As per present terminology, voltages less than 300 kV are termed as High Voltages. The voltages in the range of 300 kV to 765 kV are known as Extra High Voltages (EHV). The voltages which are 765 kV are termed as Ultra High Voltage (UHV). In India, transmission voltage ranges from 66 kV to 400 kV (RMS value of line to line voltage in three phase system). There are several advantages of extra high voltage transmission.
In EHV AC lines additional parallel three-phase line is always provided to ensure continuity of power supply and stability of line. A long EHV AC line is always double circuit line with intermediate substations at an interval of 250 km to 400 km for compensating reactive power.
Electrical energy is generated at a voltage of about 11 kV. Then it is stepped up to 132, 220 or 400 kV for transmission. Electrical power transmission is preferred at high voltages because it has many advantages. The advantages of extra high voltage transmission will be obvious from the following discussion.
Power transmitted is given by
P = √3 V I cos φ
Where, V = transmission voltage, I = load current, cos φ = load power factor.
Or Load current, I = P/( √3 V cos φ)
From above expression, it is clear that for a constant power and power factor, the load current is inversely proportional to the transmission voltage. In simple words, as we increase the transmission voltage, the load current decreases.
Advantages of EHV AC Transmission
This decrease in load current results in following advantages:
- As current gets reduced, size and volume of conductor required also reduces for transmitting the same amount of power.
- Voltage drop in line (3IR) reduces and hence voltage regulation of the line is improved.
- Line losses (3I2R) gets reduced which results in the increase in transmission line efficiency.
Some other advantages of extra high voltage transmission are as under:
- Power handling capacity of the line increases as we increase the transmission voltage. It is proportional to the square of operating voltage. The cost related to tower, insulators and different types of equipment are proportional to voltage rather than the square of voltage. Thus the net capital cost of transmission line decreases as voltage increases. Therefore, a large power can be transmitted with high voltage transmission lines economically.
- The total line cost of per MW per km decreases considerably.
- The operation of EHV AC system is simple, reliable and can be adopted easily.
- The lines can be easily tapped and extended.
Disadvantages of EHV AC Transmission
The major problems in this system are as under.
- Corona loss is a big problem at higher voltages. This may further increase in bad weather conditions.
- It increases radio interference.
- The height of towers and insulation increases with increase in transmission voltage.
- The cost of different types of equipment and switchgear required for transmission increases with increase in transmission voltage.
- The high voltage lines produce electrostatic effects which are injurious to human beings and animals.
Thanks for reading about “advantages of EHV AC Transmission”.
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