The Semicon Screen - A Most Critical Point In Any MV Cable Joint, Termination Or Connector

By Chris Dodds on 28th March, 2017

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The Semicon Screen - A Most Critical Point In Any MV Cable Joint, Termination Or Connector

Thorne & Derrick, UK leading suppliers of MV-HV Cable Installation & Jointing Equipment, are delighted to be able to provide the following excellent Guest Blog and commentary geared towards reducing cable failure rates caused by sub-standard MV cable preparation. 

Critically, the post focuses on the importance of precise and careful treatment of the semiconductor screen on medium voltage power cables by the jointer.

By: Ben Lanz - Director, Applications Engineering at IMCORP 

Witness the excellent square cut power cable semicon layer cutback (arrow) in the photograph below. The most critical point in any MV or HV cable joint or cable termination.

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.

If they do work like this during the final MV cable installation we will not be seeing any systemic semicon cutback issues when we perform a Factory Grade(R) commissioning assessment on this site in a few months.

Unfortunately, there was some 'pick off ' (circle) that required some light localised sanding. This is how IMCORP remove cable failure producing defects and eliminate future O&M through Factory Grade(R) Certification and quality control assessment enabling 100 year life, Ben advises.

  • 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.

Semi Con Screen Cut Back - Stripping MV HV Cables

To File, Or To Sand? Neither. 

Cable Jointing Semicon Cables 33kV

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," Ben clarifies. 

  • 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 or less.

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.

Jointing Cables

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 via the LinkedIn social media network.

"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

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."

Ben's Bio 

Benjamin Lanz received a BSEE degree from the University of Connecticut in 1999. Since 1997 he has worked for IMCORP in various R&D, operations, business development, reliability consulting and asset management consulting capacities and now holds the position of Director of Applications Engineering.

Ben is a voting member of the IEEE Power & Energy and Standards Societies and served as Chairman of the Insulated Conductors Committee (ICC) workgroups responsible for cable testing and cable reliability.

Ben has published over a dozen papers on power system reliability, asset management, and diagnostics and regularly presents on the topics - you can Connect With Ben via his LinkedIn profile.

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.  

IMCORP ranked for the eight consecutive year on the 2016 Inc. 5000 with Three-Year Sales Growth of 63%. 

Cable Joints Terminations Connectors 11kV 33kV

  • Further Reading

Video Tutorial Masterclass - PD Partial Discharge & Electrical Cable Breakdown

Alroc CWB18-60 Bonded Semi-Con Stripping & Removal Tool (11kV-33kV) - Video

Video : Medium Voltage Semi-Conductive Screen Removal (Bonded & Easy Strip Cables)

 

Thorne & Derrick Specialist Electrical Distributor

Established since 1985, T&D distribute the most extensive range of LV, MV & HV Cable Jointing, Terminating, Pulling & Installation Equipment - contact us today for a competitive quotation.

Key Products : MV Joints & Terminations, Cable Cleats, Duct Seals, Cable Transits, Underground Cable Protection, Jointing Tools, Feeder Pillars, Cable Duct, Earthing & Lightning Protection, Electrical Safety, Cable Glands, Arc Flash Protection & Fusegear.

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Cable Joints Terminations Tools Cleats

 

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Is Fault Locating Killing Our Cable Systems - May 2016 by Ben Lanz & Eugene Sanchez (IEEE Member





Category:  HV Cable Jointing & Terminating

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