How to calculate the SLG kA contributes from Utility

[ciy]

Hi,

I run the short circuit in SKM.

And, I fill in the SCC contribute from Utility in MVA as below.

3 phase = 8000MVA

Line to Ground= 3000MVA

I got the kA value from SKM are 20.082kA and 22.654kA base on 230kV respectively.

If(3p)=MVA(3p)/(1.732*VL-L)=8000/(1.732*230)=20.082kA---OK!

But, I am not sure how to calculate the fault current of Line to Ground.

Please advise! Thanks in advance.

 

[rbulsara]

Hint: It is a single phase circuit.
Use single phase voltage, L-N and formula.

Rafiq Bulsara

http://www.srengineersct.com

 

[Kiribanda]

ciy,
Use the same equation but with 3000 MVA/ 230,000 V.Then the 1-phase to ground fault current will be 7530.8 A.

 

[rbulsara]

Kiribanda:
You may want to try that again...with L-G voltage.

Rafiq Bulsara

http://www.srengineersct.com

 

[Kiribanda]

ciy,
Could you please give us the single phase fault current which is given out from your software program when you key-in single phase fault level of 3000 MVA?

 

[ciy]

kirbanda,
Refer to the attached from SKM program.

 

[odlanor]

MVA3sc= 8000 MVA
MVA1sc= 3000 MVA
Vb=230kV
calculation symmetrical components reactance in per unit base 100MVA-230kV:
X1= X2=100/8000= 0.0125 pu
Xg=3*100/3000= 0.1000
X0= 0.1000 - 2*0.0125 = 0.0750 pu
conclusion:
X1= X2= 0.0125 pu base 100MVA-230kV
X0= 0.0750 pu base 100MVA-230kV

 

[rbulsara]

Is a single phase amp calculation from a given MVA and voltage that complex and difficult?



Rafiq Bulsara

http://www.srengineersct.com

 

[Kiribanda]

ciy,
Sorry for coming back little late.
I donot know how your program has calculated SLG=22654 A when 1-phase fault MVA fault=3000 MVA only.For me,MVA rating is just a value and has no practical sense like short circuit current rating.Therefore,whether it is 1-phase fault MVA or 3-phase fault MVA,the equation is used with line quantities and NOT with phase qauntities.Based on above,if the 1-ph fault MVA=3000 MVA @ 230 kV,then corresponding 1-ph fault current should be 7530 A only.
Also if I use the X/R=10.618 which is given in the PDF attachment,my calculation gives R0=0.0070324 pu and X0=0.084 pu,whereas your program had given R0 and X0 values with an additional zero.Please double check the default settings in your program or contact tech support to get a final reply is my advice.

 

[jghrist]

Use the same equation but with 3000 MVA/ 230,000 V.Then the 1-phase to ground fault current will be 7530.8 A.

3000/[230/sqrt(3)] = 22.59

 

[stevenal]

First of all, I dislike using MVA ratings for fault duty because I find it not very intuitive.

But in my experience, the SLG MVA rating is based on a three phase equivalent, sort of like having three SLG faults occurring in rapid succession. In either the 3LG or SLG case, multiply the current by the line to ground voltage times 3.

I believe this method was necessary when breakers were rated by MVA, simply use the higher value.

 

[odlanor]

rbulsara,
I am sorry for that calculation.
I only use SCC contribute from Utility in MVA to calculate short circuit in MY system. So, I have to convert to impedance.
I did not see SKM attached. With X/R we can change:

MVA3sc= 8000; % MVA
MVA1sc= 3000; % MVA
Vb=230; %kV
XR=10.618;% (X/R)

% calculation symmetrical components impedance in per unit base 100MVA-230kV:
Z1= 100/8000;
Z2=Z1;
Zg=3*100/3000;
Z0= Zg-Z2-Z1;

% Z1^2 = R1^2 + X1^2;
% Z1^2/R1^2= R1^2/R1^2 + X1^2/R1^2;
% R1^2 = Z1^2 / (1 + XR^2) ;
R1 = sqrt(Z1^2 / (1 + XR^2));
X1 =XR*R1;

% Z02 = R0^2 + X0^2;
% Z0^2/R0^2= R0^2/R0^2 + X0^2/R0^2;
% R0^2 = Z0^2 / (1 + XR^2) ;
R0 = sqrt(Z0^2 / (1 + XR^2));
X0 =XR*R0;

%base 100MVA-230kV
Z1= R1+ j * X1;=== 0.0012 + j0.0124 pu
Z0= R0+ j * X0;===> 0.0070 + j0.0747 pu
Z2=Z1;

 

[Kiribanda]

odlanor,
Thanks odlanor for your calculation.It shows that my calculated values are in consistence with yours.Now the question is the pdf attachment posted by ciy gives different impedance values with an additional zero for the same X/R=10.618.Please check the pdf at post#6.

 

[rbulsara]

All,
See what jghrist posted. I have been trying to hint at that all along. Not sure why that was so difficult.

Single phase to ground voltage is =V_LL/1.732.

3-phase short circuit analysis is always done on per phase basis.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

For 8000 MVA 3-phase and 3000 MVA SLG,

EasyPower calculation gives:

3-Phase: 20,082 A

SLG: 7531 A

I used X/R = 50

The convention normally used for SLG MVA fault current is:

MVASLG=SQRT(3)*VLL*ILG

Note the line-to-line voltage is used.

I generally try to get information in amps or ohms to avoid this confusion.




David Castor

http://www.cvoes.com

 

[rbulsara]

I see that ETAP does the same as what dpc indicated. Not sure why, but I can understand the source of confusion.

But to me SLG or Line to Ground Fault MVA means a product of Single Line to Ground fault current and L-G voltage. Never gave it a second thought. I am with SKM, I guess.

I also agree that dealing with amps values is the way to go to avoid confusion.

Rafiq Bulsara

http://www.srengineersct.com

 

[stevenal]

Note that sqrt(3)*Vll*I=3*Vlg*I, so dpc, Kiribanda, and I agree.

3000*MVA/(sqrt(3)*230*kV)= 7.53 kA

Counterintuitive, but see my post above for what I believe to be the history behind it. Aspen One-liner uses this convention.

 

[dpc]

I suspect stevenal is correct regarding the link to the old MVA-based breaker ratings. I had never considered that.



David Castor

http://www.cvoes.com

 

[rbulsara]

I agree and understand that breaker MVA rating has to be SQRT(3)*VLL*Isc(where Isc is the fault line current, SLG or 3 phase), whichever is higher.

Still breaker MVA ratings and "fault MVA" could be kept separate, but as long as the manual explains what they mean by their displays I am OK.

Rafiq Bulsara

http://www.srengineersct.com

 

[odlanor]

Kiribanda,
for me there is an inconsistency in the X0 / R0 ratio.
DPC post with X/R=50 is more radical!