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Tuesday, 14 October 2014

Transmission and Distribution losses in India

Nearly all the modern generating power plants are located far from the actual electricity load centres. Transmission of generated power to major load centres and their further distribution is carried out through transmission and distribution lines. The losses occurring in the transfer of power from the generator to the end consumer is known as Transmission and Distribution (T&D) losses.

Transmission and distribution network comprises of overhead lines, cables, transformers, switchgears and other equipments to facilitate the transfer of electricity. The efficiency of these networks is improved by reducing the T & D losses. 

In India electricity is usually generated at a voltage ranging from 11 kV to 25 kV and is stepped up to a voltage of 400 kV or 765 kV (EHV range) for their transmission. Of course bipolar HVDC lines of voltages 500 kV and 600 kV are in use. The voltage is stepped up mainly to reduce the I2R losses. At the distribution end it is again brought down to a very low value. Most of the residential, commercial and agricultural consumers are supplied at a low voltage of 230/415 V. Larger commercial and industrial consumers are typically supplied at voltages of 11 kV, 33 kV or greater values.

In the process of supplying electricity to consumers, some power is dissipated in the transformers while stepping up or down the voltage levels. Some power, in the form of I2R losses, is lost in the lines and cables that carry the power. Losses occurring at various stages of power transformation and loading of transmission system at 132 kV and above are known as transmission losses, whereas losses at 33 kV and below this voltage are distribution losses. Normally the transformer losses are 0.5% of the total losses, for each voltage transformation. The line losses are nearly 2% for transmission voltage and are 4% and 5% respectively for primary and secondary distribution system.
Apart from T & D losses there are commercial losses too. Therefore the concept of Aggregate Technical and Commercial (AT & C) loss was introduced. The AT & C loss reflects the technical as well as commercial losses in the whole network and hence is the true indicator of the system efficiency.

Causes of losses
The technical losses depend largely on: 

  • system configuration, 
  • pattern of loading of transmission and distribution lines, 
  • magnitude and types of loads, 
  • characteristics of equipments. 
The losses are higher due to weak and inadequate sub-transmission and distribution lines, inappropriate sizing of conductors, lengthy transmission and distribution lines and inadequate reactive compensation in the system. The non-technical or commercial losses are component of distribution system losses and are because of unidentified and uncollected revenue arising from metering errors, shortfalls in billing and revenue collection and consumer malpractices such as meter tampering, illegal connections etc.

Comparison between networks of different countries is not straight forward and it is difficult to define an optimum level of losses for a network. The AT & C losses vary in various Indian states from 20 to 45% because of the difference in load density, type of network (urban or rural lines), voltage of supply, geographical size of the area etc. Other factors such as inadequate design, monetary investments and bad maintenance of the system over the years contribute to higher losses.