Instrument Transformers Questions and Answers

Answer: A

Metering wants early saturation to protect the instruments and high accuracy at normal current; protection wants no saturation until far above rated current and accepts a looser ratio. No single core can do both, so two cores share one primary conductor.

Answer: B

It is the standard definition of the onset of saturation, taken from the excitation curve. Above the knee the transformer can no longer force its secondary current through the burden, so a relay scheme must be designed to operate below it.

Answer: C

Multiplying the nominal ratio by this factor gives the actual ratio, so a reading scaled by the nominal ratio is corrected by the same factor. It is a little above or below unity, and precision work applies it rather than accepting the class limit.

Answer: D

Ratio error is defined as the nominal ratio minus the actual, divided by the actual, so \( (20 - 20.1)/20.1 = -0.005 \). A negative ratio error means the secondary current is a little less than the nominal ratio would suggest, which is the usual direction before compensation.

Answer: A

Its construction is that of an ordinary two-winding transformer, but the burden is only a voltmeter or relay coil, so it operates at a small fraction of the load its size would suggest. That light loading is what keeps its regulation, and hence its ratio error, small.

Answer: B

A short circuit on a voltage source draws an enormous current and destroys the winding, which is why a potential transformer secondary is fused. The rule is precisely the reverse of the current transformer rule, and confusing the two is a classic and dangerous error.

Answer: C

Both the resistive and the reactive drops in the two windings reduce the secondary voltage and shift its phase, and the exciting current contributes its own drop in the primary. The remedies are a low-reluctance core and generous copper, just as with a good power transformer.

Answer: D

A larger burden draws more secondary current, which increases the drops in the windings, so the secondary voltage falls further behind the ideal in both magnitude and phase. As with the current transformer, the class is quoted for a stated burden.

Answer: A

At four hundred kilovolts a conventional transformer becomes enormous and expensive. A capacitor stack divides the voltage down to a few kilovolts and a small electromagnetic unit then provides the standard secondary, which is far cheaper and doubles as a coupling for carrier signalling.

Answer: B

Without it the divider would behave as a source of high internal reactance and its ratio would change with every change of burden. A series inductance tuned to resonate with the stack at the supply frequency makes the source impedance small and the ratio stable.