Knee Point Voltage for PS Class CT

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anandsmm Participant Join Date: Jun 2012 Location: New Delhi Posts: 1	Knee Point Voltage for PS Class CT 06/26/2012 3:23 AM
	Dear All, Please guide me for the calculation of knee point voltage for PS Class CT (for REF Protection of Trafo) Transformer Rating:10MVA,33/11.5kV,Dyn11, CT Ratio Selected 50/5,5,5A as Phase CT and the system to be protected for Restricted Earth Fault protection) My Questions are 1) What should be CT Ratio of the NCT to be mounted in the Transformer and its knee point voltage. 2) What should be Neutral Current transformer Voltage Level (11kv or 0.415kV) 3) Should the Phase CT mounted at HT Panel and NCT should be of same Specifications? Kindly guide me. __________________ anandsmm
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	Pathfinder Tags: Knee point Voltage Restricted earth fault

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nobodysomebody47 Power-User Join Date: Nov 2011 Posts: 171 Good Answers: 1	#1 Re: Knee point voltage for PS Class CT 06/26/2012 4:59 AM
	See this, it may help you Current Transformer CT Sizing & Max Fault Level of Generator
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avnishnarula86 Member Join Date: Feb 2010 Location: Gurgaon Posts: 9	#2 In reply to #1 Re: Knee point voltage for PS Class CT 06/27/2012 1:40 AM
	@nobodysomebody47 In the link you have posted, Vs = If (RCT + 2RL) But I've come across the following formula Vs = If (RCT + RL+Rr)where Rr is the relay resistance Can you explain? If is Fault current converted to CT secoondary. What is 3200 in the calculation below? *If = (Ifp5)/3200 =39.52A**
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evangelist Participant Join Date: Jun 2012 Posts: 3	#4 In reply to #2 Re: Knee point voltage for PS Class CT 07/05/2012 4:45 AM
	[3200] is the no. of turns on secondary side of CT. i.e., 3200/5A is the CT ratio. 'Ifp' is the fault current on the primary side and 'If' is the current reflected on the secondary side which will be [5/3200] times the primary fault current. the voltage across relay Vs is the voltage across relay terminals and should not include relay internal resistance Rr. It is considered while calculating the knee point voltage of the CT.
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manindra Power-User Join Date: Jun 2010 Location: Kolkata, West Bengal, India Posts: 172 Good Answers: 1	#3 Re: Knee Point Voltage for PS Class CT 06/29/2012 8:48 AM
	Dear anandsmm, The (3) point as mentioned by you is the correct answer. Regards, Manindra __________________ Manindra
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PRS Participant Join Date: Nov 2012 Posts: 4	#5 In reply to #3 Re: Knee Point Voltage for PS Class CT 11/15/2012 12:08 PM
	Hi mr. mahindra, What if the Knee point voltage of both end cts are different.??
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vyral2311 Associate Join Date: Mar 2013 Posts: 31	#6 In reply to #5 Re: Knee Point Voltage for PS Class CT 07/15/2014 2:57 AM
	In case of through faults the secondary currents of both the end CTs will not match due to saturations and hence differential relay will trip which ultimately is unstable operation.
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