1. The burden of the instrument transformer is considered when instruments are ______
Connected internally
Connected externally
Both 1 and 2
Out of phase
Answer.2. Connected externally
Explanation:
The burden of the instrument transformer is considered to be everything connected externally to its terminals, such as monitoring devices, relays, and pilot wiring. The impedance values of each component, which can be obtained from manufacturer data sheets, should be added algebraically to determine the total load.
2. The burden value in Potential Transformer (PT) should be ______ to achieve low errors in PT.
Low
Medium
Infinite
High
Answer.4. High
Explanation:
The burden is the greatest VA load at which the PT will operate contınuously without Overheating its windings beyond the permissible limits. . Burden must be high for a potential transformer to achieve low errors.
3. The potential transformer is always a ______
Step-up transformer
Step-down transformer
Auto-transformer
Power Transformer
Answer.2. Step-down transformer
Explanation:
The Primary winding of the potential transformer 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 potential transformer is always a step-down transformer.
4. The Burden of protective Current Transformer (CT) is specified in
Percentage of load current
Percentage Ratio error
Volt-Ampere
Percentage Phase error
Answer.3. Volt-Ampere
Explanation:
The burden of CT is the maximum load (in VA) that can be applied to the CT secondary.
The burden can be expressed in two ways.
The burden can be expressed as the total impedance in ohms of the circuit or the total volt-amperes (VA) and power factor at a specified value of current or voltage and frequency.
5. The potential transformer is always a ______
Step-up transformer
Step-down transformer
Auto-transformer
Power Transformer
Answer.2. Step-down transformer
Explanation:
In the current transformer, the primary winding of the transformer is connected in series with the line carrying a high current. The secondary of the transformer is made up of a large number of turns of fine wire having a small cross-section area. This transformer is basically a step-up transformer, it’s step-up a voltage from primary to secondary. Thus it reduces current from primary to secondary.
6. The ratio of transformation in the case of potential transformers
Increases with increases in power factor of secondary burden
Remains constant irrespective of the power of secondary burden
Decreases with increases in power factor of Secondary burden
None of the above
Answer.3. Decreases with increases in power factor of Secondary burden
Explanation:
The burden across the secondary of an instrument transformer is specified as V2/I2
The ratio of transformation in the case of potential transformers decreases with increases in the power factor of the Secondary burden.
7. If the secondary burden of the potential transformer increases, the secondary current will ______
Decrease
Increase
Remain Same
Become Negative
Answer.2. Increase
Explanation:
If the secondary burden of the potential transformer increases, the secondary current will increase proportionately, and therefore, the primary current will also increase causing both the primary and secondary voltage drops to increase.
8. An ammeter of 0.1 Ω resistance is connected across the secondary of a 400 / 5A current transformer (CT). The CT burden is
0.5 VA
2.5 VA
5.0 VA
25 VA
Answer.2. 2.5 VA
Explanation:
Given that
Rs = 0.1 Ω, Is = 5 A
Vs = Secondary voltage of CT.
Rs = Ammeter resistance which is connected in the CT secondary
Is = CT rated secondary current
Secondary voltage of CT = CT rated secondary current x Ammeter resistance which is connected in the CT secondary
Vs = Is × Rs
= 5 × 0.1
⇒ Vs = 0.5 V.
CT burden = CT rated secondary current x CT secondary Voltage
= 5 × 0.5
= 2.5 VA
9. If the secondary burden of a current transformer is 20 VA, and the secondary current is 8 A, the impedance of the connected load will be
0.50 ohms
0.25 ohm
0.13 ohms
0.31 ohms
Answer.4. 0.31 ohms
Explanation:
Secondary burden of a C.T. = I2R
⇒ (8)2(R) = 20
⇒ R = 0.3125 Ω
10. What are the advantages of instrument transformers?
Cheap
High voltage insulation is not required
Low power consumption
All of the above
Answer.4. All of the above
Explanation:
Advantages of Instrument Trasnformer
A single range instrument can be used for the measurement of currents or voltages of various ranges simply by using a multi-range current or voltage transformer or several single-range transformers.
When the measurement is to be done on a high-tension circuit (transmission line), the instrument can be located at some distance from the circuit, thus giving great safety to the control room operator.
As the measuring instrument is isolated from a high-tension circuit, it need not be insulated for high voltage.
With the help of a current transformer with a suitably split and hinged core, the current in a heavy current bus bar can be measured without breaking the current circuit. T
When a.c. indicating instruments are used in conjunction with instrument transformer, their readings do not depend upon their constants viz. R, L, and C as is the case with shunts and multipliers.
It is a very cheap and moderate rating instrument that can be used to measure large currents and high voltages.
There is low power consumption in the metering circuit.
With the standardization of C.T. and P.T. secondary winding ratings, it is possible to standardize instruments around these ratings and, therefore, there is a great reduction in the costs of instrument transformers and instruments.
Replacement of instruments is easy on account of the standardization of the ratings.
Several instruments can be operated from a single instrument transformer.
11. Which of the following are the main disadvantages of an Instrument transformer?
High power consumption
Can be used only for AC circuit
Very Expensive
All of the above
Answer.2. Can be used only for AC
Explanation:
The only disadvantage of these instrument transformers is that they can be used only for a.c. circuits and not for d.c. circuits.
12. Which of the following are applications of instrument transformers?
Range extension
Isolate measuring Instrument
Used for routing Measurement
All of the above
Answer.4. All of the above
Explanation:
Applications of Instrument Transformers
Extend the range of the a.c. measuring instrument (similar to that of the multiplier used to extend the range of a d.c. meter).
Isolate the measuring instrument from the high-voltage power line.
Widely used for very precise measurements as well as routine measurements.