and cost effectiveness with low power requirement.
at these modest voltages, while it provides a guide to the presence or
absence of serious faults, is less useful to provide information about
general degradation and ageing of insulation.
effective in detecting insulation defects
harmful to aged extruded medium voltage cables
(v) Should not
be used for extruded cables.
(i) DAC method
over others is relatively less power demand
(ii) DAC voltage is sensitive in detecting
defects that cause a breakdown due to void discharge
combination of DAC and PD detection are an alternative to continuous ac test
range between 20 Hz and 500 Hz
to local problems
(ii)PD is not
detectable in the field as a result of water trees but may be detectable if
electrical trees are formed.
(iii) High attenuation of the PD
signal along the long cable length have difficult and challenging to pick the
exact PD and its location.
(iv) Some defects do not develop a PD signal (ionize) at the test voltage
while some defects produce PD, but do not necessarily lead to failure.
above 5 PC has taken as a measure of cable deterioration
problems may be difficult to see, Bulk measurement, Sensitive to moisture
capacitance parameter, it is difficult to evaluate and investigate the
performance of XLPE cables in service because the values of capacitance
versus U test voltage are inconsistent.
(iii) Data obtained at low voltages (
to moisture ingress in PILC cables, cannot be used to locate the defects
(ii) High DC
voltages create space charge accumulation if the local electrical stresses
exceed 10 kV/mm.
voltage DC on aged XLPE cables can cause the cable to premature fail after
returning to service.
(i) improvement of the detection sensibility for
signals related to water trees
(ii) Precise to determine XLPE cables connected to
GISs if pulse width is smaller than
half period of power frequency.
(i) Before and after each test, cable should be completely discharged.
The time required for a complete discharge can be four times the test
duration or longer.
(ii)cannot be used to
locate the defects
(i) Testing multiple very low frequencies can take longer time than
(ii) Hamon approximation is only valid for linear systems, there is no
voltage dependence of the dielectric response.
(iii) Small currents are affected by environment and cannot be used to
locate the defects