The Semicon Screen – The Most Critical Point In Any MV Cable Joint, Termination Or Connector
Published 08 Oct 2019
Semicon Screen Cutback
uploaded by Chris Dodds | Thorne & Derrick Sales Marketing Manager
Thorne & Derrick, UK Specialist Distributors of LV MV HV Jointing, Earthing, Substation & Electrical Eqpt, are delighted to be able to provide the following excellent Guest Blog and commentary geared towards reducing cable failure rates caused by poor workmanship and sub-standard MV cable preparation.
Critically, the article focuses on the importance of precise and careful treatment of the semiconductor screen on medium voltage power cables by the jointer or cable splicer. This insulation screen, is the semi-conductive layer of the medium/high voltage cable and provides a homogeneous even distribution of the electric field between the external insulation and metallic screen on the MV cable.
Typically, the insulation screen can be fully bonded or easy-strippable. The removal of the insulation screen is the most critical step during the cable preparation process and the the use of the correct jointing tools by a Competent Jointer can extend the service life of power cable systems and underground distribution networks.
Incorrect or sub-standard cable preparation is the primary cause for the failure of medium/high (MV-HV) cable accessories including joints, terminations and connectors.
By: Ben Lanz – Director, Applications Engineering at IMCORP
Witness the excellent square radial cut to the semicon layer of the power cable (arrow) in the photograph below.
- Pictured: Square Semicon Screen Cut On Medium Voltage Cables – the semicon cable screen was removed using Speed Systems Screen Scoring Tools. MV cable type is ICEA 35kV strippable insulation shield design.
To File, Or To Sand?
IMCORP, providers of diagnostic services for medium and high voltage power cable systems, have just finished an Accessory Performance Consultation for a critical generation facility.
Expert cable condition analysis and power cable life cycle consulting services provided by IMCORP can prevent any systemic semicon cutback issues when a Factory Grade(R) site commissioning assessment is provided. This can prevent multiple costly cable system failures on medium voltage power systems.
Unfortunately, there was some ‘pick off ‘ (circle) that required some light localised sanding by the cable splicer.
This is how IMCORP remove cable failure producing defects and eliminate future O&M through Factory Grade(R) Certification and quality control assessment enabling a predicted 100 year life cycle of the cables.
In the UK it is normal to smooth-out the step down on the screen cut from the primary XLPE insulation using a rat tail file or aluminium oxide abrasive paper for sanding.
However international standards and jointer preferences vary on the subject.
Ben comments, “this semicon cut back was made for an IEEE 386 35kV separable connector which has a built in square step in the stress cone cable adapter that ideally needs to interface with a square semicon step made with a semi con scoring tool on a ICEA type strippable insulation shield. If the cable jointer bevels the edge it would require significantly more void filling grease which can be a risk factor for substandard performance in this type of medium voltage cable installation. We have a factory comparable partial discharge test for the field that we have used to test tens of thousands of these types of separable connector terminations. I can assure you that this technique in conjunction with installing the rest of the cable termination correctly will far surpasses the manufacturer’s PD performance expectations.”
- ♦ IEEE 386 Standard for Separable Insulated Connector Systems for Power Distribution Systems Rated 2.5kV through 35kV – IEEE 386 Standard covers the definitions, service conditions, ratings, interchangeable construction features and tests established for loadbreak and deadbreak separable insulated connector systems for use on power distribution systems rated 2.5kV through 35kV and 900A.
Removal of Semiconductor “Pick Off” On Medium Voltage Cables
Regarding the removal of the semicon ‘pick off’ contaminant from the MV cable which is circled in the picture, Ben recommended the following.
“At 35kV we recommend using a little 120 grit aluminium oxide sandpaper, but only at the location of the contaminant with a small circular motion at the tip of the thumb.”
“We never recommend sanding at this voltage class with a strippable insulation shield unless the cable cleaner will not remove the contamination,” adds Ben.
Here, the semi conductive layer has been left on the 11kV XLPE insulation which can cause surface tracking and eventual flash over.
The below cable preparation images clearly shows the straight and clean cut to the black semicon screen stepping down onto the 11kV cable insulation (XLPE) – here a constant force spring provides a straight edge for copper tape screen on the 11kV cable before installing the cable termination.
The F Word
FAILURE – cable failures are disruptive and dangerous, but also avoidable.
Not all, but some.
Whatever your currency the financial cost of cable failure is overshadowed by the immeasurable cost inflicted in terms of loss of reputation.
Tony Haggis (Director at Tony Haggis Consulting Ltd) with 46 years experience in electricity distribution up to 132kV provided some professional insight.
“It looks like a very carefully executed screen cut and a nice neat job. However, one problem that occurs with this method is that the screen can lift slightly off the insulation when the upper part or the medium voltage cable screen is torn off at the depth cut. This results in a small void under the screen which can result in partial discharge.”
“In EoN Central Networks, we identified this as a root cause of MV cable failures after microscopic inspection. We changed the semicon screen cut method to using a small round file to make a circumferential groove at the screen off position which just exposed the white insulation below.”
Tony continues, “when the cable screen was pulled off there then was no danger of lifting the remaining screen. The result was a tapered transition on the screen. This eliminated failures due to discharge at the screen off position. Eventually, we moved to bonded screen and top quality bonded screen stripping tools which again provide a reliable screen off cut but more easily and quickly. Bonded screen was also lower cost than strippable.”
Delamination & PD
Ben replied on the issue of MV cable failures, “we taught the class how to remove the semicon while minimising the likelihood of delamination and the how to inspect for this issue. Fortunately, we will not have to guess if they followed our recommendations. The owner requires a third party factory comparable PD test (offline 50/60Hz PD test with better than 5pC sensitivity) on 100% of the installations and the installer is on the hook for repairs and retests. This is becoming the new standard for QA/QC!”
Tools Of The Trade
The new Alroc CWB/18-60-MVS cable jointers tool, with “stop” included, removes both the bonded semi-conducting screen without requiring lubrication while also creating a chamfer in a clean and reliable cable preparation process.
Here the cable jointer demonstrates the ease of simplicity of removing bonded semiconductor screen layer from 11kV Triplex cables with 300sqmm stranded copper conductor and XLPE insulation prior to terminating into medium voltage switchgear. The Ripley US02 is set at 1mm during this cable preparation process.
21st century power networks depend upon type-tested, trustable medium voltage cable accessories such as cable joints and cable terminations, to energise our lives.
Cable accessories are only reliable if installed in accordance with the manufacturers jointing instructions and recommended cable stripping dimensions.
Precision engineered cable jointing tools are an essential part of the MV-HV jointers toolbox ensuring the accurate removal of cable sheaths, insulation and semicon screens on MV-HV cables.
Ben goes on to explain, “I agree that the semicon cutback is the most critical part of the cable termination and I agree that a smooth transition (chamfer or slope) is better than a step if the termination design has a smooth interface. However, this cut back was made for an IEEE 386 35kV separable connector as referenced above.”
However, jointers tools alone without the knowledge, expertise and skill of a Competent Jointer to use them are not worth diddly squat.
For instance the photograph below highlights the problem of unskilled labour, even if equipped with the correct cable jointing tools, inflicting catastrophic damage to MV-HV power systems through lack of training, experience and understanding of the criticality of semicon cutback.
Non-radial, rough and jagged semicon screens with protuding points at the cutback will cause cable termination or joint failure. Outage. Blackout.
Ben Lanz is Director of Applications Engineering at IMCORP USA and has expert technical oversight of power cable life cycle consulting (5kV-500kV).
Ben has published several technical papers on power system reliability, asset management, diagnostics and regularly presents on the topics. He is a Senior Member of the IEEE Power & Energy Society, a voting member of the IEEE Standards Society, and a member of the IEEE Dielectrics and Industrial Applications Societies. He has served as Chairman of the Insulated Conductors Committee (ICC) technical committees responsible for cable testing, cable reliability and surge arresters.
Who Are IMCORP?
IMCORP is the leading provider of power cable reliability assessment services for medium and high voltage (5kV to 500kV) power cable systems for both aged and new cable installations. Our Factory Grade® assessment is non-destructive, requires no hazardous materials, and is a cost-effective alternative to cable replacement. We identify the precise cable system Rehabilitation requirements, allowing the customer to Certify their cable systems to like-new condition.
MV HV Power Cable System Reliability Solutions by IMCORP
Thorne & Derrick are national distributors of LV, MV & HV Cable Installation, Jointing, Substation & Electrical Equipment – servicing businesses involved in cabling, jointing, substation, earthing, overhead line and electrical construction at LV, 11kV, 33kV, 66kV and EHV. Supplying a complete range of power cable accessories to support the installation and maintenance of low/medium and high voltage voltage power systems:
- Slip-on Cable Terminations
- Cold-shrink Cable Terminations
- Heat-shrink Cable Terminations
- Cable Joints – Heat & Cold-shrink
- Separable Connectors (Euromold)
- Surge Arresters & Switchgear/Transformer Bushings
Key Product Categories: Duct Seals | Cable Cleats | Cable Glands | Electrical Safety | Arc Flash Protection | Cable Jointing Tools | Cable Pulling | Earthing | Feeder Pillars | Cable Joints LV | Joints & Terminations MV HV
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