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Commissioning 145-420 kV cable systems
When it comes to commissioning of cable systems at higher voltages, IEC 62067 and IEC 60840 are often used as a basis. According to these, one can either perform an AC Hipot test (20-300 Hz) or a so-called SOAK test. Both tests can be combined with PD monitoring.
AC hipot test
Special equipment is required to perform a AC Hipot testing. This is a significantly better test since you have the opportunity to test the system with overvoltage. The challenge with AC Hipot testing is that this is a very expensive and demanding process. For the same reason, this test is often abandoned.
Picture; AC Resonant test system from HIGHVOLT Prüftechnik Dresden GmbH
SOAK test
SOAK testing is the most common test on cable systems on higher voltages in Norway. With this, the system is energized for 24 hours without load. The test is passed if the system does not fail during these 24 hours. Weaknesses that need time to develop will thus not be detected by a SOAK test. To get a breakdown within the first 24 hours, the faulth must be of a critical nature in the first place.
SOAK test + PD monitoring of joints and terminations
An alternative is to combine the SOAK test with PD monitoring. This is done by installing PD sensors at joints and end terminations. The sensors can either be mounted in link boxes where applicable, or directly on the cable at the joint. The PD sensors can be left and used for testing at a later occasion.
During the actual SOAK test, partial discharges is monitored. Whether a joint or an termination has PD activity at this voltage, it will only be a matter of time before an breakdown occurs. By combining the SOAK test with PD measurements, we know that the system is PD free at operating voltage. The disadvantage is that one does not detect weaknesses that only ignite at overvoltage.
Nevertheless, this is a good alternative where one can not perform AC Hipot testing. The cost will be a fraction of what an AC test will be while making sure that the joints, terminations and cable are PD free before taking over.
Picture; PD monitoring system from Prysmian Electronics