# High Voltage Engineering Questions and Answers – Cascade Transformer

This set of High Voltage Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Cascade Transformer”.

1. What is the third winding in the transformer called?
a) meter winding
b) high voltage winding
c) secondary winding
d) primary winding

Explanation: Low and high voltage windings are the normal winding found in a transformer. The meter winding, also known as the third winding is the additional winding used to measure the output voltage.

2. Single transformers can be used for test voltage requirements of less than 500kV.
a) True
b) False

Explanation: Single transformers can be used for test voltage requirements of less than 300kV. Depending on the circuit design, the transformer must be capable of giving short circuit current for one minute or more.

3. Cascading of transformers brings the high voltage windings of all unit in __________
a) series connection
b) parallel connection
c) series and parallel connection
d) series or parallel connection

Explanation: The transformer having identical units are cascaded for high voltage requirements. Here, the high voltage windings of all units are connected in series. Cascaded transformers make the transportation and erection process simple unlike a single large transformer.

4. What is the rating of the primary winding for small units up to 100 kVA?
a) 400 or 720V
b) 10-100kV
c) 120 or 400V
d) 230 or 400V

Explanation: The ratings of the primary or low-voltage winding for small units are 230 or 400V.The supply to these units is provided from a generator-motor set. For variation in the output voltage, and induction regulator is used.

5. What is the rating of the primary winding for larger outputs?
a) 400 or 720V
b) 3.3kV, 6.6kV or 11kV
c) 3.3KV or 6.6kV
d) 6.6kV or 11kV

Explanation: The ratings of the primary or low-voltage winding for larger output is 3.3kV, 6.6kV or 11kV.
The supply to these units is provided from a generator-motor set. For variation in the output voltage,
and induction regulator is used.

6. The transformer units are enclosed by large size metal rings to prevent corona.
a) True
b) False

Explanation: Large testing transformers are used mainly for outdoor use. To prevent corona, they are enclosed by metal rings which are large in structure. The electrodes are spherical in shape.

7. How is the high short circuit current limited in the cascade transformers?
a) by using limiting reactors in output stage
b) by using limiting reactors in input stage
c) by using limiting reactors in input and output stage
d) by using limiting reactors in between the transformer units

Explanation: There are chances of the occurrence of excessive load currents. Thus to limit the high short circuit current, limiting reactors are employed at the input stage. The short circuit currents are capacitive in nature.

8. What is the capacitance value of power transformers with a power rating greater than 1MVA?
a) 10-100pF
b) 100-1000pF
c) 1000-10000pF
d) 10000pF

Explanation: Power transformers are high capacitance test objects. They have a typical capacitance value of 1000-10000pF for power rating greater than 1MVA. Whenever a high voltage apparatus is tested, it is supplied with capacitive load. The power dissipation is very low during this time.

9. What is the capacitance value of power transformers with a power rating less than 1MVA?
a) 1000pF
b) 100pF
c) 10pF
d) 10000pF

Explanation: Power transformers are high capacitance test objects. They have a typical capacitance value of 1000pF for power rating less than 1MVA. Whenever a high voltage apparatus is tested, it is supplied with a capacitive load. The power dissipation is very low during this time.

10. What is the capacitance value of high voltage power cables with solid insulation?
a) 150-200pF/m
b) 250-300pF/m
c) 10pF/m
d) 250-600pF/m

Explanation: Power cables are high capacitance test objects. They have a typical capacitance value of 250-300pF/m with solid insulation. Whenever a high voltage apparatus is tested, it is supplied with a capacitive load. The power dissipation is very low during this time.

11. What is the capacitance value of high voltage power cables with gas insulation?
a) 150-200pF/m
b) 250-300pF/m
c) 50-80pF/m
d) 250-600pF/m

Explanation: Power cables are high capacitance test objects. They have a typical capacitance value of 50-80pF/m with solid insulation. Whenever a high voltage apparatus is tested, it is supplied with a capacitive load. The power dissipation is very low during this time.

12. What is the ratio of isolating transformers used with a cascaded transformer unit?
a) 1:1
b) 1:2
c) 2:1
d) 1:3

Explanation: The ratio of isolating transformers used with a cascaded transformer unit is 1:1. They are insulated to their respective tank potentials. These transformers are light and compact. The same ac input feeds the supply to the isolating transformer.

13. Identify the following diagram.

a) equivalent circuit of one stage of the cascaded transformer
b) equivalent circuit of one stage of the resonant transformer
c) series and parallel of transformer
d) equivalent circuit of isolating transformer

Explanation: The figure shows the equivalent circuit of one stage of the cascaded transformer. N is the number of turns in the winding and Z is the leakage impedance. P stands for primary, S stands for secondary and M and for meter winding.

14. ____________ stages of the primaries of the transformers are loaded more compared with the ____________ stages in a cascaded transformer.
a) upper, lower
b) lower, upper
c) middle, lower
d) upper, middle

Explanation: The lower stages of the primary windings in the cascaded unit is loaded more. This poses as the main disadvantage of cascading the transformers. Hence, loading is done carefully.

15. The impedance of a two-stage transformer is ___________ times the impedance of one unit and impedance of a three-stage transformer is _________ times the impedance of the single unit transformer.
a) 2, 2
b) 3, 2
c) 3-4, 8-9
d) 8-9, 3-4

Explanation: The impedance of a two-stage transformer is 3-4 times the impedance of one unit and impedance of a three-stage transformer is 8-9 times the impedance of the single unit transformer. Hence, in order to keep the impedance very low, the impedance of each unit of the transformer is kept minimum.

Sanfoundry Global Education & Learning Series – High Voltage Engineering.

To practice all areas of High Voltage Engineering, here is complete set of Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]