VON MODEL SST15-832 ARC REFLECTION SECTIONALIZING SYSTEM Page 18
cable. Because of the time it takes for a reflection on the radar to recover to the zero level,
the operator must be skilled when locating faults near discontinuities in the cable such as
splices or the cable terminals. The radar is connected to the faulted cable through a
coupler(filter) and displays the low resistance at the fault as a down blip during the time of
the arc. The coupling system performs three functions.
1.
Induce the high frequency radar signal onto the faulted cable through high voltage
isolation required to protect the radar.
2.
Provide a wave trap so the radar does not see the low impedance of the impulse fault
locator with each discharge.
3.
Lengthen the impulse with a large air coil inductor so that it provides current to the arc
at the fault for a longer time so the fault position can show up on the radar. The
inductor keeps the current flowing into the low resistance arc until the charge in the
capacitor bank is dissipated. Increasing the size of the capacitor bank in the impulse
fault locator lengthens the pulse and thus the time of the arc at the fault.
When the radar signal is induced on the cable, all discontinuities in the cable such as
splices, change in cable insulation, change in neutral construction, connected transformers,
and ends show up on the radar screen.
This system:
1.
Reduces the number of thumps required to pinpoint a cable fault.
2.
Uses standard radar so operator training is simplified.
3.
Shows cable ends and splices so that an approximate location can be determined
looking at the screen.
4.
Provides the conductor distance to the fault. Only the conductor distance is displayed.
Actual ground distances are subject to variations caused by the cable route and the
cable depth. The accuracy of any distance determined by the radar is dependent on
the correct velocity of propagation and the operator's skill.
XII
CABLE DISTANCE MEASUREMENTS
The distance provided by a radar is conductor distance not ground distance. Accuracies of
2% of cable length are possible but not often achieved. Information is provided in this section
on how to get the most accuracy using the radar. For maximum accuracy use the two
terminal method where the fault distance is determined from both ends of the cable. The
fault will be between both marks made using these distances.
All distances provided by a radar are determined using time measurements based on the
speed at which the pulses move up and down the cable. The pulse speed is based on
characteristics of the cable such as conductor size, shielding type, insulation thickness,
eccentricity, and insulation material. The speed changes as the cable insulation ages. If the
neutral shield is solid, the dielectric constant of the insulation is the determining factor in the
velocity of propagation. For maximum accuracy, the speed (or time) must be determined
from a known length of cable with identical characteristics to the cable being worked upon.
This speed is entered into each radar in several forms. The speed is normally compared to
the velocity of an ideal conductor in free air of 983 feet/microsecond.
To determine the true velocity of propagation or velocity of propagation factor of a cable the
following procedure is recommended.
Содержание SST15-832
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