Replacing traditional high voltage paper cabling with polymeric equivalents means an alternative to heat shrink joints must be used. Paul Bickerdyke explains how one power utility company found a faster and safer solution.
Time is money says the old adage. This certainly holds true for cable installers and contractors in the power utility industry where the quicker the job can be done, the more jobs can be done per day. But while the actual speed that a joint or termination can be completed is important, it is by no means the only crucial factor. Safety, reliability and lifetime costs also have to be included in the equation. And the work has to be guaranteed right first time if the high costs of accessing underground cables are to be minimised, which means any human element involved in quality has to be eliminated as far as possible.

Heat shrink has been the technology of choice for joints and terminations in the UK's medium voltage, underground power distribution network. But there is a growing trend by utility firms to move away from traditional paper cabling to modern polymeric types. This, combined with a need to lower costs, simplify installation and improve reliability, has led many to seek out better ways of completing joints and terminations.

One example is Scottish Power, whose PowerSystems division is phasing in polymeric cables to replace paper types at the 11 kV level. "We are one of the first power utility companies to fully convert to a polymeric solution for our 11 kV cabling," says Clark Sherry of Underground Systems for PowerSystems.

Poorly installed heat shrink allows moisture penetration or discharge, leading to failures when the cable is energised. It also makes the cable susceptible to factors such as temperature cycling and stress. These failures mean expensive repairs because the underground cables have to be dug up, accessed, and repaired – all of which costs time and money. Then there are the problems of using heat shrink itself, most especially the safety aspects of applying a heat source in a confined space and the significant element of installer skill required to make a successful joint.

Faced with this situation, Scottish Power set about finding alternatives to heat shrink joints and terminations. Tape, resin and cold shrink were all considered, but the first two were discounted as they can be labour-intensive and difficult to apply. Cold shrink seemed to offer the right solution: it is suitable for use with polymeric cabling; does not need the application of heat; and is independent of an installer's skill to form a lasting joint. It is also quicker, easier and safer to install than conventional heat shrink technology, with its lifetime of 80 years or more, lasting as long as the cables it is designed to work with.

"We looked at several types of cold applied solutions," says Sherry. "However we found that many smaller suppliers don't have a full range of joints and terminations available, so we only seriously considered products from two main vendors."

The company began trials on these products in accordance with BS 7888, covering all of the key factors required before introducing a new type of jointing technology – such as heat cycling, insulation levels and water protection.

"Both vendors' products performed well in the tests," says Sherry. "But we chose 3M after factoring in the time and costs required to apply the joint. Not only has it developed a cold shrink technology that is safe and easy to use with polymeric cabling, but it has also created the smallest joint on the market."

The size and ease of use of 3M's cold shrink are key features that save the installer much in terms of time and money. For example, a skilled contractor could complete an 11 kV tape joint in around six hours, whereas an equivalent cold shrink joint can be finished in under three hours.

Its small size allows for further savings, because it requires a much smaller joint hole to be excavated to access the work area. By combining the need for a smaller hole with the higher speed of application, an installer can perform many more cold shrink joints per day than with any competing technologies.

As the developers of cold shrink in 1968, 3M now has around 30 years' experience with the technology (see box, above, for more details). Its silicone and epdm rubber-based system uses a system that pre-stretches the joint around a plastics spiral core, which, when removed by the installer, allows the join to shrink-fit onto the cable.

The inside of the cold shrink splice body has a built-in stress control and Faraday Cage that exerts a permanent radial pressure for the lifetime of the joint or termination, eliminating the need for heat and flame, special tools or connector adaptors and lead sweating. The single-part joint bodies and terminations also have built-in stress control, semi-conducting and insulation layers.

Once applied, the joint will not shrink or creep under prolonged loads. The choice of silicone means that it is uv-stable and resistant to water ingress up to a pressure of three bar, even with extreme changes of temperature. It has a smooth surface, preventing the build-up of contaminants – all of which adds up to a reliable, long-lasting joint or termination.

Scottish Power realised that it could benefit from specifying cold shrink technology for its installations. The simplicity of the process means that reliable joints can be made independently of the installer's skill. The installer can then make more joints in less time, with initial training made far easier. The system's permanent radial pressure avoids the need to use silicone grease and tape, and the one part construction allows for a simple, tool-free installation with lower costs.

The reliable features of cold shrink joining technology are allowing Scottish Power to rapidly introduce polymeric cabling across its network. "Our first cables are due to be installed in October," confirms Sherry, "and by this time next year we'll be almost totally using polymeric. We have given notice to our contractors that we are switching over and advised them to get familiar with it – quick."

Cold shrink technology

3M was the first to develop a cold shrink connector insulator back in 1968. This was quickly followed by the silicone cold shrink connector insulator and the silicone cold shrink termination. Today there are over ten million 3M cold shrink products installed worldwide. One of the building blocks in the range is the QS1000, a one-piece joint body manufactured from a specially formulated silicone material that provides flexibility, easy installation at low temperature and good electrical performance. The finished body is expanded at the factory and loaded onto a removable supporting core that allows the joint to be installed without heat or tools. The injection-moulded body consists of three layers: a semi-conductive electrode forming a Faraday Cage around the connector; a high-permittivity insulation layer that both insulates and controls stress; and an outer semi-conductive layer, which ensures that all screens are at earth potential. It is suitable for making polymeric and paper in-line and transition joints on three and single-core up to 15 kV. The range also includes the QSIII silicone rubber splice for 33 and 45 kV, the QS2000 cold shrink for 24 and 33 kV and QSG for 33 kV. Systems are sold in kit form comprising a base module plus a range of components such as connectors, earthing and encapsulation. They are available to join and terminate configurations from 25-630 mm2.

Source

Electrical and Mechanical Contractor