How to sum or calculate voltage or current harmonics component that are in parallel ? 1

[ELCdude]

Recently I measured voltage and current harmonics in coal mining industry. But they only have Power Quality Meter with limited current clamps (not big enough for the PCC(point of common coupling)). So I decided to calculate it load by load. Is it okay?
If it is, then How to sum or calculate voltage or current harmonics component that are in parallel ?

I want to sum some lines (every lines contains different voltage and current harmonic components).
Is it just sum every n-th component by the same component ? Or how?

Let me give a case:
I have three lines: line a and line b, and line c
in line a-> I have inverter, which has biggest distortion at 5th harmonic and also another harmonic components
in line b-> I have a device that has biggest harmonic at the 3rd
in line c-> I have linear load, so there is no Voltage or Current harmonics

So how can I calculate the sum of these three lines,(how to sum both current and voltage harmonic)

And another question if possible. Can we convert three different single-phase voltage components into three-phase voltage components?

Sorry if it is too wordy. Thanks

 

[jghrist]

You can only add the parallel harmonic currents if your measurement includes the current phase angle. They have to be added as complex values. Even then, you have to ensure that the phase angle is related to the same zero-angle. If the phase angle is related to the fundamental frequency zero-angle, and the two currents have different fundamental phase angles, then an adjustment would be needed. You might have to adjust based on the fundamental current angle with respect to the voltage.

The voltage harmonic depends on the source impedance. If you are measuring each line harmonic voltage at the same point, then the harmonic voltages will be equal.

 

[ELCdude]

thanks jghrist, I see it now
but, I have one more problems
what will happen to current harmonics if it across the impedance above it , or wye-delta transformer above it,what will happen to the summed current harmonics.
will it decreasing? how can we calculate it?

 

[jghrist]

Most non-linear loads act like current sources, so the current does not decrease when going through an impedance. Going through a transformer with a phase shift, like a delta-wye, will affect the phase angle of the current which affects the addition of currents from different feeders.

Phase shifting one-half of the load through a delta-wye transformer can greatly reduce the 5th and 7th harmonic currents in the sum of non-shifted and shifted currents. This is if the non-linear loads on both shifted and non-shifted are the same. This is the basis of Harmonic Mitigating Transformers (HMT).

You can calculate it by adding the phase shift to the sinusoidal functions. Keep in mind that the phase shift of the nth harmonic is n times the fundamental phase shift. This is why balanced 3rd harmonics are zero-sequence. Phase B is shifted 3 x 120° = 360° from Phase A. A 360° phase shift puts Phase A in phase with Phase B.

 

[ELCdude]

thanks jghrist, your answers are very helpful.
So when it's 7th and delta-wye, it need to be shifted 7x120
when it's 7th and wye-delta (harmonic current move from wye to delta) it need to be shifted -7x120 isn't it?

 

[jghrist]

With a delta-wye, there is a 30° fundamental phase shift. The phase shift in 6th harmonic current is 6 x 30° = 180°. A 180° phase shifted 6th harmonic current will completely offset the unshifted current. Balanced 6th harmonic currents are triplens and won't flow through the delta-wye transformer, so this doesn't work for 6th harmonics which are usually very small anyway. 5th and 7th harmonics don't completely offset, but are greatly reduced.

 

[ScottyUK]

Even-order harmonics are usually a result of a problem - asymmetry in the firing delay angle being a common cause. Few loads generate significant even-order harmonics in normal operation so, although what jghrist says is true, there shouldn't be much 6th harmonic to begin with.

 

[waross]

Even-order harmonics are usually a result of a problem

This gives rise to a side discussion at:
thread238-341550

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Coming in via the back door (Bill's link), so to speak.

Some equipment indicate even harmonics also when there are none. I had a problem understanding how there could be a rather high even harmonics contents where there couldn't be any. It turned out that the Dranetz 4300 I was using, and still use now and then, doesn't have an anti alias filter that can handle components at around 8 kHz. The low sampling frequency made these components appear as odd AND even harmonics. I have heard that the problem exists also in later versions of Dranetz-BMI equipment.

It is quite easy to test if your power quality analyzer has that problem. Just connect a function generator and test with a few frequencies above the analyzer's specified frequency range. If it shows even harmonics, then you have that problem.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Can you access the secondary wiring of a protection CT at the PCC?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ELCdude]

Yup, it's 30 degrees phase shift as I expected.
Thanks a lot for all your response jghrist.
Really, you are a big help