Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Errors in Current Transformers”.
1. Errors are introduced in Current Transformers.
a) True
b) False
Answer: a
Clarification: A current transformer is used for the measurement of very high currents. The use of a C.T. leads to the introduction of two errors in power measurement.
2. Ratio error is defined as ___________
a) Ratio error = Kn⁄R
b) Ratio error = Kn – R⁄R
c) Ratio error = Kn – R
d) Ratio error = 1⁄R
Answer: b
Clarification: Ratio error of a C.T. is defined as the ratio of the magnitude of the difference between the nominal and actual ratio with respect to the actual ratio.
3. Phase angle in a C.T. is defined as ____________
a) (frac{180}{π} [frac{I_m cosδ}{nI_s}]) degrees
b) (frac{180}{π} [frac{I_c sinδ}{nI_s}]) degrees
c) (frac{180}{π} [frac{I_m cosδ – I_c sinδ}{nI_s}]) degrees
d) (frac{180}{π} [frac{I_m sinδ – I_c cosδ}{nI_s}]) degrees
Answer: c
Clarification: During power measurement, there exists phase angle error in a C.T. The phase angle is defined as
Phase angle = (frac{180}{π} [frac{I_m cosδ – I_c sinδ}{nI_s}]) degrees.
where. Im is the magnetising component of the excitation current
Ic is the core loss component of the excitation current
Is is the secondary winding current.
4. Phase angle error is given by ____________
a) (frac{180}{π} [frac{1}{nI_s}]) degrees
b) (frac{180}{π} [frac{I_m}{I_s}]) degrees
c) (frac{180}{π} [frac{I_m}{n}])degrees
d) (frac{180}{π} [frac{I_m}{nI_s}]) degrees
Answer: d
Clarification: The power measurement in a C.T. leads to phase angle error. Phase angle error is given by the relation
θ = (frac{180}{π} [frac{I_m}{nI_s}]) degrees
where, θ is the phase angle error
Im is the magnetising component of the excitation current
Is is the secondary winding current.
5. Ratio error is due to _________
a) iron loss
b) C.T.
c) magnetising component
d) supply voltage
Answer: a
Clarification: We know that the ratio error in a C.T. is given by the relation
Ratio error = Kn – R⁄R = n + Ie⁄Is
where, Ie is the iron loss component of the excitation current
n is the turns ratio.
6. Phase angle error is due to _________
a) C.T.
b) magnetising component
c) iron loss
d) supply voltage
Answer: b
Clarification: We know that the phase angle error in a C.T. is given by the relation
θ = (frac{180}{π} [frac{I_m}{nI_s}]) degrees
where, θ is the phase angle error
Im is the magnetising component of the excitation current
Is is the secondary winding current
It is observed from the equation for the phase angle error that it depends on the magnetising component of the excitation current.
7. In power measurements 180° phase shift is required.
a) True
b) False
Answer: a
Clarification: For eliminating errors in power measurement, there must be a phase difference of 180° between the primary and the secondary currents.
8. Errors in a C.T. can be minimised by _________
a) making use of laminations
b) having low reactance
c) increasing the secondary winding turns
d) decreasing the primary winding turns
Answer: b
Clarification: The excitation current Io can be minimised thus eliminating the errors in a C.T. by minimising the iron loss. The core must have a low iron loss and a minimum value of leakage reactance.
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