Disc insulator

Where did you get the figure of 11KV per disk? It's not a definite voltage.

It can be what ever it's designed for.

11/1 = 11kV/disc
220/18 = 12.2 kV/disc
400/25 = 16 kv/disc

The safe operating voltage for shorter stacks is lower-per-disc. This does not seem unusual to me. I don't know the specific geometry of your application, but I suspect the field gradient for short stacks is significantly higher which increases the probability of a surface flash-over.

This is quite an interesting question. The fact that has been brought out and then as shown by you is correct.

But there is another fact.

As you string the insulators, due to stray capacitance losses the voltage gradient is not constant.

It is highest near the power line and lowest near the earth point. The difefrent designs do try to bring it as close to equality as possible but the different remains.

And this means the disk nearest to the conductor takes quite a bit more of the voltage drop across it (and is electrically overstressed).

My guess is that the factor here is not the breakdown of the insulator but rather the creep length (preventing flashover) is the one that is in picture.

As the disks string together, the upper one shades the next one and that effectively makes the creep length just a bit higher than the bottom corrugation.

Any other factors?

The number of insulator disc in a voltage system is selected by the calculation of the total minimum creepage distance. There are different types of insulators discs available in market with different minimum creepage distances.

Example: For 400KV system for heavy pollution area, from above table,

Therefore, no. of disc required = 9660 / 320 = 30 numbers

Vinu_Answers Sure_Answers

The Table-1 in your answers says that for heavily polluted atomospheres, the minimum creepage requirement is 25mm per kV. And, vide your Table 3, for hevaily polluted atomospheres, for 420kV, the minimum creepage is 9660mm. But, wherefrom you got this 320, in the calculation from? Is it the nominla creepage of one disc in a disc insulator?

Dear ee65,

This is to inform you that,

1) Table -1 reference in my previous post has been taken from IEC-60071-2 (1996), This table shows the Minimum nominal specific creepage distance in mm/kV for different pollution levels.

Specific Creepage Distance:

The overall creepage distance of an insulator divided by the highest operating voltage acrossthe insulator. It is generally expressed in mm/kV.

2) Table-3 reference has been taken from Indian Standard-731 Specification For Porcelain Insulators For Overhead Power Lines with a Nominal Voltage Greater than 1000V, which shows the value of total creepage distance against different voltage levels.

3) 320mm is the nominal creepage distance of one single disc insulator. You can verify by checking the specification of any manufacturer disc insulator.

I think that it would be sufficient to clarify your doubt.

Vinu_Answers Sure_answers

Number of disks and normal condition voltage acceptance is linear(ish), but total fixture failure possibility rate (contamination or otherwise generated) is less than additive (possibility of more than one disk to fail at the same time diminishes), so think this somewhat explains the applied unlinearity in figures, keeping equivalent safety factor. S.M.