5/19/2014 CR4 - Thread: Knee Point Voltage and Current Transformers

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B.JEYACHANDRANActive Contributor

Join Date: Apr 2009Location: MaduraiPosts: 22

Knee Point Voltage and Current Transformers09/23/2009 10:22 AM

dear friend,

kindly any one explaine me what is the effect of knee point voltage in CT.

If one PX Class CT having Vk>300V, Ia<30 mA how i calculate this CT BURDEN also what is the primary current whenth ct is satsurated,

thanking you,

yours ,

B.JEYACHANDRAN

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ManimaranVasanMember

Join Date: Sep 2009Location: ABB Limited,NashikPosts: 9

Re: Knee Point Voltage and Current Transformers09/26/2009 2:26 AM

Dear friend,

Knee Point voltage is nothing but a point at which the CT saturates. Thatsy its always better to design the CTs whichcan operate without saturation even at a voltage appears in the CT during fault condition. Pls find below the followingformula for calculating the Knee point voltage.

Vk=2If(Rct+2Rl).

where, If is the current refected to the CT secondary during fault condition,

Rct is the resistance of the CT which can be adopted by means of a thumb rule considering the CT ratio,

Rl is the lead resistance i.e. sum of the resistance of the wire connected between the relay & CT and Impedanceoffered by the internal circuitary of the relay

For PS class CT's designed considering the above,burden is not applicable.

Pls revert back if you have any further queries.

Regards,

Manimaran.V

B.JEYACHANDRANActive Contributor

Join Date: Apr 2009Location: MaduraiPosts: 22

Re: Knee Point Voltage and Current Transformers09/26/2009 2:42 AM

dear friend,

thank you for your guidence, how i calculate the fault current limit in primary side with knee point voltage

ManimaranVasanMember

Join Date: Sep 2009Location: ABB Limited,NashikPosts: 9

Re: Knee Point Voltage and Current Transformers09/26/2009 7:44 AM

Dear Friend,

Pls find below the formula for calculating the fault current,

Fault current(in KA) = Rated Curent/(% Impedance*1000)

% Impedence shall be calculated for the system from the point of source to the point of occurence of the fault.

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5/19/2014 CR4 - Thread: Knee Point Voltage and Current Transformers

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Regards,

Manimaran.V

shock007Active Contributor

Join Date: Jun 2008Posts: 10

Re: Knee Point Voltage and Current Transformers09/28/2009 4:12 AM

Dear Manimaran,

Can you please explain, how to calculate Ia and what is its significance?

AnonymousPoster

Re: Knee Point Voltage and Current Transformers09/28/2009 11:19 AM

dear friend,

thankyou friend

AnonymousPoster #1

Re: Knee Point Voltage and Current Transformers05/29/2012 1:48 AM

ayubksebMember

Join Date: Apr 2008Location: Doha QatarPosts: 8

Re: Knee Point Voltage and Current Transformers09/30/2009 12:38 PM

Dear member

For PX class CTs , the burden is not normally mentioned . There the CT is defiened based on the CT kneepoint voltage ,magnetising current etc . The burden is mentioned in class P CTs

In PX class CTs the kneepoint voltage is normally based on the max fault current and a diamensioning factor based onsystem X/R ratio

Ayub

shock007Active Contributor

Join Date: Jun 2008Posts: 10

Re: Knee Point Voltage and Current Transformers09/30/2009 9:06 PM

Understand from above formula to calculate knee point voltage. Can you please explain how to calculate magnetisingcurrent.

ayubksebMember

Join Date: Apr 2008Location: Doha QatarPosts: 8

Re: Knee Point Voltage and Current Transformers10/01/2009 12:24 AM

There is no formulae as such for calculating the magnetising current . However some a application demands minimumvalue of mag current for relay setting ( eg; REF protection ). Iin any case a magnetising current of less than 30 mAwould be sufficient for most of the application

ManimaranVasanMember

Join Date: Sep 2009Location: ABB Limited,NashikPosts: 9

Re: Knee Point Voltage and Current Transformers10/01/2009 3:31 AM

ya.. thats right.. there is no formula for calculating magnetising current. however, lesser the magnetising current,better is your CT performance.

Regards,

Manimaran.V

GovindarulParticipant

Join Date: May 2012Posts: 2

Re: Knee Point Voltage and Current Transformers05/07/2012 7:57 AM

The formula for calulating the required magnetising current for the CT =

[(IOP / CT Ratio) - IR] / n

Where,

IOP = Relay operating current (absolute current value in terms of CT primary)

IR = Relay setting current (in terms of CT secondary)

n = Number of parallel CTs forming (87) protected zone

Anonymous Re: Knee Point Voltage and Current Transformers06/20/2010 6:35 AM

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Users who posted comments:

Anonymous Poster (3); ayubkseb (2); B.JEYACHANDRAN (1); Govindarul (2); Manimaran Vasan (3); shock007 (2)

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