Scenario of Transmission and Distribution Losses
- The power plants typically produce 50 Hz alternating current electricity with voltages between 11 kV and 33 kV.
- At power plant site the three phase voltage is stepped up to higher voltage for transmission through conductors strung on towers.
- High voltage and extra high voltage transmission is the next stage from power plant to transfer AC power over long distances at voltages like, 220 kV and 400 kV.
- Sometimes for transmission of long distances, say above 100 km, high voltage direct current transmission is used , which minimizes the losses.
- Sub-transmission network at 132 kV, 110 kV, 66 KV or 33 kV constitutes the next link towards the end user Distribution at 11 kV, 66 kV or 33 kV constitutes the last link to consumer, who is connected directly or through transformers depending upon demand level of service.
- The transmission and distribution network include sub – stations, lines and distribution transformers.
- High voltage transmission is used so that smaller and more economical wire sizes can be employed to carry the lower current and to reduce the losses.
- Sub-stations containing step-down transformer reduce the voltage for distribution to industrial users. The voltage is further reduced for commercial facilities.
- There is no difference between a transmission and distribution line except for the voltage level and power handling capacity.
- Transmission lines are usually capable of transmitting large quantities of electricity over large distances.
- They operate at high voltages. Distribution lines carry limited quantities of power over shorter distances.
- Voltage drop in the line are in relation to the resistance and reactance of line, length and the current drawn.
- For the same quantity of power handled, lower the voltage, higher the current drawn and higher the voltage drop.
- The current drawn is inversely proportional to the voltage level for the same quantity of power handled.
- The power loss in line is proportional to resistance and square of current. Higher voltage transmission and distribution helps to minimize the line voltage drop in the ratio of voltages and the line power loss in the ratio of square of voltages.
- For example, if the voltage is raised from 11 kV to 33 kV, the voltage drop will be lowered by a factor 1/3 and the line loss will be lowered by (1/3)2 that is 1/9.
- Lower voltage transmission and distribution also need bigger size conductor on account of current handling capacity needed.