Line Differential Protection Susceptance Calculations - Please help!!

[Ashdgee]

Good day protection engineers,

I am struggling to see how in the SEL 411L protection manual example they managed to convert the line primary susceptance values to secondary values. I understand the primary values are derived from line characteristics like impedance, capacitance, geometry etc and short circuit program can determine it.

We have the following :

Line length : 70 miles
CT ratio (at this end) : 1600:5A
VT Ratio: 500kV:111.11V
Line voltage : 500kV
Positive sequence susceptance : 0.009 mS/mile (primary).

So the total susceptance will be : 0.009 mS x 70 = 0.000 63 S.

When reflected to secondary, they obtained 8.86 mS. Can someone explain how they got this value please. I tried multiplying the primary value by the CT Ratio/VT ratio but seems wrong. Is there anything I am missing?

 

[cranky108]

You seem to be using the right formula, if you are using Ohms, but you are using Smho, so what should the formula be?

 

[Parchie]

@Ashsdgee,
I got 12.75 milliSiemens. IDK if that's worthwhile.
The formula when referring to the secondary needs the voltage and the VA bases.
Y[sub]new[/sub]=Y[sub]old[/sub] X (kV[sub]old[/sub]/kV[sub]new[/sub])^2 X (kVA[sub]new[/sub]/kVA[sub]old[/sub])
Perhaps the conductance portion was left out causing the error. Susceptance is just the imaginary part of the admittance

 

[wcaseyharman]

I get their answer by multiplying .00063*PTR/CTR. That seems right to me - the equation for secondary susceptance should be the inverse of the normal secondary impedance calculation.

Casey

 

[Parchie]

My bad. I missed the title of the topic. Yes, the formula for calculating the susceptance from primary siemens to secondary is just multiplying the value to the PTR divided by the CTR. It is because the admittance of lines is assumed to be purely susceptance, so no conductance!

 

[Ashdgee]

Many thanks for your replies . Yes @Parchie I had swapped the CTR/VTR around as mentioned.

 

[Parchie]

@Ashdgee,
I understand SEL 411L needs to have the susceptance values so it can compensate for the charging current in the differential protection logic. IIRC, if one uses the positive susceptance alone, the accuracy of the protection is going to be a bit inaccurate. SEL has a paper on that. I have searched the web and found it HERE.