Instrument Transformers Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.