Advanced Problems on C.T. and P.T.

Electrical Measurements

This set of Tricky Electrical Measurements Questions and Answers focuses on “Advanced Problems on C.T. and P.T.”.

1. A 50 Hz, bar primary CT has a secondary with 800 turns. The secondary supplies 7 A current into a purely resistive burden of 2 Ω. The magnetizing ampere-turns are 300. The phase angle is?
a) 3.1°
b) 85.4°
c) 94.6°
d) 175.4°

2. A 50 Hz, bar primary CT has a secondary with 500 turns. The secondary supplies 5 A current into a purely resistive burden of 1 Ω. The magnetizing ampere-turns are 200. What is the ratio error at full load, if the iron loss is 1.5 W and magnetizing mmf is 100 A?
a) Zero
b) 45 μWb
c) 25.5 μWb
d) 100 μWb

3. A 1000/5 A, 50 Hz correct transformer has a secondary burden comprising a non-inductance of 1.6Ω. The flux in the core at full load is?
a) 160 μWb
b) 180 μWb
c) 200 μWb
d) 150 μWb

4. A 1000/5 A, 50 Hz correct transformer has a secondary burden comprising a non-inductance of 2Ω. What is the ratio error, if the iron loss is 3 W and magnetizing mmf is 250 A?
a) 4%
b) 5.7%
c) 3.6%
d) 4.8%

5. A 200/1 Current Transformer (CT) is wound with 400 turns on the secondary on a toroidal core. When it carries a current of 180 A on the primary, the ratio is found to be -0.5%. If the number of secondary turns is reduced by 1, the new ratio error (in %) will be?
a) 0.0
b) -0.5
c) -1.0
d) -2.0

6. A (350 A/ 7A), 50 Hz current transformer has a primary bar. The secondary burden is a pure resistance of 1 Ω and it draws a current of 5 A. The magnetic core requires 350 AT for magnetization. Find the percentage ratio error.
a) 10.56
b) -28.57
c) 11.80
d) -11.80

7. The observation when the secondary winding of a current transformer is open-circuited is?
a) The whole of the primary current produces a large value of flux in the core thereby producing a large voltage in the secondary winding
b) The large voltage may act as a safety hazard for the operators and many even raptures the insulation
c) When the large magnetizing force is taken off, it leaves a large value of residual magnetism
d) When the large magnetizing force is taken off, it leaves a small value of residual magnetism

8. Usually a CT has ____________
a) Power overload capacity than PT
b) The same overload capacity as a PT
c) A higher overload capacity than a PT
d) No overload capacity

9. A PT is a device which is ___________
a) Electrostatically coupled
b) Electrically coupled
c) Electromagnetically coupled
d) Conductively coupled

10. The CT supplies current to the current coil of a wattmeter power factor meter, energy meter and, an ammeter. These are connected as?
a) All coils in parallel
b) All coils in series
c) Series-parallel connection with two in each arm
d) Series-parallel connection with one in each arm

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