Browsing: Transmission and Distribution

Underground Cables

Underground Cables are used for transmission and electric poser in congested areas (Such as in cities, towns etc.) at comparatively low at medium voltages. Usually, the underground cables used in congested cities or towns, substations, railway crossings and where safety is very important. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltage. In this article, we will discuss about the definition of underground cable and construction of underground cables. 

What is Cable?

A conductor covered with a suitable insulation and protecting layer is called cable.

Requirement of Underground Cables

  • The conductor used in underground cable should be standard in order to provide flexibility. The conductor should have high conductivity.
  • The area of cross section should be such that the cable carries the desired load current without overheating.
  • The voltage drop in conductor should be within permissible limits.
  • The cable must have proper thickness of insulation.
  • The cable must be provided with suitable mechanical protection.
  • The cables do not react with chemicals.

Construction of Cables

The cables have various parts. There are given below.


The cable consists of one or more than one core. It depends upon the typ0es of service for which it is intended. The conductors are made of tinned copper or aluminium . The stranded conductors are used in order to increase flexibility.

underground cables


Each conductor or core covered with proper thickness of insulation. Basically thickness of insulation depends upon the voltage level. The commonly used insulating materials are impregnated paper, varnished cambric etc.

Metallic Sheath

A metallic sheath is provided over the insulation in order to protect the cable from moisture, other liquids that are present in soil and the atmosphere. The commonly used material for metallic sheath is lead or aluminium.


Over the metallic sheath bedding layer is provided which consists of a fibrous metarial like jute or hessian tape. Bedding protects the metallic sheath against corrosion and from mechanical injury due to armouring.


Armouring is provided over the bedding which consists of one or two layers of galvanized steel wire. It protects the cable from mechanical injury.


In order to protect armouring from atmospheric conditions, a layer of fibrous material provided over the armouring. This layer is called serving.

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Interconnected System

The connection of many generating stations such as hydro power plant, Thermal power plant, Nuclear power plant etc running in parallel is called interconnected system. The interconnected system may be two types (i) Integrated and (ii) Uni-integrated. An integrated interconnection results in maximum overall economy. But most of the system interconnections are unintegrated. An unintegrated interconnection the identity of individual not lost. There is no central control office like integrated interconnection system.

Advantages of Interconnection or Interconnected System

The interconnected system has many advantages. These are given below.

  1. It increases the service reliability. Maintenance and replacement of any equipment is required during its operation so if the system is interconnected it will increase the reliability. An electrical energy is generated from different sources, such as water, coal, nuclear etc. It the system is interconnected, then we will choose that plant where will have to pay less cost. During peak load we can receive or transmit energy from large capacity plant.
  2. Load growth necessitates additional transmission facilities. If the system is interconnected them we have a provision to add and replace transmission lines. It makes the system more reliable.
  3. Reserve capacity required is reduced with interconnected system we do not need reverse capacity of plant.
  4. Reduction in total installed capacity. An interconnection decrease the installed capacity needed to meet the load requirement. Different areas have different demands of electrical energy. In areas required less energy and at the same other area required more electrical energy. If two such areas are interconnected, the diversity of load would cause the maximum combined demand to be less than the some of the individual maximum demands. In this way, diversity factor improves.
  5. The interconnected system improves the efficiency of plants.
  6. Economical operation of station is ensured. We can choose that plant which has lower running cost in this way the system become economical.
  7. With system interconnection capital and maintenance cost is reduced.

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Classifications of Over Head Transmission Lines

An electrical energy transmitted from generating power stations to substations or grid through overhead transmission lines. A transmission lines has three constants resistance R, inductance L and capacitance C. These constants are distributed uniformly along the whole length of the transmission line.

The line resistance cause voltage drop (IR) and power loss (I2R) in the line, the inductance also causes voltage drop due to inductive reactance (IXL). The resistance and inductance form the series impedance. The capacitance produces charging current (2∏fcv) in the line, which quadrature with the voltage. This constant existing between conductor and neutral for single phase and or three phase line forms shunt path through out the length of the line. The capacitance effects make the calculations complex. The capacitance effects are more predominant in case of underground cables. The overhead transmission lines are classified as.

  • Short transmission lines
  • Medium Transmission lines
  • Long transmission lines
  1. Short transmission Lines

Transmission lines having length about 50km and operating voltage is comparatively low about 20KV, it is usually considered as a short transmission lines. Due to smaller distance and lower operating voltage, the capacitance effects are extremely small and hence can be neglected. Hence, performance of short transmission lines depends upon resistance and inductance only. Therefore, while studying the performance of such lines, only resistance and inductance of the transmission line are taken into account. In short transmission lines, Constants are assumed to be lumped at one phase.

  1. Medium Transmission Lines

Transmission lines having length is about 50 to 150km and operating voltage is more than 20KV and less than 100KV, it is considered as medium transmission line. Due sufficient length and operating voltage of the line, capacitance effects are taken into account. The capacitance is distributed uniformly over the entire length of the line. It is assumed to be shunted across the line at one or more points.

  1. Long Transmission Lines

Transmission lines having length more than 100KM and operating voltage more than 100KV (that is very high), it is considered as a long transmission lines. In case of long transmission lines constants are considered uniformly distributed over the entire length of the transmission line.

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