harmonics

[jcole]

Good day Guys and Gals.

My understanding of harmonics is that they over heat the power source (transformer) and the neutral (grounded) conductor. I work at a wastewater treatment plant that has numerous VFD that is supplied by 3 phase 480 volt systems. None of these systems have the neutral conductor connected to any of the loads because the neutral conductor is not needed for proper operation. I am assuming that the harmonic (triplen) currents are seen on the phase (ungrounded) conductors if the neutral is not connected. Is this a safe assumption?

I am asking because we have an ATS that one set of the contacts is burning up and causing a single phase condition every 2-3 years. We hardly ever run at full load (usually 30-50% percent of capacity) of the equipment. Could harmonics be a source of heat on the phase that is burning up and failing in the ATS? Just trying to figure out why we cannot get the life out of these ATS that we should be getting. Sound more like a connection issue but connections are torqued to manufactrure specs and monitored with thermal camera every 90 days. Over time one of the phases start to show signs of overheating on the images and soon fails. The failure does not occur at the termination point of the feeder conductor but in the ATS contact that open and closes.

Any ideas would be appreciated.

 

[masterinbama]

Are you replacing just the contacts? It might the springs that hold the contacts together that are failing

 

[ron]

Triplen (3rd order) harmonics require a neutral to collect the peaks that occur at the same time (additive).

Since the VFDs don't use a neutral, you will not have triplen harmonics.

You may however have other harmonics (just not triplen) that don't add, but may result in a net higher increase in net current than full load.

 

[Jraef]

Neutral current due to harmonics has to do with single phase loads that are non-linear, such as power supplies, HID lighting, and now LED lamps, because each one has a power supply (albeit small).

But all power electronics produce harmonics, some more than others, and harmonic current has real effects on things like contacts, especially in that the “load” that you are observing may not show the harmonic current. The VFDs would be producing odd-order non-triplen harmonics, which would be generally equal between phases. If your facility has a preponderance of single phase non-linear loads that are not balanced between the 3 phases, that could cause one phase to be carrying more harmonic current than the other two. So the first thing I would look at would be your load balancing.

Then also, do a harmonic current study. If you have more than 10% THD-I, you should be looking into some form of mitigation. If the VFDs are all mitigated already, then you might want to look into an Active Harmonic Filter installed at the PCC (Point of Common Coupling). An AHF does not “care” where the harmonics come from, it measures it and reacts to correct it, usually on each phase individually.

 

[synchro]

As mentioned above, the loading on the different phases should be balanced as well as possible.
Is the set of contacts that's burning up on the same phase each time that it has happened? If so, it might be useful to rotate the phases of the load on the ATS and document what was done. Then if the problem occurs again you can see if the failure moves with the same phase or if it stays at the same physical switch contact.

In the meantime, you might check the voltage drop across the switch contacts and the associated currents on a regular basis. This could provide an earlier warning than checking the temperature would of any degradation of the switch contacts.
If it's not already being done, perhaps some periodic use of contact cleaner and application of appropriate lubricant such as at the link below could be done. There are likely others on this forum that have specific experience in what would be the best monitoring and maintenance procedures.
Since you mentioned that it's occurring in a wastewater treatment plant, environmental factors including humiidity, etc. may be involved in degrading the contacts. And so more frequent preventative maintenance as mentioned above could be helpful.

https://www.gordonelectricsupply.com/p/Square-D-Swlub-Lubricant-Tube/5718255

 

[jcole]

Neutral current due to harmonics has to do with single phase loads that are non-linear, such as power supplies, HID lighting, and now LED lamps, because each one has a power supply (albeit small).

But all power electronics produce harmonics, some more than others, and harmonic current has real effects on things like contacts, especially in that the “load” that you are observing may not show the harmonic current. The VFDs would be producing odd-order non-triplen harmonics, which would be generally equal between phases. If your facility has a preponderance of single phase non-linear loads that are not balanced between the 3 phases, that could cause one phase to be carrying more harmonic current than the other two. So the first thing I would look at would be your load balancing.

Then also, do a harmonic current study. If you have more than 10% THD-I, you should be looking into some form of mitigation. If the VFDs are all mitigated already, then you might want to look into an Active Harmonic Filter installed at the PCC (Point of Common Coupling). An AHF does not “care” where the harmonics come from, it measures it and reacts to correct it, usually on each phase individually.

Thank for the reply Jraef.

I took these pictures of the display on the ATS. Current THD% is definitely higher than 10%. That is with just 2 pumps running. On a rainy day we could have 4 or more pumps running. What do you think about these readings?

 

[junkhound]

At 60 Hz, the arc on opening usually extinguishes within a cycle or 1-1/2 cycle.
Higher frequencies take longer (e.g 400 Hz 20 or so cycles) and 400 Hz contactors (aircraft) have much stronger springs or forced opening mechanisms.
The arc upon opening is a major factor in contact life. Thus, yes,, harmonics do impact contactor life.

as previously mentioned, if a spring gets overheated and loses temper, even 60 Hz contacts may arc for many cycles with associated contact damage.

In yur readouts, Those big 5th and 7th harmonics indicate there is a high power 6 pole rectifier on line, likely VFD inputs.

 

[ron]

Thank for the reply Jraef.

I took these pictures of the display on the ATS. Current THD% is definitely higher than 10%. That is with just 2 pumps running. On a rainy day we could have 4 or more pumps running. What do you think about these readings?

What is the ampere loading on the ATS relative to its nameplate? If the load is only a small % of the nameplate, then the total demand distortion is more important. If the load is very small, the acceptable THD can be higher.

Total demand distortion (TDD) is the per-phase harmonic current distortion against the full load nameplate of the electrical system. TDD indicates the impact of harmonic distortion in the system.

 

[jcole]

What is the ampere loading on the ATS relative to its nameplate? If the load is only a small % of the nameplate, then the total demand distortion is more important. If the load is very small, the acceptable THD can be higher.

Total demand distortion (TDD) is the per-phase harmonic current distortion against the full load nameplate of the electrical system. TDD indicates the impact of harmonic distortion in the system.

Thanks for the reply Ron.

The current rating is 800A. Thats the feeder breaker size. The ATS is rated for 800A as well. Transformer is rated a lot higher because it feeds.. several other builidngs as well. The transformer is located in a substation owned by us.

Each pump draws on average about 110 to 140 amps according to what speed it is running. We hardly ever run more than 2-3 pumps at a time.

 

[synchro]

Apparently the distortion in line current that you're measuring is typical for a 6-pulse rectifier with line reactor according to pg.5 of the document at the following link:

https://acim.nidec.com/drives/contr...s/documents/en-us/whp_harmonicmitigation.ashx

 

[ron]

Thanks for the reply Ron.

The current rating is 800A. Thats the feeder breaker size. The ATS is rated for 800A as well. Transformer is rated a lot higher because it feeds.. several other builidngs as well. The transformer is located in a substation owned by us.

Each pump draws on average about 110 to 140 amps according to what speed it is running. We hardly ever run more than 2-3 pumps at a time.

The load is small, so likely not a big deal.
This is a good video lesson and they discuss TDD around 10 minute mark

 

[jcole]

The load is small, so likely not a big deal.
This is a good video lesson and they discuss TDD around 10 minute mark

The load is small, so likely not a big deal.
This is a good video lesson and they discuss TDD around 10 minute mark

Thanks for the reply Ron.

My understanding from the video and your response, correct me if I am wrong, is that the combination of the normal load and harmonic load currents should not be an issue with overheating unless the combination of those two load currents exceeds the rated current of the equipment. Also, if that was the case, our breakers protecting the feeder would be tripping as well. Is my understanding correct? How much harmonic current would I expect to see if the normal load is around 300A total from two six pulse drives? It is obviously less that 800A or my breakers would be tripping. Correct?

 

[Strathead]

When I think of ATS's I think of emergency vs. normal power. I can't imagine an ATS cycling very often. What is the function of the ATS's? Also, I don't think I saw it mentioned here, but are the load side conductors of the VFD's sharing a conduit? That is a no-no per most manufacturer's instructions.

 

[ron]

Thanks for the reply Ron.

My understanding from the video and your response, correct me if I am wrong, is that the combination of the normal load and harmonic load currents should not be an issue with overheating unless the combination of those two load currents exceeds the rated current of the equipment. Also, if that was the case, our breakers protecting the feeder would be tripping as well. Is my understanding correct? How much harmonic current would I expect to see if the normal load is around 300A total from two six pulse drives? It is obviously less that 800A or my breakers would be tripping. Correct?

Depending on the type of circuit breaker you have, it is going to see "true RMS" for solid state or just analog heating effects.

TDD essentially indicates that if you have a lot of harmonics, but the overall load is small compared to the equipment rating, don't worry about it. If the net current (fundamental and harmonic) is high relative to the equipment rating, it may result in tripping or harmful results to the equipment contacts

 

[Joethemechanic]

n yur readouts, Those big 5th and 7th harmonics indicate there is a high power 6 pole rectifier on line, likely VFD inputs.

Yeah, six pulse VFDs

How large of a transfer switch? How many 6 pulse VFDs and what kind of motor HP?

 

[drcampbell]

... application of appropriate lubricant ...
https://www.gordonelectricsupply.com/p/Square-D-Swlub-Lubricant-Tube/5718255

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[Joethemechanic]

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[synchro]

Do you know what special attribute of Square D's Field Maintenance Lubricant justifies its $350/kilogram pricetag?

Not really. But here's info from the manufacturer, Dupont:

https://www.dupont.com/products/molykote-bg-20-synthetic-bearing-grease.html

And an FAQ from Square D which says their lubricant is Dupont BG-20:
https://www.se.com/us/en/faqs/FA124976/

 

[Joethemechanic]

Not really. But here's info from the manufacturer, Dupont:

https://www.dupont.com/products/molykote-bg-20-synthetic-bearing-grease.html

And an FAQ from Square D which says their lubricant is Dupont BG-20:
https://www.se.com/us/en/faqs/FA124976/

That''s a bit cheaper. About a hundred bucks for a 400 gram cartridge

https://www.motion.com/products/sku/00788575?utm_source=google&utm_medium=cpc&utm_campaign=performance_max&utm_content=&gclid=Cj0KCQjwoeemBhCfARIsADR2QCs06Z6uliS0YeHzRmu0OalBQeVChSgjbSfba0_ZZbCfhimFHZ00d5UaAoiOEALw_wcB