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28 Cards in this Set
- Front
- Back
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What is the difference between Transformers with the following primary winding voltage ratings?
7200 7200/12470Y 12470 Grd Y /7200 |
7200 is an example of a phase to phase connection only. Where 7200 is the voltage between two primary phases.
7200/12470Y the first number is the primary coil rating, and the second number is the system voltage. The transformer can be connected between two phases or between phase and ground. As long as the primary coil voltage rating is met. 12470 GRD Y/7200 the first number is the system voltage, and the second number is the primary coil rating. These Transformers have only one primary bushing and can only be connected between a phase and ground that will provide the rated coil voltage. |
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What is the purpose of a tap changing transformer?
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The purpose of a tap changing transformer is to be able to supply a constant output voltage to the customer. When differences in primary voltage occur it is possible to change the tap position of the transformer so that it better matches the primary voltage.
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On overhead circuits, why are the three single phase units bank together, preferred over a three-phase transformer?
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It is more cost effective to replace one single face transformer should it go bad than it is to replace 1, three phase transformer. It also offers greater flexibility because they can be installed as individual units for single phase services or banked for three phase services.
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Why do transformer banks with Delta connected secondaries, serving combination single and three phase loads, have one transformer of greater size?
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The transformer of greater size will always be the one with bushing X2 Grounded. Seeing that Delta secondaries do not share the single phase load equally the larger transformer is there to provide approximately 2/3 of the single phase load.
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By industry standards Transformers rated_____kVA and smaller, with a primary coil voltage rating of_____volts and below, will be additive. All other Transformers will be subtractive.
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200KVA
8660V |
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If the impedances of the Transformers used in a bank are not the same, what happens and or what must be done?
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Mismatched impedances between parallel units or banks require de-rating because the load is not then divided in proportion to the KVA ratings. The lowest impedance transformer will supply a greater share of the load and possibly end up being overloaded.
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Delta connections are phase to____, while Wye connections are phase to_____?
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Phase
Ground |
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What can be said about voltage and current in a Delta system?
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The voltage on the coil and on the primary line are the same. The current on the primary line is 1.73 times greater than that on the coil.
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What can be said about voltage and current in a Wye system?
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The primary line voltage is 1.73 times greater than the voltage on the coil. The coil current is the same as the line current.
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Why is it important to parallel the secondary coils of a transformer for Wye secondary connections?
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If the secondary coils are not paralleled then only one of the two coils in the transformer would be used. This will cut the KVA rating in half. Essentially only utilizing one half of the transformers capabilities.
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What does the riddle
"Alley-Cat, Bad-Dog" help a line worker with? |
It aids in remembering how to parallel the coils inside a transformer. Each letter represents the end of a coil, "A" and "C" always go together and "B" and "D" always go together in a WYE transformer.
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Open banks are approximately ______% as efficient as closed banks?
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57.7%
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When would an open bank be used?
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if one transformer of a three-transformer bamk goes bad than the bank can be temporarily be rewired to an open bank. This can be useful in an emergency situation.
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True/False. 3-phase power cannot be derived from an open bank if the secondary is connected open WYE?
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True
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Which conection requires the primary WYE-point to be floated (ungrounded)?
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A WYE primary and a Delta secondary. WYE-Delta bank.
This connection can be ahazard as the floating WYE point will have a high potential on it. |
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What is a "power leg"? Does it have any use electrically?
(Wild leg) |
The Power or Wild leg is nothing more than a biproduct of a Delta secondary. the voltage of the wild leg is 1.73 times greater than that of any hot leg, when measured to ground.
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What is the difference between a "lighter transformer" and a "power transformer", both of which are found in banks with Delta secondaries?
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Delta secondaries do not allow all 3 transformers to share the single phase load equally. Therefor one transformer must provide the majority of the lighting load. It will always be the xfrmr that has the X2 bushing grounded. It will also be larger than the other 2. The other xfrmr's share somewhat equal portions of the single phase and 3 phase load and are referred to as Power xfrmr's.
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Ture/False. In underground systems, 3 phase xfrmr's are almost always used to supply 3-phase service?
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True.
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What are the 2 objectives of xfrmr fusing?
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1. To prevent damage to the xfrmr from overloads or short-circuits on the secondary side of the xfrmr.
2. To isolate a xfrmr that has an internal fault from the circuit so that the circuit protection device does not have to clear the fault. |
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When are current limiting fuses used?
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When a xfrmr is installed on a circuit that has a very high fault current capability. It reduces the ammount of current it can rush into a xfrmr during a fault.
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Why is "Angular Displacement" important?
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Angular displacement is the relationship between the rise and fall of voltage on the primary and the rise and fall of voltage on the secondary. When the angular displacement is 0 then a rise in voltage on H1 will instantly cause a rise in voltage on x1. This is important when paralleling xfrmr banks as only banks with similar angular displacement can be paralleled.
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Which banks will yield 0 and 180 degree angular displacement?
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WYE-WYE and Delta-Delta both.
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Which banks will yield a 30 degree leading and lagging displacement?
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Both the WYE-Delta and the Delta-Wye.
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What are the 2 main reasons for paralleling xfrmrs and xfrmr banks?
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To increase capacity or to provide a temporary service while work is performed on the permanent transformer of xfrmr bank.
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What are some installation considerations regarding overhead xfrmr banks?
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Climbing space, and pole stress when mounting overhead xfrmrs.
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What are some installation considerations regarding padmount xfrmrs?
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Proper leveling and compaction of the ground, or base.
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What are some installation considerations regarding submersible/subsurface xfrmrs?
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Proper placement of the vault is critical. It should be placed so that there is adequate clearance for the cables, terminations, bending radius, and xfrmr operations.
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True/False The NESC states that only ground rods can be used as an electrode at transformer locations. Pole butt wraps and butt plates are not allowed.
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True
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