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

Abstract

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 (Schrö der 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.Keywords: cable testing; on-site testing; VLF; very low frequency; variable frequency resonant test system; testing of sea cables Today's situationTesting long and extra long HV and UHV cables has become an important issue in the last 10 years. Especially testing extra long cables, such as sea cables, brings existing test equipment to its technical and economical limit. Innovative solutions to overcome the existing limit are required.Availability and suitability of DC cables for HVDC transmission (beside the need for ...

Controller design and experimental validation of a very low frequency high-voltage test system

“…To extend the range of VLF test systems to higher voltage levels, a VLF test system based on the so-called differential resonance technology (DRT) was developed. This type of VLF system allows a mobile VLF testing of cables up to 500 kV rms and higher, see Eberharter, Kemmetmüller, and Kugi (2014) and Mohaupt and Bergmann (2010).…”

VLF Testing of High Voltage Cables, State of the Art

“…Higher frequencies, however, require a higher input power of the system which implies that the system components need to be designed for that. The patented [4,5] working principle of a DRT system is described in more detail in [1,2,3,6].…”

“…0.1Hz up to 5Hz. This also leads to a significantly lower power requirement of the test source [1]. In a resonant circuit only the losses of the generator's individual components, specifically the high voltage reactor, have to be covered by the mains.…”

“…A new testing technology based on DRT was recently published by the authors in [1,2]. This testing technology has now been implemented in a unit for field operation up to 200kV (see Fig.1).…”

“…The DRT method for the generation of low frequency high voltage is based on a high frequency voltage whose amplitude is modulated by the desired low frequency. Using a resonator, which is tuned to the high frequency, and a demodulator, the desired low frequency high voltage can be generated [1,2]. The input power required -and in direct relation the size and weight of the equipment -is significantly smaller than for other existing methods.…”