I'll just ignore the problems that will occur, so easy is the game. There will
be partial discharges at 10kV rms and not only at the end. Who tells that they
are just to find at the end? Sure, there the field is maximum inhomogenious but
the difference to the center of the wire (50m hight) is not to much.
In the power grid, say in the 20kV (up to 123kV) field you do not really have
problems with PD at the normal wires but there we are talking about a wire
radius of abt 2cm. My antenna has a wire of 1mm diameter! That is a complete
different case since E falls and rises with 1/r.
But anyway, slight PDs are no problem. And the power grids does also produce
PDs all the time. Just drive with your car under a 400kV line and listen to the
AM MF band and you know what i mean ;-)
Just let's see what we get in the next test...
73, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Alexander S. Yurkov
Gesendet: Mi 10.03.2010 01:12
An: [email protected]
Betreff: Re: LF: Possible VLF HV problems, or not?
On Tue, 9 Mar 2010, [ISO-8859-1] Stefan Sch?fer wrote:
>
> I have had some thoughts about HV problems when operating a vertical
> antenna wire at some 10kV.
Dear Stefan,
there was no such a problem (seems to me) in Markus experiment with 10kV.
May by with 20kV... But anyway corona protected rings (rather small i
think) to be usefull on the ends of wire. Remember, high E is only arround
ends of wire, not at the center of wire. Besides such a problems arises in
50 Hz systems at some about 100kV, not at 10 kV. 10 kV is rather small
voltage:-)
>
> Normally, e.g. in a 50Hz system there will flow a current into the air
> (capacitive with a restistive component) due to the ionisation of the
> air when partial discharges occur.
> This is due to exceeding the maximum E field strange on the wire (a
> function of its radius and geometry and insulation and the gap between
> wire and insulation and the epsilon r of the insulation and and and ...
> ;-) ).
> In the special case of a VLF antenna there might be some special
> conditions that can limit the influence of the partial discharges. For
> example, the Z (sqrt (L/C) of such an antenna is very high, so the
> resonance is quickly lost if C is changing slightly. The presence of
> partial discharges will apparently increase the C since the surface is
> virtually increasing. There will be additional resistive losses but
> since a change the C will bring the circuit out of resonance (what would
> reduce the voltage at the wire), there might be a limitation of the
> momentary value of the voltage. This will be anything else than a stable
> process but there might be the effect, that the voltage will be limited
> like with a varistor or a spark gap in series with a resistor. So,
> theoretically, there will be many harmonics and my signal might be
> better visible at its harmonics than on the ground wave ;-) So, have
> some of you, perhaps on LF, have experiences with theese effects??
>
> 73, Stefan/DK7FC
>
Regards,
Alexander
<<winmail.dat>>
|
