A new concept for test equipment for testing large HV and UHV cables on-site

Summary

The number of projects involving long and extra long HV and UHV AC cables as well as HVDC cable connections can be observed to increase (Cao et al., 2008; Kabouris et al., 2006). At the same time, operating voltage levels are increasing, accentuating the need for more powerful on-site test systems needed to prove the integrity of the insulation before energization. Low cost and ease of operation are other important features of such systems. As a matter of fact, traditional methods and test systems for routine and on-site testing of these cables come to their limit – technically and economically – as the lengths and voltage levels increase (Marelli, 2008). The logistics for the arrangement of tests as well as the testing itself are challenging (Schröder et al., 2006). Development of power electronics has now made it possible to realize high voltage DC interconnections that do not exhibit polarity reversal at reversal of power flow. As a result of this, extruded XLPE cables are now coming to the fore in high voltage DC application, although methods and test equipment for routine and on-site testing show some open issues so far. Space charges in the insulation, initiated by the test voltage during testing, are unacceptable, and this leads to the requirement that testing must be made at an alternating voltage, which can be very low frequency without creating space charge. Available test equipment for power frequency testing is not feasible for testing extruded DC cables, among other reasons, due to the lengths usually associated with DC cables. These open issues can be solved with a newly developed on-site test system that provides the reliability of established test methods and test equipment, but exhibiting lower weight, dimensions and power consumption compared to existing systems. At the same time, the on-site test system as well as the prearrangement of the tests and the testing itself are less cost intensive. Extra long cables (i.e. high capacitance) could be tested with the equipment, which by virtue of its smaller dimension is easier to handle and thus reducing the testing logistics dramatically. For example where one of the systems on the market would require two trailers of equipment for a test on a high voltage cable, only a single trailer would be needed with the new system, or a roll on/roll off test can be carried out without the use of any crane on-site. A reduction in weight by a factor of 3 is expected compared to today’s on-site test equipment, a gain which also is reflected in reduced volume.