The first underground power cables to be used in India were probably at Shivanasamudram hydel power station in the erstwhile Mysore sub-station in around 1902. This was Asia’s first hydro-electric generating station, commissioned in 1902. While there is no official record, it is probable that underground cables had to be used to bring the electricity from the generators to the open outside. However, it is on record that in about 1920 Shimla installed underground cables fed from the sub-station that still stands on the Ridge, and has an official Heritage status. The switchgear still stands though not in use, but some of the cables connecting to it are still in use after 90 years.
These cables are rated 2 KV and made of copper conductor, with oil impregnated paper insulation encased in a lead sheath – the venerable PILC cables. Although the construction was very susceptible to failure due to entry of moisture through any crack in the lead sheath, they were extremely resilient due to the “self-healing” nature of the oil impregnated paper. Any local electrical breakdown or partial discharge caused the insulating oil to heat and flow into the void and restore the insulating property. Initially liquid mineral oils were used. These later gave way to grease type “non-draining” oils. Cables using these were described as PILC MIND cables. They were used at voltages up to 110 kV.
However, the vulnerability to moisture was such that cable jointers were sometimes disqualified for having excessively sweaty hands, while cable jointing was done during humid monsoon conditions in temporary huts with coal stoves to heat them to keep moisture away. The jointing itself was encased in a bitumen tar compound melted at site over coal or wood fired stoves and poured into a cast iron case, sometimes protecting a lead sleeve soldered or “plumbed” to the cable. In spite of all these problems the cables were extremely robust and tolerant of overloads. CESC in Kolkata (then Calcutta) had an extensive network of PILC cables well into the 1990’s with some of them in service for over 50 years.
PVC cables started being used extensively in India in the late 1950’s following the setting up of a PVC cable factory in Mumbai (then Bombay) by a Siemens’-backed venture with CCI (Cable Corporation of India). Initially the voltage was limited to 1.1 KV, but gradually increased to 3.3 KV and 6.6 KV/11KV. While 1.1 KV cables were quite satisfactory, within the temperature and ageing limitations of PVC. The easy handling and jointing and resistance to moisture or even water immersion was appreciated. However, 6.6/11 KV cables pushed the technology to its limits. The need for screening at 6.6/11 KV was met with conductive graphite coating and carbonised paper beddings under the metallic copper screens. Electrical problems of screening and limitations of thermal short circuit withstand levels limited their popularity and success. PILC cables continued to be preferred for their reliability and longevity in spite of all the installation difficulties.
Things began to change in the end 1970’s with the advent of XLPE cables in India when Universal Cables and CCI decided to expand into manufacturing of XLPE cables. Just at that time a very major project to mine and export iron ore was being set up with funding from the Shah of Iran in Kudremukh in Karnataka. The electrical consultants were Canadian. The consultants were apprehensive about use of PILC cables given the high humidity and heavy and extended rainfall in the area. They would have preferred XLPE cables, but the project commissioning date did not permit waiting for the XLPE cable plants to be commissioned.
As luck would have it, the Shah of Iran was overthrown, and the project completion date had to be pushed back. CCI and Universal used the opportunity to push for the adoption of XLPE cables for the project. The XLPE cables also offered higher operating temperatures, higher short-term short circuit induced temperatures, and better ageing than PVC cables while continuing to offer the same ease of use.
This very visible use of the new type of cables in a very high-profile project encouraged other leading organisations such as BHEL and NTPC to also adopt XLPE cables for their project. In response to the increase in demand many other leading manufacturers of medium-voltage power cables also set up plants to manufacture XLPE cables.
Extrusion and Peroxide crosslinking technologies
All of these first-generation plants used steam curing of peroxide-based compounds in pressurised catenary chambers. Some of them employed only two-layer extrusion of conductor semi-con and XLPE insulation being extruded in one pass, while the outer semicon over the insulation was extruded in a separate run. A few began with triple extrusion from the start. But in all cases the outer semicon was of the strippable variety to facilitate jointing.
Over a period of time triple extrusion and bonded semicon screens became the norm being technically superior. Another parallel development was that the larger cable manufacturers pushed the concept of “dry curing” of the XLPE as opposed to wet steam curing. This was projected to better eliminate micro voids in the XLPE and reduce water treeing.
The next major revolution happened in the late 1980’s with the advent of Sioplas Technology for cross-linking of XLPE cables. This technology eliminated the need for expensive and complicated extruders and catenaries required for thermal cross-linking of XLPE. Simple and inexpensive modifications to PVC cable production lines enabled them to make XLPE cables. Initially Silane cross-linked cables were limited to low-voltage applications. The need for moisture curing by steam or hot water immersion to cross-link the XLPE lead to apprehensions about their use at higher voltages. But as manufacturers of compounds presented more evidence of the performance of silane compounds at higher voltages, backed by experience in Europe at 11 and 33 KV, many second-tier Indian manufacturers also adopted this technology for 11 KV cables.
Some of the cables performed well, others did not. In discussions with a leading global manufacturer of cable compounds, both conventional and Sioplas based, the manufacturer expressed their opinion that the problem with using Silane cross-linked cables at 11 KV were more related to lack of cleanliness than limitations or unsuitability of the technology. Many manufacturers of LT PVC cables who found themselves having the equipment to produce MV cables using Silane technology did not realise the importance of excluding contamination during the storage and handling of the compounds prior to feeding into the extrusion.
Another significant milestone has been introduction and evolution of FRLS cables in India. A major cable fire took place at the Obra Thermal Power Station in UP in 1985. It started in the cable gallery and probably burn unnoticed for an hour or two. By the time fire-fighting operations commenced it became impossible to approach the fire, not just because of the heat from the fire, but because of the dense smoke that made visibility zero, but the toxic fumes that caused people to fall unconscious. In fact, the toxic fumes not only affected people in the affected unit, but smoke and fumes travelled through the cable gallery to two adjoining units and necessitated the evacuation and shutdown of those units also.
A study team that visited the UK for discussions with leading manufacturers and the Central Electricity Generating Board became aware of the need for cables that limited spread of fire and generated significantly lesser quantities of smoke and toxic fumes, predominantly chlorine.
Unfortunately, the non-availability of technology in India and the unwillingness of the foreign firms to share the knowhow resulted in the drawing up of Indian FRLS cable specifications based on the then limitations of Indian cable manufacturers. These were based on improved PVC compounds. PVC is inherently flame retarded. The flame retardancy of PVC arises from the chlorine liberated in the fire. This blankets the fire and cuts off supply of oxygen. But at the same time, it is highly toxic to humans and vey corrosive to metal structures and specially reactive with copper conducting parts. The FRLS cables incorporate alternative chemicals that impart flame retardance without generating so much smoke or toxic gases.
Zero Halogen Cables
The current international standards have moved from low toxicity FRLS specs to Zero Halogen specs with very high flame retardance and very low smoke generation. Insulating Compounds and technology are more widely available globally. On the other hand, a few fires in India that caused major loss of lives have increased demand for such safer cables. The Indian cable industry is capable of producing such improved cables, and if the demand for them increases these can become the norm for internal power distribution, especially in high rise buildings. Leading cable companies can take the lead to catalyse this movement by drawing up and proposing specs that could eventually be incorporated in national Indian Standards.
Aerial Bunched Cables
The early years of the current century saw a major thrust on improving the availability of power in India, both in quantity and in reliability. Power theft was identified as a major constraint to investment in improving distribution systems. Privatisation of distribution, especially in Delhi, led to a focus on preventing theft and improving safety through the use of Aerial bunched cables in place of bare conductors, both for low voltage and at 11 kV. The increase of environmental concerns and restrictions on tree trimming also aided the adoption of an insulated network, and installation of aerial cables was found to be faster and less expensive than installing underground cables.
Experience with low-voltage aerial bunched cables have been largely satisfactory with a significant reduction in power theft. At the 11-kV level the system performance has been mixed. In part this is due to some issues of UV degradation of HDPE cable sheaths. This has been addressed through better UV stabilisation where PE is used for jacketing, or by staying with PVC jackets where the technology is well known and proven. Another major reason for problems has been the use of poor installation techniques. Contractors used to install bare conductors overhead have not taken the necessary care in handling insulated cables at the stage of stringing, tensioning, and supporting by suspension and dead-end clamps. Cables damaged during installation have experienced frequent tripping shortly after energising as the locally damaged weak spots begin to fail in an unpredictable and frequent manner. On the other hand, cables installed with due care using proper installation techniques and tools have performed reliably.
Another major cause of 11 kV AB cable failure has been poor jointing, including poor earthing of the cable screens. Improperly earthed screens result in capacitively induced voltages on the phase jackets. With voltage difference on adjoining jackets there is sparking between cores when the air between them breaks down. This sparking has been physically observed, both as audible chattering and in some cases as visible sparking. The consequence is damage initially to the cable jacket, and then progressing to damage to the metallic screen and then the insulation underneath till the cable fails. Mid-span straight joints have also been found to have a shortened life due to continuous vibration as the cable swings due to wind.
Improvements in cable screens
MV cable screens in India are specified and found to be much less in thickness compared to European practice. This makes them both mechanically and electrically weak. One contractor observed in a seminar that the screen resistance was found to increase significantly between the value on the drum and after installation. Since the author has seen an instance of partial tear of the copper tape of a screen in an apparently undamaged cable, it is postulated that the increase in resistance is due to such tearing. The likely cause of the tearing is a combination of the very thin screen and possible over-bending during installation.
Some future trends
As cable networks become more widespread the demand for improved reliability will drive future trends in India. The improved financial situation of the DISCOMs will permit funding of such improvements, and the demand for better electricity supply by consumers will drive the DISCOMs to pay more attention to the cable networks. The quality of cables will improve through use of better compounds, and greater sophistication in extruding them. FRLS Standards will tighten up on both smoke and toxicity requirements and perhaps become mandatory for closed environments such as climate-controlled buildings. Most importantly cable companies will find it necessary to extend their support services to educate and train installers in better installation techniques so that controversial cable failures do not harm their brand image. Cables will truly have to play their part in the desired improved distribution system of the country.