Transmission Line Voltage drop/rise equation 1

[shattenjagger]

Hello All,

I am having trouble rectifying an equation for voltage drop....

IEEE Red book states that voltage drop on a line can be calculated as
Vdrop = IRcos(phi) + IXsin(phi)

where R, X are the line impedance and phi represents the load angle. I is a magnitude.

I am using the short TLine model where

Vsending = Vreceiving + I(R+jX)

and it seems that I(R+jX) should be approximately equal to the Vdrop as suggested by the RedBook. However, I seem to notice quite a bit of difference between the two, especially as line reactance goes up.

Mathematically it makes sense that they are not equal as
I(R+jX) = IRcos(phi) + IXsin(phi) + j(IXcos(phi) - IRsin(phi)) where the vector I = Icos(phi) +jIsin(phi)
and it seems that IEEE is just taking the real portion of the result for their Voltage drop.

My Question is, when does the IEEE version stop being applicable and you are forced to use the Short Line Model? Or should you ever use the IEEE model? Or have I missed something and am merely having a "doh!" moment?

 

[SixOn]

I also came across EPRI AC Transmission Line Reference Book

I'll definitely be getting myself a copy.