Some Practical Examples Of Mixed Dielectrics In Cable Accessories

By Chris Dodds on 17th April, 2014

Follow @ThorneanDerrick

When a medium voltage cable is prepared for jointing or terminating the earth is removed to expose the cable core insulation. This immediately introduces air into the situation and this has to be addressed to prevent problems. The following defines the areas that must be addressed to ensure satisfactory operation of the cable joint or termination accessory.

Cable Breakout Area Of Paper Belted Cable

When removing the lead or aluminium sheath and the belt and carbon papers of an unscreened paper cable, there is air surrounding the three cores. Figure 1 shows a cross section of the cable and the obvious areas that cause problems.

Figure 1 - Cross Section Of A Paper Belted Cable

Adjacent Unscreened Cores

When three core cables are to be terminated there is always the possibility that adjacent cores will touch each other in an unscreened area. This is because the roll of the cable is such that when it enters a cable box the core positions are arbitrary in relation to the equipment bushings.

In order to achieve correct phasing it is often necessary to cross cores and they will very often touch each other at the point of the cross. This situation is not a problem with joints because the cores will be screened, or as is becoming quite common, resin will be poured around the cores.

The problems caused by core crossing of terminations are worst on paper belted cables, but screened cables can exhibit the same problem if they have been incorrectly prepared (e.g. the screens are removed too close to the breakout point of the cable).

The voltage divider effect in this situation with paper belted cable is shown in fig 2 where a capacitive divider effect is clearly seen between the two cores.

If the cores touch the problem does not go away, as the capacitive effect is merely moved to either side of the contact as shown in fig. 3.

Figure 3 - Cross Core Situation With Paper Belted Core

The End Of An Earth Screen On Polymeric Cable

Whenever a cable is prepared to accept a joint or termination there is potential for air to be trapped as shown in fig. 4. Note that if a knife has cut into the insulation a void is created that is difficult to fill and it also gives rise to a high stress point which may lead to treeing.

At the point of the screen removal there is a capacitive divider effect which leads to a voltage across the air in the region of the end of the screen and the cable insulation. This can be graphically illustrated in a darkened laboratory if a voltage is applied to the cable prepared to accept an accessory. It will be seen that as the voltage is increased a blue glow appears around the screen-cut area at a voltage less than the working voltage of the cable.

This glow is the air breaking down electrically and will be clearly visible at about 6 or 7kV on an 11kV cable. The same test on a 33kV cable will exhibit the blue glow at about 12 or 14kV rms.

Partial discharges will be detected at much lower voltages using a discharge detector and will be in the order of 4kV for 11kV cable and 8 or 9kV for a 33kV cable.

Referring now to the example in the previous chapter where air of 1 mm was positioned around 8 mm of solid as shown in fig. 7 it was shown that just over one quarter of the total voltage was dropped across the air. In other words, an applied voltage of about 8 kV would be sufficient to cause electrical breakdown of the air.

This theoretical example is not totally dissimilar to the end of the earth screen shown in fig.4 above and the result is very similar to the actual voltage at which discharging is detected on a prepared cable.

It must be pointed out, however, that the actual calculation for a cable is rather more complicated and 3 dimensional in nature, not the simple 2 dimensional example shown here.

Figure 4 - The Screen Cut On A Polymeric Cable

Area Surrounding A Connector In A Joint

Cable conductors are usually joined either using crimp connectors or mechanical shear head bolt connectors. The type of connector used is immaterial in this present discussion, but will be raised in a later section. Figure 5 shows the area, which clearly is a potential weak spot.

When joints are considered in more detail it will be clear that there are other factors to be taken into account. These include the diameter of the connector in relation to the insulation and any sharp edges that may result from crimping, but this will be covered at that time.

The discussion so far has been of a relatively theoretical nature, but it should be remembered that good cable preparation is essential to the satisfactory operation of accessories. The scenarios referred to above exist no matter how well the cable is prepared, however, poor workmanship can lead to other problems.

Therefore before continuing specifically with cable accessories the following two chapters will concentrate on the critical areas of cable preparation especially relating to polymeric cable.

Figure 5 - Air Pockets At Ends Of Connector

Nexans Euromold MV Cable Accessories Guide - Click To Buy

• Further reading

Essential Reading - Medium Voltage Cable Accessories Book Published By Nexans

Invitation

Thorne & Derrick are inviting you to join LinkedIn’s fastest growing Discussion Group : Low & High Voltage Power, Cabling, Jointing, & Hazardous Area Electricals (LV-HV).

Discussion subjects include cable installations, cable jointing, electrical substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.

Network, engage and promote your profile, company or products with over 10,000 influencers.

 

Category:  HV Cable Jointing & Terminating

Go back to blog index