×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Calculating Resistance and Reactance from positive sequence fault current

Calculating Resistance and Reactance from positive sequence fault current

Calculating Resistance and Reactance from positive sequence fault current

(OP)
Dear All,

I know following two parameters

a) Maximum three phase fault current in amperes (A) and phase angle in degrees (B).

I want to calculate the positive sequence resistance (R) and reactance (X) from above two variables for a symmetric system. I couldn't figure out a way to find R and X values using just these two parameters. However I noted somebody has calculated this using following formula for same purpose (unfortunately couldn’t verify who did this in my workplace).

R = A * COS (PI()*B/180) X = A * SIN (PI()*B/180) where A is the three phase fault current and B is the phase angle

If someone can guide me to figure out the correct formula and the relation above, please

Thanks

RE: Calculating Resistance and Reactance from positive sequence fault current

A word picture;
Construct a right triangle with hypotenuse = A (Amperes)
and
Angle = B
Hypotenuse = current = 1/Z = (1/Impedance)
Base = Real Current component = 1/R = (1/Resistance)
Altitude = Reactive Current component = 1/X = (1/Reactance)
You should be able to solve this with reciprocals, sines and cosines.

Quote (OP)

R = A * COS (PI()*B/180) X = A * SIN (PI()*B/180) where A is the three phase fault current and B is the phase angle
Is this the formula when the angle is reported in radians?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Calculating Resistance and Reactance from positive sequence fault current

Type this into google:

symmetrical components filetype:xlsx

There are a few good excel calculators out there that are very simple.

RE: Calculating Resistance and Reactance from positive sequence fault current

(OP)
Hi Waross,

thanks for the explanation
is it like , 1/Z = (current / voltage)
(do i need to have voltage as well?)

RE: Calculating Resistance and Reactance from positive sequence fault current

In my opinion, if you intend to calculate the impedance in ohm-not in p.u.-then you’ll need the voltage.
A=V/(R+Xi)=V(R-Xi)/(R^2+X^2) then:
Iactiv=V*R/(R^2+X^2) Ireact=-V*X/(R^2+X^2) TAN(B)=Ireact/Iact=-X/R
A=SQRT(Iactiv^2+ Ireact^2)
A=V*SQRT(R^2+X^2)/(R^2+X^2)=V/SQRT(R^2+X^2)
X=TAN(B)*R
Then:
A=V/R/SQRT(1+TAN(B)^2) and from here:
R=V/(A*SQRT(1+TAN(B)^2)
(1+TAN(B)^2)=1+SIN^2(B)/COS^2(B)=1/COS^2(B)
R=V/A*COS(B) X=V/A*SIN(B)
B has to be radians then if B it is done in degrees then B[rad]=pi()/180*B[degrees].

RE: Calculating Resistance and Reactance from positive sequence fault current

I agree with 7anoter4 as long as you use phase-neutral voltage for V.

RE: Calculating Resistance and Reactance from positive sequence fault current

Thank you, jghrist and sorry for the delay.
You are right. Since this is a symmetric 3 phases system [direct sequence system] the rated voltage has to be phase-to-phase voltage. Then the involved voltage here has to be V/sqrt(3).

RE: Calculating Resistance and Reactance from positive sequence fault current

Hi !!
I will explain in a reverse way , by taking an example . Just ignore the material which is not relevant to you .
Suppose you have to calculate the 3 phase short circuit at point A as shown below .


As shown in blue is how we calculated the short circuit current , but in your case you already know the SC current , thus
Z = Un/(1.732*Isc).
Once you have Z , then you can calculate , R/Z=Cos(theta)
As you said you already have theta.



I hope this will help.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources