Three Phase Transformers ( Transformers ) MCQs – Transformers MCQs
Latest Transformers MCQs
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Transformers MCQs – Three Phase Transformers ( Transformers ) MCQs
The most occurred mcqs of Three Phase Transformers ( ) in past papers. Past papers of Three Phase Transformers ( Transformers ) Mcqs. Past papers of Three Phase Transformers ( Transformers ) Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related Three Phase Transformers ( Transformers ) Mcqs. The Important series of Three Phase Transformers ( Transformers ) Mcqs are given below:
Three Phase Transformer Construction
1. Which type of winding is used in 3-phase shell-type transformer?
a) Circular type
b) Sandwich type
c) Cylindrical type
d) Rectangular type
Answer: b
Explanation: In core type of the transformer, winding is done by normal method, while in the shell type transformer, winding is sandwiched between corresponding shells of core material. Hence, it is sandwich type.
2. 3-phase transformers compare to 1-phase transformers ________
a) More economical
b) Easy in construction
c) Easy to construct
d) Easy to handle
Answer: a
Explanation: 3-phase transformers are also used to power large motors and other heavy loads. A three-wire three-phase circuit is more economical than an equivalent two-wire single-phase circuit at the same line to ground voltage because it uses less conductor material to transmit a given amount of electrical power.
3. How 3-phase transformers are constructed?
a) A bank of 3 single phase transformers
b) A single 3-phase transformer with the primary and secondary of each phase wound on three legs of a common core
c) Single 3-phase transformer or a bank of 3 1-phase transformers
d) By different method
Answer: c
Explanation: #-phase transformers can be either constructed by 2 methods. One is a bank of 3 single phase transformers and another method is a single 3-phase transformer with the primary and secondary of each phase wound on three legs of a common core.
4. Three phase transformer compare to a bank of 3 single phase transformers is ____________
a) Cheaper
b) Costlier
c) More in space
d) Can’t be determined
Answer: a
Explanation: 3-phase transformers can be manufactured by 2 methods. The three-phase transformer unit costs about 15% less than that of a bank and furthermore, the single unit occupies less space. There is little difference in reliability.
55. In mines we use _______________
a) a single unit of 3-phase transformer
b) a bank of 3 single phase transformers
c) use of 3-phase transformer is avoided
d) a single unit or a bank
Answer: b
Explanation: It is cheaper to carry spare stock of a single-phase rather than a three-phase transformer. In underground use (mines) a bank of single-phase units may be preferred as it is easier to transport these units. The bank also offers the advantage of a de-rated open-delta operation when one single-phase unit becomes inoperative.
6. In three phase bank phases are ___________________
a) the phases are electrically not connected and magnetically independent
b) the phases are electrically not connected and magnetically dependent
c) the phases are electrically connected and magnetically independent
d) the phases are electrically connected and magnetically dependent
Answer: c
Explanation: The 3-phase load bank is constructed by so that all three phases are made to be connected with together electrically. By the same time these banks form three magnetic circuits which are independent.
7. Where the tappings are provided in a transformer?
a) At the phase end of LV side
b) At the phase end of HV side
c) At the neutral side end of the HV side
d) At the middle of HV side
Answer: d
Explanation: Tappings are provided in order to maintain the terminal voltage across the loads within the permissible specified limits with respect to load variations. Tappings can be provided at phase end or neutral end of HV side. But to maintain magnetic symmetry tappings are preferred in the middle of the winding only.
8. Tappings are on _____________
a) LV side of a transformer
b) HV side of transformer
c) Not on any side
d) On both sides
Answer: b
Explanation: Generally, tappings are preferred on HV side, because the HV side is low current-high voltage side, tap changer involves low current interruption and large variations or small steps are possible due to more no of turns.
9. In core type 3-phase transformer flux path chooses how many paths to return?
a) 2
b) Single
c) 3
d) Many
Answer: a
Explanation: Flux of each limb in core type uses the other two limbs for its return path with the three-magnetic flux’s in the core generated by the line voltages differing in time-phase by 120 degrees. Thus, the flux in the core remains nearly sinusoidal, producing a sinusoidal secondary supply voltage.
10. Why shell type 3-phase transformer is used in large power transforming applications?
a) Can be made with more height
b) Can be made with less height
c) More height and less height flexibility
d) Due to other reasons
Answer: b
Explanation: The shell-type five-limb type three-phase transformer construction is heavier and more expensive to build than the core-type. Five-limb cores are generally used for very large power transformers as they can be made with reduced height.
11. A three-phase transformer generally has the three magnetic circuits interlaced.
a) True
b) False
Answer: b
Explanation: A three-phase transformer generally has the three magnetic circuits which are interlaced to give a uniform distribution of the dielectric flux between the high and low voltage windings. The exception is a three-phase shell type transformer. In the shell type of construction, even though the three cores are together, they are non-interlaced.
Three Phase Transformer Connections
1. When does star/star transformers work satisfactorily?
a) Load is unbalanced only
b) Load is balanced only
c) On balanced as well as unbalanced loads
d) Independent of load type
Answer: b
Explanation: With the unbalanced load connected to the neutral, the neutral point shifts thereby making the three line-to-neutral (i.e. phase) voltages unequal. The effect of unbalanced loads can be illustrated by placing a single load between phase (or coil) a and the neutral on the secondary side.
Excitation Phenomenon And Variable Frequency MCQs
2. When does delta/star transformer work satisfactorily?
a) Load is balanced only
b) Load is unbalanced only
c) On balanced as well as unbalanced loads
d) Independent of load type
Answer: c
Explanation: Large unbalanced/balanced loads can be handled satisfactory. The Y-D connection has no problem with third harmonic components due to circulating currents in D(delta). It is also more stable to unbalanced loads since the D partially redistributes any imbalance that occurs.
3. Scott connections are used in _______________
a) three-phase to single phase transformation
b) three-phase to two-phase transformation
c) single phase to three-phase transformation
d) all phase transformations
Answer: b
Explanation: Scott connections are used to convert three-phase to two-phase conversion, to start two phase motors and two phase furnaces. It requires two single phase transformers with adjustable tappings, one transformer is main transformer which is centre tapped through teaser transformer primary with a suitable number of turns to get a balanced two-phase supply.
4. In a three-phase star – delta transformer, what is the angle difference between primary and secondary phase voltages?
a) Delta side leads by 300
b) Delta side lags by 300
c) Star side leads by 300
d) Star side lags by 300
Answer: a
Explanation: This is a vector group and has + 30° displacement. Therefore, delta side leads by + 30°. So, it can be stated that delta side is having lead of 300 over star side because of the connections made.
5. Which can be also called as 00 /1800 connection?
a) Star/star
b) Direct star
c) Delta/star
d) Star/delta
Answer: a
Explanation: Star connection is formed on each side by connecting together phase winding terminals. The voltages of the corresponding phases (and thus of the corresponding lines) are in phase. This is known as the 0°-connection. If the winding terminals on secondary side are reversed, the 180°-connection is obtained.
6. What is the ratio of transformation of star/star connection?
a) Phase transformation x:1, line transformation x:1
b) Phase transformation x:1, line transformation 2x:1
c) Phase transformation x:1, line transformation x/3:1
d) Can’t say
Answer: a
Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/star connection we will get the same ratios, as at a particular point, voltage reading on primary will be proportional to secondary with x.
7. Delta/delta connection is also called as ____________
a) 00-connection
b) 900-connection
c) 1800-connection
d) 00/1800-connection
Answer: d
Explanation: Delta/delta connection is also called as 00-connection as seen from the phasor diagram that primary and secondary line voltages are in phase with each other. By reversing connection, we can get 1800 phase shift.
8. What is the ratio of transformation of delta/delta connection?
a) Phase transformation x:1, line transformation x:1
b) Phase transformation x:1, line transformation 2x:1
c) Phase transformation x:1, line transformation x/3:1
d) Can’t determine
Answer: a
Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the delta/delta connection also similar to the star/star, we will get the same ratios, as at a particular point, voltage reading on primary will be proportional to secondary with x.
9. Open delta connection has VA rating of _______________
a) √3 times delta/delta VA rating
b) 1/√3 times delta/delta VA rating
c) 3 times delta/delta VA rating
d) 1/3 times delta/delta VA rating
Answer: b
Explanation: When one of the transformer in delta/delta connection is removed we get open delta connection. This connection can handle the power of √3VI. While on the similar line delta/delta connection can handle the power of 3VI.
10. Star/delta connection is also called as ____________
a) 300-connection
b) 00-connection
c) -300-connection
d) 300/-300-connection
Answer: d
Explanation: Star/delta connection is also called as +/-300-connection as seen from the phasor diagram that primary and secondary line voltages are either ahead or below by 300 phases with each other. By reversing connection, we can get another condition phase shift.
11. What is the ratio of transformation of star/delta connection?
a) Phase transformation x:1, line transformation x:1
b) Phase transformation x:1, line transformation √3x:1
c) Phase transformation x:1, line transformation 3x:1
d) Can’t determine with information available
Answer: a
Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/delta connection we will get the √3 factor in ratios, as at a particular point, voltage reading on primary will be proportional to secondary with √3x.
12. x/√3:1 ratio is obtained in ______________
a) Star/delta
b) Delta/star
c) Delta/delta
d) Star/star
Answer: b
Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/delta connection we will get the 1/√3 factor in ratios, as at a particular point, voltage reading on primary will be proportional to secondary with 1/√3x.
13. Which both connections have the same line transformation ratios?
a) Star/star and delta/delta
b) Star/delta and delta/star
c) Star/zig-zag star and delta/zig-zag star
d) Star/star, delta/delta and star/delta, delta/star
Answer: d
Explanation: Star/star and delta/delta both connections have phase transfer ratio of x:1 and line transfer ratio also equal to x:1, while star/zig-zag star and delta/zig-zag star connections have line transformation ratio equal to 2/√3x:1.
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
Answer: a
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
Answer: d
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.
Efficiency And Voltage Regulation MCQs
3. When star/star connection is used?
a) Small HV
b) Small LV
c) High HV
d) High LV
Answer: a
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.
4. When delta/delta connection is used?
a) Small HV
b) Small LV
c) High HV
d) High LV
Answer: d
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
Answer: c
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
Answer: c
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
Answer: a
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
Answer: b
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
Answer: a
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
Answer: d
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
Answer: b
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
Answer: b
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
Answer: a
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.
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
Answer: a
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
Answer: a
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
Answer: a
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
Answer: b
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
Answer: c
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.
6. Auxiliary transformer is not needed in the back to back test.
a) True
b) False
Answer: b
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
Answer: d
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
Answer: a
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%
Answer: a
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
Answer: a
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
Answer: a
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.