An autotransformer differs from a two-winding transformer in that
Answer: C
Only part of the power is transformed magnetically and the rest is conducted directly through the common winding, which saves copper and reduces the losses. The two circuits are no longer electrically isolated, which is its chief drawback.
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The saving in copper in an autotransformer compared with a two-winding transformer of the same rating is greatest when the ratio is
Answer: D
The saving is \( 1 - 1/k \) of the copper for a ratio \( k \), so a ratio of 1.1 saves nearly everything and a ratio of ten saves little. That is why autotransformers are used for modest voltage adjustments and starting duty.
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An autotransformer should not be used where
Answer: A
Because a winding is common, a fault or a break in the common part can put the full input voltage on the output side. Safety isolation, and any application where the two systems must be earthed separately, calls for two windings.
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A current transformer's secondary must never be
Answer: B
With no secondary ampere-turns to oppose the primary, the core saturates and a dangerous voltage appears at the open terminals. Shorting links are fitted for exactly that reason and the earth connection is a safety requirement.
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The burden of an instrument transformer is
Answer: C
Correct answer: the impedance of the connected instruments and wiring
Burden is the load an instrument transformer is asked to drive, expressed as an impedance or as volt-amperes at rated secondary current or voltage. It matters because the ratio and phase angle errors depend on it, so an instrument transformer is only accurate within the burden range for which it is specified.
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A potential or voltage transformer differs from a power transformer in being designed for
Answer: D
Correct answer: accuracy of ratio and phase angle at a small burden
A voltage transformer exists to reproduce a measurement, not to deliver power, so it is designed so that the ratio and phase relationship it presents to the meter or relay are accurate over its specified burden. Its rating is a few tens of volt-amperes, and unlike a current transformer it must never be short circuited.
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A Buchholz relay on an oil-filled transformer detects
Answer: A
Slow deterioration liberates gas that collects in the relay chamber and gives an alarm, while a violent fault surges oil through it and trips the transformer. It is fitted in the pipe between the tank and the conservator.
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The conservator of an oil-filled transformer accommodates
Answer: B
Correct answer: the expansion and contraction of the oil
The conservator is a tank above the main tank, partly filled, that takes up the change in oil volume as the transformer heats and cools. Keeping the main tank always completely full excludes air from contact with the hot oil, which is what limits oxidation, and the small air space in the conservator breathes through the silica gel breather.
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The silica gel breather of a transformer prevents
Answer: C
Correct answer: moisture from entering with the breathing air
As the oil expands and contracts the conservator must breathe, and the air drawn in would carry moisture that would dissolve in the oil and destroy its dielectric strength. The silica gel breather dries that air. The gel changes colour as it becomes saturated, which is why it is a routine inspection item.
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A transformer designated ONAN is cooled by
Answer: D
Correct answer: natural oil circulation and natural air circulation
The four-letter code names the internal medium and its circulation followed by the external medium and its circulation. ONAN is oil, natural circulation, air, natural circulation - thermosiphon flow through the radiators with no pumps or fans. Adding fans gives ONAF and adding oil pumps as well gives OFAF, each step increasing the rating of the same unit.
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