# Power Transmission through Transmission Lines

Homework Helper
Gold Member
In the numericals based on transmission line, the wording is like this-" A transmission line feeds power to a load at 132kV, 0.8 pf lagging and so on." In practice, are pf and
current really constant? Doesn't it depend on the load? I mean, every minute someone is switching something on and something off. If the current waveform for a minute is plotted, how will it look? Will the pf be constant? By pf , I mean transmission line pf.

anorlunda
Staff Emeritus
In the numericals based on transmission line, the wording is like this-" A transmission line feeds power to a load at 132kV, 0.8 pf lagging and so on." In practice, are pf and
current really constant? Doesn't it depend on the load? I mean, every minute someone is switching something on and something off. If the current waveform for a minute is plotted, how will it look? Will the pf be constant? By pf , I mean transmission line pf.

Yes, of course the loads change all the time. That doesn't mean that you can't quote an average value or a specific value as an example.

cnh1995
Homework Helper
Gold Member
Yes, of course the loads change all the time. That doesn't mean that you can't quote an average value or a specific value as an example.
So, if the current waveform for a minute is plotted, it has to be of variable amplitude(provided the load is changning). But for the numericals, some specific value is taken for convenience, right?

anorlunda
Staff Emeritus
It is a simplification. AC analysis with P+jQ does assume an average over an integral number of pure cycles. But we also use it when the average is time variant. We simply ignore the distortions in the sinusoidal. Nevertheless, the approximations are highly accurate and often verified.

Most students never realize the apparent contradiction between "static" AC analysis and time varying averages. Good for you.

cnh1995
sophiecentaur