Power lines question for you engineers

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Mr Milk said..
A few towers supporting a high voltage line in Victoria got blown down yesterday. I'm speculating that it is due to the wind setting the lines swinging too much for them to handle the strain. Maybe even harmonics like that famous suspension bridge in the USA.
So is that a civil engineering problem which you don't deal with? Or maybe one of you knows how the wind risk is taken into account in places where extreme wind is a high probability like Tornado Alley. Do they put the towers closer together so there isn't so much metal swinging between them or make the towers stronger?

There are a couple of wind effects that need to be considered. Aeolian vibrations in transmission lines are low amplitude, high frequency vibrations that will cause the conductor to crack over time due to stress fatigue. They are limited by the addition of 'dog bone' dampers that you will see attached to the conductor at each end of a span. The larger visible type is 'galloping' (e.g. the Tacoma Narrows bridge that you referenced). Transmission lines don't suffer from galloping because the conductors are round. When large transmission towers come down, it is generally just due to the huge force created by a massive wind gust, not some oscillatory or harmonic behaviour.

The civil engineer will design the tower to meet a specified wind speed. It's not practical or economic to design for the biggest possible gust. I don't know if they specify a higher wind speed in tornado areas because I've never designed a tower for one. The examples of tower failure that I have dealt with in NSW have mostly been related to microbursts. This is where a column of cold air falls from a great height and when it hits the ground, the falling air creates a blast of around 150 km/h or more. It's quite a sight....everything in a radius of a few hundred meters is flattened and you can see it pointing outwards from the middle.

You may be interested to know that there are two main types of transmission tower....tension and suspension. A suspension tower does just what the name suggests, it holds the vertical weight off the ground. Tension towers look much the same to the untrained eye, but have much heavier steel truss members and larger foundations. Tension towers can withstand the horizontal force that is used to tension the line. Therefore, you need a tension tower whenever the line terminates at a substation or the line changes direction. If you are unlucky enough to have a tension tower fall down, then you can have a cascading effect as the suspension towers fall like dominoes, since they aren't designed to pull against the line tension after a nearby tension tower falls down and the line becomes slack on just one side of the suspension tower. This actually happened a few years ago on the 132 kV line that runs along the side of the road to Kurnell and about half a dozen towers were affected, although it wasn't due to wind - it was due to a contractor disconnecting all the conductors on one side of a suspension tower instead of gradually removing one from each side at a time to keep the forces even. Also, the next time you're at Bungendore, head north about 5 km toward the southern end of Lake George and you'll find a transmission line section where the steel transmission towers have been replace by large wooden pole structures. These are emergency structures that were used to quickly get that line back in the air when a microburst took out a string of suspension towers.

I stand with Carantoc. I'm calling BS on Mr Milk and Harrow.

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Harrow said..

Mr Milk said..
A few towers supporting a high voltage line in Victoria got blown down yesterday. I'm speculating that it is due to the wind setting the lines swinging too much for them to handle the strain. Maybe even harmonics like that famous suspension bridge in the USA.
So is that a civil engineering problem which you don't deal with? Or maybe one of you knows how the wind risk is taken into account in places where extreme wind is a high probability like Tornado Alley. Do they put the towers closer together so there isn't so much metal swinging between them or make the towers stronger?

There are a couple of wind effects that need to be considered. Aeolian vibrations in transmission lines are low amplitude, high frequency vibrations that will cause the conductor to crack over time due to stress fatigue. They are limited by the addition of 'dog bone' dampers that you will see attached to the conductor at each end of a span. The larger visible type is 'galloping' (e.g. the Tacoma Narrows bridge that you referenced). Transmission lines don't suffer from galloping because the conductors are round. When large transmission towers come down, it is generally just due to the huge force created by a massive wind gust, not some oscillatory or harmonic behaviour.

The civil engineer will design the tower to meet a specified wind speed. It's not practical or economic to design for the biggest possible gust. I don't know if they specify a higher wind speed in tornado areas because I've never designed a tower for one. The examples of tower failure that I have dealt with in NSW have mostly been related to microbursts. This is where a column of cold air falls from a great height and when it hits the ground, the falling air creates a blast of around 150 km/h or more. It's quite a sight....everything in a radius of a few hundred meters is flattened and you can see it pointing outwards from the middle.

You may be interested to know that there are two main types of transmission tower....tension and suspension. A suspension tower does just what the name suggests, it holds the vertical weight off the ground. Tension towers look much the same to the untrained eye, but have much heavier steel truss members and larger foundations. Tension towers can withstand the horizontal force that is used to tension the line. Therefore, you need a tension tower whenever the line terminates at a substation or the line changes direction. If you are unlucky enough to have a tension tower fall down, then you can have a cascading effect as the suspension towers fall like dominoes, since they aren't designed to pull against the line tension after a nearby tension tower falls down and the line becomes slack on just one side of the suspension tower. This actually happened a few years ago on the 132 kV line that runs along the side of the road to Kurnell and about half a dozen towers were affected, although it wasn't due to wind - it was due to a contractor disconnecting all the conductors on one side of a suspension tower instead of gradually removing one from each side at a time to keep the forces even. Also, the next time you're at Bungendore, head north about 5 km toward the southern end of Lake George and you'll find a transmission line section where the steel transmission towers have been replace by large wooden pole structures. These are emergency structures that were used to quickly get that line back in the air when a microburst took out a string of suspension towers.

Gee who'd have thought you could get such information on a water sports site
I just come here to get financial advice & if I should get the vaccine jab

Aeolian vibrations in transmission lines are low amplitude, high frequency vibrations that will cause the conductor to crack over time due to stress fatigue.

So that's the hum that you notice sometimes near substations?

The hum is the noise produced by the large power transformers as their cores vibrate due to the 50 Hz supply. However, the hum is actually 100 Hz, not 50, since there are two voltage peaks (+ve and -ve) within each 50 Hz cycle that each cause the transformer core to try and expand (magnetostriction).

I used to work at a very high power (biggest in oz) HF transmissiom site. The huge curtain antenna array were fed by 300 ohm feeders in a huge field in the middle of nowhere. If there was a fault in the transmission lines instead of the 300kw HF going out the curtain, you would get arc faults. The energy would ionise the air into whats called a corona and you would have these 6ft high green and blue flames on the lines. It would also demodulate the broadcast audio (i think because of the balanced audio on the lines now had a path to gnd through the air) and so you would hear bits of the transmission. Youd be out in total darkness at 2 am in the field and all of a sudden there'd be bizzare northern lights flames and tesla coil style arcing, with bits of indonesion or english religous speak (evangelical content) ringing out. Was quite an experience.

Big radios. Drew 30 amps per phase at 11kv. Size of a small apartment. Some were cooled by steam.

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decrepit said..
Totally off topic, but reminded me of a similar phenomenon. If you get an arc over on a radio transmission tower, it demodulates the RF and acts as a speaker. Not very Hi Fi, not much base, but you can hear it for a long way.

That's super cool, I'd never heard of it before.

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myusernam said..
Youd be out in total darkness at 2 am in the field and all of a sudden there'd be bizzare northern lights flames and tesla coil style arcing, with bits of indonesion or english religous speak (evangelical content) ringing out. Was quite an experience.

Big radios. Drew 30 amps per phase at 11kv. Size of a small apartment. Some were cooled by steam.

You had God speaking to you? What did he say?

can you cool things with steam?

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FormulaNova said..
I remember hearing or reading this same thing, but I am not sure I agree. Maybe it was in a TAFE class many years ago.

Now you can get apps on your phone that use the mic to analyse audio frequencies. Maybe I should actually try this to see for myself.

I could be wrong as it's been a few months since I last measured harmonic sound pressure levels in a substation.

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myusernam said..
yeah steam is a superheated gas. I didnt do chemistry but it probably takes 1 joule or energy unit (whatever it is) to heat 1 gram or kg of water by 1 degree. But once it reaches 100deg and turns into steam it is superheated and then instead of a linear scale it becomes logorithmic (i havent googled this is just what I was told years ago) and to heat steam for every degree over 100 deg it takes way more energy. so long as you are happy with the thing u want to cool being around 100degc then steam becomes a very good way to remove heat because it is so hard to make any hotter

Oh, that makes sense. I just never thought it would be used that way, but as you say, its for things that are already over 100c.

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Harrow said..

Maybe you've been hearing 100 Hz from things all along and assumed it's been 50 Hz. What kind of things are you talking about?

Hit the 4th fret on the bottom E string. Yes that's it. 100hz. (Well 103.8 if you're exactly in tune).

So I'd guess 100hz has been driven into our subconscious audio processing since birth. There you go a million dollar riff. Down tune by 40 cents compose a ditty in G sharp. People couldn't help but hum along.

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FormulaNova said..
The worst thing about those frequency generating apps is that you find that high frequency hearing gets worse as you get older...

I tried to play a 50 Hz tone and virtually nothing came out of my laptop speaker due to its poor frequency response, so I'm just gonna assume that's the reason for me not being able to hear any high frequencies in the online hearing test I did either... it has nothing to do with my old age!

Mmmmm . Love all the answers but..:: it was really really windy. 50 km up the road from me it was 130 km wind gusts and trees were snapped 30 m off the ground. A bit of a mess and it will take a few weeks to sort it out.

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FormulaNova said..If you used the same one I did, you can change it to a square wave, sawtooth, or triangle wave to get output. For some reason the sine wave doesn't output. Maybe a filter?

edit: onlinetonegenerator.com/

Maybe there is a filter and the harmonics are what makes it audible?

I'd guess it is just that your PC speaker has almost zero response at 50Hz. I can hear the 100Hz if I turn it right up, but it's very quiet. From 200Hz upwards it's okay.

I'd think the reason you can hear those other waveforms is that the 50Hz sawtooth, square and triangle contain harmonics of 100, 150, 200. 250, 300, etc, which your speaker does have a reasonable response for, so that would be what you are hearing.

Did you try this one, it starts at 150Hz and slowly drops. My speaker starts to really drop out below 70Hz and I think I'm imagining anything below 50 or 60Hz.

onlinetonegenerator.com/subwoofer.html

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Harrow said..

I'd think the reason you can hear those other waveforms is that the 50Hz sawtooth, square and triangle contain harmonics of 100, 150, 200. 250, 300, etc, which your speaker does have a reasonable response for, so that would be what you are hearing.

Did you try this one, it starts at 150Hz and slowly drops. My speaker starts to really drop out below 70Hz and I think I'm imagining anything below 50 or 60Hz.

onlinetonegenerator.com/subwoofer.html

I downloaded Spectroid, so I can at least see what the phone hears. The speakers don't seem to produce anything below 170hz or so.

It makes me wonder if the output would be better connected to external speakers. Given the tiny size of laptop speakers, I can understand if this is the case....

Oh hang on, I just realised I had some cheap headphones lying around, so I plugged them into the laptop's headphone output. I could hear down to about 30hz, but when I did it again in Spectroid, it showed unreliable readings at about that point anyway.

I don't know if low frequency hearing varies too much among people, but high frequency certainly varies a lot with age.

With the inbuilt speakers I couldn't hear any of that subwoofer test, but with the external headphones, no problem, at least until 30hz.