Why all the voltage levels are multiples of 11 kV?

All the power generation units generates energy which is the average value.
We required to use RMS value of the electrical quantities.So all the average value is multiplies by the form factor
(1.11) to give RMS value, so the all the voltage levels are multiple of 1.11.

Firstly, not all of them are.

400kV is the top voltage transmitted in GB, and it is not a multiple of 11. Similarly 13kV is commonly used by the
distribution networks, which also is clearly not a multiple of 11kV.
The rail network uses 25kV for most of its connections to the grid, also not a multiple of 11. Countries with longer connections are known to use other even higher HV values, again most of which are not multiples of 11.

The voltages in common use that are multiples of 11kV are 33, 66, 132 and 275.

These are a result of many generators- particularly in the early days of the electrical industry- working at 11kV, which when being stepped up to transmission voltages and down again to distribution voltages meant that the networks at the time would use simple transformer coil ratios- 3:1, 6:1 etc.

The reason for them working at 11kV came primarily due to the limits of such factors as insulation and eddy currents etc in the generator modules themselves. As technology has progressed many have stepped up to higher voltages. Often this is 22kV as, again, the transformer ratios are simple.
In fact not only are the transformer ratios simple, but insulator stacks can simply be doubled up on.

To change the network voltages from those that have arisen historically would be both a vast cost and incredibly disruptive, which is why it has not been done. It would also require the re-tooling of almost every factory that makes HV equipment.
However, when the higher voltage 400kV (and higher) transmission networks were added there was no requirement
for the transformer ratios to be kept so simplistic and the voltage was chosen based on other engineering factors  instead.

Contrary to the statements above, the form factor of 1.11 for sine wave power compared to transmission losses was not an influence. 
Losses are well below 10%. 1/10 losses in a 1000MVA line would destroy most transmission systems by melting the
conductors with 100MW of heat being dissipated along it. In fact the entire point of HV AC transmission is to keep the losses to a minimum.

The form factor is simply the ratio of the RMS to the average of the absolute values. This ratio gives the ability to calculate the DC equivalent power in a system by comparison to the AC voltage.
The figure of approximately 1.11 would still apply at the users end, regardless of any losses, because it is a ratio.

• The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is approximately 1.11.
• The reason is something historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side.
• This is the reason. It has nothing to do with form factor (1.11).
• Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.
• Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise.
• The Basic Idea behind a desired transmission voltage was still the form factor. In ancient times when we needed to use 10 kV at destination, simply multiplied the form factor to it Say 1.11X10=1.11 =11KV aprox.(we had taken 10% losses as standard thumb rule) similarly for 30 & 60 KV.
• Form Factor = RMS voltage/Average Voltage For AC sine wave Form Factor is 1.11.
• Now a days when, we have voltage correction systems, power factor improving capacitors, which can boost/correct voltage to desired level, we are using the exact voltages like 400KV in spite of 444KV.