Harmonics case studies

So, one of the questions we get a lot is do you have any case studies or examples of harmonic situations and how did you solve the problem?

As far as case studies go, we put together a whole power quality case study video and what we're going to do is show you the two examples of harmonic case studies that we solved. We'll explain those by using the other video.

Case Study 1

Ryan, did I ever tell you the story about the bad noise and the evil noise? No? I think I would've remembered. It went something like this -  an offshore rig was having a problem and they called us in to analyze what was going on. I remember them saying, we'll pick you up in new Orleans and we'll drop you off on the platform. Then, we'll return with a helicopter when you fixed the problem. It was kind of an uncomfortable feeling, but we got there with our PQ tools, our instruments in hand, and we asked them what the problem was. I remember them saying, well, it's not really one problem.  We have two problems – a bad noise and an evil noise. I remember Martin who was standing next to me. He looked at them and said, I don't think you need a PQ guy. I think you need a priest.

So, what did you figure out? 

Well, after we settled in and performed some measurements, what we found was a harmonic resonance problem created by a poorly designed harmonic filter. Instead of reducing the harmonics, the filter actually amplified the harmonic currents and created a 5,000 amp pulse of current through a large inductor on the platform. The sound was a consistent thump, thump, thump, which resonated throughout the platform. That was the bad noise. 

Then, when the current got severely distorted and the voltage got distorted even more on the generator bus, the generator's regulators couldn't synchronize the operation of the generator with a four 80 volt bus. And, the generator actually slipped a pole and created the evil noise. It was like thump, thump, thump, bam. And, it was pretty scary. After all, electrical equipment really isn't supposed to make noise, especially not that kind of noise. When it was all said and done, we helped the oil company, redesigned the filter and resolved the problem. We captured some pretty great waveforms and learned a lot about harmonics, but most importantly, they came back to pick us up on the platform. 

End of Case Study 1

We've seen many harmonic issues, but the bottom line is this – harmonics are not a problem unless they are a problem. 

What does that mean? 

Because it's much easier today to measure harmonics and sea wave forms. People often jump to the conclusion that all harmonics are bad and all PQ problems are harmonic problems. However, harmonics on the power system are the new normal.

Harmonics can create two fundamental issues. Current distortion, which has the symptoms of equipment overheating, audible noise and/or nuisance tripping of protected devices. And, the second issue is voltage distortion, which typically causes miss operation and equipment. Either or both problems can cost you money with damage or miss operation to your equipment.

But, every system is different and careful analysis and consideration must be made. Or, you could significantly overspend on a perceived problem that may or may not exist. So, what causes what? In this case, harmonic currents cause harmonic, voltage distortion, the amount of voltage distortion depends on how much current distortion is produced or how many nonlinear loads like VFDs, computers or other electronics you have. And, the amount of impedance upstream of that current, usually the most significant impedance is in the nearest upstream transformer, but can be increased significantly when operating on a generator. 

Case Study 2

I remember a case where a financial company wanted to save some money. So, they put a bunch of small single phase standby UPSs under every desk in an office building and purchased a 40 kilowatt generator to back up the UPS's. The incoming service transformer was much larger than the generator, so the effect of impedance was much lower on the utility than running on the generator.

So, why was that a problem?

When the power went out and the generator came on, the UPSs went back to normal, but they began to recharge the batteries and created a lot of harmonics. Current distortion, the voltage distortion on the 120 volt UPSs went from 3% distortion on the utility source to nearly 22% distortion on the generator.

Wow, isn't that high for voltage distortion?

Yes. Extremely high. The UPS had determined that it wasn't a good or acceptable voltage and transferred back to battery. Once the UPS went to battery, the voltage cleaned up again, and the UPS decided the voltage was okay. So, we went back to the generator source again, when enough UPSs came on and began charging batteries, again, the cycle repeated until the UPS batteries died and the load was dropped.

So, what did you figure out in that case?

Well, the symptom of the problem in this case was the MIS operation of the UPSs. And ,in fact, they were also the source of the harmonics interacting with the high impedance of the generator. The solution was to design and build two harmonic filters, and third and fifth harmonic filters. The voltage distortion was limited to a similar level, no matter which source the system was running.

End of Case Study 2

Hopefully those examples of harmonic situations and the resolution will give you an indication of things to look for and how to solve your own case studies for harmonic situations.