harmonics

[Ameen87]

Hi All,

I have carried out a harmonics test on a site the system is with the generator and the distribution loads the distribution load is having some communication systems.

i got values ranging between 4% to 20% current harmonics and also my neutral to ground phase harmonics measurements are all above 50% . My voltage harmonics are less than 2%.

Clearly am not having an idea of harmonics measurements can any one suggest the values are good as per standards.what are the standards for voltage harmonics and current harmonics.

 

[Skogsgurra]

Both THDi and THDu are OK in your plant. There are different standards when it comes to harmonics. For practical purposes, you can live with 8-10 % THDu and dont need to be too worried about the current harmonics.

Be sure to measure line-to-line and at the PCC. That is what most (all, I think) standards say.

Gunnar Englund

http://www.gke.org

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

 

[HamburgerHelper]

IEEE 512 gives recommendations on harmonic levels. I believe the recommendations change depending on loading and voltage level.

Edit: I meant 519.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[magoo2]

I think HH meant IEEE 519.

 

[rcw retired EE]

A good explanation of current versus voltage harmonics is the hot shower / toilet flushing analogy. Ever been in the shower when a toilet is flushed and the resulting pressure drop changes the hot/cold mixture? You get either scalded or frozen by the change in water temperature. The pressure drop (voltage drop) is the issue caused by the flow (current). IEEE 519 basically says your power system has to be stiff enough to hold the pressure at the Point of Common Connection (PCC) that your neighbor doesn't get scalded when you flush and vice versa.

Other loads on the bus don't see the chopped current harmonics. They only see the voltage distortion caused by those currents flowing in the impedance of the upstream equipment. If the voltage distortion is within limits, the other equipment should be OK. (Neglecting possible harmonic resonances if the other equipment includes capacitors. This plumbing analogy fails at that).

Harmonic currents will cause additional heating in transformers and cables. That is probably not an issue with this equipment, unless those neutral harmonics create unusual heating. Something to check.

IEEE 519 and a new Standard IEEE 3002.8-1980, "IEEE Recommended Practice for Conducting Harmonic Studies and Analysis of Industrial and Commercial Power Systems" are good guides.

Table 4 in 3002.8 lists allowable Total Design Distortion (TDD) for harmonic current as 5%-20% depending on the ratio, Isc/Il = (I short circuit at PCC)/ Load current (I) at fundamental frequency. The ratio ranges from "<20, 20<50,...>1000). Table 5 for Voltage Total Harmonic Distortion (VTHD) lists 8% as the limit for systems < 1000V with no individual harmonic above 5%. From 1 kV to 69 kV the limits drop to 5% and 3%.

 

[jraef]

Condensed version of a complex subject, as I responded in the other forum where you posted this question:

Current harmonics are what cause voltage harmonics. So the current harmonics in your facility are the source of voltage harmonics, along with the stiffness of the system as measured by the ratio of Available Short Circuit current vs actual Load current. Current harmonics are also affected by EXISTING voltage harmonics in a system in that if there is high V-THD coming in FROM the utility connection, it will make your I-THD worse and the situation can spiral out of control, causing transformers to melt down etc. So V-THD is of concern when you are connected to a utility supply because voltage harmonics travel from you to your neighbors on that common connection. IEEE 519 specifications call out the limitations for the end users AND the utilities with regard to how they affect each other in order to prevent this down hill spiral.

If you are on your own generator, the IEEE 519 rules are irrelevant. They can still be used as a guideline however, because what the I-THD will affect is the capacity or fuel consumption of your generator. The generator has to supply ALL of the power going to the equipment, whether at the fundamental frequency or at the higher harmonic frequencies. If you follow the IEEE-519 guidelines, you can mitigate the impact that harmonics will have on your generator capacity and/or fuel consumption. Or you can chose to ignore it, because it will not affect anyone but you.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Ameen87]

Thanks for the replies,
want to add one more querry:
If i am using a generator supply for the system and my load is sensitive communication system, all i understand is some current harmonics will be there for sure may be higher or lower.
My sensitive communication system is fed by the generator and its having some issues during operation and is it be the only possibility is to provide a UPS supply before the communication system for rectification?.
I also have some utility load in the same bus which is also fed from my generator.

 

[Skogsgurra]

I think that "the sensitive communication system" has other problems than the harmonics. If there are any problems with it at all.

Suppliers of different systems quite often blame their problems on the mains voltage and it is not uncommon that the user tries to make mains voltage "better".

As I said before; your harmonics are well below any limits that exist. It would be interesting to know why you did this harmonics survey. Did the supplier of the "sensitive communication system" ask for it? Or is it something you "just did"?

Gunnar Englund

http://www.gke.org

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

 

[Ameen87]

The output of my test is shown below,
Voltage
THD voltage harmonic values for all the three phases (A, B, C) are found normal, relatively below at 2.5% (average value).
 Neutral to Ground Voltage THD harmonic values are ranging between 35% and 350%.
 The even harmonic levels for all the three phases are relatively below 1%.
 The even harmonic values of Neutral to ground are ranging in between 0.8% to 20%.
 The Odd harmonic levels for all the three phases are relatively below 1%.
 The Odd harmonic values of neutral to ground are ranging in between 1% and 350%.
Current
THD Current harmonic values for all the three phases (A, B, C) are ranging in between 2% and 12%.
 Neutral to Ground Current THD harmonic values are ranging between 18% and 300%.
 The even harmonic levels for all the three phases are relatively below 1%.
 The even harmonic values of Neutral to ground are ranging in between 0.8% to 6%.
 The Odd harmonic levels for all the three phases are ranging in between 0.8% to 7%.
 The Odd harmonic values of neutral to ground are ranging in between 0.8% and 280%

 

[Skogsgurra]

Is this a system where N is grounded? Or isolated?

What voltage levels do you measure between N and PE?

Again, why all talk about the "sensitive communication equipment"? Is there a problem with it?

Gunnar Englund

http://www.gke.org

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

 

[Ameen87]

I cannot disclose the names of the equipment because its a military base, the generator neutral point is grounded inside the generator. I measured all the readings from the main breaker terminals of the generator and its only less than 10 metres cables between the generator and the main breaker.
NG voltage is less than 1 volts.

 

[Skogsgurra]

If Neutral-Ground is less than 1 V, there is no need to do harmonic analysis on that voltage.

I actually had a similar case in France. Military installation with communication problem in an "unusual" frequency band. It turned out to have nothing with mains to do. The solution was surprising. Really surprising. PM me for details.

BTW, mains harmonics don't usually go much higher than a few kHz. And definitely not into RF bands. Not even LW.

Gunnar Englund

http://www.gke.org

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

 

[Guilhermebr]

The ieee 519 will not solve your problems with harmonics.

GS

 

[ScottyUK]

Hi jraef,

You can only get active power transfer if voltage and current harmonics of the same order are present - harmonic currents on a 'clean' voltage source don't result in active power flow, so they don't increase fuel burn for the generator. Harmonics will certainly eat up alternator capacity though.

Ameen87 - you could consider an active harmonic filter rather than a UPS, but ensure that the vendor certifies it for use on a relatively weak generator source as well as a utility connected supply. Generators are generally more susceptible to voltage and frequency excursions than the utility, and some of the cheaper units don't like this. I do agree with skogsgurra that the harmonic levels present on your system should not be a concern to properly designed equipment.