# Transformers Questions and Answers – Three Phase Bank of Single Phase Transformers

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This set of Transformers Interview Questions and Answers for freshers focuses on “Three Phase Bank of Single Phase Transformers”.

1. For very high voltage transformers which connection is cheaper on primary side?
a) Star
b) Delta
c) Open delta
d) Can be star/ delta/ open delta

Explanation: In star connection with earthed neutral, the maximum voltage of the phase winding to ground is 1/√3 or 58% of the line voltage, while in delta connection this is equal to the line voltage in case of earthing of one of the lines during a fault. Thus, star on HV side is cheaper.

2. In which of the circuit given positive and negative sequence currents will flow in primary?
a) Star/delta
b) Star/star
c) Open delta/delta
d) Delta/delta

Explanation: The only positive and negative sequence currents flow in the lines on the delta side. This could also be achieved by star-connected primary provided the primary and secondary star points are grounded. But this is not recommended on account of flow of ground current for unbalanced secondary loads.

3. When star/star connection is used?
a) Small HV
b) Small LV
c) High HV
d) High LV

Explanation: This case is economical for small HV transformers as it minimizes the turns/phase and winding insulation. A neutral connection is possible. However, the Y /Y connection is rarely used because of difficulties associated with the exciting current.
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4. When delta/delta connection is used?
a) Small HV
b) Small LV
c) High HV
d) High LV

Explanation: This suits large LV transformers as it needs more turns/phase of smaller section. A large load unbalance can be tolerated. The absence of a star point may be a disadvantage. This connection can operate at 58% normal rating as open-delta when one of the transformers of the bank is removed for repairs or maintenance.

5. For using as a step-up transformer which connection is used?
a) Star/star
b) Delta/delta
c) Delta/star
d) Star/delta

Explanation: This is the most commonly used connection for power systems. At transmission levels star connection is on the HV side, i.e. D/Y for step-up and Y/D for step-down. The neutral thus available is used for grounding on the HV side.

6. At distribution level transformer with which connection is used?
a) Star/star
b) Delta/delta
c) Delta/star
d) Star/delta

Explanation: At the distribution level the D/Y transformer is used with star on the LV side which allows mixed 3-phase and 1-phase loads, while delta allows the flow of circulating current to compensate for neutral current on the star side.

7. Third-harmonic currents have phase difference of _____________
a) 00
b) 900
c) 1800
d) 270

Explanation: The phase difference in third-harmonic currents and voltages on a 3-phase system is 3 * 120° = 360° or 0° which means that these are cophasal. Therefore, third-harmonic currents and voltages cannot be present on the lines of a 3-phase system as these do not add up to zero.

8. Core flux in the transformer is _______________
a) sinusoidal
b) flat-topped
c) square wave
d) triangular

Explanation: The supply voltage provides the input current to the transformer primary, here primary current is only sinusoidal magnetizing current. Thus, this sinusoidal magnetic current will produce core flux, flat-topped.

9. In delta/delta connection flux is almost _________________
a) sinusoidal
b) flat-topped
c) triangular
d) square wave

Explanation: The supply voltage provides only sinusoidal magnetizing current so that core flux is flat-topped; but the third-harmonic emfs induced (cophasal) cause circulating currents in deltas restoring the flux to almost sinusoidal.

10. Apart from connection which of the following is different in star/delta or delta/star compare to delta/delta?
a) Flux is flat-topped
b) Impedance offered to third-harmonic currents in delta is less
c) Impedance offered to third-harmonic currents in delta is constant
d) Impedance offered to third-harmonic currents in delta is more

Explanation: Because of one delta connection the same conditions are obtained as in D/D connection except that the impedance offered to the flow of third-harmonic currents in delta is now larger and so are third-harmonic voltages.

11. In star/star connection the voltage can be correctly expressed as ____________
a) eaN = ea sin 2ωt + ea3 sin 3ωt
b) eaN = ea sin ωt + ea3sin 3ωt
c) eaN = ea sin 3ωt + ea3sin 3ωt
d) eaN = ea sin 6ωt + ea3sin 3ωt

Explanation: In the case of isolated neutrals, third-harmonic voltages are present in each phase as explained earlier. Further, since these voltages are cophasal, no third-harmonic voltages are present between lines. The voltage of phase a to neutral can now be expressed as shown in option b.

12. Rate of change of voltage in star/star connection is ______________
a) ω
b) 2 ω
c) 3 ω
d) Can’t determine

Explanation: While fundamental frequency voltages in the three phases have a relative phase difference of 120°, the third-harmonic voltages in them are cophasal (with respect to each other), but their phase with respect to the fundamental frequency (voltage changes at the rate of 2ω, twice the fundamental frequency).

13. Voltage at the neutral point oscillates at frequency of 2ω, this phenomenon is called as _________
a) oscillating neutral
b) doubling voltage
c) doubling current
d) doubling neutral

Explanation: Voltage of the neutral point oscillates at frequency 2 ω, phenomenon is known as oscillating neutral and is highly undesirable because of which the star/star connection with isolated neutrals is not used in practice. If the neutrals are connected, it effectively separates the three transformers. Third-harmonic currents can now flow via the neutrals.

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