What Produces Second Harmonic Current?

Are you using a variable speed drive or a Variable Frequency Drive?

Since you are trying to measure current, an open circuit would render that impossible.

Could you provide any more details? A sketch? A bone?

Hey Joe,

We know that it's not a drive misfiring and we know it isn't a transformer saturating. Some details about the configuration of the transformer and they type of equipment installed on the load side is as follows: The service entrance transformer is a Wye - Grounded Wye winding configuration. The load bus is commercial facility. One hint is that we only see elevated levels of the 2nd harmonic current on two of the three phases; with an average of 8% Distortion level as expressed as percentage of the fundamental, the fundamental current is measuring an average of 100 amps. This is measured at 480V at the clients main switchboard.

The measurements were taken over a two day period. A wonder is that it might be a recent client lighting retrofit with a high efficiency ballast, but this is just a guess. Whereas they do have some drives, these were not energized during the predominant period of the measurement; but when AC Drive is online and working, approximately 15 minutes out of two days profile, it provided a slightly higher 2nd order current as well. The drives were scoped and there were no problems with pulse firing sequences. That was the first place that was looked at given that 2nd order current has been historically seen to be produced by a misfiring SCR on a six pulse drive. But that is not the case here.

Thank you very much for your thoughts,

Ashley

Lighting is easy...turn it off...re-measure. You have a great suspect there with the lighting.

You can do this for the rest of the load, as well. Turn off the devices at their load distribution. You could also measure at each load's distribution. Half split your plant's utilities. You should be able to do this if the facility was designed for 24 hour operation.

Do you have a lot of 3 phase motors higher that 100 HP?

"...Nonlinear loads with asymmetrical i-v characteristics inject even harmonic currents that, when circulating through the power system, produce even harmonics in voltage waveforms. Half-wave rectifiers, half-controlled converters, arc furnaces or electrical discharge devices are examples of asymmetrical loads..." From this paper.

There's much more available by Googling on the title of your thread.

Non-linear loads that are being supplied by only 2 of the 3 phases. Prime example would be electronic lighting ballasts. You should have them relatively evenly split between all 3 phases so that they cancelled each other out, but most likely nobody paid attention.

something that is using only one diode

One of the things that can cause a 2nd harmonic in a system is a problem with the incoming line. I was asked to arbitrate a problem with a power system in a large office building owned by a major steel corporation that rented space out to others as well. This was in a large southern city in the summer. Carrier had new units in the building for air conditioning and were losing motors and drives for unexplained reasons and the building had frequent failure of air conditioning. The biggest complainers were the utility which rented space for a regional office for management in the building. I installed recorders within the building and soon found an intermittent 2nd harmonic that would come and go at odd times. It seemed to have nothing to do with loading of the building including the AC. Suspecting that the incoming power had a problem, we changed the recorders to time of day units and went to a number of other customers and other locations in the steel company's plant. This established that the problem was on the utility side. Presenting the evidence in a meeting, the management of the utility and it's holding company maintained that was impossible but the utility engineers kept really quiet and by the next morning they had located corroded crimp connections on aluminum ASCR cable at one of the main substations. The problem was solved. The crimps were on the other side of the second transformer(s) (3 phase bank of single phase transformers) in series away from the building in their system.

Bottom line, put on recorders and monitor the power, including time of day to see if it is continuous. The recorders I used were about the size of a pack of cigarettes and could store several days worth on information before being downloaded into my laptop. Once you establish whether it is continuous or changing then time of day will help track it down. If it is continuous your search will be different.

I was unclear on your system, is it Y-Y without a tertiary? Is the transformer a 3 legged or 5 legged core or a shell form? All these will affect the harmonics differently. Delta-Wye are better to isolate from utility problems but previous statements sounded like Wye-Wye.

Do you have any capacitors for power factor correction? A failure in those can sometimes cause a 2nd harmonic. I would go through an analysis of the first time the problem was noted and what was installed right before that. Is there any correlation with the lighting change as far as time?

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The question is unsettling in a sense. And it breaks down in at least two parts. And you will not get to the bottom of this by scoping, whatever that is.

1,. What is the measured harmonic current at the service entrance transformer's primary.? Is that within permissible limits? If it is, you have only a local problem. Otherwise you must mitigate.

2,. Plain old diode rectifiers and switching power supplies fed from a single phase are known to produce harmonics, starting with the 2.nd. You will see them, when you are looking for them.

3,. For your troubleshooting purpose, smart ballasts etc. are simply switching power supplies.

4,. It would be time now, for breaking out a current clamp. And use it with the scope, that you see the harmonic current. That will take you fairly straight to the offending loads. It is a pain to do, as you have to take covers off of panels to get to the cables, to measure them.

5,. Mitigation, if needed beyond load balancing, is a different story. But, why measure an individual shiny new fixture, before installing them wholesale?

Dear ashley,which industrial have you encountered with this program?