Khader Basha K., Senior General Manager, Electrical Markets Division, 3M India
With the growing demand for power in urban areas and industries, underground cable systems are becoming an increasingly indispensable part of power transmission and distribution systems. Traditionally, the high cost and complexity involved in laying underground cable systems, particularly cable jointing and cable termination, have been the key deterrents to their widespread adoption. Well-designed and well-installed underground cable systems are expected to have a service life of 30-40 years. Some power utilities that have an underground network develop their own capability including skilled installers, and special tools and equipment required for the installation of joints. However, the majority of the users lack the in-house capability to handle joint installations and thus have to depend on other agencies.
Power markets across the world are facing four conflicting challenges:
- To carry more power generated from remote sources of energy
- To augment the interconnection capacity between cities and states
- To increase system security while replacing ageing transmission assets
- To manage increased planning delays for new overhead transmission infrastructure owing to heightened public interest in environmental matters.
Utility providers, transmission companies and cable manufacturers are looking for new ways to respond to these challenges. One cost-effective solution to address all the sensitivities is underground cables, which have unique properties for transmitting power. In addition, they are out of sight, often require only a narrow band of land, emit no electric field and can be engineered to emit no magnetic fields as well, have better power loss characteristics, and can absorb emergency power loads. As a result, underground cables can assist in power transmission across:
- Densely populated urban areas
- Areas where land is unavailable or consent is difficult to obtain
- Areas with rivers and other natural obstacles
- Land with natural or environmental heritage
- Areas of significant or prestigious infrastructure development
- Land whose value must be maintained for future urban expansion and rural development.
The design and construction of underground transmission lines as compared to overhead lines has two significant technical challenges that need to be overcome. These are:
- Providing sufficient insulation so that cables can be within inches of the grounded material
- Dissipating the heat produced during the operation of the electrical cables.
These two factors may lead industry experts to think that undergrounding is more expensive than overhead power lines since the cost of laying underground cables at transmission voltages is much higher, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line. On a positive note, with improved technologies and increased reliability, the cost differential between underground cables and overhead power lines is narrowing. This means that power project developers are more frequently deploying underground cables as an economically and technically effective alternative when physical obstructions or public opinion hinder the development of networks. As a result, opportunity costs from planning delays are reduced and the expense and complexity of public legal cases are minimised. Cable manufacturers today are able to provide innovative and customised solutions for the state-of-the-art power transmission industry. Underground high and extra high voltage cables are now equipped with new design features, such as real-time monitoring which make them an effective and reliable alternative to overhead lines. In addition, for utilities, undergrounding helps in bringing down operations and maintenance costs, tree trimming costs, storm damages, day-to-day electricity sales losses, and losses of electricity sales when customers lose power after storms.
The growth of the power cable accessories business has been slow in the recent past owing to delays in various greenfield projects and cable deployment by power distribution companies. It is unfortunate that due to price competitiveness, quality has taken a backseat for power cable accessories. However, in an emerging trend, both power utilities and industrial end-users are encouraging the use of critical products like cable terminations and other cable accessories, which ensure better performance and extend the life of the cable system. The cable joint is a weak link in any cable network; therefore, it is important to invest in cable accessories to prevent failures in the cable network, thus ensuring an uninterrupted power supply.
Faults in cables can arise due to various causes. Power cables are manufactured in factories under carefully monitored and balanced conditions. However, when the cables are laid on site, the actual conditions are starkly different from the sterile factory conditions. Jointing is required to be done in trenches, which are often infested with dust, moisture, vibrations, etc. Along with these unavoidable factors, there are human factors. A joint is the weakest part in an underground cable network. It is mainly subjected to three kinds of stresses – thermal, electrical and mechanical.
As per the cable construction, a failure in a cable can occur in the conductor, insulation, or the sheath. Some common forms of failures in a cable are thermal breakdown, partial discharge and water treeing. According to a study conducted by CESC, the various factors leading to transition joint are as follows:
A sudden failure can have serious and expensive consequences for the utility provider and its customers. The first problem that arises is difficulty in finding the fault location. This may take a lot of time, leading to a long downtime and service outage, thereby affecting the efficiency, productivity and profitability of the utility provider. The rate at which the cables can be replaced is also limited by the cost.
India being a diverse country, there are no standardised voltage levels that are followed for transmission and distribution. In such a situation, standardised products fail to deliver the desired results. The accessories might fail to fit properly leading to faults in cable joints and terminations. Therefore, developing cable accessories as per varying voltage levels is a very challenging and crucial task.
Underground cables are available in various connection shapes, thus making them easy to use. Built to withstand extreme environmental conditions, they provide moisture imperviousness, good chemical resistance, good thermal ageing properties, and effective and reliable stress control. They are compact and thus enable multiple terminations. They are designed to meet international standards and have an unlimited shelf life.
The need of the hour is to offer customised configurations that can cater to the needs of clients as no two projects are the same and different projects have different requirements. Even the most innovative technology provides little benefit unless it is correctly applied. One of the primary reasons for the successful completion of a project is the network of technical support, sales and supply chain specialists. Through their expertise, customers can evaluate and select a solution optimised as per their requirements with training, installation assistance, custom configurations and more.
After decades of discussion, municipalities and electric utilities are acknowledging that there are many societal advantages of undergrounding power lines, apart from avoiding infrastructure damage due to storm events such as hurricanes and thunder storms. In fact, cost benefits accrue from reducing day-to-day operations and maintenance costs, improving reliability, enhancing public safety, and improving aesthetics and property values. In many areas of the country, public policies are being developed, that are considering cost sharing for undergrounding, as government agencies learn more about the benefits to utilities, developers, homeowners, businesses, communities and other taxpayers, as a result of undergrounding.
In the current scenario, underground cables are undoubtedly an appropriate solution for the country. Technical improvements in cable technology have increased the reliability of underground power, thus giving a much-required impetus to the widespread adoption of this technology. Meanwhile, converting overhead facilities into underground systems is in line with many utilities’ goal of providing uninterrupted power supply to their customers. Evolving technology has given the ability to power utilities to physically locate and detect any faults thus resulting in higher-than-expected production, and setting a precedent for the projects to come.