LEM A188506510, Руководство по эксплуатации

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8.4.2 Measurements on high voltage pylons
8.4.2.1 Measuring the earthing resistance without disengaging the
overhead earth wire
The measurement of the earth resistance of a single high voltage pylon usually
requires the overhead earth wire to be disengaged (lifted off) or the seperation of the
earthing system from the pylon construction. Otherwise false reading of the
resistance of the pylon earth electrode are liable to occur because of the parallel
circuit of the other pylons connected to each other by an overhead earth wire.
The new measuring method employed in this instrument - with its external current
transformer to measure the true current flowing through the earth electrode - allows
measurements of earth electrode resistances without disconnection of the earthing
system or disengaging the overhead earth wire.
As all four pylon stubs are connected to the foundation earth of the pylon, the
measuring current
Imeas is divided into five components according to the present
resistances involved.
One part flows via pylon construction to the overhead earth wire and further to the
parallelly circuited pylon earthing resistances.
The other four current components (
I1... I4
) flow via the individual pylon foots.
The addition of all currents result in a current IE going through the earthing
resistance, i.e. the resistance of the "composite"earth electrode to the soil.
If the current transformer is fixed to each pylon stub, one after the other, four
resistances have to be measured which show a behaviour inversely proportional to
the corresponding current components
I1 ... I4 . The feeding point of the measuring
current is to be left unchanged to avoid a change in the current distribution.
Accordingly, these equivalent resistances are displayed as :
R
U
I
Ei
meas.
i
=
Therefore the earthing resistance RE of the pylon is determined as a parallel circuit of
the individual equivalent resistances :
R
E
R R R R
E E E E
=
+ + +
1
1
1
1
2
1
3
1
4