THD - Ideal Values?

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DaniBo

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Any ideas on what ideal values should be for THD?

For instance, I've received a report and the number 23 appears consistently.

Any guidance on desired values would really be appreciated.

NB. All values were calculated using a Fluke 434.
 
Depends on the system you are dealing with. Some equipment does not care at all what level of harmonics are present. Can you provide more information?
 
IEEE 519 lists maximum permissible THD when measured at the service. But harmonics will attenuate across impedance, so for example measuring 23% THD at a VFD disconnect is not gonna give you the same reading you'd get measuring harmonics at the service point.

Where are you measuring and why?
 
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DaniBo said:
Any ideas on what ideal values should be for THD?

For instance, I've received a report and the number 23 appears consistently.

Any guidance on desired values would really be appreciated.

NB. All values were calculated using a Fluke 434.
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Read IEEE 519.

But when it all said and done, it depends on your entire system. I have had applications where multiple thousands HP motors were fed off generators line. we were creating 30% to 50% THDi and more than 10% THDv. These may seem too high but we were completely isolated from utility and everything on our line (VFDs, transformers, reactors, generators...) were rated for that much harmonics, so nobody cared.
 
LMAO said:
Read IEEE 519.

But when it all said and done, it depends on your entire system. I have had applications where multiple thousands HP motors were fed off generators line. we were creating 30% to 50% THDi and more than 10% THDv. These may seem too high but we were completely isolated from utility and everything on our line (VFDs, transformers, reactors, generators...) were rated for that much harmonics, so nobody cared.
Click to expand...
Bingo.

Super condensed version of IEEE 519:

"Thou Shalt Not Screw Up Thy Neighbor's Voltage!"

No neighbors, no problem!

Alternate version if you do have neighbors: or

"How Harmonics will make you fat!"

Non-linear current draw leads to THDi,
THDi leads to THDv at your PCC,
THDv travels beyond your PCC to neighbors,
Neighbors call utility, makes you angry,
Anger leads to stress,
Stress leads to Doobies,
Doobies lead to Twinkies.
 
Jraef said:
Bingo.

Super condensed version of IEEE 519:

"Thou Shalt Not Screw Up Thy Neighbor's Voltage!"

No neighbors, no problem!
Click to expand...
But if you're connected to the grid, you have neighbours. Maybe not close but the the PCC makes them neighbours in that sense.

I have posted here about measurements I made on a transformer HV side. It was 11kV/400V supplying a pumping station where we installing eight VSDs. Before and after tests were required to demonstrate that the VSDs were not causing unacceptable distortion at the PCC (11kV). Well, even none of the drives running, the voltage distortion was outside the limits of what was permissible in our contract. Mostly third which emanates for the most part from non-linear single phase loads. Residential for the most part. Nothing big. Just lots of them.

The drives were mostly 6-pulse with a couple of the larger units being 12-pulse. With the drives running the 11kV distortion was not measurably worse.
 
In one Telephone exchange, current THD was within limits, but voltage THD was slightly off limit. The POCO was trying to put the blame on us and also penalty. I am about to write an objection letter to them.......
 
Sahib said:
In one Telephone exchange, current THD was within limits, but voltage THD was slightly off limit. The POCO was trying to put the blame on us and also penalty. I am about to write an objection letter to them.......
Click to expand...
Does the telephone exchange have large rectifiers? I've come across a few that do.
 
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