Sharing neutrals in lighting circuits

[ramsy]

Mike Holt's publications offer several unqualified opinions that choose to error on the side of caution. In his 1999 NEC study guide Mr Holt advises, "3? 4-wire wye's that supply nonlinear loads ..can produce unwanted and potentially hazardous harmonic (neutral) current(s) ..as much as twice the phase conductor current. The neutral conductor must be sized to carry the excessive harmonic neutral current to prevent the conductor from overheating and possibly causing a fire or equipment damage, Fig. 5-3."

Would you agree that my assesment of Mr. Holts opinion fairly reads his choice to error on the side of caution?

The fact is neutrals are not overheating on a regular basis on non-linear loads...No mater if you use two wire or MWBCs the harmonic issue (if there is one) will still be an issue for the panel, feeder and transformer.

Would you agree, if your practical experience finds no evidence for this kind of harmonics, that should not dictate the limits of what others will encounter in the field? Especially for troubleshooters, who are called in specifically for wireing problems?

Now check me here, but I believe Delta connected loads, (ie) motors, and some devices may also dampen their own harmonics on wye systems.

I would like to assume a Wye line to delta load dampens harmonics just like a delta xfmr, but without testing this myself or finding evidence elsewhere, I will phrase it as a question for anyone who might hazard a more informed opinion. Scott35 ?

 

[infinity]

You can look on page 6 of this document for the theoretical maximum neutral current in fully loaded harmonic circuits.


http://www.apcmedia.com/salestools/SADE-5TNQZ5_R0_EN.pdf

 

[don_resqcapt19]

roger,

Would you agree, if your practical experience finds no evidence for this kind of harmonics, that should not dictate the limits of what others will encounter in the field?

Can you cite any documented cases where harmonics have caused problems in the field? There are very few. This is accepted as a large problem only because of the large amount of literature from organizations with a vested economic interest in solving the "problem".
Don

 

[iwire]

Without a reference supporting your 1.732 figure for max harmonics, Mike Holts published opinion includes one for busman, who asked for a reference.

I would't feal too much grief over this, 1.732 is close enough to 2.0. Besides, Mike Holt does not support this published opinion with any references either, so ultimately your figure and thoughtful advice could be more practical.

From APCs "Hazards of Harmonics and Neutral Overloads" (Infinity's Link)

In fact, the harmonic current alone in the neutral wire can, in theory, be up to 1.7 times larger than the full rated current of the power wiring.

Thank you Trevor. 8)

Notice that they say "can, in theory, be up to 1.7 times larger" which sounds to me like 'It can be, but ain't bloody likely'.

 

[ramsy]

look on page 6 of this doc. for the theoretical max neutral current in fully loaded harmonic circuits.

Variable speed drives and SCR's are not considered in that 1.7 figure, but I agree Mike Holt's figure of twice the phase current for max neutral harmonics, now appears over cautious without qualifying his source.

That was an excellent find. I'm shure Bob will be riding on Trevor's coat tails to whip me with this one.

 

[iwire]

Variable speed drives and SCR's are not considered in that 1.7 figure, but I agree Mike Holt's figure of twice the phase current for max neutral harmonics, now appears over cautious without qualifying his source.

Roger the engineer that provided me with the 1.7 figure is very knowledgeable.

I trust his statement to me that the theoretical max harmonic current is 1.7 no matter what non-linear loads are connected.

I'm shure Bob will be riding on Trevor's coat tails

Thats for sure, I have never had an independent thought in my life. :lol:

 

[ramsy]

This is accepted as a large problem only because of the large amount of literature from organizations with a vested economic interest in solving the "problem".

No doubt, Marketing, fear and hype will artificially generate demand and revenue.
I wonder, how legitamite this concern of neutral harmonics, twice the phase value, is with industrial plants that involve VSD's, SCR's, & constantly changing configurations.

 

[iwire]

I wonder, how legitamite this concern of neutral harmonics, twice the phase value, is with industrial plants that involve VSD's, SCR's, & constantly changing configurations.

Roger it happens tomorrow I am going to a meeting at an industrial type installation where they are having troubles with large VFDs (500 HP) and stray currents.

This has been doing damage to the motor bearings.

I have been involved in this on and off for about a year now and I believe tomorrow we will hear the EEs solutions.

None of the solutions discussed so far have been remotely inexpensive. :shock:

If your interested here is an old thread with more info.

http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=3;t=002062

 

[ramsy]

Roger the engineer that provided me with the 1.7 figure is very knowledgeable. I trust ..max harmonic (neutral) current is 1.7 (phase current) no matter what non-linear loads are connected.

Bob, I had the impression you rely on engineers heavily, and don't check their figures or spec's when discussing your practices here. I havn't read you taking measurements to verify harmonic or v-drop spec's either.

Trevor removed many doubts with his source, because the test results were publically evident to all who cared to read, not based on anecdotal claims, persoanl testimony, or relationships invisible to others.

Getting a simple point across is hard enough for natives witting in english. Without knowing me, my engineer friend, or sharing my personal experiences, people see my written word mixed with a combination of testostrone and bravado to filter thru.

But, communicating the evidence for my claims, beyond my amusement in whipping on people, is extraordinary without some collaboration. Having my friends join in to support me may help, or make it worse, but this is easily perceived as "group think," far from the clear evidence that Trevor provided.

 

[ramsy]

If your interested here is an old thread with more info.
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=3;t=002062

Ya, I'm interested. Thanks for the link, and for looking into this.

 

[don_resqcapt19]

I wonder, how legitamite this concern of neutral harmonics, twice the phase value, is with industrial plants that involve VSD's, SCR's, & constantly changing configurations.

While there are some issues with power quality as a result of VFDs and such equipment, neutral harmonics are not common as these devices are normally 3 phase in an industrial occupancy.
Don

 

[iwire]

Bob, I had the impression you rely on engineers heavily, and don't check their figures or spec's when discussing your practices here.

I do and that would depend on who it was.

I haven't read you taking measurements to verify harmonic or v-drop spec's either.

Once in a while I specifically take VD readings, I also commonly take note of the voltages I see when taking any readings.

VD does not seem to be a big problem.

I have not measured for 'harmonic spec's'

Now big guy I have been up front with you it is now your turn.

What in the field documented evidence of damage by VD or harmonic issues can you show us?

I am not interested in what 'Copper Org' monthly might have say. :wink:

 

[Gmack]

Can I chime in here for a second?

Soft start has caused bearing "burn out" Motor OEM's have responded.

IMO, harmonics by way of electronics.

Its industry wide.

Question to me is, how could a VFD "target" bearings in the first place?

I think I will attempt to answer myself.

HEAT! on soft start or soft running you will subject the motor to less voltage/fz and more current than what it was designed for.

You know what they say, you can talk to yourself, but when you start answering :wink:

 

[ramsy]

Now big guy I have been up front with you it is now your turn. What in the field documented evidence of damage by VD or harmonic issues can you show us? wink:

After reading your VFD links, jim dungar provided this evidence for harmonic caused bearing damage. The Faraday Shield research shows .."Pulse Width Modulated (PWM) drive's electrical signature is a source of EMI emissions. ..The two forms of conducted noise are differential (line to line) and common mode (zero sequence) [12]. The common mode voltage - the dominant excitation source - excites stray or parasitic coupling capacitances and contributes to Vrg and consequential bearing current discharge [5, 8-10].

After reading this, I thought a cheap fix would delta connect those motor loads and dampen some harmonics the same way a Delta transformer does.

 

[ramsy]

Before reading about your VFD issue, I would have searched for a case of harmonic damage from any of the following;

1) harmonic-induced overheating,
2) dielectric breakdown in capacitor banks/cables/transformers,
3) nuisance tripping OCD's,
4) surge suppresser failure,
5) relay malfunctions, and signal interferance in control circuits via common electrode systems/main bondings/transmission lines,
6) over voltage/excessive currents due to system resonance.

The first common menace between V-Drop and Harmonics in my mind is Power Factor. If harmonics are present, VA increases thereby decreasing power factor.

dPF       = WATT/VA*Sqrt(?)    Displacement PF
PF(THD)   = Sqrt[1/(1+THD^2)], where THD = current harmonics.
  PF(TOT) = dPF * PF(THD)

Without getting references to harmonic damage for these events, I can only assume dielectric breakdown in capacitors can occur if installing displacement (Pf) corrections before the harmonic corrections. If I wan't to convince someone that harmonics does cause damage, the references would have to be found. Same with V-drop.

The place where I found PF effecting V-drop is in the notes to NEC Tbl.9. The AC formula for effective impedance depends on PF; Ze = r*PF+X*Sin{ArcCos(PF)}

 

[iwire]

Question to me is, how could a VFD "target" bearings in the first place?

I think I will attempt to answer myself.

HEAT! on soft start or soft running you will subject the motor to less voltage/fz and more current than what it was designed for.

In this case it is not heat.

There is (or was) current passing from the motor armature through the bearings to the motor frame.

This current caused 'fluting' of the bearing surfaces.

As an immediate fix GE Motors came out and replaced the standard bearings with ceramic coated bearings. The ceramic coating electrically isolated the armature from the motor housing.

Than a grounding brush was placed against the motor shaft to ground the now isolated armature.

This will prevent the motor bearings from damage but did not stop the stray currents.

I will find out more today.

By the way Google "bearings fluted current" and you will get a ton of info on this problem.

 

[iwire]

1) harmonic-induced overheating,

No overheating, the VFDs, motors and bearings have thermocouples and the data is logged.

2) dielectric breakdown in capacitor banks/cables/transformers,

No, Cutler Hammer and the testing company verify the VFDs are associated equipment are fine.

3) nuisance tripping OCD's,

No tripping whatsoever, the motors at full speed / load are running about 80 to 100 amps under the nameplate FLA of 550.

4) surge suppressor failure,

No, see 2

5) relay malfunctions, and signal interferance in control circuits via common electrode systems/main bondings/transmission lines,

No

6) over voltage/excessive currents due to system resonance.

I don't believe so, an outside testing / power quality company was brought in and I do not recall any talk of this.

The issue is apparently common with large VFDs and motors.

Cutler Hammer says add reactors and re-cable the job using shielded VFD cables. Presently all VFD cabling is racked in common cable trays as per the construction drawings.

I should find out more today, of course all party's involved are very busy trying to cover their rears so getting a true straight answer is somewhat difficult.

 

[don_resqcapt19]

Cutler Hammer says add reactors and re-cable the job using shielded VFD cables.

The instructions for most VFDs require the use of shielded power cable between the VFD and the motor. I have never done that or even seen it installed that way, but it is required by the instructions.
Don

 

[ramsy]

I should find out more today, of course all party's involved are very busy trying to cover their rears so getting a true straight answer is somewhat difficult.

Bob, did you find out if connecting the loads in Delta configurations ever helped problems like this, would a delta load act like a harnonic canceling Delta xfmr, or help dampen any noise at all?

 

[winnie]

Bob,

I rather doubt that the bearing problems are being caused by harmonics.

VSD devices may cause harmonics on the circuits feeding them, and also cause harmonics on the circuits feeding the motors, but these are rather different beasties. The only commonality (and the reason that both are called harmonics) is because both are distortions from the desired ideal sinusoidal current flow.

On the input side you have some sort of rectifier. This might be a dumb diode bank, or a smarter SCR based 'controlled rectifier', or a high end PFC correcting rectifier, but there is something there which takes the input power and converts it to DC for internal use. The simplest system, which uses a three phase diode bank and a capacitor filter can inject quite a bit of harmonic current flow into the mains. However these rectifiers are all line-line loads, and don't push any current onto the neutral.

On the output side of the VSD you have a set of transistors which approximate a sinusoidal output. However for reasons of efficiency they do not smoothy vary the output in a nice sinusoidal fashion. Instead they work in the 'switch mode', always switching full on, either positive or negative. There are many different types of VSD, some of which operate with very low frequency switching, where the output is essentially a square wave, others of which operate with high speed switching, and which approximate a drive frequency sinusoidal output with a much higher frequency duty cycle modulated square wave.

Because the sine wave is only approximate, the output is rich in harmonics. These harmonics will generate adverse rotating fields in the motors, reduce efficiency and cause heating.

My bet is that the stray currents and bearing damage are not being caused by harmonics as are being discussed in this thread, but instead by capacitive coupling of the high frequency switching. One might call this high frequency switching a harmonic, but it is a very different beastie from triplen currents overloading a neutral.

If you take a look at the switching pattern of an IGBT based VSD (the high frequency switching kind) you will find that for some length of time all the outputs are switched to the positive DC bus. There is no voltage difference between the terminals, so the coils don't see the voltage magnetically, but the entire motor winding is elevated to the positive DC bus voltage. Then the switching enters the modulation region, where first one and then the second of the phases gets switched negative. During this period the full DC voltage is applied across the terminals. Finally the third phase gets switched negative, and the voltage between the terminals is removed Now the entire motor winding is elevated to the negative DC bus voltage. The process then reverses with one then two then all three phases being switched to the positive DC bus. The fraction of the period when there is a voltage difference between the terminals is how you synthesize voltages that are intermediate between full positive and full negative.

As far as the current flow between the terminals, the inductance of the motor acts as quite a good filter, and even though this really ugly duty cycle modulated square wave voltage is being applied to the motor, a rather good sinusoidal current will flow.

But consider the voltage between the motor winding and ground. This voltage is jumping around as a 2-20kHz, 350V square wave. It doesn't take much capacitance for a significant current to flow. If you put a clamp around the cables feeding one of the motors, my bet is that you will see significant current flow at the inverter switching frequency.

-Jon

(For what its worth, I just read the thread on Eng-Tips. I recognize now that I am only aware of the tip of the iceberg on this topic!)