3-1-14 Transformers —Connections

Question 1

The primary side of a three transformer wye-delta bank is always grounded.

Answer 1

False

Question 2

When connecting a three-transformer wye-delta bank for 120/240-volt 4-wire service, it is critical to leave the grounding straps in place on all transformers.

Answer 2

False

Question 3

The primary neutral must be tied firmly to the __?__ or else high voltages may occur on the secondary side.

Answer 3

system neutral

Question 4

If a customer’s demand for power at a new plant is 450 kilowatts, most likely three __?__transformers would be used to serve the load.

Answer 4

167 kVA

Question 5

One delta primary is 13,800 volts. Of those listed below, another delta primary is __?__.

Answer 5

2400

Question 6

If a customer requests 277/480 volts, 3-phase, 4-wire, only __?__ volt transformers can be used.

Answer 6

277

Question 7

On a two transformer bank supplying 120/240 volts, 3-phase, 4-wire, the transformer that does not supply the 120/240 volts is called the __?__.

Answer 7

power pot

Question 8

An open wye-delta bank has a kVA rating of __?__ of the two transformers.

Answer 8

86.6%

Question 9

If a 100-kilovolt-ampere transformer fails in a 300-kilovolt-ampere bank, the bank can be reconnected to an open wye-delta if the customer’s load is less than __?__.

Answer 9

173 kVA

Question 10

When connecting a 3-phase transformer bank to an open-wye-delta the primary neutral also carries the full load current.

Answer 10

True

Question 11

The phantom voltage that might be read between the three secondary phases and ground of a 240 ungrounded bank is __?__.

Answer 11

138

Question 12

If a CSP transformer is used in a three transformer bank as the lighter transformer and the CSP’s secondary breaker opens (trips), it will cause unequal voltages on the single-phase loads.

Answer 12

True

Question 13

Unlike the wye-delta connection, there is no third harmonic voltage in a __?__ connection.

Answer 13

delta-delta

Question 14

The basic rule for connecting three-transformer coils in a delta is __?__ markings from each coil are connected together.

Answer 14

unlike

Question 15

The voltage of the winding (coil) is __?__ of the voltage between conductors in a wye connection.

Answer 15

58%

Question 16

When using symbols to represent transformer banks, the first symbol is always the secondary connection.

Answer 16

False

Question 17

The 3-phase capacity of any three transformer bank is always three times the rating of the __?__ transformer in the bank.

Answer 17

smallest

Question 18

If a 3-phase bank contains a 75-kilovolt-ampere and two 50-kilovolt-ampere transformers, the capacity of the bank is __?__.

Answer 18

150 kVA

Question 19

An open-delta bank contains one 75-kilovolt-ampere and one 50-kilovolt-ampere transformer, the bank’s 3-phase capacity is __?__.

Answer 19

86.6 kVA

Question 20

Whenever you encounter a transformer, be sure to consult the __?__ for connection information.

Answer 20

nameplate

Question 21

The type of transformer with a single coil connected in series with the load is the __?__.

Answer 21

autotransformer

Question 22

The most common use of the autotransformer is __?__.

Answer 22

in a street light ballast

Question 23

On a wye-delta transformer bank connection, if the star point has been floated and there is a 3-phase motor load, the loss of one primary phase can cause high voltages on the secondary circuit.

Answer 23

True