Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Reduction of Errors in Potential Transformers”.
1. Winding resistance of a P.T. can be reduced by _________
a) using thick conductors
b) decreasing the length of the winding
c) shorting the primary and secondary windings
d) using thin conductors
Answer: a
Clarification: In a potential transformer, the winding resistance is usually minimised by using thick conductors and by making use of small length for the turns.
2. Leakage reactance is minimised by _________
a) using thin conductors
b) reducing leakage flux
c) increasing flux density
d) shorting the windings
Answer: b
Clarification: By maintaining the primary and secondary windings together in a P.T. and also by reducing the leakage flux, we can minimise the leakage reactance.
3. High flux density is due to less turns.
a) True
b) False
Answer: a
Clarification: In a P.T., a high flux density in the core, gives rise to a less number of turns. This in turn results in a lower leakage reactance.
4. Ratio error in a P.T. depends on _________
a) secondary current
b) primary voltage
c) secondary current
d) turns ratio
Answer: c
Clarification: In a P.T., the difference between actual ratio and turns ratio is given by the relation,
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 magnetising component
It is seen from the above equation that the ratio error in a P.T. depends on the secondary current, magnetising and iron loss components of current.
5. In a P.T. values of components of currents are negligible.
a) True
b) False
Answer: b
Clarification: In a C.T. the various components of current such as magnetising current, iron loss component of current are almost comparable in magnitude with the value of the load current.
6. Ratio error can be minimised by _________
a) reducing the turns
b) reducing the current
c) increasing the voltage
d) using a good core material
Answer: d
Clarification: By making use of a good quality core material, low value of flux density and following required precautions in the core assembly we can minimise the value of the ratio error.
7. Another method of eliminating the ratio error is _________
a) by reducing secondary turns
b) by increasing the primary turns
c) by increasing secondary turns
d) by reducing the primary turns
Answer: a
Clarification: In a P.T., at no load, we get
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 magnetising component
From the above equation it is seen that to reduce the ratio error, actual ratio and nominal ratio must be made equal. This can be done by reducing the secondary turns.
