Answer Posted / shreekar

Let us say transmission voltage is Vt and generated voltage is Vg.
According to some people Vt is 10% more than Vg [because 10% is transmission line drop]. Hence Vt = 1.1 * Vg ----eq1.
According to some people Vt is a multiple of form factor i.e Vt = 1.11 * a ----eq1.
where 'a' is some voltage.
When we see these two equations i.e eq1 and eq2 there is lot of resemblance.

Why form factor theory is correct?

Now in India transmission line voltages are 11kV, 33kV, 66kV, 220kV.
Here the transformer ratio is greater than or equal to 2, between two consecutive transmission line voltage, so electrical engineers use 3- PHASE TRANSFORMER. Hence line voltage is a multiple of form factor (i.e 1.11).

But this is not always true for higher transmission voltage like 400kV and 765kV (Note: 400 and 765 are not multiple form factor). The transformer ratio between 400kV and 765kV is less than 2, this is true as we move to even higher transmission voltage 800kV and so on. If transformer ratio is less than 2, then Electrical Engineers use AUTO-TRANSFORMER (this is because of economical reasons). That is why you have to take phase voltages after 220kV.
Hence, Phase voltage [The reasons to take phase voltages is because of AUTO-TRANSFORMER] of 400kV = 400/1.73 = 231.2kV
231.2kV is near to 220kV and 220 is a multiple of form factor. (this approximation because of corona discharge and it is related to Insulator string efficiency, Tower size and
dimensions of Conductors). [Note: Corona loss depends on transmission line voltage]
Similarly, Phase voltage of 765kV = 765/1.73 = 462kV which is near to 440kV and 440kV is a multiple of form factor.

In the contrary,
you cannot say transmission losses is always below 10%. Some time it may exceed 10%. In major cases it dose not exceed, but who knows what happens the whole power system is a dynamic system.

Conclusion:
Hence transmission line or phase voltage are multiples of form factor. We have to consider phase voltage only when we encounter AUTO-TRANSFORMERS and not the line voltages.

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