A metering current transformer of instrument security factor 5 is rated 100/5. On a fault of 4000 A the secondary current will be at most about
Answer: A
The security factor limits the secondary to five times its rated value, so 25 A, because the core saturates well before the fault current is reached. Without that saturation the instruments would see two hundred amperes and be destroyed.
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A protection current transformer rated 500/1 has an accuracy limit factor of 20. It reproduces the primary current faithfully up to
Answer: B
Twenty times the rated primary current is ten kiloamperes, at which the secondary carries twenty amperes. Beyond that the core saturates and the relay would see less than the true fault current, which is why the factor must exceed the largest fault the scheme must clear.
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The burden of a current transformer is quoted as 15 VA at 5 A. Its impedance is
Answer: C
Correct answer: 0.6 \( \Omega \)
Burden in volt-amperes converts to impedance through the rated secondary current: \[ Z = \frac{VA}{I^{2}} = \frac{15}{5^{2}} = \frac{15}{25} = 0.6\ \Omega. \] Dividing by the current rather than its square gives the 3 ohms offered, and the square is what makes the next two items behave as they do.
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Two instruments of 5 VA and 10 VA at 5 A are connected in series on one current transformer, together with leads of 0.1 \( \Omega \). The total burden is
Answer: D
Correct answer: 17.5 VA
The instruments add directly, and the leads must be converted from ohms at the rated current: \[ 5 + 10 + (0.1 \times 5^{2}) = 5 + 10 + 2.5 = 17.5\ \mathrm{VA}. \] The 15.1 offered adds the lead resistance as though ohms were volt-amperes, and the 15 ignores the leads altogether - which is exactly the mistake that overburdens a current transformer in the field.
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Those same 0.1 \( \Omega \) leads on a one ampere secondary would contribute a burden of
Answer: A
Correct answer: 0.1 VA
\[ 0.1 \times 1^{2} = 0.1\ \mathrm{VA}, \] against 2.5 VA for the same leads on a 5 A secondary. The burden goes as the square of the current, so a 1 A secondary imposes a twenty-fifth of the lead burden of a 5 A one. That is the whole reason 1 A current transformers are specified where the cable run between switchgear and panel is long.
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A current transformer has a phase angle error of 30 minutes and is used with a wattmeter on a load of 0.5 power factor lagging. The error in the power reading is about
Answer: B
Thirty minutes is 0.00873 radian, and the fractional error is that multiplied by the tangent of sixty degrees, giving \( 0.00873 \times 1.732 = 0.0151 \). The error grows as the power factor falls, which is why phase angle is specified for metering transformers.
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A current transformer of nominal ratio 300/5 is found to give 4.98 A when the primary carries 300 A. Its ratio correction factor is about
Answer: C
Correct answer: 1.004
The actual ratio is \( 300/4.98 = 60.24 \) against a nominal 60, and the ratio correction factor is the multiplier that turns the nominal into the true value: \[ \mathrm{RCF} = \frac{60.24}{60} = 1.004. \] Being slightly above unity means the transformer reads low, so the measured current must be scaled up - and a correction of 0.4 per cent is within the tolerance of a metering class transformer.
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True or false: the secondary of a current transformer may be safely short circuited but not opened, while for a potential transformer the reverse is true.
Answer: D
A current transformer is fed from a current source, so a short is its natural condition and an open circuit drives the core into saturation. A potential transformer is fed from a voltage source, so an open secondary is harmless and a short destroys the winding.
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True or false: the errors of a current transformer are independent of the burden connected to it.
Answer: A
A larger burden means a larger secondary voltage, which means a larger core flux and a larger exciting current, and the exciting current is precisely what constitutes the error. A class is therefore always quoted together with a burden.
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True or false: a current transformer can be used on direct current.
Answer: B
Transformer action requires a changing flux to induce a secondary voltage, so a steady current produces nothing in the secondary. Direct current is measured instead by a shunt, a Hall effect device or a saturable reactor arrangement.
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