Summary of Power Transformers Units 16-24

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OBJECTIVES

To give the learner/learner an opportunity to evaluate the knowledge and understanding acquired in the study of the previous nine units.



1. What is a step-down transformer? ____

2. What is a step-up transformer? _____

3. List the items that should be marked on the nameplate of a standard power or distribution transformer._______

4. Draw the schematic diagram of an additive polarity transformer.

5. Draw the schematic diagram of a subtractive polarity transformer.

For questions 6 through 17, select the correct answer for each of the statements.

6. The primary and secondary windings of an operating transformer are tied together

a. electrically. c. through switching gear.

b. magnetically. d. not at all.

7. The H leads of a transformer are connected to the

a. high-voltage side. c. secondary side.

b. low-voltage side. d. primary side.

8. The primary winding of a transformer is the

a. high-voltage side. c. input winding.

b. low-voltage side. d. output winding.

9. The single-phase, three-wire system is

a. 115/230 volts. c. 230 volts.

b. 120/208 volts. d. 120/277 volts.

10. An open-delta connection

a. is the same as a closed delta.

b. is an incomplete connection.

c. requires three single-phase transformers.

d. requires two single-phase transformers.

11. The line voltage of a three-phase delta system is the same as

a. the line current.

b. a single transformer voltage.

c. a single transformer current.

d. 1.73 x phase voltage.

12. The neutral of a three-phase, four-wire system is

a. grounded. c. live.

b. ungrounded. d. bonded.

13. A V connection is the same as the

a. wye connection. c. open-delta connection.

b. delta connection. d. open-wye connection.

14. Three 100-kVA transformers are connected in delta-delta.

What is the total kVA capacity.

a. 58 percent of the three ratings

b. 58 percent of two ratings

c. 100 kVA d, 300 kVA

15. Five amperes is the standard rating of a(n)

a. instrument.

b. secondary of a current transformer.

c. secondary of a potential transformer.

d. voltmeter movement.

16. It is dangerous to open the operating secondary of a

a. closed-delta transformer circuit.

b. open-delta transformer circuit.

c. potential transformer.

d. current transformer.

17. A transformer with part of the primary serving as a secondary is a(n)

a. current transformer. c. potential transformer.

b. autotransformer. d. open-delta transformer.

18. What are three standard types of cores used in transformers?

a.

b.

c.

19. Name three common methods used to cool transformers.

a.

b.

C.

20. A transformer has 1,200 turns in its primary winding and 120 turns in its secondary winding. The primary winding is rated at 2,400 volts. What is the voltage rating of the secondary winding?

21. State a simple rule to follow in connecting a transformer bank in closed delta.

22. What is one practical application of single-phase transformers connected in a delta- delta configuration?

23. What is one practical application of an open-delta transformer bank?

24. The three-phase, four-wire secondary output of a wye-connected transformer bank

can be used for two types of load. They are:

a.

b.

25. State a simple rule that may be used in connecting single-phase transformers in wye.

26. What are two practical applications for a three-phase, delta-wye transformer bank?

a.

b.

27. What is one practical application for a three-phase, wye-delta transformer bank?

28. What is one advantage of using a three-phase transformer in place of three single- phase transformers?

29. What is one disadvantage of using a three-phase transformer in place of three single-phase transformers?

30. Insert the word or phrase to complete each of the following statements.

a. A wye-delta transformer bank has _____-connected primary windings and _______-connected secondary windings.

b. A delta-wye transformer bank has __________-connected primary windings and _______-connected secondary windings.

c. A delta-wye transformer bank is used to ____ three-phase voltages to ______values.

d. A wye-delta transformer bank is used to extremely high three-phase voltages.

e. A three-phase transformer takes space than a transformer bank of the same kVA capacity consisting of three single-phase transformers.

31. List the five common three-phase connections used to connect transformer banks consisting of either two or three single-phase transformers.

a.

b.

c.

d.

e.

32. What are the two distinct types of instrument transformers?

a.

b.

33. Why must the secondary circuit of a current transformer be closed when there is cur rent in the primary circuit?

34. The transformers shown in the following diagrams have a 4 to 1 step-down ratio. Determine the secondary voltage of each transformer. In addition, determine the value that each voltmeter will indicate.

35. Determine the unknown values. What is the polarity of the transformer? (Additive, Subtractive) ___________

36. Two 75-kVA transformers are connected in open delta. Determine the total kVA capacity of the transformers.

37. A transformer is marked 37.5 kVA. Its primary is rated at 480 volts and its secondary is rated at 120 volts. Calculate the primary and secondary current ratings.

(Primary)

(Secondary)

38. Calculate the values that will be indicated by the ammeter and the three voltmeters shown in the following diagram. The transformer ratio is 26 to 1. Insert the answers in the spaces provided on the diagram.

39. Explain the purpose of a buck or boost transformer.

40. On a four-wire delta system, what is an identified secondary feeder, and how is it identified?

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