On 7/8/19 8:33 PM, Hal Murray wrote:
[email protected] said:
Free space propgation delay for 5500 m is 18.5 milliseconds - compared to
16.67 millisecond period of 60Hz. A velocity factor of about 90%
Neat. Thanks.
What's going on? I'm used to calculating the velocity from the dielectric
constant. Power lines have no obvious non-air dielectric. Is the
dielectric-constant only calculation ignoring some things that are significant
in a power line? If so, what?
Dielectric affects the C per unit length, which affects the propagation
constant.
The telegrapher's equation (now the "transmission line equation") was
developed back in the days of open wire lines with essentially no
dielectric.
It gets really interesting when looking at three phase power lines,
because you have to calculate not only the C relative to ground, but the
C between the wires. And the L for the wire by itself, and the mutual L
between wires.
Fortunately, it is a linear system and superposition holds (until
there's a fault<grin>)
This is the big challenge with stabilizing large grids - you have these
long transmission lines with "springy" generators and loads. Transients
can take a long time to die out, as they bounce back and forth along a
1000km long line, with lots of impedance discontinuities. So you wind
up with synchronous condensers and switched reactive components along
the line.
The real value of DC links is that they're MUCH easier to stabilize.
Measuring the phase along such a line could be really interesting,
especially if you had a source of switching events information. The
power companies have all this, and direct connections to do the
measurements.
The intriguing thing is to do it "non-contact" and see what you can
figure out. I suspect, also, that the local power distribution company
probably doesn't care much about it - to them, it's all loads, and what
they watch is overall power factor - in a residential area, I'd bet the
PF is very close to 1 (resistive loads). In industrial areas, there are
lots of motors and magnetic ballasts for lighting, so you get a lagging
PF, but I'll bet with all the variable speed drives and electronic
ballasts, not to say LED lighting, things are changing.
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