High Voltage (HV) Engineering

Why are power engineers interested in Very High Voltages? Can't they do it in low voltage?

According to the Ohm's theorem;
Power loss in a conductor, P' = I^2 * R
=> So the power loss is directly proportional to the square of the line current.
=> Therefore to reduce losses, 'I' should be minimised.

But,
The Power requirement is fixed. Then by looking at the equation -
Apparent power, P = V * I
=> To minimise I;
=>V should be maximised.

That leads to the conclusion that transmission voltage should be increased to the MAXIMUM possible value.
Can we do that?
Even if we produce a three phase voltage of 1,000,000,000 V for argument's sake, can our equipments withstand it? 

starting from the power cable of transformers, transformer terminals, power conductors, line towers, insulators, switchgear, and numerous things in a line SHOULD be made to withstand that same 1,000,000,000 V - Which is nearly impossible in present conditions.

So it boils down to simply an issue of trade-off.
=> bigger the voltage better; smaller the current better. for that it means a VERY big investment. So let us reduce some voltage. Then the current flow is going to increase. So as the line losses!

Three types of matter:
As matter is present in 3 kinds, so as the electrical insulation is of following types:

1. Solid
2. Liquid
3. Gas

HV breakdown:

High voltage naturally consists of an electric field pattern, and it depends on the voltage. It implies that certain insulation is capable of catering a certain maximum voltage. Beyond that, it will exhibit signs of leakage. That may sometimes lead to complete breakdown of that insulation medium. This value for still air is about 30 kV per cm.

A patterned arc is formed when a worker tries to close an Air break switch which connects to a shunt line reactor

HV insulators are of special design in order to create a longer creepage path. This avoids flashovers to some extent. But when dust particles and particulate matter get deposited on the insulator surface, and when rain water falls on that dirty surface, flashovers become frequent. Proper maintenanance of Hv insulators are thus essential for power quality.

Increasing the creepage path can reduce insulator flashovers

Corona:
We can say corona is a phenomenon which occurs near a HV conductor. This effect may have been observed by many people residing under HV lines and especially near line supports. 

An electrical field (stress) will be present around any  electrical conductor. In HV conductors, the field strength would be higher. When this value increases beyond a certian limit, it starts to ionise the sorrounding air molecules. When sufficient ions are formed around that line, a portion of air becomes conductive (or called leakage). This is called 'corona effect'.

Corona glow around a HV coil

Corona has the following features, which are clearly visible in night time.
1. Hissing sound
2. Bluish/violet glow

This certainly means there is a power loss due to corona effect.
It should be noted that the electrical field strength will become more in bent/sharp conductors. Near the pole supports conductors need to bent for spur connections/ tensioning. That's why we observe corona near pole supports.

Terms associated with corona voltages are:
-corona inception voltage
-disruptive critical voltage