reflected wave
reflected wave
(OP)
After seeing a few post on this board about VFDs and the problems with refelcted wave I've though about our utility lines here in the States. How does the utility line keep the refelcted waves at 60HZ from being a problem. In addition, what about the reflected waves on an applinace branch circuit in our homes?






RE: reflected wave
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RE: reflected wave
RE: reflected wave
RE: reflected wave
If you look at high tension lines as you drive around you will often see that inexplicably at some tower two of the wires will go thru a strange re-routing then continue until it happens again. This is because of the distributed nature of the system. These wires get swapped around to keep the distributed impedances balanced.
Someone correct me if I'm off on this.
RE: reflected wave
Going back to reflection in power systems, I think that reflection effects may be important in study of switching transients.
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RE: reflected wave
RE: reflected wave
RE: reflected wave
Some of these towers are spectacular. Most complex I have ever seen!
Lotta pictures of infrared scans with obvious problems.
Check it out.
This is link:
http://www.gorge.org/pylons/page1.shtml
RE: reflected wave
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RE: reflected wave
DC transmission lines are popular partially because there are no wave effects at zero frequency.
RE: reflected wave
RE: reflected wave
Using the distributed model of series of infinitessimal shunt capacitances and series inductances we come up with the Pi equivalent circuit.
How does that relate to voltage reflection causing increased voltage at an impedance discontinuity? Worst case increase occurs on an open circuit. Think about a pi equivalent circuit tranmission line open on one end and connected to a system at the other end. All of the vars generated by the capacitance modeled at the open end have to flow through the series inductance element to reach the connected end, in the process creating a voltage drop going from the open end to the connected end. In other words the open end will have higher voltage than the connected end. The equivalent circuit has predicted the voltage increase at the open end. I suspect that with more detailed analysis of reflected waves we could come up with the same conclusion.
My apologies for mudding the thread and thanks to bacon and itsmoked for straightening me out.
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RE: reflected wave
I know that part of what tripped me up was thinking about the smith chart, which gives periodically varying characteristics (neglecting losses) as we increase lenght of the line (the anlge of the offset vector on the smith chart rotates as we increase length). The thing about 60hz power lines is the length is very short compared to the wavelength so we don't get anywhere near one rotation. And in fact as someone pointed out we don't get anywhere near the 1/4 wavelength where I think maximum voltage amplification occurs. A line of several hundreds of mils represent only a very small rotation on the Smith chart which is approxiamtely modeled by the pi circuit model.
I suspect in practical terms the most important impact to system operators is that the associated vars from distributed capacitance affect their voltage profile. Voltage increase from power frequency at an open end I suspect is small only a few percent (could be calculated as above). However the voltage at the open end of a line may have much greater amplification of transient switching surges than of low frequency 60hz power.
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RE: reflected wave
The quarter wave length for 60 Hz is only about 1250km. Given the makeup of the North American grid, it isn't that difficult to find lines that long. BPA has east-west and north-south lines that long or longer. Power flowing from the Grand Coulee area to San Diego could approach a phase shift of 180 degrees, although it would take several lines end-to-end to make that journey.
RE: reflected wave
I suspect you are right about the voltage profiles being the biggest concern.
RE: reflected wave
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RE: reflected wave
Are you familiar with the inter-mountain west? A 500kV line can go hundreds of miles without anything else to connect to. Lines between western Montana and the Pacific Coast, or between Salt Lake City and southern Oregon pass through large expanses of very sparse territory. Long line models are quite necessary. I don't know the details of these lines, but the maps I've seen certainly suggest that the lines run very long distances without significant connections.
RE: reflected wave
"...1250km. Given the makeup of the North American grid, it isn't that difficult to find lines that long. BPA has east-west and north-south lines that long or longer."
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RE: reflected wave
RE: reflected wave
But a power grid is not like that, there is power being consumed all along its length, so it is a very "lossy" or "heavily damped" circuit. Any surge or sag will not go far, especially if the system is heavily loaded as it usually is.
RE: reflected wave
K = 2*pi*f*sqrt(L*C)*length
Using values from a typical 2 bundle 345 kV line of:
L = 9.83x10-7H/m
C =11.59x10-12F/m
R~=0
length K
50km 0.063
100km 0.127
300km 0.382
I need to correct myself that cables do not see an increase in wave behavior such as reflections and standing waves, but rather just must be modeled at shorter distances. For example a theoretical cable with:
L = 9.83x10-7H/m
C = 190x10-12F/m
R~=0
length K
10km 0.052
50km 0.258
100km 0.515
200km 1.030
The cable length would instead be limited by the shunt capacitance causing large charging currents to flow and causing the voltage at the midpoint end to change drastically depending upon the load flowing through the line.
RE: reflected wave
Bung
Life is non-linear...
RE: reflected wave
The mechanical analogy would be a lump of concrete (Durban)bouncing at the connection of two springs, one that is connected at the other end to a solid base (infinite bus) and the other to another lump of concrete (the generators). Give the "generators" a kick, and the whole thing bounces around.
In fact, all power systems are continually oscillating ever so slightly- its just not normally noticable. It's really quite extraordinary (to my simple mind) that power systems remain as stable as they do.
Bung
Life is non-linear...
RE: reflected wave
Have you ever seen the power line pylon near Orlando Florida as you approach Disneyworld? It is sculpted to look like Mickey Mouse!
http://w
RE: reflected wave
I just saw it in the last week... A picture.
That really takes the cake..
Nice analogies Bung.
RE: reflected wave
Absolutely agreed, provided you qualify with "A/C power systems". Notwithstanding my enormous admiration for the skills of those engineers who are capable of keeping a system like the North American grid even operating, I often wonder if they've just gotten too busy or too specialized to have noticed the current progress of modern electronics and DC transmission.
Pechez les vaches.
RE: reflected wave
The simplist method of limiting the amplitude of reflected wave is the installation of arrestors on the end of terminal lines. This would apply to both transmission and distribution.
I might suggest that the area that you need to address first is your underground distribution. Reflected waves that occur at the end of lines may overstress the insulation medium that is being used. Generally you would use an arrestor elbow installed on a junction box, transformer, or LRTP on a 600A non-load break elbow.
RE: reflected wave
But you need ac (or some incredible switch mode PSUs!) to convert voltage levels. If the electronics is so good, maybe we should use ac (simplest for generation), and then just do all the rest with electronics. No need for dc versus ac at all - just do whatever is simplest from a field operations (=users) switching, etc point of view.
And the reflected wave problems would be pretty mind boggling too, with all those harmonics etc!
Bung
Life is non-linear...
RE: reflected wave
The last true arguments in favour of AC over DC for any significant amount of power any distance were 1) that DC could only go point-to-point, no intermediate tapping, and 2) DC-to-AC converters needed working AC on the load side before they could work.
Both those problems have long ago been overcome, and the "economic crossover distance" has been steadily dropping, now dropping under 250 miles.
Also I'd suggest any engineer who wants to work on future superconductor transmission had better start learning their DC now.
Pechez les vaches.