11. Potential transformers are used with _________
High range voltmeter
Low range voltmeter
Intermediate range voltmeter
Very high range voltmeter
Answer.2. Low range voltmeter
Explanation:
Potential transformers are used for measurement of high voltages by means of low range voltmeters or for energizing the potential coils of wattmeters, and energy meters. 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.
12. Identify the FALSE statement with concern to the current transformer and potential transformer.
Current ratio of the current transformer is generally high.
Volt ampere capacity of current transformer is generally high compared to the potential transformer.
Voltage ratio of the potential transformer is generally high.
The protective relays are connected to the secondary of the current and potential transformers.
Answer.2. Volt ampere capacity of current transformer is generally high compared to potential transformer
Explanation:
The current ratio of the current transformer is generally high: For the current transformer, the current ratio is in order of 80 such as 400/5.
Volt ampere capacity of current transformer is generally low compared to the potential transformer. Because in potential transformer the secondary, as well as primary current, will increase but as the primary voltage will remain constant (because the primary of a PT is connected to the line), the secondary voltage will decrease.
The protective relays are connected to the secondary of the current and potential transformers: The circuit part of the relay consists of the secondary winding of CT or PT and the relay operating coil for detecting the fault and initiating the operation of the circuit breaker to isolate the defective element from the rest of the system.
13. What is the effect of the secondary burden on the ratio error in the potential transformer?
Ratio error is constant with secondary burden
Ratio error decreases with secondary burden
Ratio error increases with secondary burden
Depends on the supply voltage
Answer.2. Ratio error increases with secondary burden
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.
14. In a potential transformer High flux density is due to _____
More number of turns
Less number of turns
Equal turns
Any of the above
Answer.2. Less number of turns
Explanation:
In potential transformer high flux densities in the core will reduce the core cross-section, thereby reducing the length of the winding wound over the core. In a P.T., a high flux density in the core gives rise to a less number of turns. This, in turn, will reduce the winding resistance. An optimization in the core flux density value to be used needs to do, since too high a flux density will increase the no-load current, which is also not desirable.
The objective while designing a PT should be to keep the operational flux density in the score as high as possible without causing its saturation too early in order to achieve a reduced area of core cross-section consequently reducing the winding resistance due to a lesser length of mean turn requirements. The high flux density requirements can be achieved by using CRGO steels.
15. Potential transformer is similar in design to a _________
C.T.
Step-up transformer
Power transformer
Step down transformer
Answer.3. Power transformer
Explanation:
The design of the potential transformer is similar to that of the power transformer. The power transformers are designed for achieving maximum efficiency, and minimum voltage regulation and cost; on the other hand, PTs are designed with objectives of low ratio and phase angle error.
These requirements give rise to comparatively larger sizes of PTs compared to power transformers because using reduced core and conductor size for economic designs result in large ratio and phase angle errors.
16. As the secondary burden is increased in the potential transformer the phase angle _____
Increase
Decrease
More Positive
Remain same
Answer.1. Increase
Explanation:
As the secondary burden increases, the phase difference between Vp and φ increases because of the increased voltage drop therefore, phase angle φ increases as the secondary burden increases.
With increasing voltage drops due to increased burden, the phase difference between VP and VS reversed increases with a resulting increase in phase angle error.
17. Ratio error in a potential transformer is depends on _________
Secondary Voltage
Primary voltage
Secondary current
Turns ratio
Answer.3. Secondary current
Explanation:
The ratio error in a potential transformer is given by
where
R is the ratio error
n is the turns ratio
Is is the secondary winding current
Ie is the iron loss component
Im is the magnetizing component
In a potential transformer, the expression for ratio error depends upon the secondary current.
In addition to these, the errors also depend upon the resistance and reactance of the transformer windings. Therefore, in order to reduce these errors, the secondary current, resistance, and reactance of transformer windings must be kept low.
Such reduction can be provided by using short magnetic paths, good quality core material, low flux density in the core, and certain precautions in the design of the core like assembly and interleaving.
18. The secondary winding of a P.T. is designed for _________
220 V
2.2 kV
1.1 kV
110 V
Answer.4. 110 V
Explanation:
Potential transformer is also called voltage transformer and is a step-down transformer whose secondary is rated at 110 V.
19. What is the effect of p.f. on the secondary burden in the potential transformer?
Ip shifts towards Io
Ip deviates from Io
Ip is independent of Io
Ip and Io cancel each other
Answer.1. Ip shifts towards Io
Explanation:
When the power factor of secondary burden in a potential transformer is reduced from unity to lagging, angle φ is increased, and primary current Ip becomes more nearly in phase with no-load current Io.
The voltage Vp and Vs move more nearly into phase with (primary induced EMF) Ep and (secondary induce EMF) Es respectively.
The voltage drops in windings for a given load are very little influenced by the power factor of the loads. So the result is an increase in Vp relative to Ep and a reduction of Vs relative to Es.
20. Ratio error in potential transformer can be minimised by _________
Reducing the turns
Reducing the current
Increasing the voltage
Using a good core material
Answer.4. Using a good core material
Explanation:
The ratio error is partly dependent upon the energy and magnetization components of the no-load current. The errors are controlled by reducing the no-load current. To keep the no-load current minimum possible the reluctance of its core is made as small as possible and flux density in the core is kept comparatively low. High quality material is used to reduce core losses.
The objective while designing a PT should be to keep the operational flux density in the score as high as possible without causing its saturation too early in order to achieve a reduced area of core cross-section consequently reducing the winding resistance due to a lesser length of mean turn requirements. The high flux density requirements can be achieved by using CRGO steels.
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.
