Harmonics

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K2500

Senior Member
Location
Texas
I'm looking for a way to understand harmonics and non-linear loads.
I know a little about what they are, but not how they are created, or how they effect.

Where would be a good place to start?
 

crossman

Senior Member
Location
Southeast Texas
Here is my "electrishun's" take on the matter of harmonics. I hope this will be the spark for some discussion in which I may learn a thing or two.

First, assume we have a 208Y/120 3phase 4wire system. If A, B, and C have single phase loads connected to neutral, and have the same current flowing in each load, we have the following graph for current (Black = A, Red = B, Blue = C). In this instance, the neutral current is zero because any current flowing in equipment connected to a given phase is "returning" through the equipment connected to the other two phases. Everything is balanced, and essentially we have two loads in series between any given two phases. All of the loads are conducting current continuously.

har3sine.jpg


Now, let us look only at A:

harAsine.jpg


But if we have equipment connected from A to neutral that contains "switching mode" DC power supplies, (which in today's world would include most pieces of electronic equipment), then the situation is different. Because the DC power supply in the equipment only conducts near the peak of the sine wave, the original sine wave current is no longer a sinewave. The actual current waveform looks like this:

harA.jpg


I'll go ahead and post the graphs of B and C also, first the normal sine wave of linear load, and then the graph of current from equipment from phase to neutral with DC supplies, otherwise known as non-linear load.

B phase:

harBsine.jpg


B phase current with DC power supply as connected load:

harB.jpg


C phase with non-harmonic connected load, normal sine wave: Oops, the forum only let's me put in 8 photos. I will omit the C phase normal sine wave.

C phase with "harmonic" DC power supply as connected load:

harC.jpg


And here is the conglomerate graph of all three phases with switching mode DC power supplies:

har3.jpg


Now a key point is to recognize that the current in A phase cannot balance on B phas or C phase because those loads are not conducting at the time when A phase loads are conducting. All of the A phase current must return to the source via the neutral. This same thing holds true for B phase and C phase. The phase currents cannot balance.

And finally a graph of the neutral current:

harneut.jpg


Notice:

1. The neutral current frequency is 3 times that of a given phase. This is called the third harmonic, or "triplen".

2. The current flow in the neutral is at or near - or + peak continuously. The RMS value of current would be larger than the phase currents RMS values. This could definitely cause heating of the neutral. Note that the neutral current is hardly ever near zero like that of each phase conductor.

3. The neutral current looks more like a square wave than a sine wave to me.

Now what about the higher order harmonics? Seems to me they originate with the voltage drop that will be experienced near the peak. Take A phase and a DC supply. When current flows only at the peak, the only voltage drop experienced will be at the peak. This causes a dip in the voltage at the peak and in my mind is the source of the higher level harmonics.
 

K2500

Senior Member
Location
Texas
crossman said:
Now a key point is to recognize that the current in A phase cannot balance on B phas or C phase because those loads are not conducting at the time when A phase loads are conducting. All of the A phase current must return to the source via the neutral. This same thing holds true for B phase and C phase. The phase currents cannot balance.

So the switches are like solid state commutators, and since the peaks happen 120? out, the gate opening and closing does not coincide with any 2 phases simultaneously. So, to the nuetral it goes.

As far as transient over-voltages produced by the switching, I guess that the resitance jumps from infinite to some value, making a quick current draw, then jumps back. Causing a dip, then surge in the voltage?
 

jghrist

Senior Member
crossman said:
Here is my "electrishun's" take on the matter of harmonics.
...
3. The neutral current looks more like a square wave than a sine wave to me.

Now what about the higher order harmonics? Seems to me they originate with the voltage drop that will be experienced near the peak. Take A phase and a DC supply. When current flows only at the peak, the only voltage drop experienced will be at the peak. This causes a dip in the voltage at the peak and in my mind is the source of the higher level harmonics.
This is one of the best explanations I've seen of the third harmonic content of switched mode power supplies.

The higher order harmonics are the components that make the current look more like a square wave.
 

jghrist

Senior Member
K2500 said:
So the switches are like solid state commutators, and since the peaks happen 120? out, the gate opening and closing does not coincide with any 2 phases simultaneously. So, to the nuetral it goes.

As far as transient over-voltages produced by the switching, I guess that the resitance jumps from infinite to some value, making a quick current draw, then jumps back. Causing a dip, then surge in the voltage?
Commutation occurs in three-phase bridge rectifiers. This is when two phases conduct at the same time for a portion of the cycle, causing essentially a phase-to-phase short and a high current spike. The high current spike causes "notching" in the voltage from the voltage drop through the source impedance.
 

jghrist

Senior Member
dbuckley said:
I'd just note that those current graphs are nicely polite compared to those I mostly come across, which have far more resemblence to a spike...
If you look at just one of the phases and fill in lines going down to zero, then staying at zero until the next conducting portion, it will look more like a spike. It's the addition of all three phases in one waveform (the neutral current) that doesn't have the "spike" look.
 
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