HV/EHV Safety distances

[Moley]

While driving home from Runcorn today via mid Wales my mind started wandering. I started thinking about HV safety distances. Now I know that at 132kV the safety distance is 1.4m, 275kV it's 2.4m and 400kV it's 3.1m but I want to know what sort of safety margin has been built in and how much does the weather effect the safety margin? On a moist (don't you just love that word) day the safety margin will be reduced? It's a question I've asked a few power station electrical engineers and some grid guys but no one seems to have an answer. Keep as far away as possible is an obvious answer but it would be nice to see what the figures really are before it arcs and you turn into a kebab.

Talking of moist days, I just love wandering around a 400kV yard when it's damp. The sounds of fizzing and crackling just really give you a sense of all that power just above your head. Well, as I'm short, quite a way above my head.

 

[littlespark]

Wales is the same as Scotland. It’s never not damp!

 

[pc1966]

It is not my area but I seem to recall that air breakdown is more about pressure and voltage gradsiant, and (to some degree) localised voltage gradient at any sharp points but it is over the order of MV/m. I suspect a large part of the margin is for cable swinging and other movement/mistakes ofr gaps.

Tracking distance along insulators is another matter, that will be seriously influenced by dampness and dirt!

Maybe one for @Julie. ?

 

[Moley]

Wales is the same as Scotland. It’s never not damp!

What about July 27th 1993 then eh??

 

[Moley]

Tracking distance along insulators is another matter, that will be seriously influenced by dampness and dirt!

Ah yes, good old dirty bushings. That's one the most hated jobs we do - in a cherry picker 'pot washing'. The rubber bushings are better than the ceramic but still hated

 

[Moley]

Another question, why are they called 'sheds' ?

 

[Julie.]

So I could bore you all to death with this one!

After my first degree, the second project I did was a problem of dielectric breakdown - when it shouldn't have - basically an impulse simulating a lightning strike ~175kV on 24kV switchgear causing the air chamber to break down!

Anyway, traditionally the breakdown of insulation was measured in volts per mil giving a figure around 760V/mil however these days we would use kV per mm so air is around 3kV/mm.

At this point the air breaks down causing corona discharge - the blueish glow, and the ionising smell.

It is dependent upon air pressure and humidity, but not massively - in fact moisture can increase the dielectric strength.

The field strength is very dependent upon shape - sharp points concentrate the field and you get corona at lower voltages, hence the use as much as possible of smooth rounded shapes to shield any sharp points.

In wet conditions parts of the insulators become conductive (see below) this brings more sharper points (the edges of sheds especially) closer to each other electrically and increases the field strength, so more sharper corona, more ionised gas, and more partial discharges. (A partial discharge is where the field is sufficient to cause a sparkover between two points, but it doesn't spread over to the full extent )

Corona in itself does produce losses, but little more. However if the corona causes sufficient ionised gas that this ionised gas (which is conductive) gets close enough to earth, the corona at another point, or the corona of another phase - you can get a sparkover (NOT a flashover!).

EHV lines normally run at around 3kV/mm - that doesn't dictate the spacing because the field strength is not linear. Instead the spacing is a statistical calculation based on the swing of the line, the likelihood of 'streamers' of ionised gas linking up, birds flying between etc etc.

In terms of the insulators the field is distributed unevenly over the surface, and if just a straight piece should this get dirty then combine this with moisture and you get a conductive path - in which case the non-conductive spacing along the surface can't withstand the voltage and you get a flashover.

(Flashover is along an insulator, sparkover is through a gaseous medium).

So to keep this at bay, cup or bowl shaped shed insulators are used - the idea being that the underneath bit remains less dirty and dryer to actually provide sufficient surface length - ignoring the wet/dirty exposed portion that may act as a conductive surface. On lower voltages instead of sheds the surface is extended by wavy shapes for the very same thing.

Obviously dirt can still build up too much, and they have to be washed down to limit this build up.

I always assumed the sheds were called sheds as they are in effect like a roof over an open shed - probably using bowl or cup implies such a specific shape that it doesn't suit everything.



Edit

I should add the statistical calculation is defined in iec61472.

I will see if I still have it, I cleared a lot of my stuff out when I "retired"

 

[Julie.]

OK so I have pieced a document together showing the basis of the calculation - attached here.

 

[pirate]

I've read that post 3 times @Julie...
It's over my head (no pun intended!) but I'm now fascinated by something I knew absolutely nothing about 5 minutes ago! Thank you!

 

[Julie.]

I've read that post 3 times @Julie...
It's over my head (no pun intended!) but I'm now fascinated by something I knew absolutely nothing about 5 minutes ago! Thank you!

I said I could bore people with this rubbish!!!

I just read it myself and realised I shouldn't have said "ionising smell" - I should have said "ozone smell" (ozone is the ionisation of the oxygen in the air)

 

[pirate]

Bored? No!
Rubbish? No!
Informative? Yes!

 

[Moley]

Sorry about the delay replying, I've been busy. I've downloaded your IEC upload and I'll take a look when I've got a couple of spare minutes. Looks like it's going to be interesting reading - or a cure for insomnia ;-)

 

[Julie.]

Sorry about the delay replying, I've been busy. I've downloaded your IEC upload and I'll take a look when I've got a couple of spare minutes. Looks like it's going to be interesting reading - or a cure for insomnia ;-)

A cure for insomnia!

 

[Moley]

We'll see. I'll keep it safe until my next night in a hotel when I haven't got the family to disturb my concentration.

 

[Lister1987]

Well this certainly "sparked" my interest, nothing quite the "turn on"than a female that "nerds out" , it's certainly an interesting thing though, would have thought ESQCR (or older) would have clearances in? Maybe ENA have something (no doubt based on the IEC Julie mentioned; am sure I posted something on substation design a few posts back.