1. What is 3 Phase Transformer ?
Ans: A three phase transformer is that transformer which is equivalent to three single phase transformer but wound on one core and enclosed in one common core.
2. What is advantage has the star connection over the delta connection ?
Ans: Each Star connected transformer is wound for only 57.7 % of line voltage. In HV transmission , this admits of much smaller transformer being built for high voltage than possible with delta connection, because of less insulation.
3. What Advantage id obtained with delta connection ?
Ans: When the transformer are connected in delta, one may be removed and the two remaining units will carry 57.7 % of the original three phase load and thus maintain the continuity of supply.
4. If the polarity of one transformer is reversed on a delta bank of single phase transformers that are connected for 3 phase operation what would be the result ?
Ans: Instead of zero voltage on the tie-point there would be a voltage that is twice the proper value.
5. Is it possible to connect two single phase transformers to give a 3-phase output from a 3 phase input ?
Ans: Yes, They would have to be connected in an open delta.
6. If three single phase transformer are connected in a delta arrangement and one of the transformer get damaged , is it possible to maintain continuity of supply with the remaining two transformers ? if yes How ?
Ans: Yes, The continuity of supply can be maintained by merely disconnecting the leads to the disabled transformer.
7. When a bank of two single phase transformers are connected in open delta arrangement to supply 3 phase load, do they supply their rated output ?
Ans: Each Transformer is only capable of supplying 86.6 % of its output rating.
8. In a star star connected transformer if a delta tertiary is provided what is the major reason ?
Ans: In a star star connected transformer, a delta tertiary is provided to allow sufficient earth fault current to flow for operation of protective equipment to suppress harmonic voltages and to limit voltage unbalance when the main load is unsymmetrical.
9. Is it possible to connect phasor group I and phasor group 11 transformer in parallel ?
Ans : Yes
10. While connecting Transformers in parallel , What factors must be taken unto consideration ?
Ans: Their electrical characteristic such as voltage ratio, percentage impedance, phase displacement, phase sequence and polarity.
11. If the transformer with different electrical characteristic are connected in parallel what will happen ?
Ans: They will not share the load equally.
12. Where may autotransformer may be used ?
Ans: Auto transformer are used when transformation ratio is nearly equal to unity and where there is no objection to direct electrical connection between primary and secondary.
13. Why additional central limb with an air gap is some time provided in welding transformers ?
Ans: To increase the value of leakage flux so that its leakage reactance increases.
14. What is a booster transformer ?
Ans: Booster transformer is one which is often used toward the end of a power line in order to raise the Voltage to its desired value.
15. May an ordinary transformer be used as a booster transformer ?
Ans : Yes