By Patrick O’Halloran, chief engineer condition monitoring at City Power, Johannesburg, South Africa
Medium voltage (MV) cable accessories have changed since our grandfathers terminated and jointed cables many years ago, having evolved to keep pace with the technology changes in switchgear and cable designs. In South Africa the main driving factors that changed the MV cable accessories are NRS 053/SABS 1332 and NRS 012 / SANS 876 and the introduction of compact MV switchgear. This article provides an overview of how MV cable terminations evolved and identifies the challenges that lie ahead for the South African industry (resistance to change, lack of skills and not understanding of new technologies).
Cable termination beginnings (1940 – 1950s)
In the beginning, electrical equipment (switchgear and transformers) were designed to have compound filled metal cable boxes. This way of terminating cables was technically good, except it was very difficult and hazardous to field staff. The MV paper insulated (PILC) cables at the time had belted construction and had wiped earth connections .
The compound filled cable box was designed to have no air inside, so creepage was not a major consideration when designing the cable bushing. This explains why the bushings of compound filled cable boxes are small compared to air filled cable box bushings found in metal clad switchgear and outdoor transformers.
Compound boxes were filled with many different compounds, but mainly a hot pouring compound was used. This compound was difficult to manage and gave off harmful fumes when heated up prior to pouring. Compound filled boxes were made of metal housing, with porcelain bushings where the cables exited the compound box.
Some drawbacks of compound filled cable boxes are: compound top-up is required to ensure proper insulation (no air voids), long installation times and cable box failures cause major damage when they ruptured (hot burning compound could be expelled). However, new technology cold pouring compounds are now available, which are environmentally friendly, safe to install and re-enterable.
Air insulated MV cable terminations (1960 – 2005)
With the introduction of tapes, heat shrink and later cold shrink terminations, compound filled boxes have been replaced over time with air insulated terminations. This type of MV cable termination is used by 95% of the South African market.
Screened paper insulated cables were introduced to control the electrical stresses within the cable designs, especially where increased voltage cable ratings were required. Currently belted design paper insulated cables are limited to 12kV. Screened paper insulated cables are normally rated up to and including 36kV as per SANS 97. The screened cable design provides improved MV cable termination performance, especially where in belted cables the crutch is a high stress area.
The belt design paper insulated cable is more likely to have crutch failures than the newer screen design paper insulated cable where the complete crutch area is screened. This is because of resistivity of materials and the introduction of air between unscreened insulated conductors. Refer to Image 3 for more details on the differences between belt and screened MV paper insulated cable.
Special stress control tubes were designed to control the high electrical stress at the screen cut in air insulated MV cable terminations.
With the introduction of air in the cable boxes, we now have to consider creepage distances, tracking and erosion, and clearances (phase to phase, and phase to earth). These technical considerations must be correct if an air filled termination is to last in excess of 30 years. If inadequate creepage, tracking and erosion properties and air clearances are provided, then the MV cable termination will fail prematurely. Failure of MV cable terminations are dangerous and cause long power interruptions. Image 5 shows a typical 11kV air insulated heat shrink termination with putty and tape shrouds.
International market trends (which are mainly 24kV rated systems) required smaller and smaller switchgear. This in turn equals reduced busbar clearances and cable boxes. Air was the first insulating medium for busbars. This was replaced with oil and with the introduction of SF6 insulation, busbar clearances could be reduced tremendously. This forced the cable box sizes to reduce. Along with the reduced cables boxes came reduced clearances between phases and phase to earth. The reduction of clearances required new MV cable terminations.
When switchgear manufacturers designed smaller air-filled cable boxes with reduced clearances together with MV cable, accessory manufacturers had to redesign the bushings and MV cable terminations to make the cable box and cable termination compatible for these reduced clearance requirements.
In South Africa, the industry has standardised on a type C 630A bushing with M16 thread. This Type C bushing is found on all the new SF6 insulated switchgear that is currently only used by City Power, Eskom and Ethekwini municipalities.
The Type C bushing allowed end users to move away from traditional putty and tape shrouds to factory made fully insulated shrouds. These shrouds are installed the same way every time and ensure that cables are terminated properly on Type C bushings as well (refer Image 6). This is a product that is designed to be used on South African paper insulated cable systems.
Paper insulated cables, which are susceptible to moisture ingress that causes insulation breakdown, were also forced to find alternative cable designs. With the introduction of screened XLPE cables, heat shrink MV terminations evolved once again.
Internationally it was decided to standardise the cable interface and also introduce screened cable terminations. Screened MV cable terminations could only be used on MV XLPE cables and, once installed, eliminated the problems of creepage, tracking, erosion and clearances experienced by most air insulated MV cable terminations. The terminology ‘Screened’ means earthed. Once a cable termination is completely screened it can be completely submerged in water without flashover. Eskom use screened connectors on 24 kV systems when connected within the new compact switchgear. Furthermore, larger internal utilities have changed away from 3 core cables and now utilise single core XLPE insulated cables. This is not an easy change to make as all electrical aspects of the network must be reviewed and staff trained on how to install and terminate single core XLPE insulated cables. The South African market uses mainly 3 core cable designs (refer Image 7) for many reasons.
Image 8 shows a cross sectional view of a screen connector nonextensible and an extensible back to back double cable connection. The image also shows the international earthing design, which uses copper wires instead of copper tape as is currently used in South Africa. The design of the screened connector controls the electrical stress from the XLPE cable through the Type C bushing and into the switchgear. Because the surface of the cable and the screened connector is screened there is no leakage current along the surface of the screened connectors. With these screened connectors installed in the cable box, the cable box and all electrical clearances can be drastically reduced. The life expectancy of screened MV cable terminations is double the expected life expectancy of unscreened cable terminations, especially with reduced clearances inside new reduced cable boxes. It is not possible to use a screened connector on paper insulated cables.
South African national standard
To eliminate failures from occurring in the MV cable compartment the following national standards have been published:
- NRS 012 / SANS 876 – Cable terminations and live conductors within air-filled enclosures (insulation co-ordination) for rated AC voltages from 7,2kV and up to and including 36kV
- NRS 053 / SANS 1332 Accessories for medium-voltage power cables (3,8/6,6kV to 19/33kV)
These standards are not compulsory as yet, leaving the end user to specify them when purchasing any MV switchgear and MV cable accessories.
NRS 012 / SANS 876 covers the requirements for cable terminations and live conductors within air-filled enclosures (insulation coordination) for rated AC voltages from 7,2kV and up to and including 36kV. All critical dimensions are given in the standard for the following termination types:
Type 1 – Bare
Type 2 – Shrouded
Type 3 – Unscreened separable connectors
Type 4 – Outer cone screened separable connectors
Type 5 – Inner cone screened separable connectors
Shrouded insulation termination: air-insulated termination that has additional, unscreened, local insulation enhancement typically consisting of bushing protection or taping along with a suitable heat shrink, cold shrink or slip-on cover at the terminal connections.
Unscreened Separable Connector termination (USC): air-insulated termination that is fitted with unscreened separable connectors Screened Separable Connector termination (SSC): air-filled enclosure within which the cable cores are terminated with screened separable connectors
Type 1 Bare interfaces (air insulated)
– Cable cores terminated with stress control appropriate to the cable design and voltage
– Air being the sole insulation medium for the terminal connections
– The minimum distance from any live bare metal (e.g. bushing, post insulator, live conductor, lug, fitting etc.) to an adjacent phase or to earth determined by the impulse withstand voltage requirement
Type 2 Shrouded interfaces
– Cable cores terminated with stress control appropriate to the cable design and voltage
– Unscreened local insulation enhancement at the terminal connections
– The minimum distance from any unscreened, shrouded, live metal (e.g. shrouds and cable cores) to an adjacent phase or to earth determined by power frequency (e.g. corona inception and extinction) and impulse withstand voltage considerations
Type 3 Unscreened separable connector interfaces (USC)
– Cable cores terminated by stress control appropriate to the cable design and voltage
– USC at terminal connections
– The minimum distance from any unscreened, live metal (e.g. USC and cable cores) to an adjacent phase or to earth determined by power frequency (e.g. corona inception and extinction) and impulse withstand voltage considerations – Leakage current limited by quality of the interface between USC and bushing – interference fit
Type 4 Screened separable connector interfaces (SSC)
– Clearances determined by the mechanical clearance required to fit the SSC’s within the cable box
– Safe to touch due to surface being earthed
– Leakage current limited by quality of the interface between SSC and bushing – interference fit
– Note that traditionally PILC cables could not use SSC, sector shaped cores or loose core screen
Cable box sizes
It is also important to ensure that the correct size cable boxes are supplied, as nearly all MV cables installed are 3 core, so space is required.
Low voltage current transformers in MV cable boxes
As technologies have improved with screened cables, the use of low voltage current transformers have been utilised in MV cable boxes for metering and protection applications. It is essential that these low voltage current transformers be installed in a screened area, otherwise discharge may occur if the air clearances are not adequate.
The dimensions from the top of the low voltage current transformer to the screen curt is covered by the dimensions in the Type 2 (shrouded) and Type 3 (unscreened separable connectors) terminations.
All MV cable accessories should comply with the requirements of the NRS 053 standard, which covers accessory ranges; termination requirements; tail lengths; indooroutdoorand tri-furcating; shrouded, USC and SSC; earthing requirements; joint requirements; repair sleeves and end caps; type test requirements; range of approvals; packaging and documentation.
Challenges ahead for the South African market In the past all MV cable accessories and cable laying was done in house by electricity utilities. With privatisation and loss of skilled labour, almost all MV cable accessories are now done by subcontractors.
When wiring a low voltage house or factory the electrician is required by the department of labour to have a valid wireman’s licence (SANS 10142-1). This is not the same for MV cable terminations where the voltage cable is as high as 36kV. No national standard exists for MV cable terminating; and therefore no formalised training is offered nationally for the essential skill required by jointers who work on these MV cable systems. With the loss of skills in South Africa, it is essential for a MV wireman’s licence for MV cable accessories and cable laying to ensure the quality of supply.
STANSA’s TC66 cables working group is currently revising SANS 10198 Parts 1-14. This national standard is to become the course content for a MV wireman’s licence and will facilitate the availability of adequate skills when it is a compulsory requirement for jointers to have a MV wireman’s licence to perform MV cable accessories and laying cables. Bi annual training of MV cable accessory jointers is a must to ensure MV cable accessories are done properly the first time round.
The quality of the material in MV cable accessories is also an essential part of ensuring that the MV cable accessory lasts its expected 30 years, as a lot can happen with electrical equipment in that time. NRS 053 covers the technical requirements for MV cable accessories in South Africa, specifying the type tests that a MV cable accessory should withstand.
It is also important to review the previous track-record of MV cable accessories before buying a MV cable accessory. All MV switchgear should comply with the requirements of NR 012 / SANS 876, to ensure that adequate air clearances and electrical parameters are provided, so that jointers can install the quality terminations properly.
MV cable accessories have definitely evolved over the years. It is important when specifying a cable accessory that it is the right MV cable accessory for the switchgear and cable being installed. It is also important to ask for compliance type test reports to NRS 053/SABS 1332. If no conformance type tests are available, how would you know that the product is type tested for all conditions it may be exposed to during its expected 30 year life cycle?
Staff training should be the focus for all end-users and contractors who are daily performing MV cable accessories. Training courses are offered by the reputable MV cable accessory suppliers and are recommended even for experienced jointers who believe they are already competent.
All MV switchgear should comply with the requirements of NRS 012 / SANS 876.
Lastly, there is considerable pressure to change to what the rest of the world is using. However, we need to review the reasons why the rest of the world is promoting its new technologies. Is it because of the cable designs, voltage levels or switchgear designs?
New technologies are designed to solve current problems, reduce safety risk, meet environmental requirements and make our lives easier.