Instrument Transformers Questions and Answers

Answer: A

A multiplier for eleven kilovolts would place that voltage on the instrument terminals, which is intolerable. A transformer keeps the instrument and the operator at a low potential while scaling the quantity, and it dissipates far less power than a shunt or multiplier would.

Answer: B

Standardising the secondary means any instrument may be used with any transformer of the same class. Five amperes is the traditional value; one ampere is preferred where the leads to the instrument are long, since the burden of the leads then falls by a factor of twenty-five.

Answer: C

As with the current transformer, a standard secondary lets any voltmeter, wattmeter or relay be used with any transformer. A hundred and ten volts is high enough for a good signal and low enough to be handled safely on a panel.

Answer: D

The burden of the connecting leads is proportional to the square of the secondary current, so dropping from five amperes to one reduces it twenty-five-fold. In a large substation where the instruments are far from the transformer this is decisive.

Answer: A

For a large current a single turn is enough, and the simplest single turn is the busbar or cable itself threaded through the core window. There is no primary winding to insulate, which is a great simplification at high voltage.

Answer: B

The ampere-turns must be sufficient to magnetise the core and drive the secondary, so a primary current of a few amperes needs several turns. Above a few hundred amperes a single bar turn suffices and the wound construction becomes unnecessary.

Answer: C

The whole error of a current transformer comes from the current needed to magnetise the core and supply its loss, so both must be minimised. Nickel iron alloys of very high permeability are used for metering cores, at the cost of saturating at a low flux density.

Answer: D

It is quoted either as an impedance in ohms or as a volt-ampere rating at rated secondary current, and it comprises the instruments, relays and the connecting leads. The error of the transformer depends on the burden, so a class is meaningless without one.

Answer: A

A larger burden means a larger secondary voltage, hence a larger core flux and a larger magnetising current, and it is the magnetising current that constitutes the error. This is why the leads are kept short and why an unused instrument is removed rather than left in circuit.

Answer: B

The primary ampere-turns must supply the secondary ampere-turns and in addition magnetise the core and cover its loss. The exciting component is therefore missing from the secondary, so the actual ratio differs from the turns ratio.