Fab Foodie
hanging-on in quiet desperation ...
- Location
- Kirton, Devon.
Calling @swansonj ....
- Thread starter Fab Foodie
- Start date
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swansonj
Guru
Yup, it's warning to birds.
We tend to use them on high voltage lines (steel lattice pylons) where there's a known problem with bird strike - a known route used by birds, or, more usually, a known landing ground. As I've remarked before, large birds, swans are an example, have a very marginal power to weight ratio for take offs. They are so little above their stall threshold that they have little in reserve and can't do any last minute avoidance manoeuvres when taking off, so you need to give them as much warning as possible.
On lower voltage lines, wood poles typically, you are usually more bothered by avoiding multiple birds sitting on a line because of the bird poo issue.
At high voltages you can only put such devices on the earth wire, not one of the live conductors, because of corona. The earth wire is usually the one at the top (it's really there for lightning protection, whatever other ancillary purposes it serves). At low voltages you can put what you like on the live wires (and the earth wire tends to be at the bottom as it happens, so that if a lorry or ladder being carried hits anything it's the earth wire). You sometimes see twisted bits of spiral wire at low voltages, which are usually about aerodynamics not birds. Aerodynamics are a problem at high voltages too, but we have to find ways of solving it that don't involves sharp edges or tight radii that would create corona.
Thank you for giving me an absolutely legitimate excuse to indulge my nerdier engineering instincts
.
We tend to use them on high voltage lines (steel lattice pylons) where there's a known problem with bird strike - a known route used by birds, or, more usually, a known landing ground. As I've remarked before, large birds, swans are an example, have a very marginal power to weight ratio for take offs. They are so little above their stall threshold that they have little in reserve and can't do any last minute avoidance manoeuvres when taking off, so you need to give them as much warning as possible.
On lower voltage lines, wood poles typically, you are usually more bothered by avoiding multiple birds sitting on a line because of the bird poo issue.
At high voltages you can only put such devices on the earth wire, not one of the live conductors, because of corona. The earth wire is usually the one at the top (it's really there for lightning protection, whatever other ancillary purposes it serves). At low voltages you can put what you like on the live wires (and the earth wire tends to be at the bottom as it happens, so that if a lorry or ladder being carried hits anything it's the earth wire). You sometimes see twisted bits of spiral wire at low voltages, which are usually about aerodynamics not birds. Aerodynamics are a problem at high voltages too, but we have to find ways of solving it that don't involves sharp edges or tight radii that would create corona.
Thank you for giving me an absolutely legitimate excuse to indulge my nerdier engineering instincts
.
OP
OP
Fab Foodie
hanging-on in quiet desperation ...
- Location
- Kirton, Devon.
AwesomeYup, it's warning to birds.
We tend to use them on high voltage lines (steel lattice pylons) where there's a known problem with bird strike - a known route used by birds, or, more usually, a known landing ground. As I've remarked before, large birds, swans are an example, have a very marginal power to weight ratio for take offs. They are so little above their stall threshold that they have little in reserve and can't do any last minute avoidance manoeuvres when taking off, so you need to give them as much warning as possible.
On lower voltage lines, wood poles typically, you are usually more bothered by avoiding multiple birds sitting on a line because of the bird poo issue.
At high voltages you can only put such devices on the earth wire, not one of the live conductors, because of corona. The earth wire is usually the one at the top (it's really there for lightning protection, whatever other ancillary purposes it serves). At low voltages you can put what you like on the live wires (and the earth wire tends to be at the bottom as it happens, so that if a lorry or ladder being carried hits anything it's the earth wire). You sometimes see twisted bits of spiral wire at low voltages, which are usually about aerodynamics not birds. Aerodynamics are a problem at high voltages too, but we have to find ways of solving it that don't involves sharp edges or tight radii that would create corona.
Thank you for giving me an absolutely legitimate excuse to indulge my nerdier engineering instincts
subaqua
What’s the point
- Location
- Leytonstone
That reminds me: I was cycling over the Severn bridge yesterday, looking at this:
@swansonj I assume the height of the pylons is due to the span of the cables across the estuary?
similar ones at Dartford. apparently the tallest in the Uk at Dartford
swansonj
Guru
I feel like an alcoholic who got invited to a drinks reception....Awesome
swansonj
Guru
Yes.That reminds me: I was cycling over the Severn bridge yesterday, looking at this:
@swansonj I assume the height of the pylons is due to the span of the cables across the estuary?
Second tallest in the UK (as @subaqua says, the Dartford crossing is the tallest). The cables are basically a parabola (exactly so if you ignore self weight, if you want the exact solution it's a catenary*, but the parabola is a good enough approximation). y=x squared. So as you increase x, the span, y, the height, goes up much worse than linearly. Hence long spans end up with ridiculously tall towers. (Exception: valley crossings where the ends are on the raised land at the sides of the valley. Our line out to Dinorwic and Wylfa crosses the Aber Valley with one of our longer spans but it doesn't look so spectacular because the pylons are quite normal. ) Plus the clearance at the low point in the middle usually has to be higher for ships than the standard clearance for on-land which is based round combine harvesters.
That is our original 275kV line to Wales. When the time came to expand to a 400 kV crossing as well, we did that with a tunnel instead. They actually cross over each other in the no-mans-land of the Bristol Channel. We also tunneled under the Solent at Fawley and the Medway to avoid using these massive pylons, and there are tunnels under the Thames as well as the overhead line at Dartford.
Boy, those drinks at the drinks reception taste good
* I did catenaries from first principles in maths at school. 15 years later, having crossed a rope suspension bridge in Nepal, I lay awake in my sleeping bag that night trying to rederive the equations. And failed dismally. Now, another 15 years on, not only could I not derive them, I can't even remember what they look like...
subaqua
What’s the point
- Location
- Leytonstone
the London Power tunnels are amazing. i might or might not have been in them , depends who is asking 
Did I ever mention that I work for a company making transformers for the transmission industry. In Leicester we specialise in Earthers and reactors, at our other plant we tend to do exotics. We slso supply GE the di/dt reactors to go into their converter valves for HVDC systems.
At the moment I am developing something completely different, a 3ph triangular isolation transformer where the input sits at ground, but the output is at 260kVDC, so there is a massive resin casting over the coils. Working with the customer we are currently developing the stress ring around the output snout (bushing) as it is throwing off a corona at the triple point that none of the modelling predicted. Fun stuff.
OP
OP
Fab Foodie
hanging-on in quiet desperation ...
- Location
- Kirton, Devon.
I can't pretend I have the foggiest idea what you're talking about .... But I like the pictures ....
Well you learn something new every day
OP
OP
Fab Foodie
hanging-on in quiet desperation ...
- Location
- Kirton, Devon.
I usually learn that there's just more stuff that I simply don't understand ....
srw
It's a bit more complicated than that...
Catenaries look like heavy chains, and the equations have got coshes and sinhs in them somewhere.
I was quite please to discover catenaries, because it was the first bit of applied maths I learned where you didn't ignore an inconvenient but very real feature of the world. I'm now unreasonably disappointed to discover that in real life a parabola is an appropriate approximation.
(According to wikipedia....
swansonj
Guru
Yeah, that rings a bell, oh yes, really, I'm sure it will all come back any moment now....
Catenaries and cycloids, they were my thing back in the day, memorise the derivation of both and that was one question's worth of marks pretty well in the bag. Memorise the proof that Poissons and Binomials tend to normals for large numbers and that was a second question, all without actually doing much maths
srw
It's a bit more complicated than that...
Everything tends to a normal for large numbers. Once upon a time (more recently than my teenage years - I never met either beast until I was well into my twenties) I could prove it, and tell you intelligently what a binomial looked like. I can still just about manage a Poisson, even though for reasons I won't pretend to be able to understand quite a lot of things where the Poisson process axioms are on the face of it true (large claims, cycling casualties) don't actually follow the Poisson distribution in real life.
