A potential transformer used for protection as well as metering is designed with attention to
Answer: C
A distance or directional relay must have a faithful voltage during the fault, when the voltage may have fallen to a small fraction of normal, so accuracy at low voltage matters. Early saturation and shorting links belong to current transformers, not to these.
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The residual or open delta connection of three potential transformers is used to detect
Answer: D
With the three secondaries in broken delta the sum of the three phase voltages appears across the open corner. That sum is zero on a healthy system and rises to three times the phase voltage when one phase is earthed, which drives a displacement relay.
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A current transformer whose core has been left with residual magnetism after a direct current insulation test will
Answer: A
Remanence shifts the working point on the magnetisation curve, so the exciting current for a given flux swing is larger and the error worse. This is why a direct current test on a circuit containing current transformers is followed by demagnetising them.
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A summation current transformer is used to
Answer: B
Several primary windings on a common core, each fed from the secondary of a line current transformer, produce a single output proportional to the sum. It is used where one instrument or relay is to see the total of several feeders.
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An auxiliary or interposing current transformer is fitted between a line transformer and its relay chiefly to
Answer: C
It adapts a standard five ampere secondary to a relay wanting some other current, or gives galvanic separation between schemes. It cannot improve the line transformer's own accuracy limit, and it adds a burden of its own rather than reducing one.
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The magnetising current of a current transformer is deliberately kept small because it
Answer: D
Every ampere-turn spent on magnetising the core is an ampere-turn missing from the secondary, so the difference between the actual and the turns ratio is exactly the exciting current. Nothing else in a well made transformer contributes appreciably.
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The phase angle error of a current transformer is quoted in minutes rather than degrees because
Answer: A
A metering class transformer holds its phase angle within perhaps five or ten minutes at rated current, and quoting a tenth of a degree would be clumsy. The unit is chosen to make the class limits convenient whole numbers.
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The secondary winding of a current transformer is distributed uniformly round the core so as to
Answer: B
A concentrated winding leaves parts of the core linked by flux that does not link the winding, and that leakage adds to the error and worsens the behaviour at high currents. A uniform distribution round a toroid is the best arrangement obtainable.
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An earthing of the current transformer secondary circuit at one point is provided in order to
Answer: C
The secondary circuit is at panel level but its winding sits next to a conductor at system voltage, so a failure of the insulation between them could raise the whole secondary circuit to that potential. One earth point, and only one, holds it down safely.
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A current transformer and a potential transformer are both connected to a wattmeter. The overall error in the power reading is
Answer: D
Small fractional errors add, both the ratio errors and the phase angle contributions, so the measurement is worse than any single element in the chain. Precision work applies the correction factors of each transformer rather than accepting the accumulated class limits.
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