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Topic Title: Calculating 3 phase Transmission Line Power.
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Created On: 04 June 2012 03:21 PM
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 04 June 2012 03:21 PM
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lamusio1

Posts: 4
Joined: 12 February 2012

If in a balanced star load the Line voltage = root 3 x Phase voltage, then why when calculating power in a Power system does the Phase power P = Vphase x current x power factor (p.f) but the total power of the 3 lines = root 3 x Vline x current x power factor (p.f).

I can understand that the power lines may have came from a delta wound transformer, but I still can't understand why the total power of the 3 lines would equal root 3 times the line voltage?

Edited: 04 June 2012 at 11:48 PM by lamusio1
 04 June 2012 05:03 PM
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assenay

Posts: 1
Joined: 12 July 2011

Hi lamusino1
I am not sure if I understand your question but let me give you ask you, did u understand the difference between Symetrical component and 3-phase system if No then try to understand the difference between them and that will clear your doubt
 04 June 2012 05:28 PM
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lamusio1

Posts: 4
Joined: 12 February 2012

Originally posted by: assenay

Hi lamusino1

I am not sure if I understand your question but let me give you ask you, did u understand the difference between Symetrical component and 3-phase system if No then try to understand the difference between them and that will clear your doubt


Hi thanks for the reply. Yes I understand symmetrical component however it still doesn't explain the problem.

Sorry perhaps my initial question isn't very clear. Here's a better example.

Q) An electricity supply system developes an earth fault at a feeder cable connected to a 6.6KV busbar. The apparent power at the fault point is 26.1MVA.
Determine the current at the fault point.

Now to do this I believe you would use: Fault current = (Fault MVA) / (root 3 x V Line). Substituting in the number this would be: 26.1(x10^6) / [(root 3) x 6.6(x10^3)] = 2283 Amps.

The part I'm getting confused at is, if 6.6KV is the Line Voltage at the feeder cables why would you multiply the line voltage by root 3 when it is not a phase voltage?
 04 June 2012 11:15 PM
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williamjohn

Posts: 178
Joined: 22 November 2010

The power in each phase is phase volts x current x power factor.
Therefore the power in a three phase system = 3 x phase volts x current x power factor
But line volts = root 3 x phase volts
Therefore
The power in a three phase system = root 3 x line volts x current x power factor

Best wishes
John
 04 June 2012 11:43 PM
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lamusio1

Posts: 4
Joined: 12 February 2012

Originally posted by: williamjohn

The power in each phase is phase volts x current x power factor.

Therefore the power in a three phase system = 3 x phase volts x current x power factor

But line volts = root 3 x phase volts

Therefore

The power in a three phase system = root 3 x line volts x current x power factor



Best wishes

John


Thank you John, much appreciated.
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