Why doesn't long distance power tranmission cause sine wave distortion?

[Carultch]

Can a diode be compared to a neon indicator?

It was a break down voltage (forward voltage?__:?) and has a negative run away resistance.

Similar, in that it has a bias to the direction of voltage. Although diodes usually have nothing to do with neon.

A diode in general, is a component that is built to act as an open circuit in the reverse bias direction, and as close as possible to a direct connection in the forward bias direction. When it is a perfect open circuit in reverse bias, and acts as a perfect direct connection in forward bias, that is an ideal diode. A real diode has losses in the forward bias direction that typically follow an exponential curve, and a breakdown voltage in the reverse bias direction.

 

[GoldDigger]

Can a diode be compared to a neon indicator?

It was a break down voltage (forward voltage?__:?) and has a negative run away resistance.

Both are non linear, but the neon is usually symmetric while the diode is very asymmetric.
By using electrodes with different shapes you can get some asymmetry in a neon too, though.
The negative resistance characteristic of gas discharge is a lot more pronounced.

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[Smart $]

But what determines the uniqueness?

Uniqueness to what... other sine waves, or waveforms of a different type, such as square or sawtooth?

If other sine waves, just the x- and y-axis values. Two sine waves may even be identical, but if plotted differently, such as with different axis scaling, each will appear different.

 

[Carultch]

Can you go more into DC offset? Is this like the DC competent when a breaker interrupts fault current we must take into consideration?

What DC offset means, is that the signal is not centered on zero. When a sine wave has no DC offset, the maximum positive value is equal and opposite to the maximum negative value. The time average value is zero, the peaks are equal and opposite.

Now suppose you add a constant voltage to a sine waveform. That is the DC offset. The time-average voltage of the waveform is non-zero. The waveform is not completely reversed as it is for pure AC, but instead centered on something other than zero.

RE: superpositions and other waveforms, Any pics or links? Im confused...

The black waveform is an ideal square wave. The color waveforms show the stages to getting there from sine waves. The red wave is the fundamental frequency, which is usually the highest in amplitude. Everything else (overtones & harmonics) are higher in frequency and typically diminishes in amplitude, to distort the wave away from a pure sine. The more harmonics you include in the composition, the closer it becomes to the ideal square wave.
http://mathworld.wolfram.com/images/eps-gif/FourierSeriesSquareWave_800.gif

The family of sine waves that you add up to form another waveform, is called a Fourier series.

 

[mbrooke]

A most excellent question
If the generator windings were perfectly flat coils that rotated in a uniform constant magnetic field you would get very close to a perfect sine wave.
Instead you have non flat coils and a non uniform magnetic field (produced by other coils.). Add in the effect of hysteresis in steel cores and you move away from the perfect circular function.
Power generators (POCO style) are designed to come very close to a sine wave.

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How does a POCO gen differ from a regular one? I frequently see "utility" grade options for cogeneration turbines.

 

[mbrooke]

Similar, in that it has a bias to the direction of voltage. Although diodes usually have nothing to do with neon.

A diode in general, is a component that is built to act as an open circuit in the reverse bias direction, and as close as possible to a direct connection in the forward bias direction. When it is a perfect open circuit in reverse bias, and acts as a perfect direct connection in forward bias, that is an ideal diode. A real diode has losses in the forward bias direction that typically follow an exponential curve, and a breakdown voltage in the reverse bias direction.

And also a reverse recovery voltage :thumbsup: Never ignore that dot on a circuit diagram

 

[mbrooke]

Uniqueness to what... other sine waves, or waveforms of a different type, such as square or sawtooth?

If other sine waves, just the x- and y-axis values. Two sine waves may even be identical, but if plotted differently, such as with different axis scaling, each will appear different.

To a perfect fundamental (theoretical) sine wave. What can cause that to be different in a power system?

 

[mbrooke]

What DC offset means, is that the signal is not centered on zero. When a sine wave has no DC offset, the maximum positive value is equal and opposite to the maximum negative value. The time average value is zero, the peaks are equal and opposite.

Now suppose you add a constant voltage to a sine waveform. That is the DC offset. The time-average voltage of the waveform is non-zero. The waveform is not completely reversed as it is for pure AC, but instead centered on something other than zero.

Ok, I've seen that with fault current represented in a sine wave. But-- I have no clue what causes it.

The black waveform is an ideal square wave. The color waveforms show the stages to getting there from sine waves. The red wave is the fundamental frequency, which is usually the highest in amplitude. Everything else (overtones & harmonics) are higher in frequency and typically diminishes in amplitude, to distort the wave away from a pure sine. The more harmonics you include in the composition, the closer it becomes to the ideal square wave.
http://mathworld.wolfram.com/images/eps-gif/FourierSeriesSquareWave_800.gif

The family of sine waves that you add up to form another waveform, is called a Fourier series.

Check and check, digesting this.

 

[Smart $]

To a perfect fundamental (theoretical) sine wave. What can cause that to be different in a power system?

Practically any element or characteristic involved with generation, transmission, and consumption. GD already hit on the generation part... in part... and that's assuming you want a sinusoidal waveform to begin with... after all, both square and sawtooth waveforms would be considered AC also.

Some of the first inverters approximated a sine wave by outputting a stepped voltage square wave.

Everything in my house runs fine on my portable generator during an outage. However, I have to manually "bypass" everything connected to my two UPS's because neither UPS will accept the distorted waveform output by the generator. Yet Generac advertises it as having less than 5% total harmonic distortion.

 

[mbrooke]

Practically any element or characteristic involved with generation, transmission, and consumption. GD already hit on the generation part... in part... and that's assuming you want a sinusoidal waveform to begin with... after all, both square and sawtooth waveforms would be considered AC also.

Some of the first inverters approximated a sine wave by outputting a stepped voltage square wave.

Everything in my house runs fine on my portable generator during an outage. However, I have to manually "bypass" everything connected to my two UPS's because neither UPS will accept the distorted waveform output by the generator. Yet Generac advertises it as having less than 5% total harmonic distortion.

Yet I bet motors run just fine :thumbsup:. Fascinating topic though.

 

[GoldDigger]

Can you go more into DC offset? Is this like the DC competent when a breaker interrupts fault current we must take into consideration?

A sine wave, by convention, does not start or stop but goes on constantly.
The fault current on the other hand, starts suddenly at time t=0.
If you start with the positive half cycle the average current over s short time the current in the positive direction will go back and forth between positive and zero but never go negative. You can account for this by figuring in a decreasing DC offset superimposed on a symmetric wave.
When the OCPD finally opens, at whatever part of the cycle it happens, there will have been a non zero DC average to the current.


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[mbrooke]

A sine wave, by convention, does not start or stop but goes on constantly.
The fault current on the other hand, starts suddenly at time t=0.
If you start with the positive half cycle the average current over s short time the current in the positive direction will go back and forth between positive and zero but never go negative. You can account for this by figuring in a decreasing DC offset superimposed on a symmetric wave.
When the OCPD finally opens, at whatever part of the cycle it happens, there will have been a non zero DC average to the current.


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I don't understand this. Does the system actually see DC voltages or is it just a depiction effect of the oscilloscope?

 

[Ingenieur]

The fault waveform can be resolved into components
an ac and a dc
the dc is generally an exponentially decreasing form
using pu values
starts at say 1 and ramps down to 0 over time
the ac is asymmetric sinusoidal in form
amplitude and freq (depending on ckt parameters) decrease in time
add them together
so at 0 time the wave is offset centered on 1 in this case
in time its amplitude will decrease and it will be centered on 0 as th dc component decays to 0

 

[mbrooke]

The fault waveform can be resolved into components
an ac and a dc
the dc is generally an exponentially decreasing form
using pu values
starts at say 1 and ramps down to 0 over time
the ac is asymmetric sinusoidal in form
amplitude and freq (depending on ckt parameters) decrease in time
add them together
so at 0 time the wave is offset centered on 1 in this case
in time its amplitude will decrease and it will be centered on 0 as th dc component decays to 0

In terms of quantifiable expression- but where does said DC voltage come from?

 

[Ingenieur]

In terms of quantifiable expression- but where does said DC voltage come from?

the ac source or generator

 

[mbrooke]

the ac source or generator

As it provides the "energy" for the system, but what causes it? Look at it like this. You have a system without load, the voltage is a sine wave. I then switch on a 1,500 watt space heater. I get 12 amps of AC in a perfect since wave. Then I have a short circuit. Why do I get more current in the positive or negative half? Shouldn't the fault current also be a perfect sine wave (assuming a bolted fault with no arcing or sputtering impedance) like the load switched on?

 

[Ingenieur]

As it provides the "energy" for the system, but what causes it? Look at it like this. You have a system without load, the voltage is a sine wave. I then switch on a 1,500 watt space heater. I get 12 amps of AC in a perfect since wave. Then I have a short circuit. Why do I get more current in the positive or negative half? Shouldn't the fault current also be a perfect sine wave (assuming a bolted fault with no arcing or sputtering impedance) like the load switched on?

what if you had 1 amp flowing resistive
then switched a 100 A capacitor into the ckt
keep in mind the source gen is an inductance

 

[mbrooke]

what if you had 1 amp flowing resistive
then switched a 100 A capacitor into the ckt
keep in mind the source gen is an inductance

The two would shuffle AC VARS back and forth. A big enough cap will harm the gen.

 

[Ingenieur]

You will also have dc offset from the switching transient

inductive ckt (like most power networks)
i lags v
i = 0 when v is max (or vice versa)
i can't change instantaneously i = 1/ L int v dt, takes time to change
these are basic rules determined by field theory, Maxwell et all

a fault causes a large change in i
but if v is peak i must be 0
so a dc offset is supplied by the source to get i to 0, wave bottom is ~0
the rules are maintained

the dc offset magnitude depends on 2 things
system x/r or pf/angle
where in the v cycle the fault occurs

 

[mbrooke]

You will also have dc offset from the switching transient

inductive ckt (like most power networks)
i lags v
i = 0 when v is max (or vice versa)
i can't change instantaneously i = 1/ L int v dt, takes time to change
these are basic rules determined by field theory, Maxwell et all

a fault causes a large change in i
but if v is peak i must be 0
so a dc offset is supplied by the source to get i to 0, wave bottom is ~0
the rules are maintained

the dc offset magnitude depends on 2 things
system x/r or pf/angle
where in the v cycle the fault occurs

Clearer. What conditions cause more and which cause less? Ive seen waveforms with little DC offset and others with a lot. Am I correct that in front of a 345kv generator plant there would be more than at a resi service during a fault?


Is this DC value coming from active power or reactive power?