Before discussing the method of determination of the most economical working voltage for transmission of electrical power we will discuss here the advantages and limitations of high voltage transmission.
The advantages of adopting high voltage for transmission are given below:
(i) With the increase in transmission voltage the size of the conductor (area of x-section of the core carrying the current) is reduced. This further reduces the cost of the supporting structure materials.
(ii) With the increase in transmission voltage, line current is reduced, which results in reduction of line losses.
ADVERTISEMENTS:
(iii) With the increase in transmission voltage reduction in line losses results in higher efficiency.
(iv) Due to low current at high transmission voltage, the voltage drop in the lines is low. This leads to better voltage regulation.
The above facts can be shown very clearly as follows:
Let the power delivered by n-phase system be P watts per phase at a phase voltage of V volts and power factor cos ɸ.
where l is the length of line, a is the x-section of conductor and p is the resistivity of conductor material.
Volume of conductor material required per phase
Hence from Eqs. (2.21), (2.22), (2.23), and (2.24) it is obvious that for constant values of P, l, δ, cos ɸ and ρ
ADVERTISEMENTS:
(a) With the increase in transmission voltage line losses are reduced since line losses are inversely proportional to supply voltage, V.
(b) The transmission efficiency increases with the increase in transmission voltage, V.
(c) Resistance drop per line is constant. Hence percentage resistance drop decreases with the increase in transmission voltage V.
(d) The volume of conductor material required is inversely proportional to the supply voltage.
(e) From Eq. (2.25) it is obvious that for the same line losses and for transmission of given amount of power over a given distance through the conductors of given material and at a given power factor i.e., for constant values of W, P, I, ρ and cos ɸ conductor material required is inversely proportional to the square of supply voltage.
The above formula, besides showing the great saving of conductor material by employing a high transmission voltage, indicates also the importance of working as near unity power factor as possible. The effect of operating at, say, 0.707 power factor instead of unity, is to increase the amount of conductor material used in the ratio of 1: [1/(0.707)2 ] or an increase of 100%. A very considerable economy can be affected in the conductor material if the transmission line is operated at high power factor.
Where there are several advantages of high voltage for transmission, there are some limitations also, which are given below:
(a) With the increase in voltage of transmission, the insulation required between the conductors and the earthed tower increases. This increases the cost of line supports.
(b) With the increase in voltage of transmission, more clearance is required between conductors and ground. Hence higher towers are required.
(c) With the increase in voltage of transmission, more distance is required between the conductors. Therefore, longer cross-arms are required.
For every transmission line there is a superior limit fixed for the voltage to be employed, beyond which nothing is gained in the matter of economy. This limit is reached when the cost of conductors, cost of insulators, supports, transformers, switchgear, lightning arrestors and also the cost of erection is minimum.
The method of finding the optimum voltage is to choose a certain standard voltage and calculate the cost of:
(i) Transformers to be used at the generating and receiving stations,
(ii) Switchgear consisting of isolators, bus-bars, circuit breakers, relays etc.,
(iii) Lightning arresters,
(iv) Insulators,
(v) Supports including the cross-arms and other fittings, and
(vi) Conductors and cost of erection for the given voltage of generation, given power to be transmitted and given length of the transmission system.
The total sum of all the costs gives the total capital cost of transmission. Thus for various standard voltages the total capital cost is worked out and a curve of capital cost against the transmission voltage is plotted. The curve plotted is of the form shown in Fig. 2.6. From the curve it is observed that the optimum voltage from economy point of view will be voltage corresponding to point A on the curve. In fact higher voltage would be chosen as with large working voltage, power loss and voltage drop are reduced, load can be increased if required and control becomes easier.
In practice it is not possible to determine the economic voltage by the method explained above. To avoid the complications and labour Cable Research Hand Book gives an empirical formula for determining the optimum voltage for lines more than 30 km long.
The formula is:
Approximate economical voltage for transmission in kV
The worth noting point is that power to be transmitted and distance of transmission have been taken into account in the above formula, because both of these factors have considerable influence in arriving at economic voltage for transmission, as explained below.
With the increase in distance of transmission, the cost of terminal apparatus is reduced resulting in higher economic transmission voltage. Similarly if the power to be transmitted becomes large, the cost per kW of the terminal station equipment is reduced.
As a rough guide the voltage for transmission is chosen as 625 volts per km though in practice the voltage per km varies from about 400 to 900 volts for longer to shorter distances. The choice is usually limited in practice by the requirement of standardisation and for satisfactory regulation without excessive equipment cost.
The voltages normally adopted for transmission are given below:
The most common transmission voltages employed in India are 33 kV, 66 kV, 132 kV, 220 kV and 400 kV.