Harmonics and multiwire circuit

[GoldDigger]

Don't see that.

I don't either.
In a single phase (120/240) MWBC the triplen harmonics will cancel, just as the fundamentals cancel. Any second harmonic present for some reason would add.
In a "single phase" derived from three phase (120/208) the triplen harmonics would not add making a higher amplitude, but they would not cancel to zero either. They would add to an out of phase harmonic current of equal amplitude to that on each phase conductor, just as the fundamentals would.

 

[Ingenieur]

interesting
https://www.harmonicslimited.com/wp-content/uploads/2017/02/Harmonic-Currents-in-Building-Wiring.pdf

 

[Besoeker]

interesting
https://www.harmonicslimited.com/wp-content/uploads/2017/02/Harmonic-Currents-in-Building-Wiring.pdf

Nice find.

 

[Ingenieur]

Nice find.

the thd for some of those devices is much higher than I imagined
In this digital age I could see it fouling up the grid
especially with many motors being vfd controlled now a days

 

[Besoeker]

the thd for some of those devices is much higher than I imagined
In this digital age I could see it fouling up the grid
especially with many motors being vfd controlled now a days

Yes. The power side has been understood for many years. That's why we installed 12 and 24 pulse systems. We had standards to meet. Units under 16A didn't.
My background is, as you know, industrial power electronics.
One of the developments we came up with was the Kramer with our take on it specifically to improve efficiency and reduce harmonics.
But let's not wander off topic.

 

[RumRunner]

Yes. That was my point about the pumping station harmonics. Nothing big, just lots of little things. And it's probably a lot worse now.
One of the nice things about the location is that it was St Albans, a town dating from Roman occupation of southern Britain.
History and harmonics in the same post.........

I didn't know the Roman Emperor Hadrian--with his Roman Legion, used VFDs to build the Hadrian Wall to protect the town of St. Albans

 

[electrofelon]

So what would be the most likely scenario for overloading a neutral? I was thinking maybe a three wire three phase MWBC because the neutral is already carrying about the same current as the phase conductors. Seems to give you a pretty good "head start"?

 

[ActionDave]

So what would be the most likely scenario for overloading a neutral? I was thinking maybe a three wire three phase MWBC because the neutral is already carrying about the same current as the phase conductors. Seems to give you a pretty good "head start"?

Very clever.

 

[GoldDigger]

So what would be the most likely scenario for overloading a neutral? I was thinking maybe a three wire three phase MWBC because the neutral is already carrying about the same current as the phase conductors. Seems to give you a pretty good "head start"?

Not necessarily. It may be carrying the same current as a phase conductor without harmonics, but the harmonic current in the neutral will only be equal to the harmonic current in each phase rather than three times the phase harmonic current. Net result is that the neutral still carries the same current as each phase and the standard OCPD will protect the neutral.

 

[electrofelon]

Not necessarily. It may be carrying the same current as a phase conductor without harmonics, but the harmonic current in the neutral will only be equal to the harmonic current in each phase rather than three times the phase harmonic current. Net result is that the neutral still carries the same current as each phase and the standard OCPD will protect the neutral.

I dont quite follow. So say I have balanced full boat, and lets say the harmonics are such that the neutral is 1.25 times a phase current. I turn off one of the phases, what do I have on the neutral now?

 

[Besoeker]

Not necessarily. It may be carrying the same current as a phase conductor without harmonics, but the harmonic current in the neutral will only be equal to the harmonic current in each phase rather than three times the phase harmonic current. Net result is that the neutral still carries the same current as each phase and the standard OCPD will protect the neutral.

In three phase 4-wire, it carries the sum of the triple-n harmonics. At three times supply frequency they are bound to end up in phase. That's what contributes to overloaded neutrals.

 

[GoldDigger]

In three phase 4-wire, it carries the sum of the triple-n harmonics. At three times supply frequency they are bound to end up in phase. That's what contributes to overloaded neutrals.

You may have missed the situation that electrofelon was talking about and I was replying to: a three phase three wire MWBC, not a full boat.
There are two phase conductors and a neutral from a three phase source.

I was in fact mistaken about the added harmonic current in the neutral, it would not be three times the phase harmonic current, since only two phases are involved in the circuit. But it would be twice the harmonic current rather than identical to the harmonic current as I originally proposed.

So electrofelon is IMHO correct in his statement that the effect of harmonic current could indeed be more of a problem in a fully loaded 2 of 3 phase plus neutral MWBC than in the corresponding fully loaded full boat as long as the full boat did not use a reduced neutral.

If each phase to neutral load had a 30% (amplitude) third harmonic content the neutral of the 2-of-3 would be loaded to 130% of the phase current, just not 160% or 190%. By contrast the full boat would have a 90% neutral load, so a reduced neutral, as allowed for a linear full boat would be grossly overloaded, while the neutral of the 2-of-3 would be loaded to 130% instead of the 100% it is required to carry.

 

[Besoeker]

You may have missed the situation that electrofelon was talking about and I was replying to: a three phase three wire MWBC, not a full boat.
There are two phase conductors and a neutral from a three phase source.

I was in fact mistaken about the added harmonic current in the neutral, it would not be three times the phase harmonic current, since only two phases are involved in the circuit. But it would be twice the harmonic current rather than identical to the harmonic current as I originally proposed.

So electrofelon is IMHO correct in his statement that the effect of harmonic current could indeed be more of a problem in a fully loaded 2 of 3 phase plus neutral MWBC than in the corresponding fully loaded full boat as long as the full boat did not use a reduced neutral.

If each phase to neutral load had a 30% (amplitude) third harmonic content the neutral of the 2-of-3 would be loaded to 130% of the phase current, just not 160% or 190%. By contrast the full boat would have a 90% neutral load, so a reduced neutral, as allowed for a linear full boat would be grossly overloaded, while the neutral of the 2-of-3 would be loaded to 130% instead of the 100% it is required to carry.

Understood.

 

[junkhound]

schematic and waveforms.

schematic and waveforms.

Waveforms worth lots of words.
There are numerous free online Pspice programs where one can calculate precise answers to such as those posed by the OP
This one is Orcad free evaluation version of PSpice, others suck as LT_SPice give the same answers.

This example is simply 2 full wave rectifiers with loads matched at 1% for both 120/240 1 phase and 120/208 3 phase with 2 loads.