3-1-12 Transformers—Three-Phase Connections

Question 1

If a customer’s demand is for 450-kilowatt, 480-volt, 3-wire, each transformer in the bank must supply __?__ of the load.

Answer 1

150 kW

Question 2

When paralleling single-phase transformers, if the impedance is more than __?__ from one transformer to another, dangerous overloading of one of the transformers will occur.

Answer 2

7.5%

Question 3

There is no flexibility in selecting transformers to perform a particular job.

Answer 3

False

Question 4

If a customer requests a 240-volt, 3-phase, 3-wire service, transformers rated __?__ or 120/240 volts can be used.

Answer 4

240/480 V

Question 5

In an open bank with a delta connected secondary, the transformer with the ground at X2 is called the __?__ transformer.

Answer 5

lighter

Question 6

A three-transformer wye-delta bank can be connected as an open-wye-open-delta, but the kVA rating of the bank will be reduced to __?__ of the original bank.

Answer 6

57.7%

Question 7

A 240-volt, 3-phase, 3-wire service cannot be supplied by using two 120/240-volt transformers connected as an open-wye-open-delta bank.

Answer 7

False

Question 8

If a three transformer wye-delta bank is connected to open wye-open delta, and the primary bus neutral is left floating, __?__ of the phase to ground voltage will be present on the wire.

Answer 8

50%

Question 9

If a transformer bank is connected 480 or 240 3-phase, 3-wire delta, a __?__ voltage can be measured between the phases and ground under certain circumstances.

Answer 9

phantom

Question 10

In order to provide 3-phase, 3-wire service from three transformers, all neutral straps must be removed from the transformers.

Answer 10

True

Question 11

In a 120/240, 3-phase, 4-wire service, C phase is normally called the __?__.

Answer 11

wild leg

Question 12

The formula for calculating the kVA that is being drawn off a single transformer is kVA=__?__.

Answer 12

VI /1000

Question 13

On a three-transformer bank connected 120/240 4-wire, 3-phase, the lighter transformer carries __?__ of the single phase load.

Answer 13

2/3

Question 14

According to this lesson, a CSP transformer should not be used as the lighter transformer in a three transformer bank supplying a 120/240 4-wire, 3-phase service because if the CSPs secondary breaker opens, the neutral will become isolated.

Answer 14

True

Question 15

If two transformer fuses open on a 7,200/12,470, three transformer bank that has the primary bus floated, the voltage on the bus will be __?__.

Answer 15

7200 V

Question 16

When using three 120/240 volt transformers to supply a 120/208 3-phase, 4-wire service, the internal secondary leads must be reconfigured.

Answer 16

True

Question 17

The delta-delta connection is also called the star-star connection.

Answer 17

False

Question 18

One disadvantage of the __?__ connection is that third harmonic currents are allowed to flow.

Answer 18

wye-delta

Question 19

With the wye grounded on a three transformer, wye-delta bank, the bank will act as a(n) __?__ bank if one primary phase is lost.

Answer 19

open wye - open delta

Question 20

Transformer banks with an ungrounded primary are prone to ferroresonance.

Answer 20

True

Question 21

Single transformers used in a 277/480 bank have only one secondary coil that is rated at 480 volts.

Answer 21

False