THD - Ideal Values?
- Thread starter DaniBo
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
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?
Zero.
big john
Senior Member
- Location
- Portland, ME
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?
Where are you measuring and why?
Last edited:
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
And... are we talking VOLTAGE THD or CURRENT THD?
big john
Senior Member
- Location
- Portland, ME
At 23% that had better not be voltage.
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.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
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.
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.
Does the telephone exchange have large rectifiers? I've come across a few that do.
Yes. The telephone exchange has large SMPS rectifiers.
Large as in?
And how many?
Last edited:
19X100A DC-1NO
Not huge but in combination that could be your problem.
- Status