[time-nuts] GPS shielding by power lines?
Arnold.Tibus at gmx.de
Sun Aug 10 16:00:57 EDT 2008
On Sun, 10 Aug 2008 13:05:47 +0100, David Ackrill wrote:
>Arnold Tibus wrote:
>> As long as the system is kept balanced a 3 phase energy transport
>> system does not need a 4th wire as the sum of all the currents in the 3 wires
>> (phase shift 120deg between the wires) is zero!
>> Arnold, DK2WT
>That is true Arnold, but in the UK (and in some other countries) we
>don't always have a completely balanced load.
>There are cases where two phase 11kV lines are run out to farms and
>other rural houses and, often the equipment connected at the farm/rural
>supplies isn't properly balanced either.
>The unbalanced current is connected to earth at the HV/LV transformer,
>as described previously. :-)
David, I agree with the a.m. unbalance, but the current should not be routed
via the ground!
As you say, there is a connection to ground on the Y circuit of the secondary
side towards the load. From this point on there are then 4 wires in use
(L1, L2, L3, N). I repeat, the return current shall not be run via ground!
(At the entry of a building the neutral or return wire will be grounded to avoid
voltage differences. At this point the PE will be connected as well for
protection purposes.) Ground currents are in general very problematic
because the resulting severe corrosion effects and noise.
As I am informed, the connections of hundreds of loads will (statistically)
appear more or less as balanced at the end. Eventual still existing
unsymmetries are to be compensated at the medium (10 to 20 kV) to
end voltage (eg. 230/400V) using distribution transformers via tricky
To avoid overvoltage due to unbalance and the floating of the
HV-lines, the star point of the Y transformer at one end may be grounded
for ground reference, on the other side of the line a overvoltage protector
will be used instead of direct grounding.
Ground currents are not possible this way.
To eliminate such unbalances on the HV-lines there are possibilities to
compensate with special symmetry compensator devices (a kind of transformers).
Sorry, this was in effect somewhat off topic, but I didn't want to leave
this point uncomplete...
Now to the main concern and question:
I believe the problem with the 'blind' GPS system may have other reasons.
1. Due to very fast changing loads and pulse like loads (switcher etc.)
there are indeed some rf on the lines.
Driving partly the hv-transformers into saturation does as well result in higher
frequencies (fourier). The long lines do act as antennas...
2. The high and very high voltage lines do create very strong fields up to
several kV/m and some µT!
To bring this fieldstrength significantly down one shall position the antenna
at least 30 to 50m away from the center line.
(35 years ago I did stop with my car directly below a 380 kV line.
My feet on ground standing in the open door I put my hand on top the roof
of the car - wow what a strong feeling touching that big capacitor plane
1.5 m above ground! )
I am sure, the strong fields below the power lines do create some distribution
current in the soil.
Perhaps specially patch antennas are very susceptable to such strong fields
which may upset the amplfier shifting levels...?
How strong are the rf fields from these lines being perhaps a factor of
several thousands below the 50 Hz field strength?
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