Re: NEC-LIST: RF Voltage Levels

Following on from Ben's comments, I always use a small capacitance as close
as possible in value to the true capacitance of the insulator. This value
can sometimes be obtained from the manufacturer or by measurement. I have
built up a database of values for different types. A large Decca style oil
filled insulator typically comes out at around 30pF. If there are static
leak resistors, I add an appropriate resistance in parallel.

Also, don't forget that all voltages and currents in NEC are peak values,
not RMS.

Paul Carlier

 -----Original Message-----
From: nec-list-bounces+pcarlier=fanfield.co.uk_at_robomod.net
[mailto:nec-list-bounces+pcarlier=fanfield.co.uk_at_robomod.net] On Behalf Of
Ben Dawson
Sent: 29 December 2007 16:41
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: RF Voltage Levels

Roy Lewallen wrote:
> NEC can tell you the voltage stress on insulators. Include the guy
> wires in the model, representing the insulators with high impedance
> loads. Then find the voltage across each insulator by multiplying the
> magnitude of the load impedance times the magnitude of the current at
> the loaded segment. You'll have to increase the load impedances until
> the voltage doesn't change appreciably with changes in impedance.
>
> For corona, I think you'll have to find the magnitude of the E field
> near the points of interest using NE, as Paul Lynas suggested.
>
> Roy Lewallen
>
NecList:

Mr. Lewallen is entirely correct. We have used these methods for
decades to predict both insulator voltages and surface voltage gradients
for corona prevention on dozens of complex MF, LF and VLF antennas at
power levels up to 2 megawatts. A guy or station post insulator can be
modeled either as a high R (100 k is usually good enough) or as a small
capacitance.

Benj. F. Dawson III, P.E.
Hatfield & Dawson Consulting Engineers LLC
9500 Greenwood Avenue North
Seattle, WA 98103 USA

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