-current through secondary becomes zero hence the ampere-turns produced by secondary which generally oppose primary ampere-turns become zero.
As there is no counter magnetomotive force (MMF) to unopposed primary MMF (ampere-turns), it produces high flux in the core. This produces excessive core losses, heating the core beyond limits.
26. The commonly used shapes of the current transformer are _____
4
3
2
1
Answer.2. 3
Explanation:
The simplest form of current transformer is the ring type or window type. There is three commonly used shape in the current transformer such as stadium, circular and rectangular orifices. The core is either a nickel-iron or oriented electrical steel which is a continuously wound type.
27. In CT deep saturation will cause when if _______
Circuit is short-circuited
Circuit is open-circuited
Both OC and SC conditions
Operated at a very high supply
Answer.2. Circuit is open-circuited
Explanation:
The secondary side of a current transformer should never be kept in open condition because, when kept open, there is a very high voltage found across the secondary side. This high voltage causes a high magnetizing current to build upon the secondary side which in turn causes high flux and makes the core saturate.
If the secondary of the current transformer is made open-circuited the transformer temperature will rise to a higher value because of heavy iron losses taking place in the circuit due to high flux density.
When the secondary winding of a current transformer is open-circuited with the primary winding energized, the large voltage may act as a safety hazard for the operators and many even rupture the insulation.
28. What material is made use of for the lamination in the current transformer?
Cold-rolled steels
Hot-rolled steels
Both 1 and 2
Hot iron
Answer.2. Hot-rolled steels
Explanation:
The magnetic cores of current transformer are made up of stacks of laminations.The magnetic materials that are used for the construction of current transformer cores include
(i) hot-rolled silicon steel
(ii) cold-rolled grain-oriented silicon steel
(ii) nickel-iron alloys.
Current transformers using hot-rolled steel, are made of stacks of ring stampings which are insulated by means of end collars and circumferential wraps of press boards. These press boards protect the secondary winding from mechanical damage in addition to acting as the insulating medium.
29. Current transformers are _______
Series-connected type of instrument transformers
Parallel-connected type of instrument transformers
Series-parallel connected type of instrument transformers
Parallel-connected normal transformers
Answer.1. Series-connected type of instrument transformers
Explanation:
Current transformers (CT) are a series-connected type of instrument transformer. The primary winding of a current transformer is connected in series with the circuit whose current is to be sensed and across the secondary of the current transformer, the operating coil of the relay is connected.
30. What is the insulation material used in the current transformer?
Eephantide
Presspahn
Elephantide and presspahn
Elephantide or presspahn
Answer.4. Elephantide or presspahn
Explanation:
The primary and secondary windings of the current transformer may be on top of each other or side by side, on the same limb, or on different ones, however, properly insulated from one another. The core is insulated from the windings with the help of circumferential wraps of insulating tape or pressboard such as elephantine or presspahn.
