non linear loads and how they affect neutral

[benmin]

on a job where we are installing approximatly 18 rows of new LED lights. there are 36 lights in each row at 22watts per light and the supply voltage will be 277V. We are attempting to reuse the old lighting circuits which are multi circuit, L1,L2,L3 and neutral.

Although I am aware of non linear loads, I am not so familiar with how to calculate and divvy up the the circuits. Is there a good source of information?

Am planning on having row 3 rows per hot for 9 rows per multiwire circuit sharing a neutral. Sound right?

 

[winnie]

You need to know the 'harmonic content' of the current drawn by the loads. Linear loads draw sinusoidal current (at least when supplied with sinusoidal voltage); non-linear loads draw current that is a mix of harmonics.

_Some_ of these harmonics will balance on the neutral, just like ordinary line current. But some add up.

To proceed you need the specification for the harmonic current drawn by the lights, and then you can do the calculations.

-Jon

 

[gar]

170924-1055 EDT

You have no idea what the non-linearity of your load is, and that is to be expected.

Some new 9.5 W LEDs have very little 60 Hz harmonic content in their current waveform. There can be a lot of high frequency noise, but this is random from LED to LED and does not directly add, but in its own way might be a problem.

For the particular LEDs you are going to use look at their current waveform on a scope. Waveform distortion from a sine wave will give you an idea of the problem you may have. This will be a qualitative judgement.

Then, connect three of the fixtures to a three phase wye source and measure their summed RMS neutral current. Multiply by the number of fixtures on one phase to estimate your total neutral current.

A Fluke or HP RMS meter may not do a good measurement on high frequency noise because of their bandwidth limitation. I really have no idea how much the high frequency noise contributes to wire heating, but I have no reason to believe it will be much. However, the high frequency noise can be a real radio RFI problem. There is not much discussion on this problem.

.

 

[ActionDave]

on a job where we are installing approximatly 18 rows of new LED lights. there are 36 lights in each row at 22watts per light and the supply voltage will be 277V. We are attempting to reuse the old lighting circuits which are multi circuit, L1,L2,L3 and neutral.

Although I am aware of non linear loads, I am not so familiar with how to calculate and divvy up the the circuits. Is there a good source of information?

Am planning on having row 3 rows per hot for 9 rows per multiwire circuit sharing a neutral. Sound right?

I'm sure those guys right about how to do the calculations but I wouldn't do them personally. We've been lighting up buildings with MWBCs powering up non linear electronic lighting for a long time now.

Your plan to divide them up evenly is as good as it can get, and I wouldn't get hung up on doing that perfectly.

 

[Smart $]

I'm sure those guys right about how to do the calculations but I wouldn't do them personally. We've been lighting up buildings with MWBCs powering up non linear electronic lighting for a long time now.

Your plan to divide them up evenly is as good as it can get, and I wouldn't get hung up on doing that perfectly.

Here's my calculation...
3 rows x 36 fixtures x 22W/fixture ÷ 277V = 8.6A (per ungrounded conductor)

Worst case scenario, 3 hots x 8.6A = 25.7A, as if all the lines added up on the neutral... and the likelihood of that happening is nil. #12 wiring on a 20A circuit, you should be fine.

 

[Besoeker]

on a job where we are installing approximatly 18 rows of new LED lights. there are 36 lights in each row at 22watts per light and the supply voltage will be 277V. We are attempting to reuse the old lighting circuits which are multi circuit, L1,L2,L3 and neutral.

Although I am aware of non linear loads, I am not so familiar with how to calculate and divvy up the the circuits. Is there a good source of information?

Am planning on having row 3 rows per hot for 9 rows per multiwire circuit sharing a neutral. Sound right?

A three-phase system with non-linear single phase loads usually have a significant third harmonic content. These add arithmetically in the neutral. In fact, there will be a spectrum of harmonics and third plus all multiples of it (often called triple ) add. This can, and has, caused overheated neutrals.

But, to assess the extent of the problem, you need you need a harmonic spectrum for the LED lights. The manufacturer should be able to give you that.

 

[Ingenieur]

How many ckts? Total cb's?
what wire size? 12?
is each row home runned? or are some combined?
I assume source is 3 ph wye

 

[Besoeker]

that is why I assumed it

Fine, but I don't think an assumption was required.
Never mind. The subject is how non-linear loads affect the neutral. They do because of the triple harmonics which sum in the neutral of a 3-p+n system.
I don't know how widely known this is but certainly those in the lighting division of the corporation I worked for seemed blissfully unaware of it. It came back to bite them in the bum (butt) in a big way on a very large project - 3MW of controlled lighting and overheated neutrals.

So the OP is quite right to express concern over this issue.

 

[ActionDave]

Fine, but I don't think an assumption was required.
Never mind. The subject is how non-linear loads affect the neutral. They do because of the triple harmonics which sum in the neutral of a 3-p+n system.
I don't know how widely known this is but certainly those in the lighting division of the corporation I worked for seemed blissfully unaware of it. It came back to bite them in the bum (butt) in a big way on a very large project - 3MW of controlled lighting and overheated neutrals.

So the OP is quite right to express concern over this issue.

I'm not denying there can be an issue with neutral loads but it is rare.

 

[Ingenieur]

Fine, but I don't think an assumption was required.
Never mind. The subject is how non-linear loads affect the neutral. They do because of the triple harmonics which sum in the neutral of a 3-p+n system.
I don't know how widely known this is but certainly those in the lighting division of the corporation I worked for seemed blissfully unaware of it. It came back to bite them in the bum (butt) in a big way on a very large project - 3MW of controlled lighting and overheated neutrals.

So the OP is quite right to express concern over this issue.

Whether or not you feel an assumption is required is not relavent to the topic
this is a non issue at these current levels

without knowing
number of ckts
ampacity/wire size/cb rating
it is all idle speculation

 

[junkhound]

Back in the mid 90's when the EU promulgated PF and harmonic load requirements for consumer appliances, there were any number of IEEE papers on the subject.

On great line I heard a presenter give of a 3 phase harmonic paper give was: "You know there are a lot of harmonics if the white wire has turned brown".

 

[Besoeker]

I'm not denying there can be an issue with neutral loads but it is rare.

Neutral on a 3-p+n.
I come from a power electronics background, mainly variable speed drives and mostly three phase.
To that extent I have been involved with harmonic calculations for decades. Variable speed drives are a source of harmonics and that always had to be considered at the design stage. Lots of power compared to domestic, residential, commercial, and lighting. But about twenty years ago one project changed my thinking. I was doing harmonic measurements (we had to guarantee limits) at a pumping station. Even with none of my drives running, the 11kV supply was outside those limits, particularly the third. More and more non-linear loads, none very large, but the huge number of them add up.

It revised my view. I don't think it is all that rare. But then maybe that's my perspective because of the instances I've seen and because I got involved with harmonic measurements and mitigation.

 

[Besoeker]

Whether or not you feel an assumption is required is not relavent to the topic
this is a non issue at these current levels

What's relevant is the harmonic spectrum of the LED units.

 

[Smart $]

What's relevant is the harmonic spectrum of the LED units.

What's relevant is whether or not the harmonic content adds to more than the [actual] ampacity of the conductor. Let's say it is 20A after derating (unconfirmed actual). Given each component circuit only draws 8.6A, the additive harmonic content must exceed 20A/(8.6A x 3) = 77% of the line current. What is the likelihood of that happening? IMPO, zero.

 

[Besoeker]

What's relevant is whether or not the harmonic content adds to more than the [actual] ampacity of the conductor. Let's say it is 20A after derating (unconfirmed actual). Given each component circuit only draws 8.6A, the additive harmonic content must exceed 20A/(8.6A x 3) = 77% of the line current. What is the likelihood of that happening? IMPO, zero.

It happened on the CBRD so not zero. The neutral bars overheated.

 

[Smart $]

It happened on the CBRD so not zero. The neutral bars overheated.

We're not talking about the CBRD. We're talking about LED lighting.

 

[Ingenieur]

Very rarely an issue in cases like this: low i

most fixtures adhere to the <20% max std, many to the 10 or 15

max in = 0.6 x (x line)
as long as each line is loaded less than max line % = 100 / 0.6 = 165% all is hood
that is assuming the led is the max 20%,likely less

each row 36 x 22 / 277 = 2.9 A, assume poor pf call it 6 A or 2 A per ckt
so at 100% thd and 0 cancelling the neut will see 6 A
in reality <2
on #10 I guess based on longish runs
that is why the ckt count would help

moot

 

[Ingenieur]

What's relevant is whether or not the harmonic content adds to more than the [actual] ampacity of the conductor. Let's say it is 20A after derating (unconfirmed actual). Given each component circuit only draws 8.6A, the additive harmonic content must exceed 20A/(8.6A x 3) = 77% of the line current. What is the likelihood of that happening? IMPO, zero.

It's funny how numbers can render words meaningless in these matters

 

[Besoeker]

What's relevant is whether or not the harmonic content adds to more than the [actual] ampacity of the conductor. Let's say it is 20A after derating (unconfirmed actual). Given each component circuit only draws 8.6A, the additive harmonic content must exceed 20A/(8.6A x 3) = 77% of the line current. What is the likelihood of that happening? IMPO, zero.

With respect, what you seem to have missed is that isn't a single component. It's 3-p+n. All three phases have non-linear load. It's their combined triple harmonics that flow in the neutral.

Oh, and the CBRD was a 3MW lighting project.

 

[Ingenieur]

With respect, what you seem to have missed is that isn't a single component. It's 3-p+n. All three phases have non-linear load. It's their combined triple harmonics that flow in the neutral.

Oh, and the CBRD was a 3MW lighting project.

he mutiplied it by 3

assume 77% thd
or 6.6 A on the 8.6 load per ckt
3 x 6.6 ~ 20 A