cable mapping cable mapping - HV Diagnostic Services specialises

Condition Monitoring
PARTIAL DISCHARGE
CABLE MAPPING
Required to test a new XLPE circuit and recognise that
DC is no longer relevant, then VLF Partial Discharge
Cable Mapping could be the appropriate option.
By Greg Linton, HV Diagnostic Services Ltd
I
f you are plagued by older cables with suspected insulation
damage and standard tests are indicating problems, Cable
Discharge Mapping can determine exactly where and how
widespread the damage is.
The EA Technology Cable Mapping system was originally
designed for condition assessment of aged PILC circuits and is still
the most reliable method for gaining an accurate picture of
insulation condition along the cables entire length. Following the
discovery of DC’s negative side effects, VLF testing has become the
default standard for acceptance testing of new XLPE cable.
Figure 2 – The instrument control and data capture equipment
plus connection to VLF generator out the back of vehicle.
The test voltage can be raised to 20–30% above the normal
Phase to Earth voltage (i.e. 6.35kV RMS for 11kV) with frequencies
of 0.1 – 0.5Hz typically used. This is ultimately determined by the
Cable type and length with the frequency directly affecting the
total current available from the test set to "charge" the cable.
Figure 1 – Replacing approx. 200m of cable near End 1 would
improve circuit reliability by removing a problematic section
containing many joints.
ALTERNATING AND DISCHARGE FREE WAVEFORM
The Very Low Frequency generator creates its own sine wave at the
selected frequency starting from pure DC. The result is a smooth,
alternating and discharge free waveform suitable for energising
lengths of cable up to a voltage of 33kV. Circuit length limits are
dependant upon the cable type and construction with 3km typical
for PILC circuits. Newer XLPE runs of up to 10km have been
successfully mapped in New Zealand and it is this "solid dielectric"
insulation that does not tolerate the polarisation process brought
about by traditional DC Hipot testing.
TESTING PROCEDURE
The test equipment is mounted in a cargo van and requires access
to within 25 metres of the test subject as well as a standard 10A
mains supply. Connection is normally made via the cable
termination or spouts of Switchgear, although outdoor potheads
and pole terminations in the switchyard can be used if no other
option is available. While testing requires the removal of the cable
from service and its disconnection at each end, the mapping itself
takes only 2hrs to complete with all 3 phases being "soaked" for a
minimum of 20 minutes each. Depending on access requirements
and with efficient organisation, Mapping of 3 cables per day is
generally achieved.
www.powertrans.com.au
The benefits of this particular method are:
• The three phases can be tested individually
• Testing is between phases as well as to earth
• Inception Voltage is recorded which is often below
working voltage
• Test Voltage can be raised above working voltage to emulate
tap changes
• No need to insulate cable sheath from cable box
• Testing includes terminations at each end
For enquiries on the PD Cable Mapping Service:
In New Zealand please contact:
Greg Linton of HV Diagnostic Services Ltd
In Australia please contact:
Thew & McCann, Ph: +61 7 3821 3399 • sales@thew.com.au
HV Diagnostic Services Ltd
50 Disraeli St (PO Box 33078) Christchurch, New Zealand
Phone 64 3 962 0225 Fax 64 3 366 0680
Mobile 021 663 491
Email glinton@hvds.co.nz Website www.hvds.co.nz
June/July 2005 PT&D 39
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