TUTORIALS
Four high-level tutorials regarding power cables were given by professors during the Sunday afternoon of June 24th, 2007 for a limited number of people.
T1: Maintenance for HV cable systems (in English)
By W. BOONE, Kema, The Nederlands)
T2: Power cables: Service Ageing and field diagnostics (in English)
By F. STEENNIS, KEMA, The Netherlands
T3: Revision of qualification procedures for HV and EHV extruded underground cable systems (in English)
By Jean BECKER, BECKER Consult, Belgium
T4: Thermal Environment of Underground Power Cables (in English)
By Deepak PARMAR, Geotherm Inc, Canada
T1: Maintenance for HV cable systems (in English)
By W. BOONE, Kema, The Nederlands
Maintenance is important for any component in the electrical network, however from an international survey it has been concluded that cables receive less attention from the utilities in performing maintenance than other components, like transformers and switch gears. This may have to do with the facts that cables are usually invisible, do not have moving parts and have a low risk of explosion at least for cables and joints. As a lot of money is invested in cable systems and as proper maintenance may avoid failures in service, there is a strong need for criteria to assign the available maintenance budget according to the contribution to the overall system reliability.
On request of CIGRE Study Committee B1, Insulated cables, WG B1-04 completed a report on maintenance for HV cables and accessories in 2005. In this report the subject of maintenance on HV cable systems has been considered in detail and guidelines have been recommended for structured maintenance, tailored to the different type of cables and accessories, adjusted to system requirements and to customer needs.
In this tutorial the attention was focussed on maintenance on HV cable systems, based on the CIGRE report mentioned.
Important issues were:
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What maintenance practices are being applied at present?
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What are the causes of failure for cables and accessories?
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How can potential failures be detected and avoided?
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What is the meaning of Remaining Life Management in relation to maintenance?
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What guidelines can be given to perform effective and efficient maintenance?
T2: Power cables: Service Ageing and field diagnostics
(in English)
By F. STEENNIS, KEMA, The Netherlands
Ageing and failures of cable systems are dominant factors related to the (expected) reliability of a network. Asset management is partly based on the knowledge of ageing processes and failure mechanisms and it is known that in the present market situation, asset management of cables has become an important subject. On the one hand it can help to prevent outages and on the other hand it may give insight in the limits of a cable with the opportunities to increase cable loads where possible.
This tutorial discussed:
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The various ageing mechanisms for medium and high voltage cables and their accessories. It covered cables with extruded dielectrics and oil-impregnated paper insulation material.
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The various testing methods that are being applied in order to find this degradation. Important issues here are:
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what is the technical background of a test?
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what is measured: a bulk defect only or a local defect (weak spot)?
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can it localize any local defect (weak spot)?
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what is the practicability?
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what are the costs for testing?
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what is the effectiveness of the test and its result?
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what is the relation to asset management?
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Particular attention was given to partial discharges testing because of its potential to localize weak spots even in underground cable systems of 10 km in length.
T3: Revision of qualification procedures for HV and EHV extruded underground cable systems (in English)
By Jean BECKER, BECKER Consult, Belgium
An overview of the present test procedures and service experience in the HV and EHV field was presented first.
It is inevitable that over a period of time an approved cable system undergoes some changes, such as new cable construction, higher stress, new type of accessories, new manufacturing processes, etc. Improving the existing systems with innovative reliable solutions is of common interest for utilities and manufacturers. It is also a normal evolution of the techniques.
As a consequence of this situation it is proposed to complete the qualification procedures for the different HV voltage ranges with the goal to come quickly and economically to the market with innovative solutions but without jeopardizing the reliability of the installed system.
CIGRE Study Committee B1 launched Working Group WG B1.06 with the task of revising the qualification procedures for underground high voltage cable systems without making the full set of tests, which are presently recommended or specified in standards for evolutions of the systems.
The aim of this tutorial was to present the results of the WG B1-06 and to show practical examples.
T4: Thermal Environment of Underground Power Cables
(in English)
By Deepak PARMAR, Geotherm Inc, Canada
Insulated conductors are rated for a maximum safe and efficient operating temperature. Exceeding this temperature may result in shortening the life of the cables or in the worst case, a premature failure.
The temperature rise is a function of the type of cable, load (Ampacity), cable / trench geometry and the “effective thermal resistivity of the environment” in which the cable is buried. For directly buried XLPE or self-contained fluid-filled cables, this may consist of the native soil and the corrective thermal backfill. Cables may also be installed in a concrete duct-bank with a corrective backfill envelope. The heat generated by the cables must be dissipated through these different media up to the ground surface – the ultimate heat-sink. In order to properly rate a cable system, thermal resistivity of the native soil and that of all the different materials must be known.
This tutorial highlighted the importance of various components as well as how to measure these parameters (thermal resistivity, thermal diffusivity and thermal stability) in the field (in-situ) and in the laboratory. It was supported by view-graphs, photos and case studies.