b2 electronic PD Series, Руководство пользователя

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General Description
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3 General Description
3.1 PD Basics
A long high voltage cable behaves as a wave guide. The cable has a conductor, a dielectric and a coaxial neutral (Screen
copper tape or lead) which forms an ideal wave guide. The dielectric creates a large capacitor. The longer the cable, the more
of these capacitors are in parallel. These PD waves travel down the wave guide – one PD wave to the one end of the cable
and the other PD pulse to the opposite end. If we now place a capturing device – a coupling capacitor and a Digital Storage
Oscilloscope at the one end, it is possible to view the “time of flight” of these PD pulses.
The above echogram is displayed by the Transientrecorder (b2 Suite). The 1st PD pulse (emanating from the PD source) ar-
rives first at the coupling unit (see pulse marked 1st whilst the 2nd pulse travels to the far end, travels the full length of the cable
and arrives at the coupling unit (see pulse marked 2nd). The 1st pulse reflects out of the coupling unit and travels to the far
end and reflects back hence 3rd pulse. Δt2 is therefore the time differences between these two incoming pulses and the time
to the PD source from the far end. If we now know the velocity of propagation of the PD pulse in the cable we can calculate the
distance to the PD source ( Dist = Velocity x time )
Illustrations from Dr. Daniel Pepper
Unfortunately on very long cables the subsequent reflections may be attenuated to such an extent that they are not visible.
Joint / splices also attenuate these pulses.
PILC cables have a greater attenuation on these traveling PD waves than that of a similar XLPE cable. If the returning pulse (2nd) is not
visible it is not possible to do a location. The 1st pulse only indicates that there is a discharge on the cable.
By examining the rise times of the calibration pulse and this PD pulse it is possible to determine if it is from the near end termination or not.