# Transformers Questions and Answers – Tests on Three Phase Transformers

This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Tests on Three Phase Transformers”.

1. In three-phase transformer, the harmonic fluxes are suppressed because of ____________
a) high reluctance path
b) low reluctance path
c) any reluctance paths
d) independent on reluctance path

Explanation: In core type transformer, the third-harmonic fluxes in all the three limbs are simultaneously directed upwards or downwards so that this flux must return through air (high-reluctance path). The high reluctance path tends to suppress the third-harmonic flux.

2. Suppressing of harmonic fluxes becomes more prominent in _______
a) fifth harmonic currents
b) third harmonic currents
c) fourth harmonic currents
d) second harmonic currents

Explanation: The phenomenon gets more complex now and at core densities exceeding 1.5 T, the total harmonic content (particularly fifth) is very marked in the magnetizing current (fifth harmonic currents can flow on lines as their relative phase difference is 5 * 120° = 600° or 120°).

3. To reduce effect of 5th harmonic current ____________
a) separate path must be provided
b) no need of separate path
c) add a resistor in series
d) add a resistor in parallel

Explanation: To reduce the strong fifth harmonic in the magnetizing current for the star/star connection with isolated neutral, a path must be provided through iron for the third-harmonic flux. Hence, the use of a 5-limb core is adopted.

4. For performing back to back test on 3-phase transformer, transformers should be ___________
a) non-identical
b) identical
c) they can be identical or non-identical
d) they should not be identical nor non-identical

Explanation: According to connection of back to back test, arrangement of three-phase transformers should be done by keeping the fact into consideration that both of these transformers should be identical.

5. In back to back test two secondaries are connected ___________
a) in proper phase sequence
b) in phase opposition
c) in proper phase sequence and with phase opposition
d) in opposite phase sequence

Explanation: According to the connection arrangement for the back-to-back test on two identical 3-phase transformers. Then, two secondaries must be connected in phase opposition and in proper phase sequence.
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6. Auxiliary transformer is not needed in the back to back test.
a) True
b) False

Explanation: Auxiliary transformer is needed in the bak to back test as, we need to circulate the full-load current. This transformer is connected at secondaries or at primaries of the three-phase transformers.

7. Where the auxiliary transformers are connected in back to back test of 3-phase transformer?
a) Primaries
b) Secondaries
c) In the middle
d) Can be connected to primaries or secondaries as well

Explanation: The auxiliary transformer for circulating full-load current is included in the circuit of the two secondaries; it could also be included in the circuit of the primaries. Thus, with only losses (core-loss and copper-loss) supplied from the mains, a “heat run” test could be conducted on the transformers.

8. Auxiliary transformer connected to delta/delta transformer is of the type _______________
a) single phase transformer
b) three-phase transformer
c) two-phase transformers
d) can be of any type

Explanation: The primaries are normally excited from the mains. Each secondary delta is opened at one junction and a single-phase transformer can be employed to circulate full-load current in both the deltas.

9. If one of the transformers is removed from the bank of only delta-delta, then it will behave as power delivery transformer of ________
a) 58%
b) 78%
c) 45%
d) 100%

Explanation: It is true as the circuit will still be closed and the transformer will operate will lesser operating point. This new circuit so formed is also called as open delta circuit, which will deliver 58% of output.

10. Three units of single phase transformers and one single three-phase transformer_____________
a) will be same for one rating
b) can never be made same
c) may be same
d) depends on other factors

Explanation: Three single phase transformers and one single unit of three phase transformer will be same only, thus, will have same rating. This is done to reduce the cost and spacing, and to gain various other advantages.

11. A V-V connected transformer can be connected in parallel to delta-delta connected transformer
but not to _________
a) delta-star
b) star-delta
c) star-V
d) star-delta and star-V both

Explanation: The V-V transformer can be obtained from D-D transformer. The V-V connected transformer and D-D connected transformers have same phase displacement, so they only can be connected in parallel to each other.

Sanfoundry Global Education & Learning Series – Transformers.

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