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Anonymous Poster #1

Tap off and delta formation in isolated phase bus duct

01/17/2022 5:41 AM

Dear sirs,

Is tap-off from isolated phase bus duct means, tap off to unit auxiliary transformer? Where is delta formation done?

Regards,

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Guru

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#1

Re: Tap off and delta formation in isolated phase bus duct.

01/17/2022 6:50 AM
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Guru

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#2

Re: Tap off and delta formation in isolated phase bus duct.

01/17/2022 11:33 PM

Delta formation is done with windings. The isolated phase bus duct might be used to connect the windings in a delta configuration, and perhaps by using full current rated ‘tap offs’.

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Power-User

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#3

Re: Tap off and delta formation in isolated phase bus duct.

01/18/2022 2:19 AM

Delta formation with IPBD is relevant when the Generator step-up transformer comprises three single phase units (instead of one three phase unit). This happens in case of large generators.

The Delta formation of three single phase windings of the three single phase transformers is done externally using the IPBD.

For example, a 500MW unit in India has three single phase transformers of 200MVA each for Generator step-up transformer.

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Anonymous Poster #1
#4
In reply to #3

Re: Tap off and delta formation in isolated phase bus duct.

01/18/2022 6:20 AM

i mean, for IPBD of 2500 amps (mainbus), 100 amps (tap-off) and 1600 amps (of delta bus).

does this mean:main bus duct current of 2500 amps, current through unit auxiliary transformer of 100 amps and in delta, line current = phase current. how 1600 amps is obtained?

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#5
In reply to #4

Re: Tap off and delta formation in isolated phase bus duct.

01/18/2022 10:05 AM

It would be best to ask the Designer, whose details are on the drawing and other documents, or the Commissioning Engineer, whose name is on the commissioning records, rather than a bunch of anonymous contributors to a global internet engineering forum. After all, the latter doesn't even know which country this installation is in, for example, and which national standard is applicable; not a clue. Why one would withhold this information is a mystery too.

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#6
In reply to #4

Re: Tap off and delta formation in isolated phase bus duct.

01/18/2022 10:32 AM

The line current in delta windings is greater than the phase current by the factor of square root of 3. So if you have phase current of 1600 amperes, your bus duct that makes up the delta connections must be rated to carry almost 2800 amperes.

It is likely that the 1600A rating is a nominal rating, although it appears to be optimistic unless your windings are rated for about 1450 amperes, or some 34.5MVA at 13.8kV.

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