Electrical Distribution System: Overhead Vs Underground Cables

The article talks about how the cable technology, both for overhead and underground transmission of electrical power has gained increased attention in recent times due to changed perception of the electrical power system from the point of view of reliability, safety and economic aspects.

Overhead Versus Undergroud Cables
(Image for representation only)

The prominence of electrical energy is felt in almost every sector of a common man’s daily activities. The electrical power system is generally segmented into generation, transmission, distribution and utilisation. A distribution network is one of the key parts of an electrical power system which is generally directly connected to the load center. The generated and then transmitted electrical energy is distributed to customers through a utility’s electrical distribution network. The distribution system network consists of electrical distribution substations which step-down the transmission line voltage levels between 69 kV and 765 kV to distribution voltage levels, usually 35 kV or less. As per Indian standards, the level of stepped down voltage is generally 33 kV/11 kV. Distribution networks can consist of overhead electrical lines, as well as underground cable systems. Voltages at utility customer delivery points may require further reduction or stepping down, either by utility transformers or customer owned and operated transformers to a level of 400 V (three phase) or 230 V (single phase).

After the concept of integrating both renewable and distributed energy sources at the distribution level is conceived, the distribution system design and operation is currently of great interest for power system engineers. The demand from the end users of electricity in terms of electrical power quality and the improved reliability of the services has further made the distribution system operation more interesting and challenging. As per one of the reports of World Bank, India loses 4 per cent of its GDP due to inefficiency in power distribution, ranking it 80th among 137 economies in terms of reliability of power supply. India has the dubious distinction of losing 20 per cent of electricity in transmission and distribution, one of the highest in the world which amounts to be somewhere around Rs 8,500 crore per annuum. The changing paradigm in the field of electrical distribution system has resulted in many novel practices being adopted for the overall improvement of the system efficiency and reliability of the power supply. The issue of T&D losses can be addressed by proper transformations and implementation of novel technological innovations.

Electric power can be distributed either by overhead transmission systems or by underground cables. Hence, proper selection of the cables in the distribution network is crucial so as to assure required level of operational reliability with due consideration of the cost aspect.

Cables and their selection
Cables are different from the bare overhead conductors in the way that the cables are provided with insulation. Hence, the relative safety aspect can be assured. The design of the cables is generally based on the requirement. The cables used for the transmission and distribution of the power are referred to as power cables which is an assembly of one or more individually insulated electrical conductors, usually held together with an overall sheath. These power cables may be commissioned as permanent wiring within buildings, buried in the ground and run overhead or exposed to the atmosphere. The portable devices, mobile tools, and machineries make use of flexible power cables. The design and manufacture of these cable is as per the rated voltage, current, maximum operating temperature and purpose of applications desired by the customer.

Construction of power cables
Generally, a typical power cable consists of the following:
• Conductor
• Insulation
• Beading
• Armoring (optional)
• Outer sheath

Figure 1 shows a typical power cable. Conductors happen to be power carrying part of the cable. Though the conductors can be made of different materials copper and aluminum-based conductors are quite common due to their excellent properties conducive for better conductivity. Due to the advancements in the field of material science, several other materials like cadmium – copper alloys, phosphor bronze, galvanised steel, steel core copper and steel core aluminum are also being used as conductors in the recent times. Generally, the conductor resistance determines the conductor size. The conductors can be further classified as solid, flexible, extra flexible, and stranded conductors based on the requirement of the end application.

Overhead Versus Undergroud Cables Fig 1
Fig 1. Typical power cable (Picture courtesy:www.electrical4u.com)

The different insulating materials used in cable are selected based on the operating temperature and also the voltage and current rating of the cable.

Beading is the process by means of which a cable is provided with more mechanical strength. Sometimes, the beading is also used for earthing purpose.

Armoring is a process by which the earthing shield to the current carrying conductors is provided. Armoring is also used for earthing purpose of the cable for safety purpose. If the cable is properly earthed, the fault current gets enough paths to flow through the armor in case of any insulation failure in the conductor.

Outer Sheath
This is the outermost cover of the cable which provides the cable, mechanical, weather, chemical and electrical protection. The outer sheath is normally made of PVC (Poly Vinyl Chloride), Rubber (Various Types of Rubber).

Overhead Versus Undergroud Cables Fig 2
Fig 2: A typical overhead line
Overhead Versus Undergroud Cables Fig 3
Fig 3: Size comparison of overhead underground cables

Underground and overhead cables
The general understanding is that the underground cables are laid beneath the ground and the overhead cables are visible overhead. But apart from this, there are many significant features of both the types of these cables from the perspectives of electrical power transmission or distribution. In the changed scenario of the power system design, particularly, the distribution system, the cables and their characteristics have become highly selective and the technological advancements have also made the selection of cable for a particular application more flexible.

Overhead cables
Traditionally, the overhead cables or the bare conductors have been used for the transmission of electrical power and even for the distribution. These are simple in configuration and are usually commissioned using the towers or poles. The bare wire conductors on the line are generally made of aluminum (either plain or reinforced with steel or composite materials such as carbon and glass fiber), yet, some copper wires are used in medium voltage distribution and low-voltage connections to customer premises.

Overhead Versus Undergroud Cables Fig 4
Fig 4. Different cable types (Courtesy: Universal Cables Limited, Satna)

As the conductors are constantly exposed to the open atmosphere, there are several concerns related to the safety of the system and also reliability. The adverse conditions like heavy rain, wind, snowfall, humid and salty contents in the air could deteriorate the lifespan of these conductors and raise serious electrical safety concerns. Nevertheless, the most unique advantages like less cost of conductors due to less insulation levels required, ease of fault detection due to clear visibility of the conductors, relatively less cost of installation, and ease of expansion are some of the general advantages of the overhead conductors. These advantages are backed up by several other technical advantages as well like being independent of proximity effect, relatively smaller size of conductors, relatively higher life expectancy. However the changed perception of looking at the different premises from the point of view of aesthetic nature and the recent developments in the cable technology have made the deployment underground cables for the power distribution and even transmission more attractive in the recent times. This is also due to some of the critical limitations of the overhead cables apart from their non-aesthetic appearance. The overhead cables often cause the radio interference due to corona discharge. Overhead high voltage lines emit hiss or hum noises.
Overhead Versus Undergroud Cables Table1
Underground cables
The underground cables provide the uninterruptable power supply which is not possible with the overhead lines due to the limitations mentioned earlier. However, there are other technical factors which also have made the underground cable to have an edge over the overhead cables which include reduced risks of fault due to external factors like rain, wind and adverse climatic conditions. The underground cables are  also free from radio interference. The transmission towers are not required except for the local transformers in the system without considerable height of the tower. However, the underground cables are not free from limitations. Damage to underground cable is difficult to locate, and restoration of the system once the faults are located might take considerably long time. For underground cable system, a large number of cables is required for the same capacity of the overhead counterpart. The construction mechanism of the underground cable involving duct bank, vaults, splices and terminations not only increase the overall cost but also might reduce the overall system reliability. This problem further might increase with the increased line length with the additional necessity of the intermediate equipment. Due to the concealed operating conditions, the heat dissipation from the underground system can also be one of the major bottlenecks for the successful operation of the system. Summarizing all these points, the following table gives a brief comparison between underground and overhead cables.

Recent developments in the cable technology
As the underground cable system is becoming more relevant and popular, there have been some tremendous technological novelties developed to overcome the different shortcomings of the underground cables. The major areas of concentration of these technological novelties are voltage grade and insulation. The developments in the field material science have resulted in the better-quality polymers to achieve desirable electrical and mechanical properties of the cables. Teflon cables can withstand the temperatures up to 250C against conventional PVC cable which can generally withstand up to only 70C. The alternative conductors against the copper which suffers from high cost have already been explored. Concurrently, the operating voltage levels have also increased significantly in the range of 220 kV to 400 kV. These developments can definitely take the cable technology to the new technical dimension and make them more deployed in the transmission and distribution networks. Having mentioned about the recent technological trends in the underground cable manufacturing technology, the similar kinds of advancements have taken place even in the overhead cables. Based on the requirement of conductor types like ACSR (Aluminum Conductor Steel Reinforced), AAAC (All Aluminum Alloy Conductor), ACAR (Aluminum Conductor Alloy Reinforced), AACSR (Aluminum Alloy Conductor Steel Reinforced), AAC (All Aluminum Conductors), operating Temperature Conductors, operating voltage levels, Voltage (132 kV to 220 kV, 221 kV to 660 kV, > 660 kV), Rated Strength (High Strength (10 kN to 75 kN), Extra High Strength (76 kN to 150 kN), Ultra High Strength (> 150 kN), type of the current (HVAC, HVDC) and type of Application (High Tension, Extra High Tension, Ultra High Tension) several cable manufacturers have launched their products in the market as per the requirement of the system.

The cable technology, both for overhead and underground transmission of electrical power has gained increased attention in the recent times due to the changed perception of the electrical power system from the point of view of reliability, safety and economic aspects. Each type has its own advantages and limitations. However, the judicious selection of each type in the power system depends on the specific requirements and also based on the safety of the system.

Overhead Versus Undergroud Cables Shravankumar Nayak
Shravankumar Nayak,
Faculty, Department of Electrical and Electronics Engineering,
SDM College of Engineering and Technology,

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