THD - current relevancy with LED driver PSUs

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markjkubicki

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A long long time ago in ancient history, there was fluorescent lighting; first with magnetic ballasts, then electronic. And THD (Total Harmonic Distortion) was important. Most specs called for a THD <10%. THD has not gone away (never will), but its importance in 'LED Land' seems to have all but vanished. You can hardly find a manufacturer who publishes the data. While a <10%THD is possible with LED drivers, the norm is <20%. (<10% 'appears' to have limitations: 120V higher - input loads, 277V - lower inputs (20W / 30W seems to be the threshold), and the actual (programmed) load, vs. the maximum load of the driver has a significant impact also...)

What gives?
We had to have <10%, but OOPS(?) <20% is OK? The utility companies have gotten better at dealing with THD?

I have a larger project where the clients' boiler plate calls for <10% on ALL fixtures. While possible, it's a challenging, has additional cost...
I cannot just tell the client: "oh... nobody cares about that anymore"; I need some real back-up.

Any direction / suggestions will be greatly appreciated in advance,
m.
 
There is no regulation requiring THD limits for devices that cause it, never has been. The “10%” you are referring to is from IEEE-519 specifications, and refers ONLY to the THD at the Point of Common Coupling (PCC), which is generally the Service Entrance. IEEE-519 makes SUGGESTIONS as to what individual contributors should be limited to, but the issue is complex, based on the ratio of non-linear load current to total available current in a system. So basically it boils down to the end user having to take responsibility for harmonics within their system. But even then, that is ONLY as it relates to harmonics that they EXPORT into the utility grid, not what it might do to their own internal equipment.

There were some requirement changes in IEC rules years ago for single phase power supplies and drivers to include Power Factor Control (PFC) as an indirect way of regulating harmonics, but enforcement of that in the US is non-existent. Indirectly we could benefit from it because you would think that manufacturers would not want to make different versions of their products for the US vs the EU, but in reality the Chinese have seized on this lack of regulation here and flood us with cheaper products that would not be legal anywhere else, because our demand is high enough to warrant it. They can get a UL or ETL listing on their drivers without having the same PFC required for the EU, so they do it. The end result is that we get drivers with 20% THD and the problem gets kicked down the road to someone else.

This is what I like about Active Harmonic Filters installed at the PCC, or distributed in a facility with the switchgear. An AHF doesn’t care where the harmonics come from of how they got there, they just measure it and react to correct it where it needs to be corrected.
 
Clearly, you are reading somebody's boilerplate spec with language from the last century. Your RFI should ask them to clarify if they mean 10% THD for current or 10% THD for voltage at the supply end of the branch circuit. The first is impossible but the second is a slam dunk - and more consistent with the intent of IEEE 519.
 
I suspect it has become somewhat invisible because the amount of lighting load has become a fraction of what it used to be in a building, so the important characteristic for the building is the Total demand Distortion (TDD), which relates to THD, but puts it in perspective as to how much relative to the actual load.
 
ron said:
I suspect it has become somewhat invisible because the amount of lighting load has become a fraction of what it used to be in a building, so the important characteristic for the building is the Total demand Distortion (TDD), which relates to THD, but puts it in perspective as to how much relative to the actual load.
Click to expand...
Yes, but at the same time, with the death of incandescent lamps, almost ALL lighting is now powered from non-linear power supplies; fluorescent, CFL, LED and HID lighting all use SMPS based drivers / power supplies. Basically, now about the only loads that are still linear are resistance heating and motors running across the line.
 
(I wasn't so polite about it being from the last century - LMO)
My hunch is that it's uniformed administrative editing; NTL, I'm stuck with it. A box that needs to be checked.

The actual text of their standard reads:
General Requirements,
d. Ballasts/Drivers shall be electronic with maximum THD of <10% and a sound class A or better.
Click to expand...
 
markjkubicki said:
(I wasn't so polite about it being from the last century - LMO)
My hunch is that it's uniformed administrative editing; NTL, I'm stuck with it. A box that needs to be checked.

The actual text of their standard reads:
Click to expand...
The good news is you don't have any ballasts. Just LED drivers - not the same thing.
 
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