The phase angle error of a current transformer is the angle between
Answer: C
Ideally the reversed secondary current would be exactly in phase with the primary. The exciting current tilts it by a small angle, usually a few minutes, and that angle matters in any measurement involving phase, such as power, energy or a directional relay.
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The phase angle error of a current transformer matters for
Answer: D
An ammeter responds to magnitude alone and is untroubled by a few minutes of phase shift. A wattmeter multiplies by a cosine, so a small angle error becomes a power error, and it becomes a large one when the load power factor is low.
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Turns compensation in a current transformer means
Answer: A
Reducing the secondary turns by one or two raises the secondary current a little, which offsets the part of the primary current lost to magnetisation. It corrects the ratio error at one chosen current and burden but does nothing for the phase angle error.
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Which error of a current transformer is NOT improved by turns compensation?
Answer: B
Removing a turn scales the secondary current but does not rotate it, so the angle between primary and reversed secondary current is unaffected. The phase angle is reduced instead by improving the core material and lowering the burden.
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The errors of a current transformer are greatest at
Answer: C
The exciting current does not fall in proportion to the primary current, so at a tenth of rated current it is a much larger fraction of the whole and the error is correspondingly worse. Accuracy classes therefore specify limits at several fractions of rated current.
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The secondary of a current transformer must never be left open circuited because
Answer: D
With no secondary current to oppose it the whole primary current becomes exciting current. The core is driven far into saturation, the flux waveform becomes peaky and the induced secondary voltage may reach thousands of volts, endangering both the insulation and the operator.
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Before removing an ammeter from the secondary of a current transformer in service, the correct procedure is to
Answer: A
A shorted secondary is the safe condition for a current transformer, the opposite of the rule for a voltage transformer. Shorting links are provided on the terminal block for exactly this, and the transformer suffers nothing while shorted.
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A current transformer left with its secondary short circuited while the primary carries current
Answer: B
A short circuit is a burden of zero impedance, which gives the smallest possible secondary voltage and hence the smallest core flux and the smallest error. It is the condition of least stress on the transformer in every respect.
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The instrument security factor of a metering current transformer is the ratio of
Answer: C
A low security factor is wanted for a metering transformer, so that on a fault the core saturates early and the instruments are shielded from the enormous current. A factor of five means the secondary current cannot exceed five times rated however large the fault.
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The accuracy limit factor of a protection current transformer is wanted
Answer: D
A protection transformer must reproduce the fault current faithfully, since that is precisely what the relay has to measure. A factor of ten or twenty means the ratio holds up to ten or twenty times rated primary current, which is the opposite requirement to a metering core.
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