Electricity + Control March 2017

o prevent theft of cables in South Africa, suppliers are putting

in special markers with serial numbers. With the inclusion

of these serial numbers, end users are able to identify cable

ownership. Furthermore, end-users are also utilising these serial

numbers for their asset register.

Table 1

summarises the key differences between PILC and XLPE

insulated MV power cables.

Other factors influencing cable technologies

With the improvements in insulationmediums and cable terminations,

MV switchgear has drastically reduced in actual size. This means that

the sizes of cable boxes have been reduced and special bushings

have been introduced to accommodate the new cable terminations.

Things get really exciting on site if the wrong equipment has been

specified and purchased. Typically, most equipment has long lead

times, and instead of stopping the project, people make plans onsite

to terminate the cable into the switchgear that

is supplied on site. From day one therefore, the

installation is wrong, and premature failures can

be expected. These failures can be costly to repair,

could involve replacement of the switchgear, and

in addition staff or the public could be injured or

killed from any resulting explosion.

Figure 7:

Compound filled

cable boxes.

Figure 7

shows very old compound filled cable

boxes. These were designed for PILC belted,

unscrewed MV power cable, and they were filled

with hot pouring compound. This ensured that

there was no PD in the critical crutch of a PILC

belted unscrewed MV power cable as all air was

removed in the critical areas.

Due to a variety of reasons these compound

filled terminations are no longer preferred and

most end users prefer to install convention dry

type terminations. These are referred to as either

Heat Shrink, or cold applied products.

Evolution

of MV Power Cables and

Accessories up to 36 kV:

Part 2

Continuing the discussion on the evolution of MV power cables over the last century, some pros and cons of all the different types of insulation

materials used for MV power cables, and recommendations to ensure improved reliability of MV cable systems.

Patrick O’Halloran, City Power Johannesburg

PLANT MAINTENANCE, TEST + MEASUREMENT

Cable Construction PILC-Insulated cable

XLPE-Insulated Cable

Conductors

(either Copper or

Aluminium)

Usually shaped conductor, but

may be circular / oval

Only circular

Insulation

Wrapped impregnated paper

insulation

‘Solid’ extruded dielectric XLPE

insulation

Screen

Belted collectively or individually

screened

(Wrapped metallised paper

tapes)

Always Individually screened

(Extruded semi-conductive semicon

with either copper tapes or copper

wires)

Metallic sheath

Essential, typically lead

Optional, either lead or Al

Bedding layer

Extruded or Fibrous (if

armoured)

Extruded bedding (if armoured)

Armouring

DSTA/ SWA/ AWA (optional)

SWA/ AWA (optional)

Outer sheath

Extruded (PVC/PE) / Fibrous

Extruded (PVC/PE)

Continuous

operating

temperatures

70°C

90°C

Short circuit

temperatures

160°C

250°C

Longitudinally

water blocked

No, normally only radially due to

metallic layer

Yes, if specified, since it is a special

requirement in SANS 1339

PD free design

Yes

Diagnostic testing

possible

Tan Delta diagnostic, which is

a measurement of the overall

circuit condition.

Tan Delta and Partial discharge

diagnostic possible.

Pre-failure faults can’t be located

without breaking down the

insulation system by applying a

high voltage source

Pre-failure faults can be located

without breaking down the insulation

system

Jointer errors can be identified before

energising the cable

Table 1: Comparison between PILC and XLPE MV power cables.

Electricity+Control

March ‘17