21. How is the transformation ratio of the potential transformer is dependent on the secondary burden?
Decreases
Increases
Remains constant
Depends on the supply
Answer.2. Increases
Explanation:
With the increase in PT secondary burden, the secondary current is increased. This in turn will increase the primary current as well.
Both primary and secondary voltage drops are increased and hence, for a given value of the primary supply voltage VP, secondary terminal voltage VS is reduced with an increase of burden.
The effect is therefore to increase the actual transformation ratio VP/VS with a resulting increase in the ratio error.
22. The Ratio error in the potential transformer can also be reduced by _________
Reducing secondary turns
Increasing the primary turns
Increasing secondary turns
Reducing the primary turns
Answer.1. Reducing secondary turns
Explanation:
Ratio error in the potential transformer can be reduced by turn compensation. This is done by makıng the secondary turns increased slightly the required with the rated ratio at one particular value and type of burden. Primary transformers are wound as closely as possible to compensate for phase angle error and secondary windings in potential.
23. At constant voltage how is frequency affected by the flux in the potential transformer?
Decreases with an increase in flux
Increases with a decrease in flux
Remains constant
Depends on the current
Answer.2. Increases with a decrease in flux
Explanation:
Effect of change in Frequency:- The reduction of frequency results in an increase in the core flux, with a corresponding increase in the exciting current, and a reduction in reactance volt drops.
The flux is inversely proportional to the frequency, for a constant voltage. Thus, with an increase in frequency, flux is reduced and, therefore magnetizing current and core loss component is decreased leading to a decrease in voltage ratio.
The decrease is not so much, as with an increase in frequency the leakage reactance increases and, therefore, leakage reactance drops are increased giving an increase in the ratio.
24. How is the voltage ratio dependent on the frequency?
They are independent of each other
They aid each other
They oppose each other
Depends on the setup of the circuit
Answer.3. They oppose each other
Explanation:
Effect of change in Frequency:- The reduction of frequency results in an increase in the core flux, with a corresponding increase in the exciting current, and a reduction in reactance volt drops.
The flux is inversely proportional to the frequency, for a constant voltage. Thus, with an increase in frequency, flux is reduced and, therefore magnetizing current and core loss component are decreased leading to a decrease in voltage ratio.
25. The main requirement for the potential transformer is
Low voltage drop
Accurate attenuation ratio
Low loading
All of the above
Answer.4. All of the above
Explanation:
The main requirement for the potential transformer is
The attenuation ratio must be accurately maintained in a PT since it is being used for measurement purposes.
Voltage drops in the windings must be minimized in a PT in order to reduce the effects of phase shift and ratio error.
Voltage drops in windings can be reduced by proper design to minimize leakage reactance and using large copper conductors.
Loading in a PT is always small, only of the order of a few volt-amperes. Loading of a PT is actually limited by accuracy considerations; whereas, in a power transformer, load limitation is on a heating basis.
Overload capacity for PTs is designed to be up to 2-3 times their normal rated values. Whereas, high capacity power transformers, under special circumstances, can take up overloads up to only 20% above their normal rating.
26. Which of the following methods are used to reduce the error in the potential transformer?
Low flux density
Low reluctance
Low leakage reactance
All of the above
Answer.4. All of the above
Explanation:
In order to bring down the errors of the potential transformers.
The flux density in the core should be as low as possible.
The reluctance of the transformer core should be as low as possible.
The leakage reactance of the two windings should be made small by placing the two windings as close together as is consistent with insulation requirements.
The resistance of the winding should be made small.
Turns compensation should be done by adjusting the number of primary and secondary turns so that ‘n’ is less than the nominal ratio.
27. In a potential transformer in order to reduce error the primary winding is wound over the secondary winding on the _____ limbs.
Different
Same
Adjacent
Opposite
Answer.2. Same
Explanation:
Errors also occur due to Potential transformer winding resistance and leakage reactance. The primary winding is wound over the secondary winding on the same limbs to reduce the leakage to the minimum.
The primary and secondary windings must be kept as close as possible. Potential transformer winding resistance is reduced by keeping the mean length of the turns smallest and by using thick copper wire.
28. The insulation used in potential type transformer is
Cotton type insulation
Varnished
Both 1 and 2
No insulation is used
Answer.3. Both 1 and 2
Explanation:
Insulation choice is generally based on the voltage level handled by the transformer. Cotton tapes, varnished cambrics, and hard fiber separators are generally used as insulators.
Oil insulation is also used in PTs operating above 7 kV voltage levels, while dry insulators like porcelain are used at voltage levels above 45 kV.
29. To minimize the overall size of oil-filled PTs, oil-filled _____ are used.
Bushing
Brushing
Taping
Any of the above
Answer.1. Bushing
Explanation:
To minimize the overall size of oil-filled PTs, oil-filled bushings are used. Single bushings are used when PTs are connected between a line and a neutral wire, and two bushings are used when none of the lines are at ground potential.
30. The major error caused due to change in primary voltage of the potential transformer is
Phase error
Ratio Error
Harmonic
No Error
Answer.4. No Error
Explanation:
Variation of errors due to change in primary voltage in the potential transformer is of no importance, as the supply system of voltages does not vary widely.
