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On Line Partial Discharge (PD) Testing on High Voltage Cable.
Abstract: ‐ High voltage insulation breakdown has been the main causes of high voltage
system breakdown.
It
is
found
that
this
phenomenon
occurs
due
to
the
existence
of
partial
discharge within voids found in the insulation of XLPE cables, which is produced accidentally
during the extrusion process of the cables. The effect of partial discharge would lead to serious
breakdown of power cable systems and electrical apparatus failures. Many appropriate
techniques can be used to evaluate the condition of XLPE cables. Many appropriate techniques
can be used to evaluate the condition of XLPE cables. One of the techniques is partial discharge
testing.
Introduction: ‐ Partial discharge detection is nowadays a very well established method for
monitoring the
insulation
condition
of
an
electrical
apparatus.
The
breakdown
of
insulation
while in service can cause considerable damage to an apparatus and to the system to which it is
connected. It has been recognized that failures of this type often may be related to the
occurrence and severity of partial discharges within voids and/or on the surface of the
insulation. Partial discharge is an electrical discharge that only partially bridges the dielectric or
insulating medium between conductors.
There are four types of partial discharge in cable: ‐
1) Internal discharges: ‐ Internal discharges are discharges in cavities or voids which lie
inside the
volume
of
the
dielectric
or
at
the
edges
of
conducting
inclusions
in
a solid
or
liquid insulating medium.
2) Surface discharges: ‐ Surface discharges are discharges from the conductor into a
gas or a liquid medium and form on the surface of the solid insulation not covered by
the conductor.
3) Corona discharges: ‐ Corona is a discharge in a gas or liquid insulation around the
conductors that are away or removed from the solid insulation.
4) Discharge in electrical trees:‐ Electrical trees discharge is a discharge in the
hollows, whereby electrical trees start from defects in the insulation and after treeing
has progressed for some time, the steam and the larger branches grow hollow.
Therefore, the partial discharge technique is an appropriate technique to evaluate the
condition of the XLPE cable.
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Partial Discharge testing methods:‐
1) Off Line PD measurement: ‐ Off Line PD measurement generally consists HV source,
Coupler and
other
devices.
This
type
of
measurement
only
possible
at
manufacturing
facility for 220 Kv cables. This type of measurement is not economical at site.
2) On Line PD measurement:‐ On Line PD measurement through HFCT is generally used
at site. The HFCT sensor should be attached in away where it can intercept either the
conductor PD current (i+) or the earth PD current (i‐) there are 3 options to do this,
depending on the cable termination.
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1x HFCT around the core of each phase after earth screen has been taken off’ to measure i+,
thec onductor PD current. Connection can only be performed after isolating the supply.
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1xHFCT around the earth screen/drain wire of each phase to measure the earth PD current I‐.
The HFCT is placed around the earth screen, to detect the earth PD current i‐. Option 2 is
recommended when permanent or semi‐permanent HFCT installations are made as the HFCT
sensor in this case is not situated within the electric field of the cable termination.
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HFCT around core and 2xearth straps, the earth strap currents cancel leaving the conductor PD
current, i+. This is the best position for the sensor attachment for on‐line PD testing as thish as
a double loop’ of the earth screen. This has the effect of cancelling out the PD signal i‐ and any
noise pulses on the earth, leaving only the conductor PD current i+.
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It is possible to modify cable terminations when they are not suitable for online PD testing. For
lead plumbed terminations on paper insulated (PILC) cables it is not possible to do this easily
(without melting down the lead!). However, in many cases with XLPE cables, when there is an
insulated gland and a shorting link has been used) there are some options for simple
modifications to be made to access the PD signals, as follows.
• Option 1: Lengthen the shorting link by using a looped, braided earth cable so that an HFCT
sensor can be placed around it.
• Option 2: Remove the shorting bracket completely so the PD currents will flow down the
cable earth strap where the HFCT could then be installed.
• Option 3: Where insulated washers are not used and there is an accessible earth strap,
insulated washers could be added to the bolts that connect the termination to the cable box.
For further advice and information on On‐line PD testing of cables contact MTEKPRO
technologies Pvt.
Ltd,
New
Delhi.
Ph:
‐+ 91
11
4617
3333,
Email: