Modern trend is to use extra high voltage (EHV) and ultra-high voltage (UHV) for transmission of huge blocks of power over long distances.
The reasons for adopting of EHV/UHV range for transmission purposes are given below:
1. Reduction of Electrical Losses, Increase in Transmission Efficiency, Improvement of Voltage Regulation and Reduction in Conductor Material Requirement:
For transmission of given amount of power over a given distance through the conductors of a given material and at a given power factor as the transmission voltage increases,
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(a) Line losses are reduced since line losses are inversely proportional to the transmission voltage,
(b) Transmission efficiency increases because of reduction in line losses,
(c) Voltage regulation is improved because of reduction of percentage line drop, and
(d) Lesser conductor material is required being inversely proportional to the square of transmission voltage.
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2. Economic considerations have led to the construction of power stations of large capacity and so need of transfer of bulk power over long distances arose. Transmission of bulk power from generating stations to the load centres is technically and economically feasible only at voltages in the EHV/UHV range.
3. Generating stations (Steam-, hydro- and nuclear-power stations) are located in remote areas (far away from load centres) because of the reasons of economy, feasibility and from the point of view of safety and environmental conditions. EHV transmission is, therefore, inevitable for transmission of huge blocks of power over long distances from these power plants to load centres.
4. Flexibility for Future System Growth:
There is flexibility of future system growth.
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5. Increase in Transmission Capacity of the Line:
Power transferred is expressed as:
where VS and VR are the two terminal voltages, δ is the load angle and X is the line reactance.
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Thus the power transmission capacity of a transmission line increases with the increase in transmission voltage. No doubt the cost of transmission line and terminal equipment also increases with the increase in the transmission voltage but in general these costs are proportional to the transmission voltage rather than the square of the transmission voltage. Moreover there is also a saving in cost due to reduction in energy losses occurring in transmission lines. As a consequence the total cost of transmission decreases with the increase in transmission voltage, as depicted in Fig. 13.1.
6. Possibility of Interconnections of Power Systems:
It is practically not possible to have interconnections of two or more power systems, which is necessary to achieve sharing of installed reserves and for development of integrated systems and grids, without EHV transmission.
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7. Increase of Surge Impedance Loading:
Load carrying capability of a line is usually expressed in terms of “surge impedance loading” (SIL). Surge impedance loading (SIL) is the power that a line carries when each phase is terminated by a load equal to the surge impedance of the line.
For a transmission line, the surge impedance is given as ZC = √L/C where L and C are respectively the series inductance and shunt capacitance per unit length. The surge impedance loading (SIL), for a transmission line is given as 3V2/ZC where V is the line-to-neutral voltage.
It is evident that SIL varies as the square of the operating voltage, and, therefore, with the increase in voltage level, SIL itself increases. Thus power transfer capability of the line increases with the increase in voltage level.
The surge impedance of a line can be determined from its conductor configuration. The approximate values of surge impedances for lines with single, double, triple and quadruple conductors are 400, 300, 280 and 260 ohms respectively.
8. Reduction in Right-Of-Way:
In some countries ‘rights-of-way’ are paid for at a rate proportional to the total width of the transmission lines. Even in countries where right-of-way is not directly paid, there are usually strong pressures from the public towards fewer and fewer transmission lines.
With the passage of time right-of-way becomes either more costly or difficult to obtain and therefore it is becoming necessary to have fewer transmission lines operating at EHV/UHV. The worth noting point here is that with the increase in operating voltage, number of circuits and requirement of land is reduced considerably.