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Commissioning test on polymeric cable circuits
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Many electric network owners request tests on MV and HV
polymeric cable systems as part of the commissioning of new systems
or on existing systems after repair or modification. Since it
has been determined that DC testing of polymeric cable insulation
is not effective and may be harmful, present International and IEC
standards require DC testing of the outer sheath and AC testing of
the cable insulation.
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Outer sheath testing
According to the specification and/or the standard, DC testing
of the outer sheath is required to detect damage of the outer
sheath during installation. Damage of the outer sheath includes
damage of the sheath itself and damage of the main insulation.
Water may penetrate the cable via a damaged outer sheath. Depending
on the standard, the outer sheath must withstand 5 kV or 10 kV DC,
for 1 to 5 minutes.
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Main insulation testing of HV underground cable systems with AC voltage
The main insulation of HV polymeric cable systems may be tested
using the “series resonance” method. A reactor is placed in series
with the cable capacitance, and a variable frequency voltage source
is applied via a step-up transformer. The frequency of the AC
source is tuned to the resonance frequency of the circuit. At
resonance, the power needed from the low voltage source will be
less than 1% of the reactive power in the circuit. Voltages up to
260 kV and currents up to 80A may be achieved with this
technique.
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Experiences
DC testing of polymeric cable insulation is not effective and may be harmful as documented in the following references:
CIGRÉ reported the experiences from all over the world. It was concluded that DC testing should be avoided, and AC testing is effective.
KEMA reported their results from 1997-2002. It was concluded that:- On average 10% of all tested lengths failed
- Testing at Uo is not effective
- The test parameters given by IEC and CIGRÉ were confirmed.
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KEMA Test Set
The KEMA test set can test a cable having a capacitance of up to
2.5 µF (about 13 km). The maximum voltage is 254 kV and the maximum
current is 80 A. The frequency range is between 20 Hz and 300 Hz.
For longer lengths or higher voltages, KEMA can make a second set
available to realize double the above test parameters.
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