Paul's Posts — 15 October 2012
By Paul McGowan
In yesterday’s post we discussed how Edison and several wealthy investors electrified New York City and turned the lights on only to run into a fundamental problem; he couldn’t keep the lights the same brightness. Even an office building outfitted with Edison’s light bulbs had noticeably dimmer bulbs at the top of the building than at the bottom, perhaps by as much as half.
As sometimes happens with new technologies, the practicality of the invention comes into question when you try and apply it to a broad spectrum of users – and that’s just what happened to Edison. His investors as well as his customers demanded a fix for this problem. Unfortunately, the problem was a fundamental one that had to do with the nature of power itself.
When you send power down a wire the ride it takes isn’t free. As it is traveling down that wire it loses energy in the wire itself. The more energy you use at the end of the wire the greater the loss will be at the point you need the power. The reason for this is fundamental: the wire becomes a heater and the power along the wire is converted to heat. The more power you try and draw from the wire, the more heat is created and what’s not converted to heat is what you have left to power the light bulb. The longer the wire, the greater the loss.
So Edison had no choice but to try and figure out a way around this fundamental property of electricity running through a wire – and his choices weren’t many. Let’s use a water analogy and picture a long main water pipe feeding many homes so you can see the problem easily. There’s a certain amount of water pressure in the main pipe but as soon as any one home starts to draw water to fill a bath, some of the pressure goes to that task and the main pressure in the pipe goes down. I am sure you’ve seen this at your house when you’re watering the garden or lawn and someone inside flushes the toilet – the water coming out of your hose goes down.
One way around this problem is to put more water pressure than any of the homes together would need and then regulate or limit how much of that pressure can be used. If you have 10 homes and each home needs 10 pounds of pressure, if the main pipe has only 10 pounds available then you have a problem. But if the pipe has, instead, 100 pounds of pressure then each home can grab its 10 pounds and none of the other homes feel any difference.
Unfortunately for Edison regulating or limiting higher main voltages doesn’t work because in that day there was no way of making a specific limiter. The ability to do this wouldn’t happen for another 50 years.
He was stuck, but an answer was right around the corner – although for Edison, it wasn’t an answer he would be happy about.
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Soundminded
The problem is still with us. The water analogy is a pretty good one in this instance, at least for some aspects of it. As firefighters know, one of the worst times to fight a fire is during the Super Bowl. If they have to conect to a city fire hydrant to supply water to their hoses, every time there is a TV commercial break, there’s a drop in city water pressure from toilets being flushed simultaneously all over town.
Electric utilities design for “diversity” factor expecting that almost all of the time most customers, especially residential customers will only operate a small percentage of their electrical appliances. But then there are those scorching hot days in summer when everyone seems to have their air conditioners operating to keep themselves as cool as they can. These are often the largest electrical loads in a home (and among the largest in commercial and and even some industrial buildings.) The result isn’t just voltage drop but overloaded feeders, transformers, even generators. As a consequence, with inadequate networks that have been neglected in many cases for decades, extreme measures such as rolling brownouts, blackouts, and pleas for people to turn their thermostats up to reduce load have become the norm. Peculiarly, there are times of extreme power usage during the coldest winter months now too. Evidently use of electric heaters to supplement other heating systems is also becoming more common.
In a distributed network such as in utility feeder circuits with multiple customers on the same feeder, the voltage drop grows progressively worse as you get further from the source. Adjusting voltage to be satisfactory to the last customers on a line risks overvoltage that might damage customers’ appliances close to the source (the utility transformer in AC systems.) The cure…a larger network with greater capacity, more and larger feeders, more and larger transformers. Nobody wants to pay the cost though especially since most of the time there is no problem and the reliability of the network is taken for granted. Out of sight, out of mind.
The same problems exists on a very small scale desiging electronic equipment. It seems to me that one of the specifications that would be most useful to consumers who have the knowledge to understand it would be the size and degree of regulation of power supplies built into such equipment and the maximum load that power supply will see. Frankly I don’t see much correlation between price and conservativeness of design. It’s rather disappointing to see marginal power supplies in very expensive equipment. It shows there’s no real value for money in them. BTW, an appliance such as a “power regenerator” cannot compensate for deficiences in the audio component power supplies themselves while very well designed equipment anticipates power problems from the utility and compensates for them through what some would consider oversizing, extensive filtering, and through line and load regulation. Clearly some manufacturers feel their customers wouldn’t understand it while others have little in this regard to boast about.
Paul McGowan
Yup, and I am not sure what anyone is going to do about it either. You do bring up a really interesting point – with the Super Bowl comment. In England there is a weekly TV show that has everyone watching, some sort of soap opera. Its audience is so large that during the break enough people turn on their electric teak kettles that they overload the system. It is an interesting fact that Britain is forced to import power from France to cover the deficit for this once weekly occurrence.
We found out about it because we started seeing complaints of Power Plant problems occurring at the same time – the voltage dropped so low that the PPP’s would shut off. Go figure.
Soundminded
“Double, double toil and trouble; Fire burn and cauldron bubble.”
http://www.bbc.co.uk/learningzone/clips/monitoring-and-responding-to-the-demand-for-electricity/7977.html