Simplifying high-voltage jointing with cold shrinak technology

Simplifying high-voltage jointing with cold shrinak technologySanjay Jha, Sr. Technical Manager (Electrical Markets Division – Energy Products,) 3M IndiaPresently high-voltage (66 kV – 132 kV) cable jointing is based on pre-moulded type joints which are typically installed by field expansion or slide-on techniques. The installation may involve temporary expansion for ‘parking’ before conductor connection followed by shrinking on the prepared cable. In other methods, the pre-moulded joint body may be ‘parked’ by sliding on the cable core on one side prepared longer and subsequently pulling it back to the centre of the joint. Both these conventional techniques require use of special tools. The modern cold shrink technology makes the installation much simpler by supplying a pre-moulded pre-expanded joint on a removable core. This article describes this technology in high voltage cable jointing and how it can benefit the industry.With growing demand of power in urban areas and industry, high voltage (110 and 132 kV – hereafter referred as HV) underground cable systems is becoming increasingly indispensable part of power transmission and distribution systems. Traditionally, the comparatively high cost and complexity involved in high voltage underground cable systems, especially cable jointing, has been the main deterrent to its widespread adoption. Well designed and well installed HV underground cable systems are expected to have a service life as long as 30 to 40 years. However, some premature faults could still occur typically due to external damage caused by excavation or construction work. The complexity, time taken and capability to restore such faults has always been a matter of concern. Some power utilities that have underground HV network, have developed their own capability including skilled installers and special tools and equipment required for the installation of joints. However, a majority of the users sadly lack the in-house capability to handle the joint installations quickly on their own and therefore have to depend on other agencies. Cold shrink technology in HV cable jointing addresses this concern by simplifying the installation of HV joints and offer unprecedented advantage in terms if reliability and quicker installation time.  Installations of conventional pre-moulded HV underground cable joints remain a highly specialised job till date and are executed by specialist HV cable joint installers in a controlled environment. The authorised installers gain their expertise over years of training and hands on installation, often spending much long time as apprentices. The skill and craftsmanship of installers are critical as any errors may cause serious installation defects which may cause pre-mature failures in service. The joint bay too needs to be fully covered, with concrete floor and air-conditioned to maintain clean, dust-free and humidity controlled environment during installation work. Various cable preparation tools are commonly used to make the installation quicker, get better accuracy and reduce the effort required for installation. Some of the important tools used are cable straightening tools with temperature control and heating units, semicon and insulation stripping tools, belt sanders, dryers, compression tools and many other small tools. Apart from these common preparation tools, the conventional pre-moulded joints require some special mounting tools which are specific to the manufacturer and indispensible for the installation. All such tools are required to be available in good working condition during the installation work. While the cable preparation skills and tools continue to be important and necessary for installation, the cold shrink cable jointing technology eliminates the need for any special installation tools and thereby simplifies and substantially reduces the time required during the installation. The following paragraph discusses these aspects in more detail.The photo above shows a typical special pulling tool used for installation of one of the pre-moulded slip-on type joints. Different manufacturers have different fixtures and attachments which are custom built so as to match with the design of the pre-moulded joint body. Although each manufacturer uses customised tools for installation of their joints, principally all such tools operate almost in a similar way. The tools are used in two stages of the installation: the first stage while parking the joint body fully on the cable on one side before joining the conductors of both the cables. Subsequently, after completion of the conductor connection and associated procedures, the joint body is again pulled back at the centre position using the tool. Since this installation procedure involves sliding the rubber joint body on the extended cable core for parking, abundant precaution is needed to make sure that the inner surface of the joint body is protected from any scratches. Moreover, this installation procedure requires longer length of the joint to provide for the space required for parking position and to accommodate the pulling tool in the jointing area. Even manufacturers offering joints with 3 part joint bodies employ similar tools for installing the different parts of the joint body in position.  Due to the 3 part design, it becomes possible to eliminate the additional space required for parking of the joint body. However, the additional interfaces due to multiple parts joint body needs to be taken care of according to the manufacturers’ recommendations. Another approach for the installation is by expanding the joint body on expansion pipes having diameter bigger than the cable overall diameter to enable parking of the joint body on the cable outer sheath. This helps in limiting the length of the joint by eliminating the space required for parking on the prepared core. Such tools too are specific to the different manufacturers of joints depending on the permissible expansion diameter and the shape of the joint bodies. Here again the tools are used in two stages – firstly for expanding and parking before the conductor connection and secondly for extraction and shrinking after completing the conductor connection. Some manufacturers provide reusable two part expansion pipes whereas some other versions require the expansion pipes which need to be cut off and disposed after shrinking of the joint body. Likewise, mechanical, hydraulic and electrical operated versions are used by different manufacturers. It must be noted that this method also involves risk of internal scratches from unseen dust particles on the expansion tube and therefore, the joint manufacturers recommend doing the expansion in a clean environment. Further, the expansion operation also requires clear space for placing the equipment etc. and may become challenging in the limited space available in typical jointing bays.   Most importantly, the joint manufacturers have very strict recommendations on the number of hours that the joint body can be kept in expanded position (usually 3 to 4 hours). The installers must complete all the steps including conductor connection and shrink the joint body within the time limit. In case of failure to do so, the joint may have to be discarded and a new joint installed. This puts the installers under undue pressure and may tempt them to take short-cuts under pressure.The HV joint based on cold shrink technology utilises silicone rubber single piece joint body supplied in factory expanded form. The pre-expanded joint body is preloaded on an easily removable core which does not require any special tools for installation. The cold shrink joint body can be parked over the cable outer sheath and thus helps minimise the length of the joint as well as the length of the joint bay. The easy shrinking by pulling the loose core end (pull tab), eliminates the need for any sliding movement of the joint body and thereby eliminates any possibility of internal scratches due to friction. Heavy expenditure on purchase and maintenance of custom joint installation tools is eliminated. Last but not the least, the installation time is substantially reduced by eliminating the time spent on setting up, usage and dismantling of special tools required for installation which can save upto 20 to 30 per cent of installation time and therefore cost.  Thus adopting cold shrink joint in HV cable systems can greatly help minimise project execution timelines and installation costs. On the other hand, from maintenance point of view also it can greatly help reduce the down time and the need to have specific installation tools during the restoration work. Training for installers is also simple and easy.Summary Cold Shrink technology has made it possible to make the installation of high voltage joints simpler and faster. The advanced design and silicone rubber material provides excellent performance while minimising the complexity of installation and need for special tools. Adoption of cold shrink type joints can simplify and speed-up HV underground cabling projects as well as proves helpful for maintenance and restoration jobs.