Treating The Screen Cut On A Polymeric Cable

By on 22nd July, 2013

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One of the most common causes of failure of polymeric cable accessories is breakdown of the cable insulation at the end of the screen cut. This is the result of internal partial discharges brought about by one or more of the following:

• poor cable preparation
• poor installation

which can result in:

• cuts in the primary insulation
• the presence of voids

which in turn causes:

• partial discharges
• ultimate failure

The Problem Area

Correctly prepared cable still presents an area of potential cut - see fig. 25, especially with easy peel screens. This area is a site for voids and must be treated correctly because the stress control system usually does not exactly follow the profile at this point.

Several techniques have been devised over the years and the most common ones will now be discussed.

Note that the copper tape screen is removed about 10 mm from the end of extruded screen layer. If the cable is wire screened the wires are bent back at least 25mm from the end of the screen cut.

The use of the correct tool to remove the screen does usually present a chamferd edge, but this may not be sufficient to prevent void formation. In any event, it is preferable to use one of the following techniques as they will help address potential problems caused by poorly removed screens.

Figure  25 The End Screen Of A Polymeric Cable

Some Techniques For Eliminating Potential Voids

Conductive Paint

The use of conductive paint as shown in fig. 26 extends the screen layer and eliminates the step at the original screen cut. This technique will not lead to void formation at the end of the earth plane because the thickness of the paint is minimal.

This technique is perfectly satisfactory, but the paint can dry out during storage making it impossible to apply so there is a degree of skill required in applying it.

It is usual to smear a thin layer of silicone grease on the cable insulation and onto the paint prior to fitting the stress control system. This helps fill any minor surface irregularities caused by less than perfect cable preparation.

The use of conductive paint is now rarely used at system voltages up to 36 kV, but at higher voltages is often used in conjunction with other systems.

Void Filling Stress Relieving Tape

Tape, which is carefully wrapped around the area of the screen cut will provide a void free smooth profile - see fig. 27. This method remains the most popular at the present time.

Figure  26 - Conductive Paint Applied To The End Of The Screen Cut

Several modern accessories contain stress control, which is an integral part of the system. This can be in the form of an elastomer which is sufficiently pliable at the installation temperature that it will follow the profile of the screen cut. A coating may be applied within heat shrinkable tubing which softens during installation.

Another type is an elastomeric patch, which is wrapped around the cable core and over the screen cut. Figure 28 shows how the product fills in any voids and irregularities at the screen cut and eliminates the need for other treatments such as conductive paint or void filling tape.

Figure  27 - Void Filling Tape Wrapped Around The Screen Cut

Figure  28 - Flexible Stress Control Incorporated In The Accessory

Pros And Cons

It is fair to say that all the techniques described above will work perfectly satisfactorily if applied correctly but there is an element of skill required and care should be taken at this most important stage.

The first two methods do not give stress control in their own right. Although the void filling tape may be stress relieving, a stress control system must be installed subsequently. The most common causes of problems are poor application and the jointer forgetting to apply the paint or tape, in theory the flexible stress control system incorporated in the body of the accessory has a distinct advantage in that it does not require treatment of the screen cut.

However, the application must be carried out in the correct manner, otherwise air be trapped and resulting voids can give problems, especially at higher voltages.

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Category:  HV Cable Jointing & Terminating

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