Harmonics & transformers

Open Neutral

Senior Member
So I was pondering something the other day.

What residential loads are there these days that are sure to be steady-state? By that I mean they use the whole 60HZ sine wave cycle; not just the latter portion of each half cycle.

Illumination? Nope LED's driven by switchers.
Ovens? Well, if gas, maybe. But if there's not a switcher-driven PID range out there now, wait a bit.
Cooktop, ditto.
Microwave? No more ferroresonant tanks...
HVAC?
Washer? [Maytag Neptune, etc.]
And so forth.
But all that sinusoidal abuse creates harmonics. Somewhere I heard that NEC requires oversize neutrals under some articles.

Upstream, what about the ubiquitous pole-pig? How well do they cope with such? (No 3-phase delta windings on my street to eat them.)
And padmount units?

Is this an upcoming issue?
 

ron

Senior Member
In the late 90's and early 00's commercial market, harmonics and the associated solutions were a big deal. Everyone was talking about canceling harmonics or tolerating them with k rated transformers, double neutrals, phase shifting transformers, freq filtering, etc.

The loads have changed and now most design don't include double neutrals anymore and a typical distribution transformer can accommodate the regular amount to harmonics from the loads.

Maybe something to worry about, but maybe not.
 

Besoeker

Senior Member
Location
UK
In the late 90's and early 00's commercial market, harmonics and the associated solutions were a big deal. Everyone was talking about canceling harmonics or tolerating them with k rated transformers, double neutrals, phase shifting transformers, freq filtering, etc.

The loads have changed and now most design don't include double neutrals anymore and a typical distribution transformer can accommodate the regular amount to harmonics from the loads.

Maybe something to worry about, but maybe not.
It was certainly something to worry about in the supply to a hotel in Dubai with about 3MW, yes 3MW, worth of phase controlled dimmers.
 

Andy Delle

Senior Member
Location
Los Angeles CA
The reason that harmonics are less of a concern these days is that many switch mode power supplies now have internal power factor correction, especially larger units. And if i'm not mistaken, the EU requires power factor correction on some products.

But you can't assume that. There is plenty of legacy gear that has a pretty poor PF. I still spec oversize neutrals and do not allow MWBC, single or 3 phase, on any technical power system.
 

Besoeker

Senior Member
Location
UK
The reason that harmonics are less of a concern these days is that many switch mode power supplies now have internal power factor correction, especially larger units. And if i'm not mistaken, the EU requires power factor correction on some products.

But you can't assume that. There is plenty of legacy gear that has a pretty poor PF. I still spec oversize neutrals and do not allow MWBC, single or 3 phase, on any technical power system.
It isn't so much about PF, more about distortion.
 

GoldDigger

Moderator
Staff member
It isn't so much about PF, more about distortion.
We are most familiar with low power factor based on a reactive load (displacement power factor) that we often forget that the harmonics result in a low power factor defined in the same way, namely real power/apparent power, and it is referred to in that case as distortion power factor.
 

Besoeker

Senior Member
Location
UK
We are most familiar with low power factor based on a reactive load (displacement power factor) that we often forget that the harmonics result in a low power factor defined in the same way, namely real power/apparent power, and it is referred to in that case as distortion power factor.
Ny early life has come back to haunt me. Mercury archaic rectifiers...............
 

Open Neutral

Senior Member
Ny early life has come back to haunt me. Mercury archaic rectifiers...............
Tung-Sol lamps in battery chargers; before we upgraded to selenium rectifiers.


And how do good PF switchers keep the line pure and chaste all 360 degrees?

(I avoid using "low/high" for PF discussions because it creates confusion w/beancounters & PHB's.)
 

beanland

Senior Member
Location
Vancouver, WA
Transformer Heating

Transformer Heating

In a transformer, the harmonic current does not cause excess heating, except as far as the I^2R goes. A distorted waveform will have more heating at the peak. But, transformer core heat, often the biggest concern, is caused by excitation from the voltage. If the voltage has low harmonics, meaning it is a clean robust strong source, the harmonic currents in the load do not create much harmonic voltage at the transformer primary and so the transformer core does not see excessive heat from the harmonic flux in the core caused by the harmonics in the voltage.
 
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