Capacitor Bank

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bipc54 Member Join Date: Aug 2008 Posts: 7	Capacitor Bank 09/14/2009 7:17 AM
	we have consumers(induction motors-charger...) in oil & gas area with following spec. p=430kw I=768A V=400v we added 150 kvar capacitor bank for improve the PF but sometimes it reached to PF=0.77 Is it insufficient?how do we calculate the necessary kvar?
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wareagle Guru Join Date: Jun 2008 Location: Alabama Posts: 1604 Good Answers: 63	#1 Re: Capacitor Bank 09/14/2009 10:39 AM
	Due the figures you are showing include the capacitor? If not my calculations show that you should have a pf of about 0.91.
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msamad Guru Join Date: Feb 2009 Location: Houston, USA Posts: 946 Good Answers: 244	#2 In reply to #1 Re: Capacitor Bank 09/14/2009 11:09 AM
	The data you provided is P=430 KW I=768 A V=400 V From the data, P.F = (430 x 1000) / (1.732 x 400 x 768) = 0.81 KVA = 430 / 0.81 = 531 KVA Reactive power drawn by the motor = sqrt ( 531² – 430²) = 311 KVAR (inductive) To compensate this inductive KVAR (to make the P.F close to unity), you need almost 300 KVAR capacitor bank. With your 150 KVAR capacitor bank, the effective reactive power is 311 – 150 = 161 KVAR Effective KVA = sqrt (161² + 430²) = 459 KVA P.F = KW / KVA = 430 / 459 = 0.94. You mentioned it is 0.77 and this value is strange to me. With 150 KVAR, the P.F should be close to 0.94. -MS __________________ "All my technical advices in this forum must be consulted with and approved by a local registered professional engineer before implementation" - Mohammed Samad (Linkedin Profile: http://www.linkedin.com/in/msamad)
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wareagle Guru Join Date: Jun 2008 Location: Alabama Posts: 1604 Good Answers: 63	#3 Re: Capacitor Bank 09/14/2009 12:19 PM
	Masamad Thats the same figure I calculated. Thats why I asked if the data he posted was with the capacitors. The size he listed seems to be the correct size.
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msamad Guru Join Date: Feb 2009 Location: Houston, USA Posts: 946 Good Answers: 244	#4 In reply to #3 Re: Capacitor Bank 09/14/2009 1:54 PM
	Wareagle, You and me are in same page answering this question. I provided the detail calculation to make the OP feel better to accept the answer (as he asked for the calculation). The P.F 0.94 is pretty good improvement, and improving more than this is not economical (as we can see, improve from 0.81 to 0.94 requires 150 KVAR capacitor bank, while to improve from 0.94 to unity requires another 150 KVAR). So, 150 KVAR bank is correct size in this consideration (if the target P.F 0.94 is ok). However, if some one is interested to improve it up to unity, they need to use 300 KVAR capacitor bank in total. I am also curious about the PF 0.77 is with or without the capacitor bank. - MS __________________ "All my technical advices in this forum must be consulted with and approved by a local registered professional engineer before implementation" - Mohammed Samad (Linkedin Profile: http://www.linkedin.com/in/msamad)
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marcot Guru Join Date: Feb 2008 Location: Montreal, Canada. Posts: 1372 Good Answers: 80	#6 In reply to #4 Re: Capacitor Bank 09/15/2009 3:17 PM
	If they are using a fix capacitor bank and the load is reduced, the PF might be 0.77 leading... This is a common problem when people just slap a capacitor bank on a feeder without having a controller to take it off as the load decreases. __________________ Experienced is earned, common sense is taught, both are rare essentials of life.
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peteciliberto Member Join Date: Apr 2009 Location: Philadelphia, Pennsylvania, USA Posts: 9 Good Answers: 1	#5 Re: Capacitor Bank 09/15/2009 7:54 AM
	"but sometimes it reached to PF=0.77" ? Is this a static load or is the load varying (possibly a motor control center)? What is the range of PF's that you are logging? What is the range of power consumption in kW? As noted by the other gentlemen, based on the a PF of .77, assuming the same VA, would result in p=409kW. That's almost a 5% difference in kW. What were your initial condtitions ___V ___A ___PF & what were your conditions after adding power factor correction ___V ___A ___PF ? __________________ "How many legs does a dog have if you call the tail a leg? Four. Calling a tail a leg doesn't make it a leg ." - Abraham Lincoln
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marcot (1); msamad (2); peteciliberto (1); wareagle (2)

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