Why isn't our electricity a higher frequency?

[ShawnD1]

In North America the electricity is only 60hz. That low frequency is why people get strained eyes from fluorescant lights like they have in offices and schools. If we had our power running at a higher frequency like 120hz, we wouldn't get eye strain as much from that type of lighting. It's sort of like increasing the refresh rate on your monitor from 60 to 100 - big difference on the eyes.

Is there any logical reason for keeping only 60hz?

 

[Lynx516]

It is imperetive that you keep the frequency as close to what you choose as posible.Any deviation even by 1% can cause huge output loss. 120hz is a lot harder to maintain then 60hz.

 

[ShawnD1]

Why would it be harder? All they have to do is add more magnets on the generator. Can the electricity somehow lose frequency in the lines or something? I don't see how it could but there's a lot I don't know about electricity.

 

[Mark R]

Because transmission would be a big problem - the inductance of long transmission lines would present a higher impedance and need considerably more power factor correction in order to transmit power. This would mean greater difficulty in ensuring power stability.

Although you could use smaller transformers you'd need more shunt reactors or capacitor banks, or operate more power stations in 'power factor correction mode' in order to limit voltage fluctuations from deficiencies in 'reactive power'.

 

[PowerEngineer]

Originally posted by: Mark R
Because transmission would be a big problem - the inductance of long transmission lines would present a higher impedance and need considerably more power factor correction in order to transmit power. This would mean greater difficulty in ensuring power stability.

Although you could use smaller transformers you'd need more shunt reactors or capacitor banks, or operate more power stations in 'power factor correction mode' in order to limit voltage fluctuations from deficiencies in 'reactive power'.

Yes, the impedance of a transmission line is largely inductive and therefore increases with frequency. Increased impedance lowers the amount of power that can be transmitted down the line. The increased inductive impedance will also increase the phase lag for current following voltage (i.e. consuming "reactive" power) that would have to be countered by more shunt (or series) capacitor banks (to produce "reactive" power). "Power factor correction mode" is normally defined as a control mode where the voltage excitation on the generator is adjusted to maintain a constant ratio between real and reactive power output. It may be more accurate to say that generators would also have to increase their reactive outputs (i.e. their excitation volatge), and maybe let their outputs vary with system voltage (rather than holding them to some amount fixed by real power output).

Another issue is power losses. Excitation losses in transformers and motors are also a function of system frequency.

Of course, no one was thinking about refresh rates on monitors back in the early 1900's when the frequency standards were being established. The bigger concern then was what made sense for running motors. Now that we've built continent spanning electrical grids of equipment all designed to operate at either 50 Hz or 60Hz, there'd be a HUGE cost to making a change.

I think we're much more likely to get better light bubs!

 

[Geniere]

High Powerengineer, or should I have said ?Hi?. Yuck, yuck, pretty bad eh?

Any how, here?s a couple of questions for you.

I recall being told that one could determine the voltage of high voltage transmission lines by counting the number of (ceramic?) insulators from which the wire was suspended i.e., ~3000 volts per insulator. Any truth in that?

At peak usage hours, what would an individual, average capacity, generator output as far as current?

What is the ?usual? phase to phase voltage produced by a generator?

I recall reading somewhere about Italy installing a DC transmission line. Any truth to that?

Are power companies investigating super-cooled. low voltage transmission lines?



Thanks

 

[Mday]

Originally posted by: ShawnD1
In North America the electricity is only 60hz. That low frequency is why people get strained eyes from fluorescant lights like they have in offices and schools. If we had our power running at a higher frequency like 120hz, we wouldn't get eye strain as much from that type of lighting. It's sort of like increasing the refresh rate on your monitor from 60 to 100 - big difference on the eyes.

Is there any logical reason for keeping only 60hz?

take a few semesters worth of math, physics, and then circuits.

 

[uart]

Yes transmission loss, particularly [/b]iron losses[/b] in transformers, is a major reason for not using higher mains frequency.

Also note that with a 60Hz supply that the lowest frequency in the light output is not 60Hz but is in fact 120Hz

BTW, Here in Australia we only use 50Hz.

 

[NeoPTLD]

There are MANY reasons that makes high frequency unpractical.

I. Skin effect. Power will only flow through the outer layers of wires reducingwires' current carrying capacity.
II. Iron loss. Higher the frequency, higher the loss in transformer core
III. Inductive resistance. High frequency current experiences more resistance in wires known as impedance.
IV. Capacitive coupling. high freq. can flow through insulators through capacitive coupling. A safety hazard at user end and increase in transmission loss at utility side.

The only benefit I can come up with for 120Hz transmission is an ability for direct drive AC machinery to achieve 7200RPM. On 60Hz power, the maximum direct shaft speed you can get from any AC rotating machinery is limited to 3600RPM

In regard to the problem you mentioned, something have been done specifically for it already. Some new fluorescent ballasts are inverter driven. They take 60Hz and run lamps at 20,000 to 40,000Hz virtually eliminating flicker.

 

[PowerEngineer]

Originally posted by: Geniere
High Powerengineer, or should I have said ?Hi?. Yuck, yuck, pretty bad eh?

Any how, here?s a couple of questions for you.

I recall being told that one could determine the voltage of high voltage transmission lines by counting the number of (ceramic?) insulators from which the wire was suspended i.e., ~3000 volts per insulator. Any truth in that?

At peak usage hours, what would an individual, average capacity, generator output as far as current?

What is the ?usual? phase to phase voltage produced by a generator?

I recall reading somewhere about Italy installing a DC transmission line. Any truth to that?

Are power companies investigating super-cooled. low voltage transmission lines?



Thanks

"High" Geniere,

It is true that the length of the insulator strings on a transmission line is an indicator of its voltage. What complicates this is that not all ceramic insulators (or "bells") are created equal. Their size, shape, and number are designed to enough open air space between the conductor and anything grounded (like the tower itself), and to have a longer surface path (traced along the surface of all the bells) between the conductor and the tower. This surface path needs to be much longer because its insulation value is always being compromised by water (which is why they look a bit like umbrellas to provide some dry areas) and by surface deposits like dirt, etc. You can see that an engineer might decide to pick a different "bell" design depending on the location. For instance, a "bell" with a longer surface path near a cement factory (where you can expect a lot of dust in the air).

Another factor that comes in play is that the insulators are intended to hold against more than just line voltage. Line switching and lightning strikes can send voltage surges along transmission lines, and the designer must decide how much more voltage he/she wants the line to handle before it will "flash over" the insulators and fault the line. This is usually described as the Basic Insulation Level (BIL), and is expressed as a multiple of nominal line voltage.

Taking a nominally-sized generator of 500 Mw, it's likely to have a phase-to-phase output voltage of anywhere from 13.8 kV to 24 kV. Power output for the generator is: sqrt[3]*voltage(l-l)*current(phase)*cosine[theta], where theta is the phase angle between the voltage and current sine waves. This works out to something in the neighborhood of 20 kA. This high current is one reason why generators always connect to the grid through a "step-up" transformer that ups the volatge and drops the current so that the line losses don't eat us alive.

Yes, there are examples of DC transmission in many places, including the North America. You can imagine the size and expense of the "converter" stations needed to integrate this DC transmission into the AC network! This means that it is only applied to situations where power needs to be transmitted over very long distances (where higher power transmitting ability over the same conductors gives lower line costs and losses to offset converter costs).

The industry continues to do some research into equipment made from superconducting equipment. Again, it becomes a question of whether or not the increased costs of building and operating this equipment can be more than offset by savings in better utilization of lines and reductions in losses. Some work is being done on generators with superconducting rotor windings. Superconducting underground cables are also being considered. My personal view is that it will be along time before this technology is widely applied.

NeoPTLD's point about "skin effect" is one I missed. Good point! As he says, there's a tendancy for current to flow more densely near the surface of the conductor than near its center. This tendancy is an effect of the alternating electromagnetic fields, and therefore becomes more pronounced as the frequency rises. This means that you need a larger conductor than would otherwise be requires if the current density were uniform throughout the conductors cross section.

 

[TRUMPHENT]

Thomas Alva Edison wanted nothing to do with frequency at all. When the first powerplants were to be built, he wanted them to generate DC, no frequency at all. Tesla discovered AC and apparently it had some benefits over DC. I think the big advantage was that AC travelled farther with less loss than DC making it choice for power.

Tesla may have experimented too much with his discovery, things like electro shock therapy etc. He was looney as kite in his later years.

 

[zephyrprime]

That low frequency is why people get strained eyes from fluorescant lights like they have

Actually, with modern electronic ballast rather than magnetic ballasts, the amount of peak to peak fluctuation in luminosity of flourescent bulbs is much less than in even incandescent bulbs. If you have problems with that at your workplace, ask your employer to upgrade to electronic ballasts.

 

[uart]

Originally posted by: TRUMPHENT
Thomas Alva Edison wanted nothing to do with frequency at all. When the first powerplants were to be built, he wanted them to generate DC, no frequency at all. Tesla discovered AC and apparently it had some benefits over DC. I think the big advantage was that AC travelled farther with less loss than DC making it choice for power.

Tesla may have experimented too much with his discovery, things like electro shock therapy etc. He was looney as kite in his later years.

No it's actually the other way around. At any given voltage level DC should have lower transmission losses than AC. There are many other problems with DC however, and this was even more so in the early days of grid power generation before the advent of solid state switching devices. Big problems with DC are (or more particularly were) switching and voltage transformation.

 

[rneff50]

The real history of Edison and Tesla and AC vs. DC. It is actually quite interesting, while depressing at the same time. [link="http://inventors.about.com/library/weekly/aa102497.htm"]http://inventors.about.com/library/weekly/aa102497.htm[/link]

 

[LanEvoVI]

The skin effect does help out a bit in substations though. The fact that current is more dense on the surface allows them to use hollow aluminum bus bars which save on weight. This in turn helps out with the supports and foundations needed for suspending the bus.

And uart is right, I was just reading up on HVDC transmission and today's technology has made it THE best choice for long distance transmission. You can transfer more power over less conductor area and have less loss. The only reason it isn't used in distribution (as opposed to transmission) is once again the fact that it is much easier to transform AC voltage.

 

[NeoPTLD]

A few people mentioned DC transmission. There is one right here in Oregon in a rural town called Celilo.

It is called the Celilo converter station and its DC line connects all the way to California. I visited the station about two years ago and they still use vacuum valves to rectify and convert DC to AC.

 

[DrPizza]

An interesting sidenote: The nerve impulses in the human body are most susceptible to AC current at 60 Hz... we unfortunately chose the frequency that is the most dangerous to ourselves. 70 Hz or 50 Hz would be less likely to cause ventricular fibrillation.

 

[uart]

Originally posted by: DrPizza
An interesting sidenote: The nerve impulses in the human body are most susceptible to AC current at 60 Hz... we unfortunately chose the frequency that is the most dangerous to ourselves. 70 Hz or 50 Hz would be less likely to cause ventricular fibrillation.

I've also heard that mentioned DrP. It was only anecdotal but when the story was passed on to me it was given as "50Hz" (our local supply frequency) being the worst case.


Anyway, back to the issues of AC power distribution and associated losses versus frequency and I?ve been re-thinking my earlier assessment that transformer Iron loss would be a significant problem with using higher mains frequency. Actually I now believe that transformers would operate better, not worse, with somewhat higher frequency. The reason is that although the losses definitely increase for a given working magnetic flux level (often caused ?excitation? level), with higher frequency you can reduce this magnetic flux level and ,given the same volume of Iron, still maintain the same power throughput (and even increase it somewhat).

Eddy current losses are proportional to the square of (flux_level * frequency), so if you double the frequency but half the excitation then you are no worse off, and that's even if you don't change to transformer materials more suitable for higher frequency operation. Now the AC power that a transformer can convert (that is, the throughput) is also proportional the working flux_level * frequency, so doubling the frequency while halving the working flux level doesn?t hurt you there either.

Now the case of hysteresis is a little more interesting. I now believe you are actually going to be better off here at higher frequencies, even though this may at first seem paradoxical, given that the hysteresis power loss equals the area of the hysteresis loop multiplied by the frequency.

However, the area of the hysteresis loop is proportional to the flux level to some power greater than 1 but less than 2 (this is an empirical formula). From memory an exponent of about 1.6 is typical for common transformer steel. So if the frequency were doubled but the excitation (that is, flux level) halved then the hysteresis loss would be something like (0.5)^1.6 * 2 which is only about two thirds the as much as at the lower frequency.


So what's the reason we only use 50/60 Hz. Hmmm, I'm now starting to think that it may be more to do with ease of generating, particularly in relation to required turbine speeds and synchronization, than it is with transmission problems. Though admittedly I haven't looked closely at just how significant skin effect is in the transmission side of things. Given the improvements in magnetic materials, conductors and aspects of the power generation I am now inclined to think that if the power grid system were just now being redesigned from scratch then they probably would go for a slightly higher frequency (and possibly an AC DC hybrid)..

 

[Navi]

I believe you are incorrect; 50 and 70 Hz are more dangerous. From what I have read over the years and can validate from phyiscs.about.com:

1890?s: Edison, seeking to discredit AC power, to the benefit of his DC power system, employed an engineer, named Harold Brown to tour the country. In the guise of ?Professor Brown?, Brown set up his traveling show at fairs markets and in town commons. His show consisted of the electrocution of animals with both AC and DC. At certain frequencies, even low power AC confuses the electrical impulses in the heart, killing the subject quickly, whereas low power DC simply gives mild burns and stuns. Brown went so far as to conduct the first execution by electrocution ? Axe Murderer William Kemmler of New York was ?Westinghoused? ? with a Westinghouse generator secretly acquired by Edison ? in a particularly inhumane manner. The electrocution was not instantaneous, and Kemmler was slowly cooked by the current. In actuality, the 60Hz frequency Tesla implemented in the United States is does not affect the heart as strongly as other frequencies, such as the 50Hz standard inexplicably adopted by Australia.

 

[Navi]

The main reason why 60 Hz was chosen by Tesla has little to do with science. The biggest theory, and most widly adopted, is that Tesla believe that 60 Hz was the "Fundamental Frequency of the Universe".

Some other information that may be of use.

Orignally Westinghouse wanted 133 Hz because of their current generator designs. But while Tesla was negotiating with Westinghouse house, Telsa managed to show that 60 Hz was more practical (details of this I have never been able to find).

 

[Jiggermyster]

Prodigal Genius: The Life of Nikola Tesla.
Good read... Check it out.

 

[DrPizza]

Originally posted by: Navi
I believe you are incorrect; 50 and 70 Hz are more dangerous. From what I have read over the years and can validate from phyiscs.about.com:

1890?s: Edison, seeking to discredit AC power, to the benefit of his DC power system, employed an engineer, named Harold Brown to tour the country. In the guise of ?Professor Brown?, Brown set up his traveling show at fairs markets and in town commons. His show consisted of the electrocution of animals with both AC and DC. At certain frequencies, even low power AC confuses the electrical impulses in the heart, killing the subject quickly, whereas low power DC simply gives mild burns and stuns. Brown went so far as to conduct the first execution by electrocution ? Axe Murderer William Kemmler of New York was ?Westinghoused? ? with a Westinghouse generator secretly acquired by Edison ? in a particularly inhumane manner. The electrocution was not instantaneous, and Kemmler was slowly cooked by the current. In actuality, the 60Hz frequency Tesla implemented in the United States is does not affect the heart as strongly as other frequencies, such as the 50Hz standard inexplicably adopted by Australia.

While I don't doubt that it's possible that 50Hz is most dangerous, I thought I'd double check some sources; it's been a few months since I've seen the issue discussed (the last time was at a seminar on demonstrations to enhance student learning). I searched Google for the terms: +"dangerous frequency" +heart +AC
The links agree with the 60Hz figure.
Stanford
Berkley
newton.dep.anl.gov

However, after searching for fibrillation and 60 Hz vs. fibrillation and 50 Hz, there seemed to be a lot more information on the 50 Hz frequency. Unfortunately, I don't have enough time to read right now to determine which of the two is more accurate.

Edit: regardless of arguing over the 50/60 Hz danger, I think a good point to this is that what we and the Europeans use is dangerous to humans.. Higher frequencies would decrease the danger, but would prove to be much more difficult to implement.

 

[RU482]

just in case anyone is curious "which countries have which standard"

here is a page off of my company's website
PS- we specialize in international power components

 

[grant2]

Originally posted by: Mday
take a few semesters worth of math, physics, and then circuits.

that's a pretty lousy answer! if you don't know or don't want to explain then why not remain silent?

 

[wacki]

Originally posted by: redly1
just in case anyone is curious "which countries have which standard"

here is a page off of my company's website
PS- we specialize in international power components

Just wondering, why do so many counties prefer round plugs? Is there an advantage with round prongs over american sheet like prongs for our electrical plugs? I would think it would be easy to reverse the polarity with the european style plugs and fry everything you own.