3-1-15 Troubleshooting Three-Phase Banks

Question 1

A three-transformer bank serves a mixed single- and 3-phase load at 120/240 volts. Clip on amp meter readings on the secondary are: a phase=195, b phase=175, c phase=105. The total kVA bank load is __?__. (Round the final answer to one decimal place.)

Answer 1

65.7 kVA

Question 2

In a secondary connected delta, winding current multiplied by __?__ is equal to line current.

Answer 2

1.73

Question 3

The best way to calculate total bank load on a delta bank is to calculate individual transformer loads and add them together.

Answer 3

False

Question 4

After determining which transformer is bad in a 3-phase bank, there are __?__ things you should check for on the nameplate before calling for a new transformer.

Answer 4

4

Question 5

Three 37.5-kilovolt-ampere transformers connected as a 3-phase bank have a full load capacity rating of __?__ at 240 volts. (round off)

Answer 5

271 A

Question 6

The formula kVA X 1,000 divided by line to line voltage X 1.73 is used to calculate full load current __?__.

Answer 6

per phase

Question 7

If a faulted transformer is cleared and the remaining two transformers are connected as an open-wye-open delta, the bank only has __?__ capacity of the original bank.

Answer 7

58%

Question 8

Calculating load checks on an open delta bank is the same as calculating load checks on a closed delta bank.

Answer 8

False

Question 9

If a faulted transformer is cleared and the remaining two transformers are connected to open-wye-open-wye, the bank has __?__ capacity of the original bank.

Answer 9

50%

Question 10

In an open wye-open delta bank, you have an inside the delta winding current reading of 286 amperes. The bank is providing service at 480 volts. The kVA loading of each transformer is __?__.

Answer 10

137.2

Question 11

Two factors this lesson points out that might account for a low voltage at a customer’s meter are bad connectors and the __?__.

Answer 11

distance of the service drop

Question 12

When troubleshooting a transformer bank, the transformer that does not produce secondary voltage is always the bad one and needs replacing.

Answer 12

False

Question 13

If two 25-kilovolt-ampere transformers are connected into an open-wye-open delta bank, the total bank capacity would be __?__.

Answer 13

43.5 kVA

Question 14

When checking a 3-phase bank, the problem may be that someone installed primary side fuses that are too small.

Answer 14

True

Question 15

A transformer rated at 75 kilovolt-amperes can carry __?__ at 240 volts.

Answer 15

312.5 A

Question 16

Overloaded transformer banks can cause __?__ to/for the customer.

Answer 16

voltage fluctuations

Question 17

The common connection point on a 3-phase wye connected bank is also called the __?__.

Answer 17

star point

Question 18

The formula to calculate an individual transformer load in kVA is kVA= __?__.

Answer 18

current I x E /1000

Question 19

Bank capacity is __?__ of total nameplate capacity in an open wye-open wye bank.

Answer 19

75%

Question 20

Current and voltage checks are an integral part of analyzing circuit problems.

Answer 20

True

Question 21

One procedure this lesson points out to go through when trying to find a bad transformer in a 3-phase bank has __?__ steps.

Answer 21

6