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Right option is (B) varies over a WIDE range of normal operation

The explanation is: In a potential transformer, the excitation current REMAINS constant under normal operation. While in a current transformer, the excitation current varies over a wide range of OPERATIONS.

The correct CHOICE is (a) independent of secondary CIRCUIT

To EXPLAIN: A C.T. is used for the measurement of HIGH magnitude of currents in a circuit, while a P.T. is used for the measurement of high magnitude of voltages in a circuit. Primary current in a C.T. is independent of the secondary circuit conditions.

Right choice is (d) 110 V

To elaborate: A potential TRANSFORMER is generally used for the measurement of the very high magnitude of VOLTAGES in a circuit. But the SECONDARY WINDING of a P.T. is designed for a capacity of 110 V.

Right OPTION is (a) high voltages

Easiest EXPLANATION: Potential transformers are ALSO known as P.T. and are used in the measurement of high magnitude of voltages.

Correct option is (c) Power TRANSFORMER

The explanation: In terms of design, the potential transformer resembles a power transformer. Potential TRANSFORMERS have a very low LOADING capacity of the ORDER of a few volt amperes.

The CORRECT answer is (B) low RANGE voltmeter

For explanation I would say: In general, potential transformers are used with low range voltmeters. They are also used for energizing the potential COILS of low range energy meters and WATTMETERS.

The correct CHOICE is (b) C.T. is demagnetised

Explanation: In a C.T., when the secondary winding is opened by MISTAKE when the PRIMARY current is flowing, the C.T. must be demagnetized first fully and then only must be USED again in the circuit for any measurements.

Correct ANSWER is (b) False

For EXPLANATION I would SAY: In a current transformer, even when it is not in use for any measurement PURPOSES, the secondary CIRCUIT must be closed when the primary current is flowing.

Right choice is (a) can be shorted

Easy explanation: When a C.T. is used in measurements, it ACTS as a SHORT circuit. This is because the impedance of the load is negligible DUE to the ammeter or the wattmeter COIL. Thus the secondary winding of a C.T. can be shorted.

Right CHOICE is (d) residual magnetism

For EXPLANATION: In a C.T, the high magnetizing forces on the core MAY leave behind residual magnetism in the core and ratio and phase angle errors.

Correct CHOICE is (a) a very HIGH flux density is produced

To elaborate: When current is flowing in the secondary CIRCUIT which is kept OPEN in a C.T., a very high flux density is produced in the core as a RESULT of the absence of demagnetizing ampere turns.

Right choice is (b) False

The best I can explain: In a CURRENT transformer, the PRIMARY ampere turns is fixed. When the secondary CIRCUIT is OPENED, ASSUMING a constant primary current we get a fixed number of primary ampere turns.

The CORRECT choice is (c) medium permeability, high loss

To explain I would say: Mumetal is an ALLOY of NICKEL, iron and copper. It has a high permeability, low loss and small retentivity. It has a MAXIMUM permeability of 90,000 at a FLUX density of 0.35 W/m^2.

The CORRECT option is (a) transformation ratio reduces by the same value

Explanation: In a C.T., the transformation ratio is given by

R = n + ^Ie⁄Is

where, n is the turns ratio

Ie is the core LOSS component of the excitation current

Is is the secondary winding current

As the number of secondary turns is reduced by a factor of 1%, the transformation ratio (R) is reduced by approximately the same percentage.

Correct choice is (a) True

To explain: The current transformers are enclosed in a tank at high voltages of the order of more than 7 kV. The tank consists of a SOLID insulating compound. Since the COOLING is POOR, the compound introduces DIFFICULTY.

The correct choice is (c) wound over ONE another

The explanation: Using CYLINDERS made of Bakelite, the windings of a C.T. are wound over one another. This SETUP is mounted on a steel tank SHEET filled with transformer oil.

Right option is (a) True

Explanation: A C.T. is used for the measurement of high magnitudes of current. In order to get lower line VOLTAGES, the COILS of a C.T. are WOUND SEPARATELY and insulated through tapes and VARNISH.

Correct answer is (b) 14 S.W.G copper wire and copper strip respectively

Easy explanation: For a current transformer the most popular FORM USED is the bar primary and ring core CONSTRUCTION. Hence 14 S.W.G copper wire is used for the secondary winding WHEREAS a copper strip is used for the primary winding.

Correct choice is (b) having low reactance

The explanation: The excitation CURRENT Io can be minimised thus eliminating the errors in a C.T. by minimising the iron loss. The core MUST have a low iron loss and a MINIMUM VALUE of LEAKAGE reactance.

The correct OPTION is (a) True

Easiest explanation: For ELIMINATING errors in power measurement, there must be a phase difference of 180° between the primary and the SECONDARY currents.

Correct choice is (a) iron LOSS

Best explanation: We KNOW that the RATIO error in a C.T. is given by the relation

Ratio error =^Kn – R⁄R = n +^IE⁄Is

 where, Ie is the iron loss component of the excitation current

n is the turns ratio.

Right option is (d) \(\FRAC{180}{π} [\frac{I_m}{nI_s}]\) degrees

The best I can EXPLAIN: The POWER measurement in a C.T. leads to phase angle ERROR. Phase angle error is given by the relation

θ = \(\frac{180}{π} [\frac{I_m}{nI_s}]\) degrees

where, θ is the phase angle error

Im is the magnetising component of the EXCITATION current

Is is the secondary winding current.

The correct choice is (c) \(\frac{180}{π} [\frac{I_m cosδ – I_c sinδ}{nI_s}]\) degrees

For explanation I would say: During power MEASUREMENT, there EXISTS phase angle error in a C.T. The phase angle is defined as

Phase angle = \(\frac{180}{π} [\frac{I_m cosδ – I_c sinδ}{nI_s}]\) degrees.

where. Im is the magnetising component of the excitation CURRENT

Ic is the core loss component of the excitation current

Is is the SECONDARY WINDING current.

Correct CHOICE is (b) Ratio error =^Kn – R⁄R

Best explanation: Ratio error of a C.T. is DEFINED as the ratio of the magnitude of the DIFFERENCE between the nominal and ACTUAL ratio with respect to the actual ratio.

Right choice is (a) True

Explanation: A current transformer is USED for the MEASUREMENT of very high currents. The use of a C.T. leads to the introduction of two ERRORS in POWER measurement.

Correct choice is (a) n = ^NP ⁄ Ns

To explain: For a potential transformer, the turns ratio is defined as the ratio of the NUMBER of turns in the PRIMARY WINDING to the number of turns in the secondary winding.

n = ^Np ⁄ Ns

where, n is the turns ratio

Np is the primary turns

Ns is the secondary turns.

Correct option is (d) N =^NS ⁄ Np

For explanation I would say: For a current transformer, the turns RATIO is defined as the ratio of the NUMBER of turns in the secondary winding to the number of turns in the primary winding.

n =^Ns ⁄ Np

where, n is the turns ratio

Ns is the secondary turns

Np is the primary turns.

The correct choice is (c) ratio of transformation ratio to NOMINAL ratio

To explain: The ratio correction factor for an instrument transformer is DEFINED as the ratio of the transformation ratio to the nominal ratio.

R.C.F = ^R ⁄ Kn

where,

R.C.F is the ratio correction factor

R is the transformation ratio

Kn is the nominal ratio.

Correct ANSWER is (a) Kn = \(\frac{I_{p(rated)}}{I_{s(rated)}}\)

For explanation I would say: In a CURRENT transformer, the transformation ratio is GIVEN by the ratio of the magnitude of the actual PRIMARY current to the magnitude of the actual secondary current.

Kn = \(\frac{I_{p(rated)}}{I_{s(rated)}}\)

where, R is the transformation ratio

Ip(rated) is the rated primary WINDING current

Is(rated) is the rated secondary winding current.

Correct answer is (d) RATIO of rated PRIMARY value to secondary value

Explanation: In an instrument transformer, nominal ratio is defined as the ratio of the rated primary current or VOLTAGE to the rated secondary WINDING current or voltage.

Correct CHOICE is (c) R = \(\FRAC{V_p}{V_s}\)

The explanation is: In a potential transformer, the transformation ratio is given by the ratio of the magnitude of the actual primary CURRENT to the magnitude of the actual secondary current.

R = \(\frac{V_p}{V_s}\)

where, R is the transformation ratio

Vp is the actual primary WINDING VOLTAGE

Vs is the actual secondary winding voltage.

Right CHOICE is (B) R = \(\frac{I_p}{I_s}\)

Easy explanation: In a current transformer, the transformation ratio is given by the ratio of the magnitude of the actual PRIMARY current to the magnitude of the actual secondary current.

R = \(\frac{I_p}{I_s}\)

where, R is the transformation ratio

Ip is the actual primary WINDING current

Is is the actual secondary winding current.

Right CHOICE is (a) RATIO of primary to secondary phasor

For explanation I WOULD say: For an instrument transformer, the TRANSFORMATION ratio is defined as the ratio of the magnitude of the ACTUAL primary phasor to the magnitude of the secondary phasor.

The correct ANSWER is (c) 5 A and 110 V

The explanation: Current transformers are STANDARDIZED at 5 A of secondary winding current, WHEREAS POTENTIAL transformers are standardized at 110 V of secondary winding voltage.

Correct answer is (b) electrical isolation from high RATED winding

Explanation: In an instrument transformer, the LOW rated SECONDARY windings provide electrical isolation from the high rated primary windings.

The correct answer is (c) readings are INDEPENDENT of component values

The EXPLANATION is: Instrument transformers can be used for extending the range of the A.C. instruments. In instrument transformers, readings are independent of the values of R, L, and C. But in the case of shunts and multipliers, the readings depend on the values of CIRCUIT constants.

The correct answer is (a) True

Explanation: The SECONDARY WINDINGS of C.T. and P.T. are standardized. As a RESULT, instruments can be standardized with the ratings of C.T. and P.T. HENCE the instrument transformers can be replaced with ease.

The correct answer is (d) a few turns

Best explanation: SECONDARY WINDING of a P.T. has a few number of turns. A low RANGE voltmeter is CONNECTED in parallel with the secondary winding.