Continuation of line voltage waveform discussion (60Hz, 120Hz, ...)

[AB2EZ]

I thought it might be useful to continue the discussion of the waveform of the voltage provided by the power company... which I raised in a comment to an earlier thread that was started yesterday.

I have no first hand knowledge of how one designs power distribution grids... so I am only applying my knowledge of a few basic electrical engineering principles to produce the following "thought experiment".

Suppose the power grid operator finds itself in the following situation:

A. There is a portion of the grid that is under stress because of a high level of power consumption by the end users in that geographic location.

B. The limiting factor in this (hypothetical) situation is heating of the high voltage or medium voltage lines... caused by i^2 x r losses in the wires.

C. Line voltages in the effected areas have been reduced to manage the situation

D. The power companies cannot increase the peak voltages on their high voltage and/or medium voltage lines (beyond the present levels) because of physical limitations imposed by electric field levels between conductors

E. Implicit in this scenario is the ability of the power companies to (in effect) adjust the step down ratios of some of the higher voltage to lower voltage transformers.

What to do?

Here is a hypothetical approach... for something to do to alleviate the overload.

I am going to propose that the power companies start delivering voltage waveforms that are more like square waves than sine waves.... and to make things easier to describe, I'll go to the extreme case of square waves.

Proposed solution:

The power company starts delivering 60 Hz square waves to the affected geographical area. These square waves are adjusted to have a peak voltage (the top or the bottom of the square wave) that is 0.7 as large as the peak level (amplitude) of the sine waves that the power company was delivering.

All of the resistive loads (e.g. electric heaters, incandescent light bulbs, ....) will perform exactly the same... because they will draw 0.7 times as much peak current as before... but the heating produced by a square wave current of peak amplitude 0.7A is the same as the heating produced by a sine wave current of peak amplitude A.

All of the electronic loads (things with modern power converters inside) will perform almost exactly the same. The power converters run at high efficiency, and deliver their specified output voltages (e.g. 12 volts, 5 volts) almost independently of the AC input voltage level, frequency, or waveform shape.

Some loads (e.g. Johnson Ranger transmitters) will operate at reduced power consumption, or increased power consumption, or not at all... if the line voltage waveform is changed to a square wave with 0.7 times the amplitude v. the sine wave that is normally delivered... and this needs to be considered for various types of loads.

However, the impact is a 30% reduction in the peak voltage on the high voltage and medium voltage power lines.

Now, the power company can increase the voltage on the high voltage and medium voltage power lines (without exceeding their peak voltage limits)... and, correspondingly, increase the step-down ratios of the transformers leading to the end users.  The net result is a reduction of the heating in the high voltage and medium voltage power lines. I.e. for a given amount of power delivered by those lines... higher voltage operation results in lower current operation... and, therefore, reduced i^2 x r losses in those lines.

Bottom line: using the above, greatly simplified analysis... it would seem that power companies and grid operators would be inclined to move from the production and delivery of sine waves toward square waves (harmonics in the delivered waveforms) during periods when their high and medium lines are under stress.

Stu

[N4LTA]

Stu,

I think the problem with using square waves is that there is not easy way to generate them. Almost all utility generators are rotating synchronous generators and they produce a near perfect sine wave.

DC Is probably the best way to transmit electricity on the main high lines and is already being done in a few cases. The technology is becoming reliable to switch 500 KV voltages to allow the conversions. With DC - You have only IR losses and reactance problems (reactive current causing IR losses) are not in effect.

The other problem - and you are more of a mathematics person than me by a long shot - Is what happens when you sum the three phase currents  (balanced phases) and don't get zero in a wye circuit. Are the existing neutral conductors up to the task?

Also - When you put a square wave into a transformer - when it comes out - what does it look like. With most large power transformers - the frequency response is what? I have no idea but would expect it not to be very high and every transformation might tend to convert things back towards a sine wave.

There are quite a few transformation before it hits your house - typically generation is done at low medium voltage  - 2.4, 4.16 or 12.4 KV and them up to 220or 500KV - sometimes 1000 KV - then usually back to 220 KV or similar and then sometimes down to 100KV and possible down again to 44KV - It then is dropped to 4.16, 12.4 or 22 KV for distribution and then again down to 240/120 or other utilization voltage.

These are just a few things to throw in. I think it might be easier to convert all 500 and 100 KV transmission lines to DC as there has to be a solid state conversion mechanism to make square waves or DC.
DC  has no problem with line reactance and AC has lots of problems.

One of the proposed Hydrogen generation solutions ( I think we are a very long way from a hydrogen economy and are wasting precious time on it) was to build many nuclear reactors in the western desserts and build an extensive DC transmission line to the east and west coasts possibly with superconductors to make the line essentially loss-less. The energy would be used to produce Hydrogen electrolysis.

Pat
N4LTA

[AB2EZ]

Pat

Ok... I was assuming that the power company was intentionally squaring up the waveform.

Based on your clarifications... it looks like any deviations from a sinusoidal waveform in what I measure at my house are unintentional consequences of the loads on the local grid.

Thanks
Stu

[WD5JKO]

  Good discussion thus far...

We need to keep in mind that many power supplies that turn AC to DC in the simplest forms, i.e. rectifier and some form of filter such as choke input L-C or capacitor input C-L-C depend on a sinewave input to get the intended DC output voltage. Think of the F-W voltage doublers in the big Linear amps that try to take the AC peak level and then double it. This will not work with a square wave drive where the peak level is 117v. These supplies need a peak level around 160V to work.

Consider the DC to AC inverters these days where the more common and inexpensive ones use a 'step-sine-wave' where the output is a square wave, but around 6ms wide each half wave, and centered in the normal 8.33ms period where the sine wave would have been. These use a peak level of the square wave at about 150vdc. So the shorter on time, and higher peak level give approximately the same peak level and RMS level as the usual sine wave.

I have one of these inverters, and hooking to some of my boat anchor receivers resulted in lots of noise, and in some cases blown fuses.

Jim
WD5JKO

[N4LTA]

If you look at the loads that were typical in the late 60s - which is when the power grid was pretty much mature, there was really nothing but linear loads - motors, lighting was incandescent or discharge lighting using magnetic ballasts - I can't think of any non-linear loads except solid state DC drive systems and solid state welding systems and these were just coming on line.

Now - a very large percentage of the grid electrical load is non-linear and it is causing the utilities considerable troubles.

Non linear power supplies are showing up everywhere - not just in computers and non linear lighting ballasts are become code required. Even home HVAC systems have non linear motor drives to increase efficiency.

Maybe Edison was correct and Tesla and Westinghouse were wrong after all?

Pat
N4LTA

[WD5JKO]

Maybe Edison was correct and Tesla and Westinghouse were wrong after all?

I believe that electrocution originally was done with DC. Later when AC was used, early attempts resulting in the victim frying in a prolonged and painful death. They lived until they caught fire. Remember the movie, The Green Mile" where the victim never got his head sponge wetted?

The account at the link posted below differs from what I recall reading in a Tesla book. As I recall (and relying on memory is getting dangerous), the AC to the electric chair had a peak level equal to the prior DC level. Therefore the RMS voltage was only 0.707 times what they had before, and the victim was "Westionhoused" 

http://inventors.about.com/od/hstartinventions/a/Electric_Chair.htm

Jim
WD5JKO

[k4kyv]

Maybe Edison was correct and Tesla and Westinghouse were wrong after all?

Recently, Tesla was the subject of a 1-hour radio talk show on NPR. Click on the link to listen.

Despite being interesting, informative and IMO well worth the listen, the show is marred by a couple of outstanding errors that stuck out to me like a sore thumb.  From the outset, host Tom Ashbrook mispronounces Tesla's name as "Tezla".  One of the guests on the show, who obviously doesn't know a lot about electricity, incorrectly explains the skin effect, claiming that one avoids getting burnt or electrocuted by a Tesla coil because it puts out "high voltage and  low current".  Everyone should know that the skin effect really occurs because the a.c. frequency is high enough to maintain most of the current on the surface of the conductor. This guest even uses the wrong term, calling it "surface effect”.

http://www.onpointradio.org/2010/01/nikola-tesla-and-innovation-today

[N4LTA]

I think Edison and Tesla were pretty much arch enemies. Edison's distribution system used DC and Tesla was an AC proponent.

He killed an elephant in a public show to demonstrate the dangers of AC

http://www.wired.com/science/discoveries/news/2008/01/dayintech_0104

Both men were proponents of the Electric Chair - each pushing the others type of current - to make it seem more dangerous. AC was chosen as the official electric chair current and Westinghouse refused to sell them AC generators.

Several executions were held in front of a large audience.

Pat
N4LTA