Smith Chart

[mbrooke]

Does the Smith impedance chart developed in 1939 by Bell Telephone have any use in the power EE field? I see it occasionally in electronics text books but must admit that I can not think of a single scenario where it would be used at 50/60Hz... Just wondering if its worth studying in depth.

https://en.wikipedia.org/wiki/Smith_chart

http://www.slideshare.net/amitmeghanani/smith-charta-graphical-representation

 

[charlie b]

I have never heard of a Smith Chart. I can't think of a use for one in the arena of Power Systems Engineering.

 

[mbrooke]

I have never heard of a Smith Chart. I can't think of a use for one in the arena of Power Systems Engineering.

Its rare that its mentioned outside of RF engineering, but none the less pretty much what I suspected as well.

 

[K8MHZ]

I have never heard of a Smith Chart. I can't think of a use for one in the arena of Power Systems Engineering.

I had to learn how to use them in order to pass my Extra class test for my ham radio license.

They are used in radio to determine reactance and standing wave ratios. I just learned enough to pass the test and have never actually used one in the real world.

 

[GoldDigger]

I have never heard of a Smith Chart. I can't think of a use for one in the arena of Power Systems Engineering.

A Smith Chart is basically a graphic tool for calculating the impedance seen at one end of a transmission line based on the length of the line in wavelengths of transmitted power and the complex impedance which loads down the other end of the line.

It tells you, among other things, that a dead short at the end of a 1/4 wave line looks like an open circuit at the other end (ignoring resistive losses) and that an inductor at the end of a 1/4 wave line looks like a capacitor at the other.
This can be important in doing transient analysis on long lines.

The charts were very useful in RF work when calculators and computers did not have the power they do now.
If 60Hz transmission lines are long enough for this all to matter, these days we use analysis and modelling programs on computers instead of breaking out a pad of Smith Charts.

 

[mbrooke]

A Smith Chart is basically a graphic tool for calculating the impedance seen at one end of a transmission line based on the length of the line in wavelengths of transmitted power and the complex impedance which loads down the other end of the line.

It tells you, among other things, that a dead short at the end of a 1/4 wave looks like an open circuit at the other end (ignoring resistive losses) and that an inductor at the end of a 1/4 wave line looks like a capacitor at the other.
This can be important in doing transient analysis on long lines.

The charts were very useful in RF work when calculators and computers did not have the power they do now.
If 60Hz transmission lines are long enough for this all to matter, these days we use analysis and modelling programs on computers instead of breaking out a pad of Smith Charts.

Around what length to you get into that?

 

[GoldDigger]

Around what length to you get into that?

The wavelength of a 60Hz electromagnetic wave in a vacuum is about 3100 miles.
In a transmission line with a propagation speed of 66% of light speed that would be closer to 2000 miles.
1/4 wavelength will then be about 500 miles. Very large effects!
At less than 50 miles you can probably ignore the effects in a calculation.
At exactly 1/2 or 1 wavelength the transmission line acts just like a simple wire (again ignoring resistive losses).

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[mbrooke]

The wavelength of a 60Hz electromagnetic wave in a vacuum is about 3100 miles.
In a transmission line with a propagation speed of 66% of light speed that would be closer to 2000 miles.
1/4 wavelength will then be about 500 miles. Very large effects!
At less than 50 miles you can probably ignore the effects in a calculation.
At exactly 1/2 or 1 wavelength the transmission line acts just like a simple wire (again ignoring resistive losses).

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But at 2000 miles you in essence have a large television antenna. I guess this is why something like 400Hz and above is not good for power transmission.

 

[Ingenieur]

There is a simple graphical method involving basic hand calcs to determine transient magnitudes at the junctions of a transmission line
it's called a lattice or Bewley chart
basically graphs incident, refracted and reflected waves (or sums/differences) vs a time interval (usually the pu time to travel between the subject nodes)j

 

[mbrooke]

There is a simple graphical method involving basic hand calcs to determine transient magnitudes at the junctions of a transmission line
it's called a lattice or Bewley chart
basically graphs incident, refracted and reflected waves (or sums/differences) vs a time interval (usually the pu time to travel between the subject nodes)j

But they can not tell the midpoint? I remember reading a while back in the transactions of American Electrical engineers that transmission line is almost like a violin string. Transients will travel in waves and resonant in certain parts of the line. Although I doubt it has much bearing in typical POCO settings.

 

[Ingenieur]

But they can not tell the midpoint? I remember reading a while back in the transactions of American Electrical engineers that transmission line is almost like a violin string. Transients will travel in waves and resonant in certain parts of the line. Although I doubt it has much bearing in typical POCO settings.

That is more involved
but usually damping is low so in the center is ~ the same
Especially for the time/distances under study

but we usually only care about the equipment at each end, not the conductor in the middle
if the line has equipment in the middle it is made another node on the chart

the refracted and reflected will add up sometimes (sometimes subtract) depending on the line surge Z
and it will travel as a wave down the line ie magnitude varies with time/position
again though, we are usually only concerned with max for sizing equipment

 

[mbrooke]

That is more involved
but usually damping is low so in the center is ~ the same
Especially for the time/distances under study

but we usually only care about the equipment at each end, not the conductor in the middle
if the line has equipment in the middle it is made another node on the chart

the refracted and reflected will add up sometimes (sometimes subtract) depending on the line surge Z
and it will travel as a wave down the line ie magnitude varies with time/position
again though, we are usually only concerned with max for sizing equipment

I beg to differ :angel: Insulators mid line see the effects too...

 

[mbrooke]

That is more involved
but usually damping is low so in the center is ~ the same
Especially for the time/distances under study

but we usually only care about the equipment at each end, not the conductor in the middle
if the line has equipment in the middle it is made another node on the chart

the refracted and reflected will add up sometimes (sometimes subtract) depending on the line surge Z
and it will travel as a wave down the line ie magnitude varies with time/position
again though, we are usually only concerned with max for sizing equipment

Max in terms of peak voltage spikes?

 

[Ingenieur]

Max in terms of peak voltage spikes?

You can use the magnitude (plus tower parameters, surge Z, gnd Z, etc) you can size the insulators for flashover

from a fault ~ 2 to 3 x sqrt2/sqrt3 x Vph
lightening, there is a std rise/decay (it's on surge arrestor charts) iirc magnitude depends on region and derived from a geographic chart

 

[mbrooke]

You can use the magnitude (plus tower parameters, surge Z, gnd Z, etc) you can size the insulators for flashover

from a fault ~ 2 to 3 x sqrt2/sqrt3 x Vph
lightening, there is a std rise/decay (it's on surge arrestor charts) iirc magnitude depends on region and derived from a geographic chart

Do those fancy mid air shunt arrestors help?

 

[jumper]

I had to learn how to use them in order to pass my Extra class test for my ham radio license.

I had to learn how to use them for a communication (RF) class in college. I remember having to go to the store and buy a grammar school protractor.

 

[K8MHZ]

I had to learn how to use them for a communication (RF) class in college. I remember having to go to the store and buy a grammar school protractor.

There are still questions about Smith Chart use on the Extra exam. I have no idea why, no one I know locally has ever used them. Of the two or three people I know that do use them for ham radio, they are electrical engineers or high level RF techs (one works at HAARP in Alaska). They don't use them because they have to, they do it for fun.