Megger PFL22M1500 Series, User Manual

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M
AVTMPFL22M Rev 1 April 2011
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voltage as defined by the breakdown voltage noted during the
previous step. Increase the test voltage slowly, noting that the longer
the cable , the greater the cable capacitance, hence the Arc Reflection
breakdown voltage can be higher than the breakdown voltage. If the
fault appears to be unstable, try increasing the discharge voltage
slightly.
b. If the fault does not consistently breakdown, or is unstable, at the
maximum allowable voltage, select the Proof/Burn function on the
PFL. Raise the voltage to either the maximum allowable voltage or
until the fault breaks down in a relatively stable manner as indicated
by stable current and voltage. Continue this proof/burn function
until the discharge current is stable, after a few minutes of stable
discharge, return to Arc Reflection. Do not use proof/burn
excessively as you could create a “dead short” to earth/ground that
would be extremely difficult, if not impossible to pinpoint using
acoustic methods.
c. Another effective method of HV Prelocation is the Impulse Current
method also known as Impulse Surge, or Voltage Surge. This
method is effective for pre-locating high-resistance faults (arc
resistance greater than 200
Ω ) where the Arc Reflection method does
not work effectively. The Impulse Current method is similar to the
Arc Reflection method in that both methods send high energy pulses
down the cable which are used to break down the fault. When using
the Impulse Current method, a current coupler is switched into the
surge return circuit and is used to measure the high frequencies
transients, seen as a series of spikes each separated by the time taken
for the transients to travel time from the fault back to the PFL.
It should be noted that the first displayed pulse includes the “ionisation
delay” and should not be used for measurement. In general the
second or third pulses can be used, later pulses can distort the
measurement as they have been attenuated by the cable during the
multiple reflections.
d. Once the fault has been pre-located by using any of the above
methods, the fault can be pinpointed either by acoustic or electro-
acoustic methods. Set the PFL to Impulse Current and set the
discharge voltage to a voltage similar to that used previously. Note:
The lowest possible voltage should be used (as long as it is high
enough to ignite the fault and create a flashover) as this ensures that
the maximum energy is available, making pinpoint location easier. Set
the discharge rate as desired and use the MPP2000 pinpoint receiver
to pinpoint the exact location of the fault.