TDD without THD - Harmonics

[elecshop]

What does it mean to have current harmonics beyond the TDD levels defined by IEEE 519 guidelines, but THD within those levels?
Those measurements were recorded on the main incoming service between the utility transformer and the main switchgear.
Does this necessarily imply that loads downstream of the utility transformer are causing these distortions?

 

[jim dungar]

I thought TDD was calculated based on the available short circuit amps at the service and the measured THD.

I do know it is possible to have excessive Current THD (THDi) while having acceptable Voltage THD (THDv)
I once evaluated a 3000A power monitor reading of 30% THDi. It happened on a holiday morning when the emergency lighting battery packs were the only substational load at about 40A.

 

[elecshop]

So you believe that this is caused strictly by the emergency lighting battery packs?

In my scenario, I have a 1500kVA transformer powering a hospital, and some power issues where observed in the hospital i.e., voltage dips with no corresponding current peaks, and those dips were repetitive during the almost same time frame in the afternoon. Our first thought was that it has to do with some switching at the utility/substation. But since current harmonics are also excessive and beyond the TDD limit, we are starting to doubt that the loads are causing this issue i.e., nonlinear loads.

Any thoughts? I still cannot tell how to relate the voltage dips to the excessive current harmonic distortions but no voltage harmonic distortions - the voltage dip observed was mostly in phase B, but the current harmonic distortions were observed in all phases.

 

[Jraef]

This was continued in the other thread which had more relevant information.

Isolation Transformer for "Dirty" Power

I am working on a hospital project which has "dirty" power issue i.e. several small brownouts that affect the equipment and HVAC. The electrician installed a meter between the main switchgear and the two ATS's serving the equipment branches. Voltage sags were noticed continuously on the B leg...

forums.mikeholt.com

So I am closing this one.

 

[jim dungar]

So you believe that this is caused strictly by the emergency lighting battery packs?

In your case, I absolutely do not.

My point was that a single piece of information, like just having a THDi, is not enough information to know if a problem even exists much less how to solve it.