Neutral voltage

[gt englin]

I have been reading the papers and comments about linear and nonlinear loads for 3 phase 4 wire systems. This has been very good, refreshes the memory and teaches something new. I have one question that maybe someone can answer.

Why is it such an accepted practice to apply the 3phase 4 wire clacs to a 2 phase 3 wire system with little regard to linear and nonlinear or harmonics. I cannot get it thiough my guys heads that this is why there are so many brown (used to be white) neutrals found in panels. Can someone give me suggestions?

Thanks,

Jerry in so. ca.

 

[ron]

If you mean 120/240V single phase, there will be no triplen harmonic problems on the neutral because they cancel out (180 degrees out of phase).
They add on 3 phase 4 wire because the three phases are 120 degrees out of phase.

 

[iwire]

I cannot get it thiough my guys heads that this is why there are so many brown (used to be white) neutrals found in panels.

I think you may be assuming it is a harmonics issue without any real proof.

 

[brian john]

I have been doing harmonic investigations since the issue first became the next "HOT TOPIC". In my experience this has been way over blown much like the often mentioned "BAD GROUND CONNECTION TO EARTH".

Where I have seen 3 phase 4 wire neutral harmonics issues, have been call centers, casinos and large data centers with poor planning. In a typical office building when the electricians or designers take into account the number of PCs on a circuit I seldom to never have seen issues, additionally most typical office building have over sized distribution transformers loaded in the 25%-50% loading.

Check your connections, measure phase and neutral currents, calculate neutral current and see if the calculated values is substantially lower than the measured value, this assumes you are using true RMS meters, measure RMS current and peak currents the crest factor should be in the range of 1.4143, use a scope to check the wave forms, use an analyzer to measure voltage and current harmonic distortion.


In some sort of order to their occurrence, this may change tomorrow. STRICTLY BASED ON MY EXPIERENCE
1. Loose connections
2. Voltage drop on branch circuits for copiers rated 10-16 amps. Few if any electricians give any thought to conductor length.
3. Neutral to ground issues.
4. Thinking about 4-9
5.
6.
7.
8.
9.
10. Harmonics

 

[Physis 3]

I agree with everything you've said Brian John. I've been wacthing people chasing phantoms since I was a teenager. And those people have never caught a phantom nor repaired whatever the issue was.

I'm not sure whether this applies, but I've seen a lot of garbage electronics distributors try to blame a totally simple branch circuit for the inability of their product to work as they said it would.

 

[Physis 3]

use a scope to check the wave forms, use an analyzer to measure voltage and current harmonic distortion.

Why? I understand that you want to be sure your power's cool, but does anybody really expect to find a bunch of extra tall triangles instead of a sine wave? I mean, really.

Even if there's some garbage on your waveforms, should the multi-tens of thousand of dollars worth of, whatever, not have a $100 dollar power supply that would work under those conditions anyway?

It's almost never the electricians fault that funky electronics don't work.

 

[brian john]

The major issues we saw with harmonics were in the early years of this issue and was not with 3 phase 4 wire single phase loads and the resulting triplen harmonics but with 3 phase 3 wire loads normally a byproduct of 6 pulse rectifiers (5,7, 11,13 ect...). The issue was when the site would be on generator, and the governor or voltage regulator cold not respond to a distorted voltage wave form a result of high current distortion.

3 phase triplen harmonic current snapshot note the neutral wave form 180 HZ, I should have posted the voltage snapshot to show a non-distorted voltage waveform. From a previous post of mine end users name changed to protect the innocent...

 

[Physis 3]

What in the world is that waveform from? And what's causing it to be so distorted?

This is beyond abnormal if it's looking like that for more than a millisecond.

Edit: Is there a resister in series with the load? What is that?

 

[sreeja]

Some equipment installation specifications list extremely small values for acceptable N-G voltage, such as 0.5V to 1.0V. To meet these stringent requirements, you would usually install a dedicated transformer at the load, and/or run large gage wiring directly from the source N-G bond to the load

 

[ptonsparky]

IMO, with limited experience, it seems that we should not worry about the current wave forms, as in Brians picture, as long as they are uniform and can be attributed to the type of load on the system. The problem is when the voltage sine wave is distorted because of current. This would indicate to me undersized (overloaded) conductors and/or transformer.

The snapshot of neutral current is a proplem only if the neutral is undersized. I do not know how this affects a properly sized transformer.

PLEASE correct me, but be gentle.

 

[nakulak]

the reasons would be the additive harmonic multiples that he mentioned. I could be wrong, but I would assume that the currents were driven by similarly formed voltage waveforms (?).

 

[ptonsparky]

the reasons would be the additive harmonic multiples that he mentioned. I could be wrong, but I would assume that the currents were driven by similarly formed voltage waveforms (?).

We start out with a typical looking 60hz voltage sine wave as the POCO sends us, nice, neat and clean. The loads we add, intended or not, are what will shape the voltage wave form after it gets to our facility. A large load that distorts the voltage waveform by causing voltage drop anywhere on the sine wave, will then affect what voltage other loads see. Causing even more grief.

Brian surley has a picture, I will have to search for one.

 

[ptonsparky]

Search "distorted voltage waveform"

 

[ELA]

We use to build a large piece of manufacturing equipment that was feed by a 208V -3 ph. Wye connection at 200A. We built about 20 of these a year for a number of years. This equipment contains a large number of non-linear loads.

The power entry circuitry incorporated a phase monitor. For a long time these tools ran just fine. Over time newer tools started to fail due to the phase monitor. At first we suspected undervoltage conditions but line monitoring proved this was not the issue. We found current waveforms much like the pictures John posted.

The voltage waveform contained a high level of distortion due the large current peaks that only flow for a small portion of the half cycle.

We ended up installing a better quality phase monitor that did a True RMS calculation on the voltage waveform and the problem went away.

 

[mayanees]

another 2 cents on Harmonics...

another 2 cents on Harmonics...

I do Power System Fault and Coordination Analyses, which is the typical Power Study required to develop breaker settings for a coordinated system.
On about 5 occasions the specification for the Power Study has included Harmonics Analysis. The requirement is that the system be in compliance with IEEE-519. (all of these have been Wastewater Treatment Plants)

IEEE-592 defines allowable harmonics levels at th e Point of Common Coupling (PCC) based on the facility load versus the strength of the Utility (Utility Short Ckt contribution)

In each of the 5 Harmonics Analyses I've performed the systems have had numerous VFDs. And in each of the 5 cases, none have been out of compliance with the PCC requirements of IEEE-519. And there were widely varying Utility strengths - from close to a substation, to 10 miles out in the country.

I think it's because the drives are getting less intrusive on the incoming waveform, and because the PCC is remote enough from the VFDs that it's sufficiently buffered.

The whole point of my rambling is to agree with the general tone of this thread - that the problem with Harmonics is likely overstated.

JM

 

[steve66]

Even in Brian's example, a full sized neutral shouldn't have any problems.

Steve

 

[Physis 3]

What in the world is that waveform from? And what's causing it to be so distorted?

This is beyond abnormal if it's looking like that for more than a millisecond.

Edit: Is there a resister in series with the load? What is that?

Oops, I was thinking those were voltage waves.

Edit: Error A

 

[brian john]

TV Studio, fairly typical for this type of installation.

 

[brian john]

This is NOT neutral current it is distortion on one phase (all 3 were similar) to a drive for a motor while connected to a generator, the generator will not run properly GUESS WHY?

Black is the voltage sine wave

Red is Current sine wave.

 

[ptonsparky]

and GUESS it will be. The generator is undersized or the conductors to the drive are to small.

The voltage seems to peak a long time before current even begins but I see you are phase to phase. Mmm. Better to see and learn here than somewhere else.