Harmonic sources, flowing from load, why?

[OliskaP]

I have been thinking of this and can not seem to understand why nonlinear loads can be tought of harmonic current sources injecting current back into the grid?

For example if you connect a passive filter in parallel to attenuate the 5th harmonic, the 5th harmonic would flow between the filter and the load, and the supply do not need to supply 5th harmonic current.

I can not seem to understand why? How can the load produce harmonic currents? I have always been thinking that current flows from the source to the load, but then again its AC the current flows back and forth all the time.

Still, I can not figure out why non linear loads can be tought of current sources injecting into the grid instead of drawing harmonic current from the source.Best regards

 

[anorlunda]

The more accurate word would be "causing" harmonic currents. Injecting or withdrawing are more like a glass half full or half empty, the depend on the speaker's point of view.

The words we chose in natural language are not always the most scientifically accurate ones.

 

[Windadct]

This is a good use for LT Spice - considering a rectifier (diode or thyristor) only conduct when the device is forward biased, so current only flows when the line voltage is higher than the DC voltage. -- Look at the waveform images part way down this page. The load current is controlled by the load, not the source - and it is not a sine wave of current.

You will typically add a line reactor to attenuate this first -as the reactor will inhibit the rapid current rise and fall as the diodes "turn on " and off.

From the grid perspective - the term injected is misleading(but is is just a term), however the various inductive elements in the grid have to react to this load current. The impact is seen from the LOAD back towards the source.

 

[OliskaP]

Thanks for the explanations @anorlunda and @Windadct , but I am still abit confused when a passive shunt filter is connected in parallel, see figure.

So if a nonlinear load causes flow of fundamental + harmonics, but the filter is implemented in such a way that the source only has to supply fundamental. This is confusing to me, unless we can think of:

1. The nonlinear load to "inject" harmonics back into the grid, and the filter is present to absorb it, so that the supply only have to provide fundamental current.
OR
2. The filter providing the harmonic currents to the load, thus again the supply only has to supply fundamental current.

 

[anorlunda]

Both 1 and 2 sound equivalent to me. Feel free to choose either.

 

[Windadct]

Realizing that the shunt filter is effectively a "short" to current above some frequency ( 5th of 60 = 300 Hz) ... I think part of the issue is we typically prefer to see think of circuits from a voltage source perspective, but in reality any circuit can be seen from a voltage or current thinking.

If you were to look at the current for each of the 4 branches - coming into the node they would each be different. The filter would ONLY have current above some frequency and Ideally, look like an open ckt to 60 (50) Hz - in Voltage or Current. The Non-linear load has Fundamental and High Harmonic current, the source can accept the harmonics but has a higher impedance than the filter, so the harmonics to the source are reduced but not eliminated.