1. What will happen if secondary of a current transformer is open-circuited?
a) hot because of heavy iron losses
b) hot because primary will carry heavy current
c) cool as there is no secondary current
d) depends on other parameters
Answer
Answer: a [Reason:] If secondary of current transformer is made open-circuited the transformer temperature will rise to higher value because of heavy iron losses taking place in the circuit due to high flux density.
2. The secondary winding of which of the following listed transformers is always kept closed?
a) Step-up transformer
b) Step-down transformer
c) Potential transformer
d) Current transformer
Answer
Answer: d [Reason:] Current transformer works on the principle of shorted secondary. It simply means that burden on the system Zb is equal to 0. This transformer produces a current in its secondary which is proportional to the current in its primary.
3. Current transformers are used for _____________________
a) to provide to measure voltages
b) to measure high value of currents
c) to short-circuit the unwanted instruments
d) to measure low value of currents
Answer
Answer: b [Reason:] It is the current ratio transformer meant for measuring large currents and provide a step-down current to current measuring instruments like an ammeter. Such instruments present a short-circuit to the CT secondary.
4. The secondary of a current transformer is always kept short-circuited while operating because it _______________________
a) avoids core saturation and high voltage induction
b) is safe to human beings
c) protects the primary circuit
d) to keep temperature within limits
Answer
Answer: a [Reason:] Secondary side of current transformer is always kept short circuited in order to avoid core saturation and provide high voltage induction, so that current transformer can be used to measure high values of currents.
5. In CT deep saturation will cause when _________________
a) if circuit is short-circuited
b) if circuit is open-circuited
c) in both OC and SC conditions
d) if operated at very high supply
Answer
Answer: b [Reason:] Most important precaution in use of a CT is that in no case should it be open circuited (even accidently). As the primary current is independent of the secondary current, all of it acts as a magnetizing current when the secondary is opened. This results in deep saturation of the core which cannot be returned to the normal state and so the CT is no longer usable.
6. Current transformers are __________________
a) series connected type of instrument transformers
b) parallel connected type of instrument transformers
c) series-parallel connected type of instrument transformers
d) parallel connected normal transformers
Answer
Answer: a [Reason:] Current transformer (CT) is a series connected type of instrument transformer. They are designed to present negligible load to the supply which is being measured and also have an accurate current ratio and phase relationship to enable accurate secondary connected metering.
7. Voltage transformers are designed to have _____________
a) high leakage reactance
b) high magnetizing current
c) high magnetizing reactance
d) low magnetizing reactance
Answer
Answer: c [Reason:] Voltage transformers often known as potential transformers are designed with minimum errors and to achieve this they are constructed with low leakage reactance, low loss and high magnetizing reactance.
8. Which of the following can be considered as error for Potential transformer?
a) Magnitude errors
b) Phase errors
c) Unit errors
d) Magnitude and phase errors
Answer
Answer: d [Reason:] V2 /V1 differs from the desired value (N1/N2) in magnitude and phase resulting in magnitude and phase errors. Such errors are required to be kept within the limit defined by the precision required. In order to achieve this a PT-potential transformer is designed specially.
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