Bus Differential High Impedance

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FADHEL Commentator Join Date: Sep 2012 Posts: 58	Bus Differential High Impedance 10/31/2012 5:55 AM
	Dear all why high impedance busbar protection need same ct ratio Regards
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PeterT Guru Join Date: May 2010 Location: Liverpool, NY Posts: 961 Good Answers: 131	#1 Re: bus diffrential high impedance 10/31/2012 8:26 AM
	In high impedance differential protection, all the CTs of the various branches of the circuit are tied in parallel into one input of the relay. The currents in the CTs produce a voltage that the relay senses. When all the currents add up under normal conditions, they total to zero (sum of all the ins equals sum of all the outs), so the voltage is also zero. Under a fault, some of the current flows in a path not through a CT, so the total is non-zero and you get a finite voltage that the relay can detect. If you have different CT ratios, they won't produce the same voltage for the same current, so there is a mismatch right from the start. There is no way to get your total to add to zero for normal bus condition. __________________ To get the right answers, first you need to ask the right questions.
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FADHEL Commentator Join Date: Sep 2012 Posts: 58	#3 In reply to #1 Re: bus diffrential high impedance 10/31/2012 5:54 PM
	Dear sir Thanks for the prompt reply but also in low impedance differential protection if CT ratio of the various branches not same the sum of ID not equal to zero.so why in low impedance the same ratio not necessary Regards
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PeterT Guru Join Date: May 2010 Location: Liverpool, NY Posts: 961 Good Answers: 131	#4 In reply to #3 Re: bus diffrential high impedance 11/01/2012 7:26 AM
	It depends on the type of relay used for a low impedance differential. You may not need to have the same CT ratio if the relay is programmable. On a low impedance diff, each CT has its own separate input to the relay, which are then summed in the relay. If the currents are summed electrically in the relay, you would need to match the CT ratios. But if the relay is a microprocessor type, with everything done digitally, then it probably can use different CT ratios and the settings of the relay let you enter the CT ratio so the input can be scaled to sum properly. You have to check what the relay that you are using expects for inputs. __________________ To get the right answers, first you need to ask the right questions.
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FADHEL Commentator Join Date: Sep 2012 Posts: 58	#5 In reply to #4 Re: bus diffrential high impedance 11/04/2012 4:48 PM
	Dear sir can use matching transformers with high impedance differential, and if the high impedance differential relay is programmable are we need to have the same CT ratio Regards
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nobodysomebody47 Power-User Join Date: Nov 2011 Posts: 171 Good Answers: 1	#2 Re: Bus Differential High Impedance 10/31/2012 9:31 AM
	You can read some of these High Impedance Protection
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