For transmitting electricity to consumers, utilities use overhead and underground cables. Conventionally, bare overhead conductors are used since they are the cheapest and easiest to build. They utilise air as natural insulation and there is no need for additional insulation, and have specially designed isolators present only on line supports. When installed using efficient fitting techniques, these conductors help in achieving a relatively low cost
of construction of the overhead electricity network. However, they are difficult to erect in complex network areas and are not safe, especially in densely populated areas.
On the other hand, underground cables offer a safer alternative. Their other advantages include lower maintenance costs and lower risks from harsh weather conditions as compared to overhead cables. However, underground cables are costlier to build than overhead systems as the ground needs to be excavated. This can be difficult when passing through geographic obstructions such as hills, marshes and rivers. Special trenches need to be constructed when passing through loose soil. Besides, heat dissipation in underground cables is an issue. Therefore, the conductors need to be thicker and require expensive insulation.
To address these issues, utilities are increasingly switching to covered conductors that ensure reliable transmission of power. The technology was first used in Finland back in 1970 and has since been adopted in several countries. The main advantages of covered conductors over bare overhead and underground cables are safety and lower costs respectively. In countries such as Japan and Korea, as well as Scandinavia, the fault rates of covered conductor lines have been as low as one-tenth of those for comparable bare conductor lines. Covered conductors are also used to improve safety against accidental contact with live conductors. In Japan and Korea, covered conductors are mandatory in urban areas, while in the US, these are used where lines pass through trees. For new lines, this technology is preferred as the additional cost of using covered conductors is only about 25 per cent more than that of using bare conductors.
In India, the first state to use this innovative technology has been Karnataka. State transco Karnataka Power Transmission Company Limited deployed this technology for a pilot project involving the construction of a medium-voltage transmission line in Yelahanka, a suburb of Bengaluru. The 66 kV line was erected alongside a 220 kV multi-circuit line to transfer power from Hoody to the 220 kV station in Yelahanka to improve the quality of power supply to the suburb, which is close to the international airport. Thus, while installing the new line, power consumers around these areas did not face power cuts. It also resulted in significant space savings. While 66 kV lines occupy 18 metres of space, covered conductors take only 5 metres.
Covered conductors use an insulating material as protection against accidental contact with other covered conductors or with grounded parts such as tree branches. This covering is sufficient for temporarily withstanding the phase-to-earth voltage. The types of covered conductor systems that are used at various distribution voltage levels include XLPE (cross-linked polyethylene)/HDPE (high density polyethylene) covered conductors (single or multiple sheathed), aerial cable systems and spacer cables.
XLPE/HDPE and EPR-insulated covered conductors
XLPE and HDPE are the most commonly used sheath materials for covered conductors. The conductor material can be high conductivity copper or aluminium or aluminium conductor steel-reinforced cable (ACSR) for achieving a balance between strength and conductivity. XLPE is preferred to HDPE since it has about twenty times the environmental stress crack resistance and about five times the impact and tensile strength of HDPE insulation.
Covered conductors can have one, two or three sheath layers at medium voltage (6.6-33 kV), while at 66-132 kV level, the conductor may have up to five layers. Single sheath conductors commonly use aluminium alloy with an XLPE or HDPE sheath of 2.3 millimetre (mm) thickness. These conductor systems are also produced with 1.6 mm and 1.8 mm thick sheaths for ACSR or aluminium alloy conductor steel reinforced conductors and copper conductors. The thinner sheaths reduce the overall diameter and thus, the wind resistance, leading to lower vibration levels and lower snow loads. Copper is used in highly salt-polluted environments. To improve long-term phase-to-phase contact performance at 33 kV, sheath thickness of up to 3.3 mm can also be used.
Single sheath covered conductors have certain disadvantages such as lower impulse strength than multiple sheath conductors. Further, the electrical stress caused by trees on the line or conductors on the cross-arm can erode the sheath in a few months depending on the system voltage. A typical three-layered covered conductor usually consists of a semi-conducting sheath close to the metal conductor, an insulating polyethylene sheath and finally a hard abrasion-resistant outer layer of HDPE.
Ethylene propylene rubber (EPR) is another type of insulation used on covered conductors. It is resistant to heat, oxidation, ozone owing to the compound’s stable, saturated backbone. Its advantages include less thermal expansion as compared to tree-retardant XLPE, reduced sensitivity and higher flexibility. Generally, EPR insulations retain their breakdown (dielectric) strength over the life of the installation, given proper storage and handling of the cable prior to its installation. The superior flexibility of the EPR-insulated medium-voltage cable is an important factor in larger areas where cables must be trained and coiled in vaults and other enclosures. Further, EPR insulation offers high reliability and strong performance in wet applications, as well as improved flame retardancy over tree retardant-XLPE insulations.
Spacer cable systems
Spacer cable systems essentially consist of three covered conductor phases in a polymeric support cradle supported by a “messenger” cable. The system typically comprises a messenger-supported three layer cable construction in a close triangular configuration, which has completely coordinated parts. Spacer cable lines have a compact design that tolerates intermittent contact with trees. Besides, trees can be allowed to grow much closer to a spacer cable system as compared to a bare wire system. Such arrangements reduce the tree clearing required to install a circuit. Its other benefits include reduced operational costs due to a much lower failure rate, improved personal safety, easy installation and operation, fewer right-of-way issues and the provision for installing more than one circuit on a common pole. Further, the system possesses mechanical strength to weather severe storms and electrical strength to prevent faults due to phase-to-phase or phase-to-ground contact, tree contact or animal contact. However, the use of spacer cable systems is not recommended in heavily polluted areas, such as industrial areas and sea coasts, to avoid accelerated aging of the electrical equipment and accessories.
Aerial cables are fully insulated three-core cables with an earth screen used for overhead applications. These cables are less susceptible to lightning, but also the most expensive. These are also more reliable as compared to bare overhead distribution lines, since power and neutral conductors are insulated with a better dielectric medium. Aerial-bunched cables (ABCs) are an innovative concept for overhead power distribution. As compared to conventional bare conductor overhead distribution systems, ABCs provide more safety, reliability and system economy, reduce power losses and entail lower installation and operations and maintenance costs. These are ideal for rural distribution as well as for installations in difficult terrain like hilly areas, forest areas and coastal areas. ABCs are the preferred choice for power distribution in congested urban areas because of their rerouting flexibility.
ABCs have lower fault rates on account of their protection against line and ground faults as well as higher insulation resistance. In addition, these cables have better adaptability and can run concurrently with the existing overhead bare conductor systems without any interference. Their high capacitance and low inductance lead to lower line impedance and better voltage regulation.
New technologies are emerging in the market. These are aimed at improving reliability and reducing faults on low and medium tension lines, thus eliminating theft by direct tapping and preventing the overloading of distribution transformers. In recent times, the demand for covered conductors among utilities has increased steadily, owing to their cost and safety advantages over bare overhead and underground cables. As the needs of various distribution and transmission utilities evolve, there will be more designs and variants of cables and conductors on offer in the market.