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Author Topic: Led Talks - Paralleling Two Generators  (Read 21653 times)

Guy Holt, Gaffer

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Led Talks - Paralleling Two Generators
« on: July 15, 2014, 08:36:33 am »

Of course generators in parallel mode are at 0 degree phase angle (so no triplen harmonics), and even if they were running at 180 degrees for a true 240/120 volt output, there's still no triplen currents.

Triplen harmonic currents do in fact exist in the neutrals of single-phase systems. In paralleling applications, the most troublesome of the triplen harmonics, the 3rd, can even exist when there is no load. Since some of these terms might be foreign to your readers, let’s start with some basics: there is available on the web a very informative article by Gary Olson, Technical Support for Cummins Power, on “Paralleling Dissimilar Generators.” To summarize that article: the voltage waveform shape created by a generator is not an ideal sinusoid and no two machines are the same. Furthermore, its' shape is also effected by its' load. The resulting waveform may be described in terms of its fundamental frequency and voltage magnitude and the magnitude of the harmonic voltages and their frequencies that make it up. This harmonic voltage distortion, while small in the case of inverter generators, may still be significant, particularly in paralleling applications.


(Harmonic Content of a Generator's Voltage Waveform)

The illustration above shows the relationship of first-order (fundamental frequency waveform) to third- and fifth-order harmonic waveforms of the resulting waveform. The harmonic voltages are effectively added to the fundamental waveform, resulting in the pure sinusoidal shape of the fundamental being somewhat distorted. For example, the resultant voltage at time A in the figure above will be the sum of the blue (fifth-order), green (third-order), and red voltage magnitudes. So, the instantaneous voltage at that instant in time would be somewhat higher than the voltage of the fundamental.

When generators are paralleled, the voltage of the two machines is forced to the exact same RMS voltage magnitude. Differences in the harmonic make up of the voltage waveforms result in current flowing in the common neutral conductor between the two machines even when there is no load. This is referred to as circulating neutral current (also called “cross current.”) Its’ source is illustrated below.


(How 3rd Harmonics are generated)

In this illustration, two voltage waveforms of the same RMS value (the red and blue lines) are superimposed upon each other. Note that even though these voltage waveforms have the exact same RMS magnitude (they would read the same on a true RMS meter), at different points in time the blue voltage is higher than the red, and vice versa. Since there exists potential (voltage) between the two machines at these points, when the machines are connected together on a common bus, current will flow between the machines (cross current) even if there is no load. Note that because the blue and red voltage lines cross each other three times in each half cycle, the cross current includes a 3rd harmonic component (this current is represented by the green line.)

The process of mathematically deriving the frequency components of a distorted periodic waveform is achieved by a technique known as a Fourier Transform. Microprocessor based test equipment, like power quality meters, can do this mathematical analysis very quickly using a technique known as an FFT (Fast Fourier Transform) which it displays as a bar graph. Each bar represents the magnitude of a harmonic frequency, be it voltage or current. Below is a power quality meter reading of the neutral cross-current that circulates between two Honda EU6500s operating in parallel mode with no load.


Even with no load on the generators (not even our transformer/distro) there is roughly two amps of neutral current with a significant 3rd harmonic component. So, there do exist zero sequence triplen currents (the 3rd harmonic) when generators are operating in parallel at 0 degree phase angle. And because the neutral systems of the two machines are tied together into a common neutral bus, this cross current will continuously circulate (as illustrated below) between the two generators.


(The illustration above is for a 3-Phase System - the same principles apply to Single-Phase Systems)

Though not significant without a load, this cross current can become a problem under certain circumstance if we add to it the triplen harmonics dumped into the neutral by non-linear loads such as non-power factor corrected HMIs, Kinos, & LED lights. Because these harmonic currents (the triplens and the 3rd harmonic of the cross current) are in phase with one another they do not cancel in the neutral as fundamentals do, but instead build one on the other to create elevated cross current with a large 3rd harmonic component that circulates continuously on the neutral conductors. And, because the elevated 3rd harmonic cross current is at a higher frequency (180Hz) it generates a lot of heat, resulting in the overheating of conductors and the generator's inverters (more on this latter.)

I believe what gets Mike’s “bullshit-o-meter” reading high is the commonly held assumption that zero sequence triplen harmonic currents can’t exist in single phase systems as they do in three phase systems – they do. Under balanced load conditions they are minimal, but in out of balance conditions they can be significant. Let’s start with where this assumption comes from.



In a power quality workshop I developed for IATSE Local 481 (New England Studio Mechanics) I demonstrate the source of elevated neutral return currents using a non-linear load that is used by the hundreds on motion picture stages for CGI production: the Kino Flo Image 85.


As you can see in the Power Point slide from the workshop above, Image 85s are rich in harmonics with a THD of 77% and a large triplen component (3rd, 9th, & 15th.)


In the workshop I demonstrate how and why these non-linear lighting fixtures can lead to elevated neutral returns in 3-phase systems by watching what happens on a power quality meter as fixtures on separate legs of the service are turned on one at a time (set-up pictured above)


As you can see in the power quality readings above, absent phases B & C, the Image 85 on phase A returned 9A to the neutral (left picture.) When the Image 85 on phase B is switched on, and it returns another 9A to the neutral conductor, the current on the neutral climbs to 12.71A (center picture.) And, finally, when the Image 85 on phase C is switched on, and it returns another 9A to the neutral conductor, the current on the neutral climbs to 15.86A (right picture.)  Even though the three phases are perfectly balanced (9A on each) the current on the neutral is 176% of any one of the individual phase legs. Clearly, there is some cancellation between the phase legs going on (otherwise the neutral would be carrying 27A), but it is not complete cancellation. Why?

When we dump return current from each leg into the “stew pot” of the common neutral, out of phase current cancels. The Fundamentals (A1,B1,C1 ) cancel each other out. The positive sequence harmonics (4th,7th, etc.) cancel each other out. The negative sequence harmonics (2nd, 5th etc.) cancel each other out. But, the zero sequence harmonics (3rd, 9th, 15th, etc.) do not cancel each other out.  Instead they add. Why?

If, for a moment, we consider only the 3rd harmonic (180 Hz) of each phase as they return on the neutral, you will notice in the illustration below that in each positive half-cycle of any of the fundamental waveforms, there are exactly two positive half-cycles and one negative half-cycle of 3rd harmonic. 


The net result, as illustrated above, is that the 3rd-harmonic waveforms of three 120 degree phase-shifted fundamental-frequency waveforms are actually in phase with each other and so stack on one another rather than cancel out as the fundamentals, positive, and negative sequence harmonics do. The phase shift figure of 120 degrees generally assumed in three-phase AC systems applies only to the fundamental frequencies, not to their harmonic multiples. A closer look at the harmonic currents making up the neutral return of our demonstration setup reveals that, though made up primarily of the third harmonics from each phase stacking one on another, the high neutral current also consists of the, 9th, and 15th harmonics from each phase stacking one on another as well.

Due to their significance in three-phase power systems, the 3rd harmonic and its zero sequence multiples have their own special name: triplen harmonics.  All triplen harmonics add with each other in the neutral conductor of a 4-wire wye-connected load. In power systems containing substantial nonlinear loading, the triplen harmonic currents may be of great enough magnitude to cause neutral conductors to overheat. If returned to a generator they are induced into the generator’s Stator & Rotor coils where they circulate until dissipated as heat.


As is evident in the illustration above, it is an altogether different situation in single-phase systems. Since the phase angle between legs is 180 degrees rather than 120 degrees, 3rd harmonic currents are also out of phase and will for the most part cancel just as the Fundamentals do. This is evident in the power quality meter reading below of the neutral of a single-phase system for first just one Image 85 on one leg (left) and then a 2nd Image 85 on the other leg which is 180 degrees out of phase (right.) As you can see here, when the two legs of a single-phase system are perfectly balanced (9 Amps on each) the triplen currents generated by a non-linear load will nearly cancel out – but not completely.


It is worth noting that the total harmonic distortion (THD) of the single-phase neutral current (98.3%) is as high as the three-phase neutral current (100%) – it just consists of higher order harmonic currents (other than the 3rd) at lower amplitudes. As we will see in a moment, even at these low amplitudes, the higher order harmonic currents will contribute to inverters overheating in paralleling operation.

While not canceling completely in the neutral, the triplen currents generated by non-linear loads do nearly cancel out. For this reason the harmonic currents generated by Cailen’s rig (switching power supply amplifiers, switching power supply console, powered wedges, and LEDs) may or may not elevate the cross current to dangerous levels – it depends on how evenly he loads his system. How much is too much will be discussed in a moment, but one thing is certain, because motion picture lighting crews, more often than not, can’t evenly load their systems the harmonic content of cross current reaches hazardous levels.
 
Things like this get built to meet a particular need, not to sell bullshit to the unsuspecting.


The segment of the motion picture production industry using Honda generators (regional commercial spots, historical documentaries, and indie films - the market for which our system is designed), quite often are using them to power just one big light while they plug all their small lights into the location house power. The reason for this is that, as long as there is a sun and moon in the sky there is a need for a large HMI on interior and exterior sets because small HMIs, Kino-Flos, & LED light panels that can be plugged into wall outlets don't come close to balancing direct sunlight in day light scenes or covering deep background in night scenes. The go to fixtures in these segments of the market are HMIs ranging from 1.2 to 4kw. In this country the majority of HMIs in this range are not power factor corrected (as they are in Europe) and so they generate significant harmonic currents. For example, the power quality meter readings below are the distorted voltage and current waveforms created by a 4k HMI with non-PFC ballast operating on an EU6500is and their corresponding Fourier Transforms (note that the harmonic currents encountering the impedance of the generator create harmonic voltages at the same frequency.)
 

(L-to-R: Test Set-Up, Distorted Voltage (top) and current (bottom) waveforms, Corresponding Fourier Transformations (Voltage left and Current right)

These low budget productions compound the problem presented by the harmonics generated by large non-PFC lights by operating them on a single leg of a “Splitter Box.”


The Splitter Box pictured above breaks out the 240V power of a generator into two circuits
with film style “Bates” receptacles as well as Edison receptacles.

Since Honda manufactures the EU6500 super quiet generator primarily for RV/home standby power, and not the film lighting market, their power output panels are not compatible with the larger motion picture lights. For this reason lighting rental houses have had to find ways to work around the limited power distribution panel that Honda puts on these generators. They do so by wiring custom distribution panels called “Splitter Boxes” that access more 120V power from the 240V twist-lock receptacle on the generators. While this approach worked well enough when the lighting loads placed on generators consisted predominantly of incandescent lights (a linear load), Splitter Boxes are inherently unsuitable to carry non-linear loads consisting predominantly of non-power factor corrected HMIs. To understand why this is the case, we must first appreciate why 240V circuits are provided on the generators in the first place (it is not to power motion picture lights) and how they work.

240V outlets are on generators to power common residential or industrial single-phase 240V loads. The most common are air conditioners, dryers, ranges, heaters, large motors, and compressors. If you look at the breaker of a 240V circuit on a building service panel that serves these loads, you will notice that they use two pole breakers - either 30A or 50A. Each pole of the breaker is in a sense an independent 30A or 50A 120V circuit. That is, if you measure the voltage from each pole of the breaker to ground it will be 120 volts, and if you measure the voltage between the two poles of the breaker you will notice that it is 240 volts. As illustrated below, the 120 volts of the two poles add up to 240V because the 120V circuits are on opposing legs of a single-phase service and 180 degrees out of phase of each other. In residential settings, this is how higher voltages are supplied to household appliances like dryers, electric ranges, air conditioners, motors, etc. that require more power than can be reasonably supplied by a single 120V circuit.


The voltage of opposing legs of a single-phase circuit add while the current carried on the legs subtract.

Many of these household 240V receptacles, in fact, use a three-wire system (Hot, Hot, Ground, but no Neutral) because they are designed to power single-phase linear loads (like heating elements) that draw a perfectly balanced load and return no current.  Other 240V circuits use a 4-wire system (Hot, Hot, Ground, Neutral.) They include a single Neutral wire to provide a safe return for small 120V accessories in stoves and dryers like oven lights, clocks, and timers that throw off the balance. There need be only one Neutral wire for the two Hot wires because under normal applications the current on the Neutral is the difference between the Hot legs because of the phase cancellation. Since oven lights, clocks, and timers don’t draw much power, there is minimal current to return on the Neutral under normal applications. For this reason, the Neutral wire is typically the same size (10 Awg in this case) as that of the hot legs (remember the heating element of the stove/dryer operates a perfectly balanced single phase load and hence there is no return for the wire to carry from it). To service these same residential and industrial 240V loads, the 240V receptacles of portable gas generators are wired in a similar fashion with just one Neutral of the same size as the two hot legs (see wiring schematic below).


Generator Wiring Schematic

A Splitter Box works around the limitations of the generator power output panel, and provides additional 120V circuits, by splitting out the two 120V circuits that make up the 240V outlet. Splitter Boxes worked well enough back when the load on the 240V circuit consisted of only incandescent lights. As long as you roughly balanced your load between the two legs of the generator, phase cancellation between the legs resulted in the Neutral return being the difference between the legs and without a harmonic component. Things get a bit more complicated with non-linear lighting loads, especially in paralleling applications.

As discussed previously, harmonic currents (particularly the 3rd) are troublesome. When they are dumped into the return of even a balanced single-phase distribution system, they do not entirely cancel and in fact add to the 3rd harmonic component of the no-load cross current that circulates continuously between the generators. And, when the load includes a large non-PFC HMI fixture that throws the system out of balance, the 3rd harmonic component contributed by the load, on top of that inherent in the no-load cross current, can result in cross current with a severely elevated 3rd harmonic component circulating between the generators.

I can’t demonstrate what would happen if one ran paralleled generators in an out of balance condition by operating the non-pfc 4k HMI mentioned above on one side of a single phase Splitter Box because, not being a big believer in Splitter Boxes, we don’t have any in our rental inventory. But, I can give you an idea of what would happen by running comparably size linear and non-linear loads on one side of a smaller Splitter Box on one generator.


(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.)

Let’s compare the cross currents circulated by two common loads (a 2kw quartz Fresnel and a 1.2kw HMI Par) under two different situations: when power is supplied by a Splitter Box with a common neutral (the yellow distro box on the left in the picture above) and when it is supplied by one of our Transformer/Distros without a neutral (the 60A Transformer/Distro supplying a 60A gang box on the right side of the picture above.)


(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 picture below, powering a 2kw Quartz Fresnel by means of a Splitter Box does not add to the 3rd harmonic content of the cross current because it is a linear load.


(The 3rd Harmonic Content of cross current is not increased by the addition of linear loads
like our 2kw Quartz Fresnel)

But, as is evident in the power quality meter reading below, it is an altogether different situation when the paralleled generators are powering a non-power factor corrected 1.2kw HMI. Even though the 1.2kw HMI draws roughly the same current as the 2kw Fresnel, because it is a non-linear load rich in harmonics it adds 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 will 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. And, because a greater percentage of this elevated cross current is at a higher frequency, it generates more heat (more on that latter.)


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

If this is the result of powering just one non-PFC 1.2kw HMI, powering a 4kw HMI on one side of a Splitter Box, without an equally large non-linear load on the other side to cancel it, will create severely elevated 3rd Harmonic current circulating between the two generators. 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 no harmonic current contribution by the incandescent lights 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.)

The theory makes sense to me but they don't discuss the ampacity of the loads created by their indicated conditions, and I question whether it is enough to matter....

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 # )


(Note: the following passage was edited on 7/17 to improve its' clarity) 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. In this discussion, we are focusing on the 3rd harmonic because, between the inherent no-load 3rd harmonic component of cross current and the contribution by non-linear loads, it can reach elevated levels. But, it is also worth noting that, as indicated in the equation above, higher order harmonics will generate as much heat as the 3rd, but at much lower amplitudes. For instance, according to the equation above 1 amp of the 5th harmonic will generate as much heat as 1.7 amps of the 3rd, and 1 amp of the 9th harmonic will generate as much heat as 9 amps of the 3rd. For this reason, the higher order harmonics (5th, 7th, 9th, etc.) of the generator cross current will also contribute to inverters overheating.

 

In fact, if we run the numbers on the heat generated by all the odd harmonics (the 3rd, 5th, etc., all the way through the 21st harmonic) returned on the Neutral by Image 85s operating on a single-phase service (see power quality meter reading above left) to that generated by the elevated 3rd harmonic of Image 85s operating on a three-phase service of the neutral return (above right), all the odd harmonics, because they are at much higher frequencies, will generate just over half (54%) the heat as that generated by the 3rd in a three phase system even though they are considerably lower amplitudes. It doesn’t take much 21st harmonic current to generate heat when, according to the equation above, the harmonic frequency (h) is squared for a multiplier of 441x (212) compared to the multiplier of 9x (32) for the 3rd harmonic. 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.


(With a common neutral bus, cross current circulates continuously between generators operating in parallel)

It is also worth emphasizing 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 (as illustrated above), causing heat to build up to a catastrophic level.
 

(As the illustration of the prototype of Honda’s paralleling kit for the new EU7000 indicates, the generators are
meant to provide only 240V power in paralleling operation.)

How much is too much harmonic content in cross current for inverter generators in parallel operation?  Honda is very tight lipped about their research, but we can infer from their design of the new paralleling capable EU7000is that even at low levels, harmonically rich cross current will cause inverters to burn out over time. For this reason, in the design of their new paralleling EU7000is, Honda incorporates noise filters where there are none in the EU6500is which was not designed for paralleling operation (see wiring schematic comparison below for details.)


(A comparison of the red shaded areas of the wiring schematics of a EU6500is (top) verses that of a EU7000is (bottom)
indicates that Honda engineers deemed it necessary to incorporate noise filters in the EU7000 even though it is meant to provide
only 240V power in paralleling operation.)

It is worth noting that Honda’s paralleling system for their new EU7000is supplies only 240V power (Honda’s prototype paralleling box, illustrated above, includes only a 50A/240V Twist-lock receptacle.) That Honda’s engineers deemed it necessary to incorporate noise filters in generators that are designed to provide power to only 240V loads in parallel operations, loads that by their nature will not return harmonic currents on the neutral, is an indication of how damaging to inverters harmonically rich cross currents can be even at low amplitudes.

(Note: the following passage was edited on 7/17 to improve its' clarity) One way to avoid the problems associated with even low-level harmonic currents continuously circulating on the Neutral is to simply break the link between harmonic generating 120V loads that require a Neutral and the Neutrals of the paralleled generators. For this reason, in our system we use a 240V-to-120V step-down transformer to supply 120V loads. Since transformers are, in the parlance of the NEC,  a “separately derived” system (meaning they are a new source of power) there is no direct electrical connection between the transformer’s primary and secondary windings, 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.


By removing the Neutral connection between the harmonic generating 120V loads downstream of the transformer’s secondary and the generators upstream of the transformer’s primary, we remove the path on which, not only the most disruptive current (the 3rd harmonic) can flow in a paralleling set up, but also higher order harmonic currents as well. On our system, HMIs 6kw and larger operate single-phase 3-wire (Hot, Hot, Ground, No-Neutral) and hence return no harmonic currents to circulate as cross current. To operate 120V lights that require a Neutral (HMIs smaller than 6kw, Kinos, and Quartz lights), you would use either our 60 or 84 Amp Transformer/Distro to step-down the combined 240V of the generators to 120V as illustrated below. As such, 240V loads (HMIs larger than 4kw) operate on one “separately derived” power system (i.e. the generators), while 120V loads operate on a second completely independent “separately derived” power system (i.e the secondary of the Transformer/Distro.)


(Our Transformer/Distros isolate triplen harmonics from the generators so that they can not elevate cross current to a hazardous level.)

Put another way, the delta/wye configuration of our Transformer/Distros both create the required Neutral connection for smaller 120V loads, but at the same time isolate the generators from the 3rd harmonics created by these loads that would otherwise lead to elevated cross current, hot conductors, and overheated inverters. The harmonic currents generated by small HMIs, Kinos, & LEDs will cause heating of the primary of the Transformer/Distro (it can take it - it's a K-rated transformer), but the disruptive effect of their flow in the system Neutral of the paralleled generators is eliminated so the generator inverters remain cool. By eliminating a neutral connection between the gen-set bus and the loads, a Transformer/Distro eliminates the reason (overheating by harmonic currents) that prevented the successful parallel operation of EU6500s for 120V loads.


(Powering the same non-PFC 1200W HMI by means of a Transformer/Distro instead virtually eliminates
the 3rd Harmonic current circulating between the two generators.)

For example, when we remove the path on which the disruptive 3rd harmonic current generated by the 1.2kw HMI in our example above can flow back to our paralleling set-up by using our Transformer/Distro to power the light instead of the Splitter Box, the 3rd harmonic content of the cross current (as can be seen in the power quality meter reading above) is substantially reduced (by a factor of 150x) and so the generator’s inverters will operate a lot cooler and not melt down. By eliminating a neutral connection between the gen-set bus and the loads, a Transformer/Distro eliminates the reason (overheating by harmonic currents) that prevented the successful parallel operation of EU6500s for 120V loads - making it now possible to operate more lights, or larger lights, on portable Hondas than has ever been possible.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
rentals@screenlightandgrip.com
« Last Edit: July 17, 2014, 06:15:31 pm by Guy Holt, Gaffer »
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Mike Sokol

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Re: Led Talks - Paralleling Two Generators
« Reply #1 on: July 15, 2014, 08:52:29 am »

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

lightsne - this is great stuff, but according to the rules of the forum you need to post using your real name. So please go to your control panel and make that change. And welcome to the forum.
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Mike Sokol
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Guy Holt, Gaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #2 on: July 15, 2014, 09:00:34 am »

Aspects of safety, warranty, and liability aside, I can see a few reasons to go for parallel/synchronized generators:

I would like to expound upon several of Jonathan’s points. First:

  • Size: you may need to transport the generators in a vehicle where two small gennys can fit but a larger one won't.

For indie movie productions, one of the biggest budget obstacles to obtaining good production values is the cost of an 18kw HMI Fresnel and the blimped movie generator to power it. Even though the blimped diesel tow plant required to power an 18k HMI are not that expensive (about $795), they come with hidden costs that put the production values that can be gained with a large HMI beyond the reach of most indie productions.

Since rental trucks like those from Ryder or Penske are not equipped to tow, indie productions have to hire a rental house's grip truck to tow them. And, since most rental houses require that one of their employees drive their trucks (for insurance reasons), the production has to hire a driver at roughly $575/10hrs - which is probably more than anyone else on a typical indie crew is getting paid. All of this makes the use of an 18kw HMI very expensive.

Fortunately, there have been several recent technological developments that put the production values that can only be obtained with a HMI within the reach of indie productions. The first is Arri’s introduction of their new M90 HMI.


(The light generated by the CAD designed Max Reflector of the new M90/60 is incredibly bright and sharp.)

Utilizing a new 9 kW HMI lamp, and Arri’s unique MAX reflector, the M90 creates diverging parallel rays to produce a crisp light with even distribution through a wide spot/flood range. The result is a lens-less open face fixture with a quality of light close to that of a Fresnel. The elimination of spread lenses like those used on HMI Pars, makes the ARRI MAX reflector lamp heads comparable in output to par configurations of a higher wattage. In fact, the M90 is brighter than some 18K Fresnels on the market, yet weighs 67lbs less.


(The Active Line Filtration (ALF) of the new ARRI EB 6000/9000 ballast makes it an incredibly efficient and clean load.)

To power the new M90 head, ARRI has engineered a dual-wattage ballast. The EB 6000/9000 will operate either the traditional 6kw SE globe in the M90 head, or the new 9kw SE globe, on supply voltages ranging from 195-250V. With Active Line Filtration (ARRI's system of Power Factor Correction) built in, the EB 6000/9000 ballast is incredibly efficient and generates virtually no harmonic noise.


The combined  100A output of paralleled Honda EU6500s is sufficient to operate the new Arri M90 as well as additional set lighting.

Drawing only 84 Amps, the M90 can operate on the combined 100A output of our paralleled Honda EU6500s. This approach is a lot less expensive because it eliminates the hidden costs of transporting a diesel tow plant to set. Not only is this approach a lot less expensive, it is a lot more manageable for a small indie crew and offers more versatility. It is a lot more manageable because you don’t have to run heavy feeder cables a long distance. It is more versatile because when you don't need the punch of a 18kw Fresnel, you can swap the 9kw globe for a 6kw globe making more power available to run additional lights on the paralleled EU6500s. For example, you save 27 Amps when you swap out a 9kw bulb for a 6kw bulb. The 27 Amps you save by burning the smaller 6kw globe will power quite a few more lights when you consider that both the ARRI L7 LED Fresnel and Kino Flo Parabeam 400 use approximately 2 Amps.  In fact, such versatility now enables the operation on portable putt-putt generators of just about all the lights needed to shoot a night exterior on a digital cinema camera.  This combination of smaller, brighter, more efficient lights, powered by paralleling portable generators makes it now possible for indie productions to achieve, at a fraction of the cost, the big budget production values that in the past required 18k HMIs and diesel tow plants.

  • Price: you may be just starting your business and only need one generator, and eventually need to expand to larger capacity and don't want to dish out for a generator of the proper size.

We have designed our HD Plug-n-Play Gen-set system so that one can build-up their power generation capacity in an incremental fashion as their productions require. In fact, you don’t even need a generator to increase your production values with our system.


Left: Transformer/Distro plugged into a 30A/240V dryer outlet. Right: 4K HMI Par under rain protection powered by Transformer/Distro

Our Transformer/Distros can create larger 120V circuits, capable of power larger HMIs & Tungsten lights, from common household 240V receptacles like range and dryer outlets (use this link for more details: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorSingle%20Large%2060A/120V%20Circuit.)
 

Left: Arri AS18 1800W Par powered from Transformer/Distro. Right: 4Kw and 1800W HMI ballasts powered from Transformer/Distro

When house power is not available, our Transformer/Distros will provide access to the full power available from a 4500-6500W rental generator by stepping-down it’s 240V output to a single large 120V circuit (the size of the circuit depends on the size of the generator.) When you need the most power that can be obtained from a single generator, and it makes sense to invest in one, the same Transformer/Distro will provide a 60A/120V circuit, capable of powering HMIs up to 4kw and Tungsten lights up to 5kw, from the enhanced 7500W output of our modified Honda EU6500is.


A sunny feel is created on an overcast day by a Honda EU6500is powering a 4kw HMI Par and a 1800W ArriMax for a commercial.

When you need a larger daylight source to balance the sun on day exteriors, or to replace the sun on day interiors, our Paralleling Control Box will combine the output of two Honda EU6500s to generate an unprecedented 100 Amps of power from portable generators (use this link for more details: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorParallelingEU6500s)


A sunny feel is created on an overcast day by a Honda EU6500is powering a 4kw HMI Par and a 1800W ArriMax for a commercial

A complete paralleling system consists of two Honda EU6500s, a Paralleling Control Box, and one of our Transformer/Distros (either 60- or 84- Amps.) All you need to start generating 100 Amps on set, is the Honda EU6500s (which you can rent since no modification is necessary) and our Paralleling Control Box.


Our new HD Plug-n-Play Paralleling System can power the new 9kw Arri M90 HMI as well as 2000W of additional lights

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 #3 on: July 15, 2014, 09:03:02 am »

My guess is that for a generator with 120/240V 100A output (24kVA), you are limited to 100A per leg. If you have a lighting rig that requires a single 120V connection in excess of 100A, that output won't work for you. However, if you have a step-down transformer with an input of 240V 100A, you can have an output of 120V 200A. (Well, a little less, to account for transformer losses.) You would be able to connect your 120V 150A singel-connection lighting rig to the output of that step-down transformer. Such a setup would allow you to avoid renting or buying a 50kVA generator (25kVA per 120V leg).

Jonathan’s logic is correct but he is off in magnitude. Honda’s continuous load rating for both the unmodified EU6500is and the new EU7000is is 5500VA, which translates to roughly 46A at 240V. In parallel operation the combined output of two machines will be 92A at 240V. The larger of our two Transformer/Distros (10kVA) will step down this combined 240V output to a 84A/120V circuit that is capable of powering a larger load (a 10kw incandescent lamp), or more small loads, than has ever been possible on portable generators.

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 #4 on: July 15, 2014, 09:17:55 am »

Guy Holt, I've split this thread into our first official Led Talks, where we invite guests to discuss their particular technology to our members.

We named this "Led Talks" for Loud, Entertainment, Design as a play on "Ted Talks" which stands for Technology, Entertainment, Design. Plus I like the Led Zeppelin tie in.

Everyone - Please post your comments and questions to Guy in this thread so we keep everything in one place. This is a learning forum, so let's all learn some new stuff. And let's play nice.  :D

And Guy, thanks for posting your information and answering questions. We'll only keep you here a few days, then I'll lock the thread so you don't have to watch it forever. This is great stuff and you'll find a bunch of really smart guys on this forum asking a lot of questions about your technology.
« Last Edit: July 15, 2014, 09:28:44 am by Mike Sokol »
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Tom Bourke

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Re: Led Talks - Paralleling Two Generators
« Reply #5 on: July 15, 2014, 12:31:59 pm »

Jonathan’s logic is correct but he is off in magnitude. Honda’s continuous load rating for both the unmodified EU6500is and the new EU7000is is 5500VA, which translates to roughly 46A at 240V. In parallel operation the combined output of two machines will be 92A at 240V. The larger of our two Transformer/Distros (10kVA) will step down this combined 240V output to a 84A/120V circuit that is capable of powering a larger load (a 10kw incandescent lamp), or more small loads, than has ever been possible on portable generators.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
rentals@screenlightandgrip.com
5500VA/240V=23A for each Honda unit and 46A at 240 for 2 in parallel. With the step down transformer that gives us 92A at 120V.  However that is 11KVA, over the 10KVA transformer rating.  With an 84A load at 120V every thing is withing its operating range.  Have I missed something?  I am thinking Jonathan’s logic was based on having a larger generator and transformer to handle the 150A 120V hypothetical load.  Also, do you derate the transformer to accommodate the extra heating from the triplin harmonics?
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Stephen Swaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #6 on: July 15, 2014, 12:34:44 pm »

A frequent topic on this forum is ground-neutral bonding as it relates to proper operation of OCPDs.  In this drawing, if we have a hot-neutral or a hot-ground fault, then the circuit breaker for the left generator will trip as intended, but with no fault path for the second generator,that circuit breaker will not trip-leaving the N-G tied to one leg and effectively  putting 240 volts between the neutral and whatever loads are connected to that leg.

How do you address this issue?  Obviously, shunt trip breakers could be used to make sure both  breakers open on a fault or is there another method you use?     
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Guy Holt, Gaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #7 on: July 15, 2014, 02:22:47 pm »

... do you derate the transformer to accommodate the extra heating from the triplin harmonics?

We don't need to de-rate the Transformer/Distros because we start with an "encapsulated" transformer with a high K-rating. What is a K-Rating? It is a value used to determine how much harmonic current a transformer can handle without exceeding its’ maximum temperature rise level.

There has been dramatic growth in the use of production equipment that generates harmonic distortion over the last several years. Examples are the AC power supplies of video cameras, lap top computers, video display terminals, battery chargers, and electronic lighting ballasts (HMI & Kino.) These electronic devices distort  the current waveform and  generate  harmonics because they use switching power supplies called SMPSs (an abbreviation for Switch-mode Power Supplies.) SMPSs generate harmonics when they rectify AC line current to DC, and back again in supplying current to the load. In the process, a capacitor is charged then discharged in each half-cycle of the AC line current. This process is repeated 120 times a second. This action of recharging capacitors causes AC current to flow only during the peak portion of the AC voltage wave, in abrupt pulses. These abrupt pulses distort the fundamental wave shape and create harmonic currents, which in turn generate heat in distribution equipment and neutral conductors. Harmonics can cause distribution transformers to heat up considerably because the harmonics cycle in their Primary windings. The heat harmonics generate can cause non-K-rated transformers to overheat - possibly causing electrical insulation failure and electrical arcing. K-rated transformers are designed to handle this additional heat and are tested to rigid UL standards. Design features K-rated transformers use to handle the adverse effects of harmonics is double sized neutral conductors, multiple conductors for the coils, more core and coil material, different designs, and different construction techniques like epoxy encapsulation.

Encapsulation is used in K-rated transformers because it greatly improves thermal and electrical performance and consequently the transformer’s K-rating. The mixture of silica sand resin and epoxy compound used for potting has a high coefficient-of-thermal conductivity and is very effective at dissipating heat away from the windings and core; while the heavy gauge steel casing serves as a heat sink. This thermal management reduces winding temperature differentials and allows for the generation of more heat without exceeding allowable temperatures for the insulation class.

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 #8 on: July 15, 2014, 06:23:04 pm »

A frequent topic on this forum is ground-neutral bonding as it relates to proper operation of OCPDs.  In this drawing….

Don’t get too hung up on that drawing, it was borrowed from the article mentioned above by Gary Olson on “Paralleling Dissimilar Generators” to make another point and is not representative of exactly how our system in configured. The illustration below is a more accurate representation of our system:


(Our Transformer/Distros isolate triplen harmonics from the generators so that they can not elevate cross current to a hazardous level.)

There has been and continues to be a lot of debate in our industry regarding the question of  bonding and grounding portable generators. To quickly summarize that debate: some Gaffers believe that Honda portable gas generators, like Crawford Studio units, do not require grounding, and that GFCIs will function regardless of the grounding arrangement. Others, myself included, believe Honda portable generators need to be grounded in order for GFCIs to operate reliably. What fuels this debate in regard to Honda portable generators is, I think, a general ignorance that there are two distinct types of portable Honda generators – those with Floating Neutrals and those with Bonded Neutrals. Which type of generator you are using determines whether it should be earth grounded with a ground electrode and what grounding arrangement is required to make GFCIs operational.

“Floating Neutral”, “Grounded Neutral”, “Bonded Neutral”, “Floating Ground”, “Earth Ground”, “Ground Fault”: if you are unfamiliar with the meaning of these terms I would suggest you first read an article I wrote for our company newsletter on the use of portable generators in motion picture lighting before proceeding further. Neutral Bonded generators have their Neutral bonded to the frame of the generator. Neutral Bonded generators offer a high degree of protection against Ground Faults (if there was a fault to the frame via the Equipment Grounding System, the generator’s circuit breaker would trip eliminating the fault), and so they can be operated without an Earth Ground in what is called a Floating Ground arrangement.  Crawford Studio generators are Neutral bonded, but most Honda portable gas generators are not. In most Honda portable gas generators the neutral circuit is not bonded to the frame of the generator or to the earth ground lead. These generators are commonly called Floating Neutral generators. The floating neutral configuration is common for applications such as connection to a recreational vehicle and connection to home power where the transfer switch does not switch out the neutral to ground connection. When used as a stand-alone power source (a “Separately Derived System” in NEC parlance), OSHA requires that portable generators, like the Honda EU6500is, have a bonded Neutral and GFCI protection for reason we will explore (use this link for the complete guidelines.)   

Tests conducted by The Construction Safety Association of Ontario (CSAO) uncovered significant problems in using GFCIs on ungrounded portable generators with Floating Neutrals. While the CSAO conducted their tests to determine the effectiveness of GFCIs used on portable generators in typical construction scenarios, their findings are equally applicable to motion picture and event staging production applications (use this link for the complete report.) In short, what the CSAO found was that simply using a GFCI on a Floating Neutral generator will not ensure a safe system. A GFCI will only operate reliably if one side of the generator winding is grounded to the generator frame because otherwise fault current has no path back to the winding to complete the circuit. Only when Neutral is bonded to ground, will current go to ground to complete the circuit when there is a current leak. In other words, a complete circuit is required to create an imbalance and cause the GFCI to trip. A combination of grounding the generator winding (bonding the Neutral) and adding a GFCI is necessary. For this reason OSHA requires both on all generators used on jobsites. When these requirements are satisfied OSHA permits the generator frame to serve as ground, and a ground rod does not have to be used, as long as the generator can be completely isolated from earth ground. However, absent a Neutral ground bond, a ground rod, even though it provides no protection for personnel and can even create a hazardous situation, should be used with Floating Neutral generators because it is the only means of establishing a ground fault circuit for electricity to travel back to its source – creating the imbalance required to trip a GFCI device.

So that they can provide an industrial generator that will pass OSHA job site inspections, manufacturers like Honda provide special industrial generator lines that meet these requirements. The EB generators are Honda’s “Industrial Generators.” The EB3800, EB5000, EB6500, and EB10000 generators are neutral bonded and GFCI protected to meet OSHA jobsite regulations. Unfortunately the Honda EB generators are AVR type (prone to voltage waveform distortion from dirty loads) and quite load because of their open frame design. The super quiet Honda generators are Inverter type, making them less susceptible to voltage waveform distortion, but unfortunately they are not Neutral Bonded and do not offer GFCI protection and so do not meet OSHA guidelines for use on job sites.


1) A fault in a metal fixture energizes the entire housing.
2) Since, electricity seeks the path of least resistance back to its’ source, the
bulk of the fault current will travel through the grounding wire, instead of the
individual making contact with the housing, because it is of a much lower resistance
than the individual.
3) Because the ground wire and the neutral wire are bonded on the secondary side of the
transformer, the current carried by the ground wire back to the transformer creates a dead
short (over-current situation), and the breaker trips.
4) If for some reason the breaker does not trip (the short is high resistance), enough current
flows through the ground wire to create an imbalance between Hot and Neutral to trip a  GFCI  (if one is in line)
– shutting off power to the distribution system as a fail safe system.



As illustrated above, to meet OSHA requirements we use a transformer to step-down the 240V output of the Honda EU6500s to a single large 120V circuit. Since Transformers bond the Neutral to Ground on the secondary load side they provide a low resistance path for fault current back to the Transformer windings and and a breaker that will trip from the over-current situation.


A 100 GFCI with our 60A Transformer/Distro meets OSHA requirements
for use of a Honda EU6500is generator on work sites.


And, since the Neutral and Ground are bonded in the Transformer, GFCIs will operate reliably even when power is being generated by a Floating Neutral generator like the Honda EU6500is. To completely comply with the OSHA requirements for the use of a EU6500is on job-sites all you need to do is put a large amperage GFCI, like a Shock Block, inline after the transformer.


Left: Ready for rain on the set of "Gasp." Center: Two 4kw Pars operate on a 10kw Honda EB10000 Generator through our 84A Full Power Transformer/Distro. Right: 100A Shock Block GFCI downstream of Full Power Transformer/Distro offers Ground Fault Protection for entire 100A distro system.

For more detailed information on using shock blocks to provide ground fault protection with portable Honda generators, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting and Grip Rental & Sales in Boston
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Stephen Swaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #9 on: July 16, 2014, 09:41:22 am »

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

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 was not hung up on the diagram-but it is misleading and an unsafe setup.

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?

I agree that your complete system-at least downstream of the transformer is code compliant and safe.   But your brochure seems to imply that it is acceptable to supply various 240 volt loads from your parallel setup.

However, 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(in fact the conditions where a floating neutral is allowed to exist are very limited and specific).  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.  Ina 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.

I am not trying to be argumentative, but I find that there is enough confusion among end users (welders, homeowners, gaffers, pro sound techs), as you noted even in your industry, that it is imperative that we be precise in discussions so as to not add to the confusion confusion.
« Last Edit: July 16, 2014, 10:10:47 am by Stephen Swaffer »
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Steve Swaffer

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Re: Led Talks - Paralleling Two Generators
« Reply #9 on: July 16, 2014, 09:41:22 am »


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