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 Topic Title: Calculating 3 phase Transmission Line Power. Topic Summary: Created On: 04 June 2012 03:21 PM Status: Post and Reply Linear : Threading : Single : Branch
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 04 June 2012 03:21 PM 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 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 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 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 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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