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Author Topic: Led Talks - Paralleling Two Generators  (Read 21664 times)

Guy Holt, Gaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #10 on: July 16, 2014, 07:27:14 pm »

I am not trying to be difficult-but I find there is enough confusion in this matter-especially when end users (welders, gaffers, pro sound, homeowners) start trying to speculate about how systems work that details and accuracy are vital.

I agree. Unfortunately the rule in marketing copy is brevity. It is very difficult to communicate these concepts in few words, so we have a tendency to touch on issues in a superficial fashion. Forums like this are great because it affords us an opportunity to get into the details.

Your drawing shows no neutral connection at the generator, but 250.34 (C) refers to 250.26 which requires that in a single phase 3 wire system (using neutral) the neutral must be bonded.  In a single phase 2-wire system (no neutral or your 240 volt supply) it still requires one conductor to be bonded.  This is to insure proper operation of  OPCDs-circuit breakers.  Other wise you have the potential (however unlikely) that a short will occur to ground that may or may not be adequate to trip a breaker in the event of a second fault.

You are correct. This is one of those instances where we are guilty of touching on issues in a superficial fashion.  Previously I detailed the electrical system downstream of the transformer secondary and we are in agreement that it is code compliant for over-current protection. Now let’s discuss in detail the system upstream of the transformer’s primary, but first it is important to note that transformers are what, in the NEC Parlance, is called a “separately derived” system meaning that they are a new impedance point or source of power.  As such, there is no direct electrical connection between the transformer primary and secondary windings of the transformer, which means that the electrical distribution downstream of the secondary is completely independent of the generator system upstream of the primary.  As illustrated below the only conductor that connects these two systems is the  equipment grounding conductor.


Given this fact, it would be more accurate for us to say that “by using a transformer to supply 120V loads we do not carry the Neutral of the secondary distribution system through to the generators” rather than saying that we just “drop the Neutral.” As Stephen correctly notes, by code we can’t drop the Neutral upstream of the transformer primary which is why we have the potential problem of harmonically rich cross currents burning out inverters. If you will recall,  I said above that “It is also worth emphasizing that generators are particularly prone to overheating when used in parallel operation… because when generators are paralleled their neutrals are tied together into a common neutral bus.”

In one place you say the Honda EU6500is has a bonded neutral-later you call it a generator with a floating neutral.  I'm confused?....

Your confusion stems from the fact that the neutrals of the generators are not bonded to the equipment grounding conductor of the generator or the generator’s frame – it has a “Floating Neutral.”  However, in paralleling operation the neutrals of each generator are carried through to the Paralleling Control Box and tied together into a common Neutral Bus and that Bus is bonded to the equipment grounding conductor as required by the NEC to assure proper operation over over-current protection devices.  As such, 240V loads (HMIs larger than 4kw) operate on one code compliant “separately derived” power system (i.e. the generators), while 120V loads operate on a completely independent  code compliant “separately derived” power system (i.e the secondary of the Transformer/Distro.)

… if you refer to NEC 250.34, whether or not you are required to earth ground a generator is determined by the loads (plug and cord connected) not by whether or not the generator has a floating neutral.

You are correct as far as the NEC. It is OSHA that goes a step further when they state in the “Guidelines for Grounding of Portable Generators” (below) that isolating the generator from earth ground is only permissible when the equipment grounding system, consisting of the non-current carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles, is bonded to the generator frame and the Neutral conductor is also bonded to the frame as illustrated below (use this link for the complete guidelines: https://www.osha.gov/OshDoc/data_Hurricane_Facts/grounding_port_generator.pdf).


According to these OSHA specifications, if the generator is mounted on a vehicle, the frame of the generator must also be bonded to the frame of the vehicle, and the frame insulated from making contact with the earth via it’s rubber ties (if a tow trailer a piece of wood must be under the jack and tow chains lifted off the ground.) When this is the case, the figure below, illustrates how a Floating Ground arrangement offers protection against ground faults.


1) A fault in a metal fixture energizes the entire housing as soon as the circuit is turned on.
2) Since, electricity seeks the path of least resistance back to its’ source, the bulk of the fault current will travel through the equipment grounding wire, instead of the individual making contact with the housing, because it is of a much lower resistance than the individual. The individual offers
the current a higher resistance path because it can not complete a circuit through them since the system is insulated from the earth.
3) Because the ground wire and the neutral wire are bonded at the generator bus with a grounding jumper, the current carried by the ground wire back to the generator bus creates a dead short (over-current situation). The fuse or circuit breaker pops in response.


Even with their Ground floating,  Bonded Neutral generators offer a high degree of protection. With the generator winding connected to the equipment grounding conductor, a low resistance path is established to carry fault current back to the generator winding to create an over-current situation and trip the breaker.

The link to the article in your company newsletter appears to be broken?

It appears that the auto coding  function of the forum is adding 2s and % to the front of the address. The correct address is: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorElectrical%20Hazard%20Protection

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
rentals@screenlightandgrip.com

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Stephen Swaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #11 on: July 17, 2014, 01:46:34 pm »

Thank you for the clarifications.  I do understand that often times marketing demands brevity-probably part of the reason I am not a very good salesman.  However, when I read a brochure that appears to be inconsistent, or have incomplete information, my natural reaction is to think "they are just trying to sell snake oil, what are they trying to hide?" -perhaps that is not always a fair reaction. Then too, there is always the possibility that someone will try to pirate your ideas and do something that is crazy unsafe.

Thank you for your contribution to this forum.
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Steve Swaffer

Mark Cadwallader

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Re: Led Talks - Paralleling Two Generators
« Reply #12 on: July 17, 2014, 10:11:08 pm »

Thank you for sharing your knowledge with us, Guy. A lot of your stuff is a bit over my head, but I'll keep reading and learning. Good stuff in there. Mark C.
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Mike Sokol

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Re: Led Talks - Paralleling Two Generators
« Reply #13 on: July 17, 2014, 10:24:12 pm »

Triplen harmonic currents do in fact exist in the neutrals of single-phase systems.

Guy, while I understand that both even and odd-order harmonic currents do indeed exist in single and split-phase systems, especially those using Triac/SCR dimmers, I'm wondering if you're misusing the term "triplen" in explaining your technology. From my understanding, triplen currents are a 3-phase phenomenon which is a result of the 120 degree phase shift between the three legs. This is what allows the odd-order harmonics (mostly 3rd harmonics of 180 Hz) to be additive rather than subtractive as in any standard split-phase (180 degree) system. And those tripen currents are what cause neutral overheating.

So while your system might cancel harmonic currents of both even and odd flavor, I don't think these are technically "triplen" currents. Or maybe I'm off base. Please elaborate.
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Frank DeWitt

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Re: Led Talks - Paralleling Two Generators
« Reply #14 on: July 18, 2014, 01:56:52 pm »

I am me not him (Grin) but I thought the same thing.  I looked it up and found that the word refers to the 3rd harmonic, not to 3 phase.

Triplen Harmonics – The triplen harmonics are defined as the odd multiples of the 3rd harmonic (ex. 3rd, 9th, 15th, 21st etc.). Triplen harmonics are of particular concern because they are zero sequence harmonics, unlike the fundamental, which is positive sequence. The consequence of this fact is that the magnitude of these currents on the 3 phases are additive in the neutral. This can lead to very large currents circulating in the neutral, and unless the neutral is sufficiently oversized this can present a fire hazard. These currents can also circulate in the transformer causing significant overheating there too. Single-phase power supplies for equipment such as electronic ballasts and PCs are the most significant source of Triplen harmonics.
http://www.hersheyenergy.com/harmonics.html

 Triplen harmonics

Electronic equipment generates more than one harmonic frequency. For example, computers generate 3rd, 9th, and 15th harmonics. These are known as triplen harmonics. They are of a greater concern to engineers and building designers because they do more than distort voltage waveforms. They can overheat the building wiring, cause nuisance tripping, overheat transformer units, and cause random end-user equipment failure.
http://ecmweb.com/content/fundamentals-harmonics

And
http://www.theiet.org/forums/forum/messageview.cfm?catid=226&threadid=43165
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Guy Holt, Gaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #15 on: July 19, 2014, 02:55:50 pm »

I'm wondering if you're misusing the term "triplen" in explaining your technology. From my understanding, triplen currents are a 3-phase phenomenon which is a result of the 120 degree phase shift between the three legs. This is what allows the odd-order harmonics (mostly 3rd harmonics of 180 Hz) to be additive rather than subtractive as in any standard split-phase (180 degree) system. And those tripen currents are what cause neutral overheating.
 

I don’t believe I am misusing the term triplen.  You are correct that it is commonly understood to pertain to the harmonic currents that do not cancel when dumped into the neutral conductor of a three-phase service.  But if we look at the origin of  the term in the mathematical model developed by the engineer Charles Fortescue in 1918 to make asymmetric fault analysis tractable, we see that it reflects something more.
 

Where the math involved in comparing two or more alternating waveforms gets pretty complicated, Fortescue developed a spacial model, called the rotating power vector, that makes it much easier to visualize angular or phase differences between two or more sinusoidal waveforms.


Power vectors are a simple means of representing the position of an alternating quantity, be it voltage (V) or current (I), at some particular instant in time using the Phasor Diagram depicted here.


A vector representation of the relationship of Voltage and Current that are out of phase by 30 degrees

Power vectors are also a very effective means of modeling alternating waveforms, especially when we want to analyze the interaction of two or more different waveforms on the same time axis, such as the relationship between the out of balance legs of a three phase service.

According to Fortescue’s model any unbalanced set of 3-phase phasors (rotating vectors) could be represented by the sum of three sets of balanced phasors. (Balanced phasors are much easier to work with mathematically.) He called these the positive, negative and zero sequence phasors. As illustrated in the Power Point slide below, the phase shift of harmonic currents can be broken down into these three categories.
 

Harmonics such as the 7th, which “rotate” with the same sequence as the fundamental, are called positive sequence.
 
Harmonics such as the 5th, which “rotate” in the opposite sequence as the fundamental, are called negative sequence.
 
Harmonics which don't “rotate” at all because they're in phase with each other, are called zero sequence (3rd and 9th shown in this table.)
 
By definition the positive, negative, and zero sequence phasors are of equal amplitude between legs.
 
As such, any unbalanced set of 3-phase phasors can be represented as the sum of these three sets of balanced phasors using Euclidian Geometry as illustrated below.


When we think in terms of these three categories of harmonic currents (positive, negative, and zero), we notice something interesting in the example of the Image 85s above.  When we dumped the current from an Image 85 on each leg into the “stew pot” of the neutral, the out of phase current cancels. That is the (+) sequence harmonics (4th,7th, etc.) cancel each other out. The (-) sequence harmonics (2nd, 5th etc.) cancel each other out. The zero sequence harmonics (3rd, 9th, etc.) do not cancel each other out.  Instead they add for the reasons discussed above.

The 3rd, 9th, & 15th  harmonics are called the triplen harmonics, not because there are three of them in the Neutral of a 3-phase system, but because you can divide them by three and come up with an integer. In fact, if you look at the sequence of positive, negative, and zero sequence harmonics in the chart above, you will note that all of the zero sequence harmonics (no matter how high the order) are integer multiples of the 3rd which makes the 21st harmonic also a triplen.  Because of their bad-boy behavior as zero sequence harmonics in the Neutrals of three-phase systems, Electrical Engineers gave all of the zero sequence harmonics (not just the 3rd, 9th, & 15th) a nickname that denotes their notoriety: “the troublesome triplens,” which was eventually shortened to just ”triplens.”    Even though they gained their notoriety as zero sequence harmonics in three phase systems,  the 3rd, 9th, & 15th triplens exist also in the Neutral return of single-phase systems as well.


Left: harmonics making up Neutral current generated by Image 85s in a single-phase system. Right: harmonics making up Neutral current generated by Image 85s in a three-phase system.

To use a historical analogy, when William McCarty, Jr. was buried in July of 1881 at the Fort Sumner cemetery between his two friends, Tom O'Folliard and Charlie Bowdre, many of those in attendance knew him first as William McCarty and not “Billy the Kid.” Even though he gained notoriety for his crimes under the moniker of “Billy the Kid”, it was William McCarty’s body lying in the casket.
 
In the short hand of marketing copy, are we guilty of exploiting the notoriety of the 3rd .9th, & 15th triplen harmonics? To a degree, but that doesn’t change the fact that the 3rd .9th, & 15th harmonics exist in the Neutrals of single-phase systems and that they contribute significantly to the generation of heat in the common Neutral of paralleling generators.

On a level, we are quibbling over semantics. Charles Fortescue and Jean Bapiste Joseph Fourier (the 19th century mathematician that developed the Fourier Analysis) developed means of mathematically modeling non-sinusoidal waveforms.  Their models enable us to describe how non-sinusoidal waveforms combine and recombine. But it is important to always remember that their mathematical models are just that: models.  They are two lenses through which we can analyze and predict the combination and recombination of non-sinusoidal waveforms. Behind the math we are simply dealing with the shape of waves.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
rentals@screenlightandgrip.com
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Guy Holt, Gaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #16 on: July 21, 2014, 03:36:12 pm »

I do know a lot about triplen harmonics, having blown up the power for an entire million square foot warehouse and packaging plant back in the 70's. Ouch...  ::)

If you are looking for a way to eliminate elevated triplen current on the Neutral of 3-phase systems, I would suggest you look into using Harmonic Mitigating Transformers (HMTs). HMTs are specifically designed to minimize the high neutral returns, voltage distortion, and power losses that result from the harmonics generated by non-linear loads like the electronic power supplies of HMIs, Kinos, and LEDs, as well as by Triac/SCR Dimmers. By means of a zig-zag secondary winding and phase shifting, a Harmonic Mitigating Transformer can accomplish substantial improvement in power quality.


For example the power quality reading on the left above is the Neutral return with one Image 85s on each leg of a three phase system. The Image on the Right is the same situation but with a HMT in–line. As you can see in this example a 30 degree angular displacement of the three phases in combination with secondary zig-zag windings results in a 26x reduction in the amount of current carried on the neutral and an even greater reduction in harmonics.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
rentals@screenlightandgrip.com
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Mike Sokol

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Re: Led Talks - Paralleling Two Generators
« Reply #17 on: July 21, 2014, 08:56:31 pm »

Guy,

I'm sorry I haven't read all your postings at length, but I'm recovering from eye surgery last Monday, so I'm having trouble reading computer screens. However, I've been mulling over the idea of triplen harmonic currents in a single-phase system (actually, 2-phase with 180 degrees on each leg) and wonder if 3rd order neutral harmonics are relevant on single/two phase systems. That is, if this was an old 2-phase 90 degree system like originally existed at Niagara Falls, or one of the other experiments of up to 4, 5 and 6-phase systems, wouldn't there be other harmonics involved in non-cancelling neutral currents other than 3rd?  My point is that while triplen currents are important in 3-phase systems because of neutral current buildup, and triplen currents exist in split phase 120-240 systems,  why would they create additive current buildup on legs with a phase shift of 180-degree?

In any event, I'll try to read over everything and go through the math in the next few days once my vision clears up, but I'll have to keep doing this in my head until then.     
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Mike Sokol

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Re: Led Talks - Paralleling Two Generators
« Reply #18 on: July 24, 2014, 06:51:02 pm »

Guy,

I've looked over all your postings on this thread, and really want to thank you for some very solid information that everyone on this forum can use and benefit from. While some of it may be over the heads of newbies here, I think I can speak for everyone that all your postings were concise and on point. Thanks for being our first guest on the Led Talks. I'm going to let you have the last word before locking this thread.

Once again, thanks for participating on the ProSoundWeb Led Talks, and we wish you success with your business.
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Mike Sokol
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Guy Holt, Gaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #19 on: July 25, 2014, 11:44:51 am »

I'm going to let you have the last word before locking this thread. Once again, thanks for participating on the ProSoundWeb Led Talks, and we wish you success with your business.

Mike,

Thank you for inviting me to contribute to this discussion and for the opportunity to have the last word.  I would like to take this opportunity to respond to a couple of comments related to this thread that were raised in another thread: “Re: EU-6500 running 3500 watts of sound - Volume lowers during heavy bass”.

A a major question mark for me regarding triplen harmonics in single phase systems.  Guy offered no emperical evidence because they don't believe in setup that might cause them… I am reminded of the man standing on a street corner in a midwestern US city waving a pitch fork around.  When asked what he was doing he replied, "I'm keeping he elephants away, doing a good job ain't I?"  You need to show me the elephants before you prove that you got rid of them.

While I couldn’t show you the bull elephant because we don’t have a splitter box large enough to power a 4k non-PFC HMI on one leg of the 240V single-phase service (It would require a 60A/120V circuit) I did show you a baby elephant in the form of the 1.2kw non-PFC HMI.


(Note the yellow Splitter Box on the left provides only two 20A circuits from the 30A/240V receptacle on the generator.
By comparison our 60A Transformer/Distro provides the 3 – 20A circuits of the black gang box in the center and a 60A circuit
from the same 30A/240V receptacle.)

As you may recall, in my post above, I compared the cross currents circulated between two generators before and after a 1.2kw HMI load (1.2kw is the largest light we can power on one leg of the splitter box we have.)


(The 3rd Harmonic Content of neutral cross current with no load.)

As you can see in the power quality meter reading above, there is approximately 2.08A of cross current circulating between the two machines on a continuous basis even without a load. Of that 2.08A nearly a quarter of it (.39A) consists of 3rd harmonic current.  As you can see in the power quality meter reading below, the 1.2kw HMI added an additional 4.11 Amps of 3rd harmonic current to the continuously circulating cross current.


(Powering a non-PFC 1200W HMI by means of a splitter box increases the 3rd Harmonic current
circulating between the two generators by a factor of 150X.)

This 3rd harmonic content generated by the 1.2kw HMI is problematic because it did not cancel with the 3rd harmonic of the cross current generated by paralleling the generators. Where before, with a linear load, the 3rd harmonic made up 4% of the current circulating on the combined neutral, with the contribution of the 1.2kw HMI, the 3rd harmonic makes up nearly 60% of the current. Not a big elephant, but an elephant all the same.


Left: amplitude of 3rd harmonic content of non-pfc 4k HMI. Right: amplitude of 9th harmonic content of the same non-pfc 4k HMI.

From these results we can infer what would happen were we to power a 4k HMI on one leg of a larger splitter box without a corresponding  non-linear load on the other leg to cancel it (a common situation in motion picture lighting.) As can be seen in the power quality readings above,  in a situation where a non-PFC 4k HMI is operating on one side of a splitter box, significant 3rd, 5th,7th , and  9th harmonic currents will be contributed to the neutral by the 4kw HMI with possibly no harmonic current contribution by incandescent lights on the opposing leg to cancel it out. The net result will be nearly 30 amps of primarily harmonic currents circulating continuously on the neutral with 94% of that consisting of Triplen harmonics (3rd= 26.2A, 9th= 2.1A.) Now that is a bull elephant for the reasons I will explain below.

I'm still not convinced that triplen harmonics in a 120/240-volt split phase system will contribute to overheating in the neutral conductors… However, I'm going to discuss this with one of my EE buddies.

While the 10 AWG neutral conductor of this system could handle (but just barely) a 30A cross current at 60Hz, it is an altogether different situation when the current includes higher frequencies. Similar to the “Eddy Currents” that will circulate in the secondary of a Delta/Wye transformer, cross current that consists primarily of harmonic currents will cause overheating of the Neutral (copper losses) and the generator’s inverters. The relationship of this “Eddy Current” heat loss to harmonic frequency is as follows:


(Where: PEC = Total eddy current losses, PEC-1 = Eddy current losses at full load based on linear loading only.

Ih = rms current (per unit) at harmonic h , and h = harmonic # )

What is significant about the relationship of Eddy Current heat loss as a result of harmonic currents expressed in this equation is that the harmonic current (Ih) and harmonic number (h) are squared which means that instead of increasing in a linear fashion they increase exponentially. Put another way, the heat generated by harmonic currents just doesn’t increase gradually at higher harmonic frequencies, but it jumps drastically. For example, it doesn’t take much 21st harmonic current (which by the way is a triplen) to generate heat when, according to the equation above, the harmonic frequency (h) is squared for a multiplier of 441x (212). For this reason the heat generated by the harmonics returned on the neutral of even a perfectly balanced single-phase system should not be taken lightly in paralleling set-ups.

It is the fact that the neutrals of generators in a paralleling set-up are tied together in a common bus, so that these harmonic currents circulate continuously, that make them much more hazardous than even those in the neutral of a 3-phase system. It must be emphasized that generators are particularly prone to overheating when used in parallel operation. That is because when generators are paralleled their neutrals are tied together into a common neutral bus and so the harmonically rich cross current generated by a non-PFC 4k HMI (consisting of not only the 3rd harmonic, but also the 5th,7th, & 9th) will circulate continuously between the two generators, causing heat to build up to a catastrophic level. When he talks to his EE friend, Mike needs to emphasize that we are not talking about how “triplen harmonics in a 120/240-volt split phase system will contribute to overheating in the neutral conductors”(as he states above),  but rather how triplen harmonics in the continuously circulating cross current of  120/240-volt split phase systems of paralleling generators (whose neutral buses are tied together) will contribute to overheating in the neutral conductors and inverters. Which is a completely different situation than that of the 120/240-volt split phase system of a single generator.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
rentals@screenlightandgrip.com

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Re: Led Talks - Paralleling Two Generators
« Reply #19 on: July 25, 2014, 11:44:51 am »


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