I noticed when walking home the other day that the high voltage (200kv I believe) lines running through here were hissing in the rain. What is causing them to hiss?
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\$\begingroup\$ Buzz or tssss? The lines are usually quite warm, so they might rapidly evaporate the water that falls on them. \$\endgroup\$– Nick TCommented Dec 2, 2010 at 18:48
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\$\begingroup\$ You're right, it was more of a 'hissing'. Edited \$\endgroup\$– pfyonCommented Dec 2, 2010 at 19:09
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\$\begingroup\$ those birds must have some tough feet then \$\endgroup\$– Matt WilliamsonCommented Dec 2, 2010 at 19:49
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\$\begingroup\$ are these the big bad cross-country lines? how many conductors? \$\endgroup\$– JustJeffCommented Dec 2, 2010 at 22:16
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3\$\begingroup\$ @Matt As an electronics and bird enthusiast, I'll comment that birds do, in fact, have tough feet. Their corneum is the outermost layer of skin; keratinous scales. It's worth noting that birds will avoid high voltage lines when they are uncomfortable due to temperature, electric field, etc. Some birds can sense direction magnetically (magnetoception), and HV lines likely disturb this sense. \$\endgroup\$– JYeltonCommented Dec 11, 2012 at 16:56
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6 Answers
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High-voltage partial discharges across the insulators.
answered Dec 2, 2010 at 18:51
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\$\begingroup\$ What is a partial discharge? \$\endgroup\$ Commented Dec 2, 2010 at 19:26
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6\$\begingroup\$ en.wikipedia.org/wiki/Partial_discharge ? \$\endgroup\$– Nick TCommented Dec 2, 2010 at 20:16
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8\$\begingroup\$ I'm not going to upvote an answer consisting of only 7 words, however correct. A bit more explanation would be welcome. \$\endgroup\$– stevenvhCommented Jul 15, 2011 at 17:24
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\$\begingroup\$ OP didn't say the hiss was only at the insulators. If these are high-voltage power lines, the cables themselves wouldn't be insulated. Also, I'd expect powerlines to be designed not to have PD issues. From the same article: "PD prevention and detection are essential to ensure reliable, long-term operation of high voltage equipment used by electric power utilities." \$\endgroup\$– RJRCommented Apr 28, 2014 at 0:32
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I was taught that this is due to the Corona effect. Basically, the power lines ionize the air around them, causing audible hum, along with havoc in the EM spectrum. This is why really high voltage lines and transformers will sometimes have a slight aura around them.
Generally, the effect is undesired, because it robs the transmission lines of energy (the hum/light/heat dissipates energy), so a lot of equipment is manufactured to try and stop this effect.
The Wikipedia article will do this subject much more justice than I can.
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\$\begingroup\$ If it's due to this, wouldn't I hear the hum/buzz whether it was raining or not? \$\endgroup\$– pfyonCommented Dec 8, 2010 at 16:43
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2\$\begingroup\$ I seem to recall that it was more likely in humid air, but I'm not particularly sure. \$\endgroup\$ Commented Dec 8, 2010 at 17:56
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Good stuff in the answers so far, but I work with power lines and want to put in my two cents.
This isn't technically a partial discharge; occasionally you may be hearing the crackle of what is usually called a corona discharge. I admit the phenomenon is related, but it is not the same.
See, all uninsulated lines show corona. Its just not a big deal until you're dealing with a pretty high voltage. As the voltage goes from a very big positive to a very big negative, the air around it gets ionized, so about 50 or 60 times a second, it switches direction. This is the normal mains hum discussed in another answer.
Water is much, much heavier than air, and it ionizes just as easily. So on a rainy or humid day, the corona is pulsing with water in it. This gives it momentum, so the heavier water particles travel out farther. But they themselves are ionized, which means they can ionize more air than the line could normally reach on its own, and ionized air is conductive.
And there's almost always 3 of these lines pretty close together. The sound you're hearing is a million teeny tiny electrostatic discharges from all the charged up water particles interacting with each other with nearby lines or grounded objects. This is actually the worst time to be anywhere near them; the air is supposed to be their insulator, and at that moment it isn't working as well.
Occasionally you might see a full corona discharging with the naked eye; it looks like a tiny bit of lighting crawling up the line. If it gets really bad, you'll see a momentary line to line or line to ground short, which looks exactly like a real lighting bolt, just not from the sky.
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They'll actually do it when it's not raining too. It's called mains hum. Power lines carry AC voltage at either 50 or 60 Hz which is at the low end of the audible range of most humans. In the presence of an electromagnetic field (like the one generated by AC power), the molecules of ferromagnetic materials (the metallic conductors inside of power lines) will not only try and align themselves with the field but sometimes change or distort their shape if the applied potential is strong enough. This alignment/change in shape can cause collisions between the molecules comprising the power-lines which, given enough of them, can be heard by an observer.
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3\$\begingroup\$ ...but if it gets annoying, you could just think of it as a million electrons clapping their hands & cheering you on in your journey home \$\endgroup\$– Joel BCommented Dec 3, 2010 at 1:11
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\$\begingroup\$ as per my comment to mjcarroll, why do I hear the hum/buzz only when it's raining? \$\endgroup\$– pfyonCommented Dec 8, 2010 at 16:44
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\$\begingroup\$ @pfyon - I might guess that though rain makes some noise it also drowns out much more ambient noise such as rustling leaves or tall grass by weighing down and suppressing otherwise noisy things. A sound of a bunch of dry leaves can integrate into one loud ambient noise. I go hiking on the Appalachian Trail and there is always one spot that we go under power lines and I can hear them even when it's not raining. Also, when it rains, the air is more saturated with water vapor (making it less of an empty vacuum) and can most likely carry sound further. \$\endgroup\$– Joel BCommented Feb 10, 2011 at 19:51
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My wild guess would be that the hanging water droplets might be causing corona discharges. Corona is usually worst around points of sharper curvature, where the electric field gradient is most intense. Higher voltages, like the 200kV you mention, would make this more pronounced.
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\$\begingroup\$ You beat me to it by one minute. Cheers. \$\endgroup\$ Commented Dec 2, 2010 at 22:05
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There are several causes but the most common is high voltage discharge along surface contamination on the insulators. This has a characteristic "hissing" noise, whereas most other phenonomon generate noise primarily at the line frequency or 2nd harmonic.
During dry weather dust builds up on insulators and it always contains some quantity of salt. When weather gets damp, enough water gets into the mix to allow it to become conductive, so small quantities of current start flowing. Once that happens any organics in the current path quickly become carbonised and the arc becomes more-or-less permanent (heavy rain will wash the insulators clean and the cycle then starts again)
The noise peak for hissing discharges is ultrasonic, so detecting where the actual arc is located is done using a small parabolic microphone with a viewfinder. (see http://www.arrl.org/files/file/Technology/PLN/Ultrasonic_Pinpointer.pdf for an example)
The hissing you hear is the least of it. Radio hams HATE arcs because they generate a lot of radio interference - so they tend to be the most enthusiastic purchasers/builders of ultrasonic arc locators.
Dust buildup is the primary reason why high voltage insulators aren't smooth - the typical "stack of discs" design makes it very hard for enough dust to build up to let current flow under damp conditions, so dust-based arcing is usually found only on 11kV or lower tension poles, where the insulators are a much simpler design.
Corona effects are usually only found on very high voltage lines (over 220kV)
