Line resistance
Line resistance
(OP)
Gents,
I have a line impedance of 0.9+j0.1(positive phase sequence) on a 15km transmission line, can i ignore the 0.9 resistance in the calculation of fault current using per unit method? or is this still relevant given that length of transmission line?
many thanks,
Danilo
I have a line impedance of 0.9+j0.1(positive phase sequence) on a 15km transmission line, can i ignore the 0.9 resistance in the calculation of fault current using per unit method? or is this still relevant given that length of transmission line?
many thanks,
Danilo






RE: Line resistance
When the R is so small that X and Z are essentially the same it can be easier to ignore the R when doing hand calcs, but since modern tools, such as MathCAD, handle complex numbers easily there's no reason to ignore either part.
RE: Line resistance
David is right for the lines at transmission voltage level.
With the modern scientifIc calculators, it has become very easy to compute the complex numbers like
simple arithmatic operation. So now a days I stopped using per unit method.
RE: Line resistance
The "15km transmission line" probably led most of us to believe otherwise.
RE: Line resistance
RE: Line resistance
its 132kv line and the impedance is .9+j1.1 ohms..sorry for the error.. im doing a per unit method at the moment to initially compute the fault currents.
is it recommended to ignore the R component of the line?
regards,
Danilo
RE: Line resistance
RE: Line resistance
RE: Line resistance
RE: Line resistance
RE: Line resistance
Unless the X/R ratio is less than at least 3-5 times, one should not ignore the value of resistive component.
RE: Line resistance
Is it possible to have at the 132kv source an available MVA (positive phase sequence) with an angle 10deg less than zero phase sequence angle, say 1000MVA at 70deg (positive sequence) and 1200 at 80deg (zero phase sequence)? Would it have an impact on the setting of protection relays?
regards,
Danilo
RE: Line resistance
Just a side note - angles of 70 and 80 deg. for zero and positive seq. fault conditions are pretty typical. Do some reading up on Maximum Torque Angle (MTA). The best way to really understand distance protection is to learn how the old electromechanical relays were implemented. The same philosophies are still used today for our electronic relays.