True or false: a metering current transformer should have a high accuracy limit factor.
Answer: C
The requirement is the opposite: a metering core should saturate early so that fault current is not passed on to delicate instruments. That property is expressed as a low instrument security factor, and it is the protection core that wants a high accuracy limit factor.
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Assertion: the secondary of a current transformer is short circuited before an ammeter is removed. Reason: an open secondary would drive the core into saturation and produce a high voltage.
Answer: D
With no secondary current the whole primary current becomes exciting current, the core saturates and a peaky induced voltage of thousands of volts may appear. Shorting the terminals removes the hazard, and it is exactly for that reason that the practice exists.
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Assertion: a one ampere secondary is preferred to a five ampere secondary in a large substation. Reason: the burden imposed by the connecting leads falls by a factor of twenty-five.
Answer: A
Lead burden is proportional to the square of the secondary current, so a fivefold reduction in current reduces it twenty-five-fold. In a large station where the instruments are far away that is the decisive consideration, and it is the reason given.
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Assertion: a potential transformer works at a very small fraction of the load an ordinary transformer of its size would carry. Reason: its core is made of a material of unusually high permeability.
Answer: B
The light loading follows from the burden being only a voltmeter or relay coil, not from the core material. Good core material does help the accuracy, but it is not the reason the transformer is lightly loaded, so the two true statements are unconnected.
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Match the quantity with the type of transformer it belongs to.
1. Knee point voltage 2. Instrument security factor 3. Tuning inductance
P. Metering current transformer Q. Capacitive voltage transformer R. Protection current transformer
Answer: C
The knee point defines where a protection core stops reproducing fault current; the security factor says how early a metering core saturates to shield its instruments; and the tuning inductance makes a capacitor divider behave as a stiff source at power frequency.
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Match the fault with what it causes.
1. Open circuited current transformer secondary 2. Short circuited potential transformer secondary 3. Excessive burden on a current transformer
P. Increased ratio and phase angle error Q. Destruction of the winding R. Dangerous secondary voltage
Answer: D
Each fault follows from which kind of source feeds the transformer. A current source drives its core to saturation when denied a path; a voltage source pours current into a short; and an excessive but finite burden merely raises the flux and with it the errors.
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The steps of removing a wattmeter from a current transformer circuit in service are listed below. Put them in order.
1. Disconnect the instrument leads 2. Close the shorting link across the secondary terminals 3. Remove the instrument 4. Check that the shorting link is making good contact
Answer: A
Correct answer: 2, 4, 1, 3
The shorting link goes on first and is then checked, because an open-circuited current transformer secondary is dangerous: the primary current becomes pure magnetising current, the core saturates hard and very high voltages appear across the open terminals. Only once the short is proved may the instrument leads be disconnected and the instrument removed. This ordering is a safety rule, not a preference.
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Consider the following statements about current transformers.
1. The errors are worst at low primary current 2. Turns compensation corrects the ratio error 3. Turns compensation corrects the phase angle error
Answer: B
The exciting current is a larger fraction of a small primary current, so the percentage error is worst there. Removing a secondary turn scales the secondary current and so corrects the ratio, but it cannot rotate it and so leaves the phase angle untouched.
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Consider the following statements about potential transformers.
1. The secondary is fused 2. Increasing the burden increases the errors 3. The secondary may be safely short circuited
Answer: C
A fuse is fitted precisely because a short must be cleared quickly, so the third statement is false. The first two are correct: the burden current produces the winding drops that constitute the ratio and phase angle errors.
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Which of the following is NOT a reason for using an instrument transformer?
Answer: D
Correct answer: to improve the accuracy of the instrument beyond its own class
An instrument transformer can only add its own errors of ratio and phase angle to those of the instrument; it cannot make a class 1 instrument better than class 1. The three genuine reasons are all present: isolation from system voltage, the use of standard 5 A and 110 V instruments at any system rating, and a small burden drawn from the circuit measured.
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