Line Charging Susceptance 2

[blacklabs]

I have to come up with a bunch of data for the System Operator. We've managed to pull most of the data together except for the "Line Charging Susceptance" of an underground cable. I'm not an Electrical Engineer so any help on how I can come up with this will be greatly appreciated. I've checked all existing documents on site to no avail.

 

[dpc]

The SO is just being a PITA - as usual.

They are(basically)looking for the charging capacitance.

If you know what complex impedance is ( Z = R + jX), then the "Admittance" is the inverse of this. The "Susceptance" is the imaginary portion of the "Admittance". They are looking for the "shunt" admittance of the line, as opposed to the series impedance you've probably already figured out. The admittance is a complex number - the real part is conductance and the imaginary part is susceptance. The shunt conductance of an overhead line is negligible, so that leaves the susceptance, which reduces to the inverse of the capacitive reactance.

For short overhead lines, this value is not of major concern. For long underground lines and very long transmission lines, it is of more interest.

Not sure how you came up with the other data, but a book like Stevenson's Elements of Power System Analysis, or similar text should help you out.

Cheers,

Dave

 

[jghrist]

Capacitance is:

C = E*10^-9/(18*ln(Di/dc)) in farad/m

Where E is the relative permeability (3 for EPR, 2.5 for XLPE)
Di is the external diameter of the insulation excluding the screen
dc is the diamter of the conductor including screen

Susceptance = 2*pi*f*C

 

[blacklabs]

In the Susceptance formula, what does the "f" represent?
Thanks

 

[jghrist]

frequency

 

[blacklabs]

You have been extremely helpful.One more question. In the first formula,is that "in" for inches?
Thanks

 

[davidbeach]

ln as in natural log aka log base e.

 

[royclh]

thanks for posting this interesting question.. I have another question in relation to this. If the transmission line is equipped with a series capacitor compensation, how the line charging capacitance will likely be affected? How the magnitude of the charging and discharge current are likely going to be? Will they increase or decrease?

Your answers are greatly appreciated!