Underground Cable: Technological Development & Market Opportunities

Underground cables offer an affordable and justifiable solution for critical parts and in some cases the entire length of power transmission and distribution lines. With appropriate technology used in appropriate places, the environment impact of underground cables can be minimized…

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Electric power is transmitted and distributed either by overhead system or by underground cables. A cable is basically an insulated conductor and is used for underground transmission and distribution of electricity. Cable is an assembly consisting of one or more conductors with their own insulations, individual coverings, assembly protection and protective coverings. Insulation is used to cover the conductor and provides isolation from the surroundings. Underground cables are generally used in densely populated areas such as cities and metros, where there is high density of automobiles; high rise commercial, residential buildings and places where vital installations of uninterrupted power supply such as water supply system, hospitals and IT services etc. Underground Cables (UG) are also preferred in industries, sub-stations, railways and road crossings, servicing of residential installations and other similar locations.

UG cables help in ensuring uninterrupted power supply, which are uncommon in the overhead (OH) system. The main hindrance in ensuring uninterrupted power supply in the OH system is the unavailability of space in developed urban areas. Falling of tree branches on the OH lines and short circuiting of conductors due to heavy winds and gales, rains, accidents due to bare conductors, failure of supporting structures, etc. Underground cables offer an affordable and justifiable solution for critical parts and in some cases the entire length of power transmission and distribution lines. With appropriate technology used in appropriate places, the environment impact of underground cables can be minimized. Underground cables help in ensuring uninterrupted power supply that are uncommon in overhead systems.

Underground cables are employed for transmission and distribution of electric power where it becomes impracticable to make use of overhead construction. Such locations may be congested areas where right of way cost would be excessive or local ordinances prohibit overhead lines for reason of safety, or around plants and substations or crossings of wide bodies of water which for various reasons would not permit the overhead crossings. The type of cables used will depend upon voltage and service requirement. Recent improvements in design and manufacturing have led to the development of cables suitable for use at high voltages. This has made it possible to use underground cables for transmission of electric power for short or moderate distances.

Advance design and technology trends in cables are centred on reducing the right-of-way requirement as well as increasing the current carrying capacity of transmission and distribution lines. As space is often constrained in urban areas, transmission and distribution utilities increasingly installing compact transmission and distribution lines, which have lower RoW requirements and small sized towers. Further, new conductors are being deployed for carrying higher currents while allowing higher temperature ratings. The initial heavy cost is the only factor which discouraged the use of underground cables for the purpose of transmission and distribution of electric power.

Characteristics & Design Perspective

Cables are designed and manufactured as per voltage, current to be carried, operating maximum temperature and purpose of applications desired by customer. Cables are designed according to number of conductors they contain and their size. The larger the diameter, the greater the wire’s capacity is to carry current. The most commonly used metal for wires is copper due to its efficiency as a conductor. Underground cables consist of one central core or a number of cores of tinned stranded copper conductors (sometimes use of aluminum conductor is also made) insulation from each other by paper or varnished cambric or vulcanized bitumen or impregnated paper. Insulation should possess high resistance, high dielectric strength, high mechanical strength and long durability. A metallic sheath of lead or alloy or of aluminum is provided around the insulation to protect it against ingress of moisture.

Electrical cables can be made more flexible by stranding the wires. Electrical cables are also used for wiring in households for solving the purpose of lighting for power and control circuits that are permanently installed. Cables are securely fastened and organized by using cable trays, cable ties or cable lacing. To limit the spread of fire along cable jacketing use of cable coating materials with jacketing having inherently fire retardant is used. The plastic covering on metal clad cables is stripped off at installation to reduce the fuel source for fires. Inorganic coatings and boxes around cables also safeguard the nearby areas from the fire threat associated with unprotected cable jacketing. This fire protection also traps heat generated from conductor losses so the protection needs to be thin.

Technological Trends

Several technologies have been introduced to enable utilities to augment their capacities without battling right-of-way (RoW) clearances. An emerging technology trend has been the adoption of underground cables. For high voltage transmission network, cross-linked polyethylene (XLPE) and Gas Insulated Cables (GILs) have been developed.

Cross-linked Polyethylene (XLPE)

Cross-linked polyethylene (XLPE), high-density polyethylene, aerial bunched cables and spacer cable system are the most commonly used covered cables. XLPE cables use cross linked polyethylene as the main insulating material and can operate at higher temperatures, both for normal loading and under short circuit conditions. Extruded XLPE cables are increasingly being deployed by utilities such as PGCIL for setting up transmission infrastructure in difficult terrains.

XLPE cables use cross-linked polyethylene as the main insulating material. Cross-linking inhibits the movement of molecules under the stimulation of heat and this gives these cables greater stability at high temperature, as compared to thermoplastic materials. XLPE cables can operate at higher temperatures, both for normal loading and under short-circuit conditions. These cables have a higher current rating than an equivalent polyvinyl chloride counterpart.

XLPE-insulated cables are also useful in direct current power transmission. Traditional DC power cables include oil filled or mass impregnated non-drain cables that have limitations for long distance power transmission. While the former requires frequent oil refilling, the later type suffers from the low operating temperature. Extruded XLPE cables are increasingly being deployed in new underground transmission throughout the world. Extruded XLPE cables have a high transmission capacity, which is not limited by route length.

Gas Insulated Cables (GILs)

Gas insulated lines are also gaining momentum in the international market, through the domestic market is yet to witness their widespread adoption. GILs serve as a viable alternative to overhead lines where RoW is not available for the transmission of electricity. This is primarily because GILs can be installed under the ground as well as in tunnels and trenches. Moreover, the resistive losses of GILs are lower than overhead lines and they offer greater reliability with no risk of fire. GILs have nitrogen and sulphur hexafluoride as the insulating medium. It comprises aluminum conductors supported by sealed tubes pressurised with gas nitrogen and sulphur hexafluoride in 80:20 proportion as the main insulation. These lines are ideally suited for metropolitan areas and cities where there is limited RoW for overhead lines. GILs can also be installed in agricultural areas and the ground above is still viable for growing crops. The installation of vertical GILs is popular in hydropower plants as there is no fire hazard associated with them.

GILs come with several benefits. Firstly, the resistive losses of GILs are lower than of overhead lines and other types of underground cables due to the larger size of conductors and lower resistance. Secondly, these lines offer greater reliability with no risk of fire and have electromagnetic fields that are 15 to 20 times smaller than those of conventional power transmission systems. Moreover, GILs are unaffected by high temperatures, high solar radiation and pollution. Issues related to GILs include deterioration in the insulation properties owing to particle contamination and limited protection from seismic activities / earthquakes and limited maturity of the technology.

Hybrid Cable

Hybrid optical and electrical cables are used in wireless outdoor fiber to- the-antenna (FTTA) applications. The optical fibers carry information and the electrical conductors are used to transmit power. These cables are placed in several environments example to serve antenna mounted on poles or towers. In order to increase the current carrying capacity and scale down the transmission and distribution losses, utilities are turning to high temperature superconductors. As compared to conventional conductors cable, these conductors cables have 5 to 10 times the current carrying capacity, are compact in size and have a lower RoW requirement.

Demand Drivers

The development and strengthening of the country’s transmission and distribution network has always been a key focus area. To this end, number of government initiatives and programmes have been undertaken such as the Deendayal Upadhyay Gram Jyoti Yojana (DDUGJY), the Integrated Power Development Scheme (IPDS) and the recently launched Sahaj Bijli Har Ghar Yojana (Saubhagya). All these schemes have created a market for cables and conductors in the country.

Overall, the cables and conductors industry has grown significantly in the past few years with investments infused in the power and infrastructure sectors by the government. While the cable segment witnessed a positive growth, the conductor segment experienced sluggish demand last year. One of the major drivers for the cable industry is the need of integration of the new renewable-based capacity being added in the grid. With the development of large-scale renewable energy plants and solar parks, there is a need to lay down lines in order to connect them to the existing grid. For wind power plant customers generally require flexible and UV protected cable with mechanical tough sheath.

Meanwhile, investment in the coming years will continue to be driven by the Central Government through schemes such as DDUGJY, IPDS and Saubhagya. Even though the DDUGJY scheme is nearing its target, the focus on household electrification through Saubhagya will continue to sustain the demand in the cable and conductors segment. Further, with most of the states adopting Ujwal Discom Assurance Yojana, the state discoms are expected to improve their T&D infrastructure through renewed capital expenditure.

Apart from the impetus provided by the government, another key driver for the growth of the cable segment is the shift towards high voltage transmission lines. This demand is essentially driven by the creation of high voltage long distance corridors to deliver electricity to high demand regions and the development of green energy corridors for integrating the increasing share of renewable energy into the grid. The future growth in the cable segment is likely to be driven by 132 kv and 400 kv lines. Underground cabling too is gaining increased acceptance among state and central transmission utilities as it provides greater safety as compared to overhead cables. The increase in government infrastructure spending, particularly, under the smart cities mission, is expected to propel the demand in the country and present new opportunities in the cable segment.

Challenges

The biggest challenge for players in the cables segment is input price volatility. Delay in the execution of projects is another major impediment. Most projects get delayed due to the requirement of multiple clearances and approvals, and the shortage of manpower. Obtaining a RoW clearance is particularly, difficult given the space constraints in cities. Thus, it is important for the government to develop mechanism to facilitate a single window clearance for all the players.

One issue that is often voiced by some players is that there is a lack of a level playing field in the industry. The tenders released by utilities often specify a pre-qualification criterion pertaining to a particular manufacturing process in the tender that tends to restrict wider participation, especially, from the smaller manufacturers. Lack of standardization of the end product is also a challenge for the sector. The cables market is largely unorganized, as a result of which several manufacturers do not comply with product guidelines and the end result is an asymmetric product quality. There is, therefore, a need to develop standard guidelines for the industry as a whole. Moreover, the cable segment is dependent on imports from other countries and is thus highly sensitive to exchange rate variations.

Operating Limitations

Cables require high charging current and reactive power for operation. The reactive power is capacity in nature and can affect at lightly loaded conditions due to Ferranti effects of rising of sending end voltage of the cable. Flow of charging current causes heating of cables and reduces the current carrying capability. Sometimes, higher dielectric loss may further rise in the temperature. Switching of cable capacitive current may give rise to over voltages at receiving end. Use of UG cables is also limited due to large charging current for long distance transmission. Underground cables expensive compared to overhead lines and difficulty in fault detection. Restoration of power supply takes longer time during breakdown in underground cables.

Benefits

The underground cables have several advantages such as less liable to damage through storms, lightning, low maintenance cost, less chances of faults, smaller voltage drop and better general appearance. Cables system provides high reliability, least interruptions due to line faults and good safety. In addition to this, the I2R losses are quite low compared to OH system due to the absence of steel wires. Its advantages such external protection is provided against mechanical injury, moisture entry and chemical reactions. Less subjected to damage from severe weather conditions (mainly lightning, wind, freezing). Underground cables pose no hazard to low flying aircraft or to wildlife. Underground cables have much less danger of conductor theft, illegal connections, sabotage, and damage from armed conflict. Most of the distribution utilities are using cables for reduction of AT&C losses.

Way Forward

With the growth of smart grid technologies and the increase in renewable energy capacity, specialized cables are being designed. For instance, medium voltage smart cables, which integrate optical fiber cable with power cables, are preferred for smart grid applications. Similarly, specialized DC cables in which installation is crossed linked through electron beam irradiation are being deployed for solar power projects. These cables are required to work in a harsh environment and withstand direct sunlight, humidity, rodent attack etc.

Conclusion

Underground cables offer an affordable and justifiable solution for critical parts and in some cases the entire length, of overhead high voltage power lines. The type of cable to be used at a particular location is determined by the mechanical considerations and the voltage at which it is required to operate. The cables market looks positive owing to the continuing investments by the government through its various schemes and increasing renewable energy capacity addition.


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