Harmonics and multiwire circuit

[2Broke2Sleep]

Out of curiosity, if you were to take two equal loads from equivalent equipment that give off harmonics and put them on a shared neutral, would that have any neutralizing effect on the return path of the circuit??

 

[ptonsparky]

Out of curiosity, if you were to take two equal loads from equivalent equipment that give off harmonics and put them on a shared neutral, would that have any neutralizing effect on the return path of the circuit??

Some one with more knowledge will came along shortly.

FWIU, that is the problem with harmonics, they can be additive. (If that’s a word). They don’t cancel, they add at certain frequencies. You may end up with an overloaded neutral.

 

[ron]

Harmonics at certain frequencies cancel and other add.

For example in a 3 phase, 4 wire system, "triplen harmonics" add on the neutral. Google that term.

 

[Besoeker]

Harmonics at certain frequencies cancel and other add.

For example in a 3 phase, 4 wire system, "triplen harmonics" add on the neutral. Google that term.

More than a few people have been caught out by that - some you wouldn't expect to be. I shouldn't complain - too loudly.
It has been a source of revenue for me......

Most of my working life has been in power electronics in the industrial field. Large VSDs, high current rectifiers etc. Areas where you expect and cater for harmonics.
You tend to think of it as a power problem. I was disabused of that insular opinion a couple of decades ago.

We did the installation on a pumping station with it's own dedicated 11kV/400V transformer. This included a number of variable speed drives. Part of the contract was to do a "before and after" harmonic analysis to ensure that we were compliant with the G5/3 limits, the standard in use at the time. I did this using the VTs and CTs on the supply to the station transformer.

It didn't comply even with none of our drives in operation. That got me thinking. The pumping station was in a residential area. Nothing heavy on the supply - just the sheer number of homes with an increasing proliferation of non-linear electronic gadgets.

 

[ron]

More than a few people have been caught out by that - some you wouldn't expect to be. I shouldn't complain - too loudly.
It has been a source of revenue for me......

Most of my working life has been in power electronics in the industrial field. Large VSDs, high current rectifiers etc. Areas where you expect and cater for harmonics.
You tend to think of it as a power problem. I was disabused of that insular opinion a couple of decades ago.

We did the installation on a pumping station with it's own dedicated 11kV/400V transformer. This included a number of variable speed drives. Part of the contract was to do a "before and after" harmonic analysis to ensure that we were compliant with the G5/3 limits, the standard in use at the time. I did this using the VTs and CTs on the supply to the station transformer.

It didn't comply even with none of our drives in operation. That got me thinking. The pumping station was in a residential area. Nothing heavy on the supply - just the sheer number of homes with an increasing proliferation of non-linear electronic gadgets.

The pumping station was 3 phase 4 wire, where the neutral likely received the triplens from everyone else?

The standard in the US is a utility standard. Measurement made at the point of common coupling. They don't care what happens inside the building.

 

[Besoeker]

The pumping station was 3 phase 4 wire, where the neutral likely received the triplens from everyone else?

The standard in the US is a utility standard. Measurement made at the point of common coupling. They don't care what happens inside the building.

The harmonics were measured on the supply to the 11kV transformer.
The transformer fed the pumping station only.

 

[infinity]

Out of curiosity, if you were to take two equal loads from equivalent equipment that give off harmonics and put them on a shared neutral, would that have any neutralizing effect on the return path of the circuit??

If you had two equal loads on a WYE system the current on the neutral would be nearly equal to the phase current even if there were no harmonics present.

 

[Jraef]

If you had two equal loads on a WYE system the current on the neutral would be nearly equal to the phase current even if there were no harmonics present.

The key point being EQUAL loads. In a balanced 3 phase system, 3rd harmonics (divisible by 3) in the phases cancel each other out. In all systems, even order (divisible by 2) cancel each other out. Odd order non-triplen harmonics are additive. But in SINGLE phase systems (and loads that are not balanced out in the other phases in a 3 phase system), the 3rd order harmonics will also add.

So if your question, as it appears, is in reference to a MWBC in a single phase system, all of the odd order harmonics will be additive and additional current will flow in the shared neutral.

 

[infinity]

If you had two equal loads on a WYE system the current on the neutral would be nearly equal to the phase current even if there were no harmonics present.

The key point being EQUAL loads. In a balanced 3 phase system, 3rd harmonics (divisible by 3) in the phases cancel each other out. In all systems, even order (divisible by 2) cancel each other out. Odd order non-triplen harmonics are additive. But in SINGLE phase systems (and loads that are not balanced out in the other phases in a 3 phase system), the 3rd order harmonics will also add.

So if your question, as it appears, is in reference to a MWBC in a single phase system, all of the odd order harmonics will be additive and additional current will flow in the shared neutral.

I'm assuming that the bold part is in reference to the OP?

 

[Besoeker]

The key point being EQUAL loads. In a balanced 3 phase system, 3rd harmonics (divisible by 3) in the phases cancel each other out..

My experience is that they add rather than cancel. Think about three equal phase-controlled dimmers for example.

 

[hbendillo]

Does anybody have any experience analyzing harmonics on more modern systems? I think one of the problems when the harmonic problems arose was because of poorly made power supplies for computers and electronic equipment. Has the harmonic problem been reduced to a great degree in modern electronics because advancements in construction of VFD's and power supplies?

 

[Jraef]

I'm assuming that the bold part is in reference to the OP?

Oh, yes, it was. I meant to address that part to the OP, I forgot.

 

[Jraef]

Does anybody have any experience analyzing harmonics on more modern systems? I think one of the problems when the harmonic problems arose was because of poorly made power supplies for computers and electronic equipment. Has the harmonic problem been reduced to a great degree in modern electronics because advancements in construction of VFD's and power supplies?

To a small extent, SOME power supplies and such for small single phase systems now include a certain attention to harmonics. But for the greater part, nobody has wanted to increase the cost of THEIR widgets compared to others, so as long as nobody is forcing the issue, it's not changing significantly. Then to make things worse, SMPS (Switch Mode Power Supply) technology has made its way into almost EVERYTHING we use now; LED lamps, Fluorescent and CLF ballasts, HID lighting, UPS systems, PC Power Supplies, TVs, all manner of digital devices and chargers for them, appliances, even kids toys. So yes, maybe individually the THD they create is a little lower that decades ago, but the ubiquity is now making the cumulative situation much worse.

As far as VFDs go though, Low Harmonic VFDs with Active Front Ends are gaining a foothold, but that adds to the cost significantly; you are essentially buying two VFDs for one motor. But because VFDs often represent the largest loads in a facility, the users must address harmonics in some way and that way is becoming more and more accepted, especially at 250HP and up.

 

[Besoeker]

To a small extent, SOME power supplies and such for small single phase systems now include a certain attention to harmonics. But for the greater part, nobody has wanted to increase the cost of THEIR widgets compared to others, so as long as nobody is forcing the issue, it's not changing significantly. Then to make things worse, SMPS (Switch Mode Power Supply) technology has made its way into almost EVERYTHING we use now; LED lamps, Fluorescent and CLF ballasts, HID lighting, UPS systems, PC Power Supplies, TVs, all manner of digital devices and chargers for them, appliances, even kids toys. So yes, maybe individually the THD they create is a little lower that decades ago, but the ubiquity is now making the cumulative situation much worse.

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.........

 

[2Broke2Sleep]

To answer your question, yes my OP was referring to a single phase MWBC with equivalent loads on each circuit....

 

[Jraef]

To answer your question, yes my OP was referring to a single phase MWBC with equivalent loads on each circuit....

So yes, in that case then the triplen harmonics will add.

 

[Ingenieur]

on a mwb ckt
L1 0 deg
L2 180 deg
identical loads

what is the phase relationship generated by each line?
is the relative polarity of the associated thd the same as the line?
if L1 is pos and L2 neg are the harmonics generated by each of opposite sign?
180 deg out of phase?

or is it random like noise with mag/ph varying from the fundamental?

 

[electrofelon]

To a small extent, SOME power supplies and such for small single phase systems now include a certain attention to harmonics. But for the greater part, nobody has wanted to increase the cost of THEIR widgets compared to others, so as long as nobody is forcing the issue, it's not changing significantly. Then to make things worse, SMPS (Switch Mode Power Supply) technology has made its way into almost EVERYTHING we use now; LED lamps, Fluorescent and CLF ballasts, HID lighting, UPS systems, PC Power Supplies, TVs, all manner of digital devices and chargers for them, appliances, even kids toys. So yes, maybe individually the THD they create is a little lower that decades ago, but the ubiquity is now making the cumulative situation much worse.

As far as VFDs go though, Low Harmonic VFDs with Active Front Ends are gaining a foothold, but that adds to the cost significantly; you are essentially buying two VFDs for one motor. But because VFDs often represent the largest loads in a facility, the users must address harmonics in some way and that way is becoming more and more accepted, especially at 250HP and up.

Im not sure I agree Jraef. Everytime I have measured the neutral of a MWBC serving electronic loads ballasts or drivers, Ivhave gotten no more than the about a quarter of the balanced load line current. I just really doubt this is much of an issue. Now granted my testing sample size is pretty low, maybe I just missed the bad ones, but I do suspect this is a whole lot of hand wringing over a problem that doesnt exist.

 

[Ingenieur]

Im not sure I agree Jraef. Everytime I have measured the neutral of a MWBC serving electronic loads ballasts or drivers, Ivhave gotten no more than the about a quarter of the balanced load line current. I just really doubt this is much of an issue. Now granted my testing sample size is pretty low, maybe I just missed the bad ones, but I do suspect this is a whole lot of hand wringing over a problem that doesnt exist.

I tend to agree with you
IEEE std thd 5%, no single freq > 3%

assume 20 A x 2 mwb loaded to 80% or 16 A
8 devices at 2 A ea per ckt
assume no cancellation
5% x 2 A x 8 devices x 2 ckt = 1.6 A, 10% of line i
the fundamental cancels

even if the devices do not comply to the IEEE std and make 20% thd (unlikely)
6.4 A or 40% of line i

I guess issues could arise if only 1 line is loaded
5% thd: 16 + 0.8, not bad
20% thd: 16 + 3.2, marginal
but wouldn't f1 be increased from the source since the thd power must be supplied?
or conversely the rtn f1 is reduced since p in = p out

now for large devices like util facts components, ie, tsc or tcr, filters are required

 

[Besoeker]

So yes, in that case then the triplen harmonics will add.

Don't see that.