Megger SG32-1500M, User Manual

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SG32-UG-EN-V01 Dec 2016
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Description of Voltage Decay (not integrated in the SG32)
Voltage Decay is only used in case of high resistance and high flash-over type
faults, if the voltage of the surge capacitor is not sufficient any more to reach the
flash-over voltage. Because the Decay mode uses the cable as the HV capacitor,
the charging voltage (provided by a HV DC source) should be in excess of 40kV
in order to have a reasonable amount of energy stored in the cable (E=1/2 C x
U2 ). The available surge energy for a cable with 1 µF cable capacitance and a
charging voltage to 40kV results in an energy of 800J; this shows that the Decay
method is very ineffective at voltages below 40kV. The MTDR100 is operating
in both the ICE and the Decay method as a transient recorder of the Return
current / voltage, which is de-coupled from the HV Return either via current or
a capacitive coupler.
HV DC is applied and the voltage increased until the fault breaks down, and a
flashover occurs. During this flashover (the point of fault) the resultant
transients are reflected back and forward between the fault and the dc source.
These transients are detected by the voltage divider and fed to the MTDR where
they are subsequently displayed.
Typical Trace
Distance / 2 = Fault Distance
Surge
Generator
with DC
source
>40kV