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sachin sawant
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Join Date: Nov 2011
Posts: 1

Regarding Selection of PS Class Current Transformer

11/15/2011 6:20 AM

Dear,

Im using generator differential protection relay CAG-34 of Areva make for genset application (500 KVA DG Set )To order the current transformer of PS class i need to furnish details like Knee voltage Vk , Rct & Rl value.How do i can calculate all this value.i need the formula with example for genset application.Kindly forward all formula to find the values.fault current,constant for relay etc etc.

Regards,

sachin.
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msamad
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Join Date: Feb 2009
Location: Houston, USA
Posts: 946
Good Answers: 244
#1

Re: Regarding Selection of PS Class Current Transformer

11/15/2011 8:22 AM

You may find this thread helpful:

http://cr4.globalspec.com/thread/56148/Burden-for-PS-Class-CTs

- MS

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 "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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electricalexpert65
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Join Date: Oct 2009
Posts: 2061
Good Answers: 169
#2

Re: Regarding Selection of PS Class Current Transformer

11/15/2011 8:32 AM

First of all, 500kVA Generator DOES NOT require differential protection. But, may be, your application is so critical that you don't mind the huge investment.

Well! KneePoint Voltage Vkp > K If(s) (RCT + 2 RL) ,

where, K = Constant of Cushion (generally taken as 2)

If(s) = System Fault Current as reflected on the secondary side of the CT

= Primary fault current / CT Ratio

Primary fault current = Generator Rated Full load current / Generators sub-transient

reactance

RCT = tobe given by the CT Manufacturer

RL = Two way resistance of the cable lead between the CT & the relay (this you should know as you only know the size and length of the cable you are going to provide)
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