Harmonics Testing

Status
Not open for further replies.
aelectricalman

aelectricalman

Senior Member
Location
KY
I have a customer which we found had heavy harmonics (on the utility side of the XFMR). We started testing to find the harmonics source by placing PQ equipment on the primary side of the customers xfmr and by placing a second PQ analyzer on the load side of the customers drive.

In hind sight, I'm wondering if it would have been better to place the second PQA on the line side of the customers 24 pulse drive. I am looking for some opinions on this matter. I understand I am currently behind the drive looking at the motor contribution ( which should be none). Just looking for a some other opinions.
 
aelectricalman said:
I have a customer which we found had heavy harmonics (on the utility side of the XFMR). We started testing to find the harmonics source by placing PQ equipment on the primary side of the customers xfmr and by placing a second PQ analyzer on the load side of the customers drive.

In hind sight, I'm wondering if it would have been better to place the second PQA on the line side of the customers 24 pulse drive. I am looking for some opinions on this matter. I understand I am currently behind the drive looking at the motor contribution ( which should be none). Just looking for a some other opinions.
Click to expand...
A 24-pulse drive should not produce appreciable harmonics. Lowest order is 23rd and just a few percent at that.
Do you have actual harmonic data that you can cite?
Have you checked the supply side without the 24-p drive running?

I have come across situations where the distortion on the utility side of a site transformer feeding VSDs was actually indistinguishable with or without the drives running.
The increasing proliferation of non-linear loads, not just industrially but in commercial and residential areas has a knock-on effect on the utility. It isn't that any one load is huge, just that there are huge numbers of them.
 
aelectricalman said:
testing to find the harmonics source .......24 pulse drive..
Click to expand...

Just mentioning the 24 pulse drive says you have found the harmonic source.
PQA at input to the TR will show the harmonics generated.

My guess is the 17th or 19th will be the biggest.
 
GoldDigger said:
1. Service is three phase?
2. What are xformer primary and secondary winding types?
3. What are the dominant harmonic numbers?
Click to expand...

1. 3 phase
2. 13.8 kv/7900V :4160D
3. That's what we are testing for. The Vthd was 8.5%

My question primarily pertains to proper position of equipment more so than the wiring types.
 
Besoeker said:
A 24-pulse drive should not produce appreciable harmonics. Lowest order is 23rd and just a few percent at that.
Do you have actual harmonic data that you can cite?
Have you checked the supply side without the 24-p drive running?
I have come across situations where the distortion on the utility side of a site transformer feeding VSDs was actually indistinguishable with or without the drives running.
The increasing proliferation of non-linear loads, not just industrially but in commercial and residential areas has a knock-on effect on the utility. It isn't that any one load is huge, just that there are huge numbers of them.
Click to expand...


I agree it should only have a small order of 23rd and 25th order harmonics, along with multiples of these orders. My concern is strictly equipment placement.

I didn't think about checking supply voltage with or without. That seems to be a good way to see what the drive contribution is.

Can you explain this -> "The increasing proliferation of non-linear loads, not just industrially but in commercial and residential areas has a knock-on effect on the utility. It isn't that any one load is huge, just that there are huge numbers of them."

thanks.
 
These days, the source of voltage harmonics is just as likely to be the POCO grid, caused by the large number of high harmonic current loads as it is any harmonic load your customer is imposing on POCO.
 
aelectricalman said:
I would think the 17th and 19th would be more associated with an 18 pulse? Otherwise they should cancel out with a 24 pulse. Can you explain?
Click to expand...
The harmonics associated with a multi-pulse system follow a rule.
For example, with 6-pulse you get 6n?1 where n is the integer series 1, 2, 3, etc...
So you get 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th..........you see the pattern I'm sure..........
For a 24-p system you thus get 23rd, 25th, 47th, 49th......

Then the harmonic amplitude is about proportional to the reciprocal of the harmonic number. For 5th it would be 20%, 25th would be 4% and so on.
Of course other factors affect this but it is a rough guide and explains why multi-pulse systems are used for larger drives.
 
aelectricalman said:
Can you explain this -> "The increasing proliferation of non-linear loads, not just industrially but in commercial and residential areas has a knock-on effect on the utility. It isn't that any one load is huge, just that there are huge numbers of them."

thanks.
Click to expand...
I can try.
Most of the electronic gizmos you have in your house will have an electronic power supply. Computers, televisions, chargers, printers, LED and CFL lights.......and the whole jing bang.
These draw non-linear currents and that distorts the supply. Not much individually but in their thousands or millions...........

I looked at the current being drawn by the supply in my house.

Correctedcurentwithmagicaldevice_zps35340807.jpg

Voltage:

Dirtysinewave_zps64d5c8e4.jpg

Not nice but not unusual.
 
Besoeker said:
The harmonics associated with a multi-pulse system follow a rule.
For example, with 6-pulse you get 6n?1 where n is the integer series 1, 2, 3, etc...
So you get 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th..........you see the pattern I'm sure..........
For a 24-p system you thus get 23rd, 25th, 47th, 49th......

Then the harmonic amplitude is about proportional to the reciprocal of the harmonic number. For 5th it would be 20%, 25th would be 4% and so on.
Of course other factors affect this but it is a rough guide and explains why multi-pulse systems are used for larger drives.
Click to expand...

I understand what you are saying and I understand the formulas for determining harmonic orders in a drive system... What I don't understand is why you said that 17th and 19th order harmonics would be predominate in a 24 pulse drive system. Shouldn't we be worried about only the 23rd and 25th as a possible source of significant harmonics ( also multiples of the 23rd and 25th)?
 
Last edited:
aelectricalman said:
I understand what you are saying and I understand the formulas for determining harmonic orders in a drive system... What I don't understand is why you said that 17th and 19th order harmonics would be predominate in a 24 pulse drive system.
Click to expand...
I don't think I did.
 
Besoeker said:
I can try.
Most of the electronic gizmos you have in your house will have an electronic power supply. Computers, televisions, chargers, printers, LED and CFL lights.......and the whole jing bang.
These draw non-linear currents and that distorts the supply. Not much individually but in their thousands or millions...........

I looked at the current being drawn by the supply in my house.

View attachment 10784

Voltage:

View attachment 10785

Not nice but not unusual.
Click to expand...
To expand on this concept a little more (if I may)...

Non-linear loads are mostly anything with a Switched Mode Power Supply (SMPS), especially if you accept that the converter of a VFD is essentially such. VFDs, being the "big dogs" in this realm tend to get a lot of attention and 25 years ago when very few other devices had SMPS on them, that was the most efficient use of one's time in investigating a source. But that long ago ceased to be the case. Most lighting ballasts are now SMPS instead of magnetic (linear) devices, so that includes not only HID lighting, but also "energy efficient" fluorescent lighting as well, even down to the myriad forms of CFL and LED bulbs! Add to that the removal of incandescent lighting as a portion of your non-linear loading and the effect is even more dramatic. Then to that add the proliferation of PCs and pseudo PCs such as "smart" appliances, tablets, TVs, music systems and the like, anything that plugs into a "wall wart" or charges from one, and the total percentage of the power we use is actually now shifted to being mostly non-linear.

Harmonics, like lighting, is cumulative, like filling a bucket. You can fill a room with light by having one huge lamp, or you can fill it with 1000s of little Christmas lights in strings, the total lumens can be exactly the same in the long run. So too with harmonics. You can have distortion from one large 1000kW VFD, or a cumulative effect of 10,000 SMPS units, each drawing only 100W each. In fact it might even be WORSE with the 10,000 little SMPS units because they are likely single phase and not balanced, so much of the phase shift offsets are not there either compared to a 3 phase VFD. Your customer may not have all 10,000 of the SMPS inside of his facility, they might be with his neighbors, but the effect will show on the utility side.

By the way,
... We started testing to find the harmonics source by placing PQ equipment on the primary side of the customers xfmr and by placing a second PQ analyzer on the load side of the customers drive.
Click to expand...
Measuring harmonics on the LOAD side of the VFD was pointless, that is ALWAYS going to be nasty. You want to measure on the LINE side of the VFD, the CONTRIBUTION of the rectifier to the PCC. What happens on the load side is only relevant to the motor itself.
 
Last edited:
Thanks for the detailed and well organized offering on the subject, Jraef.
I hesitate (but not very long) to add two quibbles:
1. Since the discharge tube itself is a non-linear element, even fluorescents with magnetic ballasts that tried to smooth the current were somewhat nonlinear.
2. I think you meant to refer to removing incandescents as reducing the linear part of the load. Filaments are non ohmic with their high temperature coefficient of resistance, but close to linear on the cycle time scale.
 
Jraef said:
By the way,
Measuring harmonics on the LOAD side of the VFD was pointless, that is ALWAYS going to be nasty.
Click to expand...
Or perhaps not.
Depends on what you are measuring. With good PWM modulation and a reasonably high switching frequency the current will be a pretty good sine wave.

Voltage?
Well, we currently have a project where output sine wave filters are in the contract.
I had to make the technical case, the right one in my opinion, for doing so for this particular application and I was aware that competitors would, could, and did, offer a commercially more attractive proposition.
Risk/reward perhaps. But we were awarded the contract.
 
Jraef said:
To expand on this concept a little more (if I may)...

Non-linear loads are mostly anything with a Switched Mode Power Supply (SMPS), especially if you accept that the converter of a VFD is essentially such. VFDs, being the "big dogs" in this realm tend to get a lot of attention and 25 years ago when very few other devices had SMPS on them, that was the most efficient use of one's time in investigating a source. But that long ago ceased to be the case. Most lighting ballasts are now SMPS instead of magnetic (linear) devices, so that includes not only HID lighting, but also "energy efficient" fluorescent lighting as well, even down to the myriad forms of CFL and LED bulbs! Add to that the removal of incandescent lighting as a portion of your non-linear loading and the effect is even more dramatic. Then to that add the proliferation of PCs and pseudo PCs such as "smart" appliances, tablets, TVs, music systems and the like, anything that plugs into a "wall wart" or charges from one, and the total percentage of the power we use is actually now shifted to being mostly non-linear.

Harmonics, like lighting, is cumulative, like filling a bucket. You can fill a room with light by having one huge lamp, or you can fill it with 1000s of little Christmas lights in strings, the total lumens can be exactly the same in the long run. So too with harmonics. You can have distortion from one large 1000kW VFD, or a cumulative effect of 10,000 SMPS units, each drawing only 100W each. In fact it might even be WORSE with the 10,000 little SMPS units because they are likely single phase and not balanced, so much of the phase shift offsets are not there either compared to a 3 phase VFD. Your customer may not have all 10,000 of the SMPS inside of his facility, they might be with his neighbors, but the effect will show on the utility side.

By the way,

Measuring harmonics on the LOAD side of the VFD was pointless, that is ALWAYS going to be nasty. You want to measure on the LINE side of the VFD, the CONTRIBUTION of the rectifier to the PCC. What happens on the load side is only relevant to the motor itself.
Click to expand...

Your customer may not have all 10,000 of the SMPS inside of his facility, they might be with his neighbors, but the effect will show on the utility side.

I can understand that to a certain degree with the V THD but wouldn't the utility line impedance between multiple customers tend to dampen these harmonics at the different service entrances?
 
Status
Not open for further replies.
Top