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Associate

Join Date: Sep 2008
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### Transformer Voltage Designation

07/09/2012 2:04 AM

In one of the projects, the transformer ratio actually is 33/6.6 kV. But it is specified as 33/6.93kV why?

Similarly, downstream transformer is 11 kV/415 V (actually) but it is specified as 11/0.433 kV. I analysed and found the relation to be 6.6 x 1.05 = 6.93 kV. i.e., 5% more than the actual voltage. Does this has anything to do with tap position?

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Power-User

Join Date: Apr 2009
Location: Australia, sunny North Queensland
Posts: 383
#1

### Re: Transformer Voltage designation

07/09/2012 2:43 AM

Sounds like that the entity given the task of rating the transformers has kept to a given scale...that of KV...."11 kV/415 V (actually) but it is specified as 11/0.433 kV". Nothing unusual there.

Also, the KV labelled rating that differs from the physical winding ratio
("33/6.6 kV. But it is specified as 33/6.93kV")
may have something to do with actual operating voltage within a given temperature range suited for the geography the transformer is to be used in.
Perhaps the upstream source has an output voltage that sets the label value 5% higher than it's theoretical rating based on winding ratio and lablled accordingly,
The 5% difference you mention may even just be a "buffer" safety value.
If it's a value used for creating a safety buffer/margin-of-error, unusual to have the KV rating higher....perhaps the entity labelling the transformer made a boo boo

...you may have also answered your own question, "anything to do with tap position?"

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CraziestOzzy
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#2

### Re: Transformer Voltage Designation

07/09/2012 10:29 AM

I think you need a refresher on transformers. The voltage designation given for any transformer is normally the open circuit voltage. But, when a transformer is loaded to its rated capacity, rated current would flow in the transformer windings and the current flowing in the secondary windings would cause a voltage drop within the seconadry windings due to the impedance of the windings. This impedance voltage drop is also a very important specification for a transformer and is specified in the transformer name plate. The open circuit voltage minus the internal impedance voltage drop within the transformer secondary windings is the voltage available to the loads, when the transformer is loaded to its rated capacity.

For example, for a 11kV/433V, 500kVA transformer, the percentage impedance is, say, 4%, this means that when the transformer is loaded to 500kVA, then 4% of 433 V (i.e.) about 17.32V would be dropped within the transformer secondary windings and only 415.68V would be available to the loads. Thats why all LV loads are rated for 415V.

Supposing this transformer is designed with a voltage ratio of 11kV/415, then under the above conditions, the load voltage would only be 398V and a load deisgned for operating at 415 V would not operate properly at this reduced voltage.

Same thing is true for 6.6kV vs 6.93kV too.

Hope it is clear.

Power-User

Join Date: Apr 2009
Location: Australia, sunny North Queensland
Posts: 383
#3

### Re: Transformer Voltage Designation

07/09/2012 11:01 AM

...looks like you know your stuff and might have beaten me to the job soon to be on offer the OP's way
GA matey

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CraziestOzzy
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#5

### Re: Transformer Voltage Designation

07/10/2012 9:39 AM

Thx for the GA, Buddy! You can take the job. I have no plans to move to Australia.

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#4

### Re: Transformer Voltage Designation

07/10/2012 12:00 AM

when you say 415 actually do you mean that this is what you have measured or what you think it should be?

Open circuit voltages V loaded voltages do differ and the appropriate tapping points would help to achieve the desired output for a given load. The output voltage will vary with loading anyway and it is up to the engineer to find the correct compromise.

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