3-1-15 Troubleshooting Three-Phase Banks Flashcards
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.)
65.7 kVA
In a secondary connected delta, winding current multiplied by __?__ is equal to line current.
1.73
The best way to calculate total bank load on a delta bank is to calculate individual transformer loads and add them together.
False
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.
4
Three 37.5-kilovolt-ampere transformers connected as a 3-phase bank have a full load capacity rating of __?__ at 240 volts. (round off)
271 A
The formula kVA X 1,000 divided by line to line voltage X 1.73 is used to calculate full load current __?__.
per phase
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.
58%
Calculating load checks on an open delta bank is the same as calculating load checks on a closed delta bank.
False
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.
50%
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 __?__.
137.2
Two factors this lesson points out that might account for a low voltage at a customer’s meter are bad connectors and the __?__.
distance of the service drop
When troubleshooting a transformer bank, the transformer that does not produce secondary voltage is always the bad one and needs replacing.
False
If two 25-kilovolt-ampere transformers are connected into an open-wye-open delta bank, the total bank capacity would be __?__.
43.5 kVA
When checking a 3-phase bank, the problem may be that someone installed primary side fuses that are too small.
True
A transformer rated at 75 kilovolt-amperes can carry __?__ at 240 volts.
312.5 A
Overloaded transformer banks can cause __?__ to/for the customer.
voltage fluctuations
The common connection point on a 3-phase wye connected bank is also called the __?__.
star point
The formula to calculate an individual transformer load in kVA is kVA= __?__.
current I x E /1000
Bank capacity is __?__ of total nameplate capacity in an open wye-open wye bank.
75%
Current and voltage checks are an integral part of analyzing circuit problems.
True
One procedure this lesson points out to go through when trying to find a bad transformer in a 3-phase bank has __?__ steps.
6
