Power Transmission System

 

Power Transmission is the link of electricity supply from Generating Stations to its consumption point. This is often described as transmission network due to presence of interconnected lines with multiple Step up and Step-Down sub Station Units.

Transmission Lines carry high voltage electricity in the form of Alternating as well as Direct current. It can be a three-phase alternating current power transmission line or a single phase as in railway electric supply lines.

There are two types of power transmission system based on modes of transmission-

1.    Overhead Transmission System: -

 In overhead Transmission System, high voltage electric current is transferred through ground-based towers and line networks. This is the most common mode of power transmission due to its low erection cost and easy maintenance. This network is so widespread that you will find it in almost every cities, towns, and rural areas.

      2)  Underground Transmission System: -

 The underground power transmission system is an alternative to overhead system. The armoured power cables used in underground power transmission have several advantages over overhead system. But the cost of erection is far more than overhead transmission lines.

The underground cables are not exposed to outdoor weather conditions. So once installed it has longer life and requires very less maintenance. There are several types of cables used in underground transmission system depending on its type of usage. Normal XLPE and dielectric liquid filled cables are used for short distances. But HVDC cables are heavy duty cables used for very long distances.

Underground Transmission system has limited use due to lack of its ability to upgrade and very hard fault location and its repair.

 The power transmission Systems are classified into two, based on voltage levels of transmission-

                 1)     Primary transmission: -

At Generating Stations, the electricity generated is at a voltage level of 6 KV to 11 KV at 50 Hz.  This voltage is not ideal for transmitting to distribution networks due to losses over long distances . So, electricity generated at Generating Stations are sent to a Step-Up Power Unit through transmission lines. The step-up voltages can be 66 KV, 132 KV, 765 KV or 1000 KV depending on distance of transmission.  The transmission lines carrying one of these step-up voltages terminate at a step-down power unit where voltages are converted to 132 KV or 66 KV. This is primary transmission in power system.

2)     Secondary Transmission: -

The secondary transmission lines emerge from receiving ends of primary transmission lines and connect to distribution systems. The voltage is further step down from 132 KV or 66 KV to 33 KV or 11 KV.