Commissioning tests on cable systems.
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.
At KEMA we have special test facilities available for commissioning tests of power cables. This equipment is installed on a special trailer and can easily be transported. Our test rig can perform on-site AC voltage tests and PD measurement on cable systems.
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.
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.
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 that AC testing is effective.
KEMA reported their results from 1997-2002 It was concluded that:
- On average 10% of all lengths tested failed
- Testing at Uo is not effective
The test parameters given by IEC and CIGRÉ were confirmed.
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 test voltage is 260 kV and the maximum current is 80 A. The frequency range is between 18 Hz and 300 Hz. For longer lengths or higher voltages, KEMA can make a second set available to realize double the above test parameters.
More information? Contact us.