Three-Phase Transformers -- Summary/Quiz

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¦ Three-phase transformers are constructed by winding three separate trans formers on the same core material.

¦ Single-phase transformers can be used as a three-phase transformer bank by connecting their primary and secondary windings as either wyes or deltas.

¦ When calculating three-phase transformer values, the rules for three-phase circuits must be followed as well as the rules for transformers.

¦ Phase values of voltage and current must be used when calculating the values associated with the transformer.

¦ The total power output of a three-phase transformer bank is the sum of the rating of the three transformers.

¦ An open-delta connection can be made with the use of only two transformers.

¦ When an open-delta connection is used, the total output power is 86.6% of the sum of the power rating of the two transformers.

¦ It is common practice to center-tap one of the transformers in a delta connection to provide power for single-phase loads. When this is done, the remaining phase connection becomes a high leg.

¦ The NEC requires that a high leg be identified by an orange wire or by tagging.

¦ The center connection of a wye is often tapped to provide a neutral conductor for three-phase loads. This produces a three-phase four-wire system. Common voltages produced by this type of connection are 208/120 and 480/277.

¦ Transformers should not be connected as a wye-wye unless the incoming powerline contains a neutral conductor.

¦ T-connected transformers provide a better phase balance than open-delta connections.

¦ The T connection can be used to provide a three-phase four-wire connection with only two transformers.

¦ The Scott connection is used to change three-phase power into two-phase power.

¦ The zig-zag connection is primarily used for grounding purposes.

¦ Harmonics are generally caused by loads that pulse the powerline.

¦ Harmonic distortion on single-phase lines is often caused by computer power supplies, copy machines, fax machines, and light dimmers.

¦ Harmonic distortion on three-phase powerlines is generally caused by variable-frequency drives and electronic DC drives.

¦ Harmonics can have a positive rotation, negative rotation, or no rotation.

¦ Positive-rotating harmonics rotate in the same direction as the fundamental frequency.

¦ Negative-rotating harmonics rotate in the opposite direction of the fundamental frequency.

¦ Triplen harmonics are the odd multiples of the third harmonic.

¦ Harmonic problems can generally be determined by using a true-RMS am meter and an average-indicating ammeter, or by using a true-RMS ammeter and an ammeter that indicates the peak value.

¦ Triplen harmonics generally cause overheating of the neutral conductor on three-phase four-wire systems.


1. How many transformers are needed to make an open-delta connection?

2. Two transformers rated at 100 kVA each are connected in an open-delta connection. What is the total output power that can be supplied by this bank?

3. How does the NEC specify that the high leg of a four-wire delta connection be marked?

4. An open-delta three-phase transformer system has one transformer center-tapped to provide a neutral for single-phase voltages. If the voltage from line to center tap is 277 V, what is the high-leg voltage?

5. If a single-phase load is connected across the two line conductors and neutral of the transformer in Question 4 and one line has a current of 80 A and the other line has a current of 68A, how much current is flowing in the neutral conductor?

6. A three-phase transformer connection has a delta-connected secondary, and one of the transformers has been center-tapped to form a neutral conductor. The phase-to-neutral value of the center-tapped secondary winding is 120 V. If the high leg is connected to a single-phase load, how much voltage will be applied to that load?

7. A three-phase transformer connection has a delta-connected primary and a wye-connected secondary. The center tap of the wye is used as a neutral conductor. If the line-to-line voltage is 480 V, what is the voltage between any one phase conductor and the neutral conductor?

8. A three-phase transformer bank has the secondary connected in a wye configuration. The center tap is used as a neutral conductor. If the voltage across any phase conductor and neutral is 120 V, how much voltage would be applied to a three-phase load connected to the secondary of this transformer bank?

9. A three-phase transformer bank has the primary and secondary windings connected in a wye configuration. The secondary center tap is being used as a neutral to supply single-phase loads. Will connecting the center-tap connection of the secondary to the center-tap connection of the primary permit the secondary voltage to stay in balance when a single-phase load is added to the secondary?

10. Referring to the transformer connection in Question 9, if the center tap of the primary is connected to a neutral conductor on the incoming power, will it permit the secondary voltages to be balanced when single-phase loads are added?

11. What is the frequency of the second harmonic?

12. Which of the following are considered triplen harmonics: 3rd, 6th, 9th, 12th, 15th, and 18th?

13. Would a positive-rotating harmonic or a negative-rotating harmonic be more harmful to an induction motor? Explain your answer.

14. What instrument should be used to determine what harmonics are present in a power system?

15. A 22.5-kVA single-phase transformer is tested with a true-RMS ammeter and an ammeter that indicates the peak value. The true-RMS reading is 94 A. The peak reading is 204 A. Should this transformer be derated? If so, by how much?

ON THE JOB… You are working in an industrial plant. A three-phase transformer bank is connected wye-delta. The primary voltage is 12,470 V, and the secondary voltage is 480 V. The total capacity of the transformer bank is 450 kVA. One of the three transformers that form the three-phase bank develops a shorted primary winding and becomes unusable. A suggestion is made to reconnect the bank for operation as an open-delta. Can the two remaining transformers be connected open-delta? Explain your answer as to why they can or why they cannot be connected as an open-delta.

If they can be reconnected open-delta, what would be the output capacity of the two remaining transformers?

ON THE JOB… You are a journeyman electrician working in an industrial plant. You are to install transformers that are to be connected in open-delta. Transformer A must sup ply its share of the three-phase load. Transformer B is to be center-tapped so it can provide power to single-phase loads as well as its share of the three-phase load. The total connected three-phase load is to be 40 kVA, and the total connected single phase load is to be 60 kVA. The transformers are to have a capacity 115% greater than the rated load. What is the minimum kVA rating of each transformer?

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