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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-
- Overhead Transmission System: -
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.
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.
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