Home » POWER SYSTEM » ECONOMIC CHOICE OF TRANSMISSION VOLTAGE

Economic choice of transmission voltage:

Higher transmission voltage causes reducing the conductor size. Rising voltage n times mean reducing the conductor size to 1/n2 that of the original.
So, (a) conductor cost decreases with the increase of transmission voltage.
(b) But by increases transmission voltage, the cost of transformer also slowly increases.
(c) Cost of switch-gear also increases with the increase of transmission voltage.
(d) Lighting arrestors cost largely increases with the increase of transmission voltage.
(e) Supports and insulations cost sharply increases with rising transmission voltage.
So, there are several items to be considered for best economic condition. By plotting a curve of total cost at different voltages, we can get the economical transmission voltage.

Here point O is the economical transmission voltage. As per modern American practices based on empirical formula, economical line to line voltage is
V=5.5√ (0.62L+3P/100)
Where L = distance of transmission line in km and P = maximum KW per phase to be delivered over a single circuit.
Another empirical formula used is
V=5.5√(0.62L+KVA/150)
L = length of transmission line in Km. and KVA =total power.

Requirements of satisfactory electric supply:

The electric power transmitted from generating station to consumer via transmission and distribution system. This electric supply should be free from
(i) Low voltage fluctuation– because voltage fluctuation causes lower efficiency of electrical apparatus.
(ii) Dependability– which declares uninterrupted power supply.
(iii) Frequency– it should be maintained as a constant, because varying frequency vary motor speed.
(iv) Balanced voltage– in 3 phase system, balanced voltage should be delivered because unbalanced voltage may cause lower efficiency in induction or synchronous motor.
(v) Minimum harmonics– by using good design generators and high flux densities in transformers, harmonics may be avoided.
(vi) Avoiding inductive interference</em>- by proper transposition of both power lines and telephone lines electrostatic and electromagnetic field disturbances may be reduced.