Transformers Questions and Answers – Tap Changing Transformers – 1

This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Tap Changing Transformers-1”.

1. Why tapping is necessary?
a) To provide a neutral point
b) To vary secondary voltage
c) To control real and reactive power flow in the network
d) For neutral point, variation in secondary voltage, controlling power flow
View Answer

Answer: d
Explanation: Voltage variation in power systems is a normal phenomenon owing to the rapid growth of industries and distribution network. System voltage control is therefore essential for many purposes as listed above.

2. Common range pf tap changing is __________
a) 5%
b) 7%
c) 15%
d) 10%
View Answer

Answer: a
Explanation: Adjustment is normally carried out by off-circuit tap changing, the common range being 5% in 2.5% steps. Daily and short-time control or adjustment is carried out by means of on-load tap changing gear, which makes this application easier.

3. How tap changing is achieved?
a) Voltage variation with constant flux and constant voltage turn
b) With varying flux
c) Either by voltage variation or by flux variation
d) Can’t be done by variation in voltage or flux
View Answer

Answer: c
Explanation: Tap changing may be achieved in one of the three conditions, viz.
(i) voltage variation with constant flux and constant voltage turn,
(ii) with varying flux,
(iii) a mix of (i) and (ii).

4. Negative tapping means ________________
a) less turns than positive tap
b) less turns than principal tap
c) less turns than zero tap
d) more turns than principal tap
View Answer

Answer: b
Explanation: The principal tapping is one to which the rating of the winding is related. A positive tapping means more turns and a negative tapping implies less turns than those of the principal tap. Tap changing may be achieved by three conditions.

5. Tapping is done at _________________
a) primary only
b) secondary only
c) primary or secondary side
d) on both sides
View Answer

Answer: c
Explanation: Tapping can be done to either primary or secondary sides. The taps may be placed on the primary or secondary side which depends on construction. Tapping is thus so useful for various applications.

6. Bushing insulators are required when ____________
a) tapping is done near the neutral end
b) tapping is done at the line ends
c) tapping is done
d) they are not required
View Answer

Answer: b
Explanation: The taps may be placed on the primary or secondary side which partly depends on construction. If tapping is near the line ends, fewer bushings insulators are required. If the tappings are placed near the neutral ends, the phase-to-phase insulation conditions are eased.

7. For achieving large voltage regulation tapping should be done at _____________
a) at the upper ends
b) at the lower ends
c) at the centre
d) anywhere
View Answer

Answer: c
Explanation: For achieving large voltage variation, tappings should be placed near the centres of the phase windings to reduce magnetic asymmetry. It is generally done on that winding which is placed outside.

8. In case of large voltage variations transformers are tapped at _____________
a) LV
b) HV
c) Both LV and HV
d) Can’t be tapped
View Answer

Answer: b
Explanation: Centre tapped arrangement cannot be put on LV windings placed next to the core (as in core type transformer) because of accessibility and insulation considerations. The HV winding placed outside the LV winding is easily accessible and can, thus, be tapped easily.

9. Tapping of HV winding is advantageous for _____________
a) step-up transformer
b) step-down transformer
c) both type of transformers
d) never advantageous
View Answer

Answer: b
Explanation: It is not possible to tap other than an integral number of turns and this may not be feasible with LV side tappings. An example can illustrate this, 250 V phase winding with 15 V/turn cannot be tapped closer than 5%. It is therefore essential to tap the HV windings which is advantageous in a step-down transformer.

10. Tap changing do not causes change in losses of a transformer.
a) True
b) False
View Answer

Answer: b
Explanation: Axial mmf unbalance is minimized by thinning out the LV winding or by arranging parts of the winding more symmetrically. For very large tapping range, a special tapping coil can be employed. Tap changing causes changes in leakage reactance, core loss, I2R loss and perhaps some problems in parallel operation of dissimilar transformers.

11. Off-circuit tap changer is used when transformer is ________________
a) deenergized
b) energized
c) can be used in either energy conditions
d) never used
View Answer

Answer: a
Explanation: The cheapest method of changing the turn ratio of a transformer is the use of off-circuit tap changer. As the name off-circuit tap changer indicates, it is required to deenergize the transformer before changing the tap.

Sanfoundry Global Education & Learning Series – Transformers.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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