Commissoning 145-420 kV cable systems with Damped AC testing

When it comes to final inspection of cable systems at higher voltages, IEC 62067 and IEC 60840 are often used as a basis. According to these, either an AC Hipot test (20-300 Hz) or a so-called SOAK test can be carried out. Both tests can be combined with PD monitoring.

AC hipot test

To perform AC Hipot testing, special equipment is required. This is a significantly better test since you have the opportunity to test the system with overvoltage. The challenge with AC Hipot testing, however, is that this is a very expensive and demanding process. For the same reason, this test is often given low priority.

Pictures; AC Resonant test system from HIGHVOLT Prüftechnik Dresden GmbH

Alternative to AC hipot test: Damped AC (DAC) test with PD diagnosis

DAC (Damped AC) is a very good alternative to traditional AC resonance testing because the method provides the same type of AC voltage stress as the operating voltage, but with significantly smaller and more portable test equipment. The result is an effective voltage test up to 1.7*U0 with very sensitive PD measurements.

Advantages of DAC compared to AC resonance test:

• Slightly destructive

• Uncovers and locates weaknesses without the need for breakdown

• Significantly lower price, typically 40-60% lower cost

• Significantly lower weight and less equipment required.

• Easier logistics and faster rigging in the field.

• Suitable for long cable systems with high capacitance.

• Provides both voltage testing and PD diagnostics in one test.

• Lower power requirements and less strain on available power supply.

• Documented method according to international standards for cable diagnostics.

For many cable installations, DAC therefore provides an optimal combination of reliable insulation verification, advanced condition diagnostics and efficient field implementation , while reducing mobilization and resource use compared to traditional AC resonance testing.

SOAK test

The SOAK test is the most common test to be carried out 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 therefore not be revealed by a SOAK test.

For an error to occur within the first 24 hours, the error must be of a critical nature to begin with.

SOAK test + PD monitoring of joints and end terminations

An alternative is to combine the SOAK test with PD monitoring. This is done by installing PD sensors at joints and terminations. The sensors can either be mounted in Link boxes where applicable, or directly on the cable at joints. The PD sensors can be left behind so that they can be used for testing at a later time.

During the SOAK test itself, the system is monitored for partial discharges. If a joint or termination has PD activity at this voltage, it will only be a matter of time before a failure occurs. By combining the SOAK test with PD measurements, one knows that the system is PD free at operating voltage. The disadvantage is of course that one does not uncover weaknesses that are ignited by an overvoltage.

However, this is a good alternative where AC Hipot testing is not possible. The costs will be a fraction of what an AC test would be while ensuring that joints, terminations and cable are PD free before takeover.

Pictures; PD monitoring system from Prysmian Electronics

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