I didn’t find an answer in the archives so here goes… I live in Northern California and a few years ago I purchased a house that backs up to county park land. About 100 feet from my house are huge towers carrying high tension power lines. While not the prettiest things in the world to look at, I don’t really notice them anymore, except when they become loud. (Yes, I know that some people think they can cause cancer but let’s not go there now). During the summer months, especially on warm dry evenings, I hear them pop and hiss quite loudly. I rarely if ever hear them during the wetter months. Now assuming that power is going through them all year long why would they be loud during the summer and quiet during the winter? Does it have something to do with the humidity? Could it be that a lot more power is going through them in the summer because of the loading caused by air conditioners? Any ideas?
What you are hearing sounds like Corona discharge. Hearing more of it in dry conditions than in moist sounds backwards since there is more corona discharge in times of high humidity. It could be that although there is less discharge during low humidity you hear it more because it is occuring at higher voltages and is thus noisier.
In high humidity the breakdown voltage of the air is lower than is the case in low humidity. At this lower breakdown voltage the noise made when the air does start conducting might be less than is the case at the higher voltages at breakdown in low humidity.
Other than that I can’t explain it.
On thinking about this a little more, are you sure that the humidity is lower in your area in summer than in winter? It seems to me that summers in northern California are relatively cool and damp* and the winters relatively warm and sunny.
*The coldest winter I ever spent was a summer in San Francisco. - Mark Twain
I used to live in Sunnyvale, right next door to Cupertino. That area is cold and wet in the winter, but the summer has much lower humidity than, e.g. the midwest. (and San Francisco is only warm in the fall.)
Does the fact that the lines are usually wet during the winter make any difference? Our summers are pretty dry where I live.
The answer may be simpler than that. The line may simply not be in use as often druing the winter. The California ISO load in the Summer of around 40,000 -45,000 MW In the winter the peak is probably closer to 25,000 to 30,000 MW. Also, Hydro power is actually less available in the winter in California because it is basically all frozen in the mountains (there is, of course hydro all year long) peak hydro times are generally May and June and then falling throughout the summer, with reserves building up as snow over the winter. Your lines simply may not be used very much for transporting much power during the winter. It is hard to say.
I think transmission lines are normally operated near their maximum voltage capacity to reduce line power losses. If you regulate load with voltage you increase the percentage of power lost en route, since power loss goes with 1/current[sup]2[/sup].
Yes, the line voltage stays the same, but the breakdown voltage of the air with high humidity is lower than it is with low. So the discharge with high humidity occurs at a lower voltage point on the voltage cycle.
I wonder also if during winter the rain washes off contaminant deposits. In summer they aren’t washed off so a bit of moisture creates a conductive path for the current to escape.