Answer / raghavendran

Burden CAn be denoted in VA or in Ohms.Consider that to a PT
secondary we have a relay connected to it.Now the Total
impedance on the PT secondary is given by Zt = Zsec + Zlead
+ Zrelay.where the Zsec is the impedance of the PT
Secondary,Zlead be the impedance of the lead between the PT
secondary and the Relay,Zrelay be the Impedance of the relay.

Now from the VA capacity of the PT,under normal operating
condition of the PT,calculate the secondary current, Isec.

burden interms of VA can be obtained as Zt*Isec.
Burden interms of Ohms can be had from - Rated
VA/((Isec)*(Isec))

As Burden increases beyond the prescribed value, the
accuracy decreases below the allowable value.

Answer / tuktuk

Dear Mr.Raghavendran,
Thank you for your answer, I have the same problem but after reading your answer I still have some concerns.
I am really confused. In this case, we do not know the rated burden of the VT. Now we have to calculate the total burden and then selecte the rated value from IEC standards.

__Ztotal, which you mentioned above, is in Ohm or VA?
__Zsec and Zrelay we can get the value from manufacturer but Zlead, is it calculated by this formula: Zlead= (p*L)/S
(where: p is specific resistivity of copper in ohm.mm^2/m; L is length of lead between the PT secondary and the relay in meter; S is the section of lead in sqmm)? Is this true?
__I dont know how can we get secondary current Isec? (I guess Isec=Usec/Zt, isn't it?)
__And Burden in VA = Zt*Isec. As Ohm law, we have P=VI=RI^2=U^2/R, I do not understand the formula that you used to calculate Burden of VT in VA
Hope that you could help me to solve this stuck in my head.

Thank you.

Answer / tuktuk

Dear Mr.Raghavendran,
Thank you for your answer, I have the same problem but after reading your answer I still have some concerns.
I am really confused. In this case, we do not know the rated burden of the VT. Now we have to calculate the total burden and then selecte the rated value from IEC standards.

__Ztotal, which you mentioned above, is in Ohm or VA?
__Zsec and Zrelay we can get the value from manufacturer but Zlead, is it calculated by this formula: Zlead= (p*L)/S
(where: p is specific resistivity of copper in ohm.mm^2/m; L is length of lead between the PT secondary and the relay in meter; S is the section of lead in sqmm)? Is this true?
__I dont know how can we get secondary current Isec? (I guess Isec=Usec/Zt, isn't it?)
__And Burden in VA = Zt*Isec. As Ohm law, we have P=VI=RI^2=U^2/R, I do not understand the formula that you used to calculate Burden of VT in VA
Hope that you could help me to solve this stuck in my head.

Thank you.

Answer / santhoshkumar

Hello Mr.Raghavendran CT purpose to reduce the current rating of the system to measuring purpose and The Sy will be opened they will higher current carrying capacity in SY so conductor size will be high and step up high high voltage generate higher harmonics it will create serious hazardous in CT CKt

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