Harmonics in polyphase power system

[mnada]

"Harmonics in polyphase power system"

Dear All,
I have a problem in understanding how the harmonic currents exist in a load with no corresponding harmonic voltages?
The following figures with questions under explain what I mean

This understood figure no issues about.

In above figure, 2 question arise:

How the line currents are missing the triplen harmonics
How the phase voltage is missing the other odd harmonics except the triplen

In the above figure:

How is the phase voltage missing all the harmonics while it's present in current.
Correct me if I am wrong: we can say that because triplen harmonics are in phase they cancel each other in the line cuerrent

Thank you

 

[mnada]

Any suggestion?

 

[jim hardy]

Re fig2,

How the line currents are missing the triplen harmonics

3rd harmonics are in phase as you noted.

Therefore third harmonic voltage cannot be produced between phases of a wye machine.
In your fig2 write KVL between any two phases and observe 3rd harmonic in one phase cancels the 3rd harmonic from other phase.

Since there's no 3rd harmonic phase to phase voltage and your load is connected phase to phase, that harmonic has no voltage to drive it. So with nothing to push it, that current cannot flow. (assuming linear load).
Another way to think of it is - since third harmonic current is in phase, it'd be coming out of all 3 phases in synch. How would it get back to generator to satisfy KCL ? Kirchoff's Current Law shows it could only return through neutral which has a high impedance volteter in it, so no current flows there. It is blocked by absence of neutral connection.

But if you replace that voltmeter with an ammeter it will indicate substantial 3rd harmonic current because each phase of load is in parallel with a phase of generator and 3rd harmonic voltage DOES exist phase to neutral. It can return via a neutral connection if one exists, like it would with an ammeter there.
That's why you have to be careful connecting a small 3 phase transformer to a large three phase source - what's insignificant harmonic content to the source might be real significant to the poor little transformer.

How the phase voltage is missing the other odd harmonics except the triplen

Sorry - i don't see that indicated on your fig2.

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Re last figure, which is unnumbered:

How is the phase voltage missing all the harmonics while it's present in current.

That one's kinda sneaky.
Look at that delta source . The three phases comprise a series connected loop. Since 3rd harmonic voltages are all in phase, they do not add to zero as does the fundamental.
So what you have is a short circuited supply for 3rd harmonic. 3rd harmonic current will flow around that delta winding in amount E_3rd/Z_machine and the vast majority of the 3rd harmonic voltage is dropped across the machine's internal Z.. So your 3rd harmonic basically all stays in the machine.
But i believe the 5th and 7th would appear in phase voltage.

Correct me if I am wrong: we can say that because triplen harmonics are in phase they cancel each other in the line current

Again - without a return path they can't come out all 3 wires at once so they're blocked. Only place they can flow is around a delta winding.
I think the point of the exercise is probably to demonstrate that a delta winding shorts out third harmonic voltage effectively removing it. It'll clean up a sinewave for you.

hope this response triggers a bettre one from a genuine power engineer. I'm a lowly instrument guy.

 

[mnada]

Thank you. That really helped me to clear some points

 

[alphysicist]

for your question. Harmonics in polyphase power systems can be a complex topic to understand, but I will do my best to provide a clear explanation.

First, let's define what harmonics are in this context. Harmonics are frequencies that are multiples of the fundamental frequency in a power system. In a polyphase power system, there are multiple phases of power, typically three, that are out of phase with each other. This creates a balanced system where the sum of all the currents and voltages is zero.

Now, to address your questions:

1. How the line currents are missing the triplen harmonics?
The line currents are missing triplen harmonics because they are canceled out by the other two phases. In a balanced system, the sum of all the currents is zero, so any triplen harmonics present in one phase will be canceled out by the other two phases.

2. How the phase voltage is missing the other odd harmonics except the triplen?
Similarly, the phase voltage is missing the other odd harmonics because they are also canceled out by the other two phases. This is due to the balanced nature of the system.

3. How is the phase voltage missing all the harmonics while it's present in current?
The phase voltage is not completely missing all harmonics, it is just missing certain harmonics. This is due to the fact that the harmonics present in the current are canceled out by the other phases, while the fundamental frequency remains unchanged.

4. We can say that because triplen harmonics are in phase they cancel each other in the line current.
Yes, that is correct. Triplen harmonics are in phase with each other and therefore, when they are summed with the currents from the other two phases, they cancel out.

I hope this helps to clarify the concept of harmonics in polyphase power systems. It is important to note that harmonics can still cause issues in power systems, even if they are canceled out in the line currents. This is because they can still affect the voltage levels and cause problems with equipment. Therefore, it is important to properly design and maintain power systems to minimize the impact of harmonics.