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Author Topic: Non Power Factor Corrected Pro Audio Gear  (Read 12207 times)

Keith Broughton

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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #20 on: November 01, 2014, 07:23:55 am »

Thanks for that explanation Guy.
This is something new to me and that last "demo" got the point across.
I still need to do more reading but at least now I have a clue :D
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Guy Holt

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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #21 on: November 05, 2014, 01:52:19 pm »

This is something new to me and that last "demo" got the point across.
I still need to do more reading but at least now I have a clue :D

If it helps I will relate the third part of the IA demonstration. To further demonstrate that skin effect was the cause of the more than 100% increase in voltage drop, in the next part of the demonstration we put a Harmonic Mitigating Transformer (HMT) in line at the end of the 400' cable run and before the dimmer pack. 


(From front to back in the PP slide above are the Strand Dimmer Packs, Cam Spider Boxes, an 400A 3-Phase Dual Output HMT,
and finally the Dimmer Room Company Switches)

As you can see from the side-by-side comparison of the voltage waveform without the HMT (left) and with the HMT (right), the HMT greatly reduces the voltage waveform distortion and virtually eliminates the voltage drop caused by the harmonics drawn by the dimmer packs encountering the system impedance.


How do HMTs accomplish this? As you may recall from my earlier post, the voltage distortion is caused when each harmonic current drawn by a non-linear load flows through the impedance of the distribution system resulting in a voltage drop at that harmonic frequency. Because the combined impedance of the typical Delta-Wye service transformer, and lengthy distribution cable is high at the load, voltage distortion often exceeds the 5% maximum voltage distortion limit recommended by IEEE Std. 519-1992 when just lightly loaded with non-linear loads. At just one-half of full-load RMS current service transformers can produce critically high levels of voltage distortion and flat-topping at their outputs and at the downstream loads.


To minimize the voltage distortion rise due to impedance of the service transformer and cable, HMTs are deployed close to non-linear loads and thereby reduce the impedance encountered by harmonic currents. HMTs accomplish this by zero sequence flux cancellation and phase shifting.

Cancellation of zero sequence flux is accomplished in HMTs by winding two phase legs on each core leg connected in opposite directions. This zigzag winding has a beneficial effect on triplen harmonics (3rd, 9th, 15th,…) that have a similar phase angle.



(Trace winding of Phase A. It is left to right on the first leg and the opposite direction (right to left) on the middle leg. The same is
true of Phase B and C)


From the vector analysis in the Power Point slide above, we can see that with this winding configuration the triplen harmonic currents that were in phase are now 180 degrees out of phase and thereby produce ampere-turn fluxes that cancel each other so that triplen currents are not induced in the upstream distribution system. On top of that, the flux cancellation created by the zig-zag windings of HMTs create a low impedance path for triplen harmonics to return to the load, thereby off-loading the upstream neutral of triplens, satisfying the loads requirement for triplens, and reducing the overall current drawn by non-linear loads. Finally, with a 30 degree phase shift between its’ outputs, HMTs cancel 5th and 7th harmonic currents.  The net effect is that the harmonic currents drawn by non-linear loads encounter very low impedance and, as can be seen in the Power Point slide above, cause virtually no voltage waveform distortion and voltage drop from skin effect.

Besides greatly reducing voltage distortion and voltage drop, HMTs can also greatly reduce current on the system neutral. As can be seen in the lower half of the Power Point slides above, by off-loading the harmonics currents and returning them to the loads as described above, the HMT in our demonstration reduced current on the system neutral by over 500% (from 153.9A without the HMT to 4.36A with the HMT.)

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

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Guy Holt

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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #22 on: November 06, 2014, 03:44:26 pm »

power is delivered on a 3 phase distro. Load is tested on 3 120 volt recepticals , 1 on each "leg".

scenario 1: 1kw par plugged into outlet 1, current on neutral and hot the same.
                 1 kw par plugged into outlet 2, current on neutral drops
                 1 kw par plugged into outlet 3, current on neutral 0
All as expected with a resistive load and I understand why.

scenario 2: the same process with 3 identical video projectors causes current on neutral to INCREASE as each load is added.

Please explain what is happening here. Is it something to do with the type of power supply the projectors are using?

I am posting this question here even though it is the start of a separate thread (http://forums.prosoundweb.com/index.php/topic,150582.10.html) because it likely has to do with the projectors using non-power factor corrected switch mode power supplies (SMPSs). See the other thread for a full explanation and discussion.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightsandgrip.com


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Guy Holt

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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #23 on: November 15, 2014, 01:41:45 pm »

Since this response is slightly off topic, I would like to respond to this post http://forums.prosoundweb.com/index.php/topic,152268.0.html titled "Re: 25kw diesel generator is enough to power 65kw sound system" here since this thread seems to be the more appropriate.

I would like to know if a 25kw Diesel Generator is power enough for a 65kw sound system:
8 x jbl vt4888
6 x Jbl vt4880
3 x crown itech 1200hd
6 x crown itech 9000hd
2 x crown itech 5000hd

Plus monitors, 2 digital consoles and accessories.

After doing a little research into Crown amps, I see harmonics is not a de-rating factor in Maidson’s case because the I-Tech series that he uses features Power Factor Correction. But if say he were using the XLS or XTI series of non-PFC amps harmonics would be a limiting factor to be taken into consideration along with the dynamic load. Since as I gather you guys use portable generators for your smaller rigs, you might be interested in the results of a series of tests I did of linear and non-linear loads on different power supplies


Left: Honda EU6500is (L) Honda EX5500 (R) Center: Test Set-Up w/60A Full Power Transformer. Right: P@L PFC 1200W Elec. Ballast (L), Arri Non-PFC 1200W Elec. Ballast (C), Arri 1200W Magnetic Ballast (R)

The test consisted of running different lighting loads (quartz, kino, magnetic HMI, non-PFC Electronic HMI, PFC Electronic HMI) on two different types of generators and grid power as a sort of control. I then took pictures of the resulting waveform on an oscilloscope. The two types of generators were a conventional AVR generator (the Honda EX5500) and an inverter type (the Honda EU6500is.) You guys might find interesting the results I got for lights that use non-PFC and PFC Switch Mode Power Supplies (SMPSs.) In the side-by-side comparisons of load types below, the frame on the far left is always grid power (our control), the center frame is always the EX5500 power, the right frame is always the EU6500is.

The middle oscilloscope shot below shows the severe adverse effect a non-PFC SMPS (1200W HMI Electronic ballast) can have on the power waveform of a conventional AVR portable generator. Given the large sub-transient impedance of conventional portable generators, even the draw of a small amount of harmonic currents will result in a large amount of distortion in its’ voltage. The adverse effects of the harmonic noise exhibited here, can take the form of overheating and failing equipment, circuit breaker trips, excessive current on the neutral wire, and instability of the generator’s voltage and frequency.


Left: Grid Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw Arri non-PFC Elec. Ballast.

The power waveform below right of a PFC SMPS of the same wattage (a PFC 1200W HMI Electronic ballast) demonstrates that when your load consists predominantly of non-linear equipment, like switch mode amps, power wedges, and projectors, it is essential to have PFC circuitry in them and to operate them on an inverter generator. The combination of improved power factor and the nearly pure power waveform (1-2 %THD) of the inverter generator creates clean stable set power (like that in the power waveform below right).


Left: Grid Power w/ 1.2Kw P-2-L PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw P-2-L PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw P-2-L PFC Elec. Ballast.

These tests clearly demonstrate that with smaller non-PFC loads an inverter generator is typically a better option than conventional generators twice the size.  If say you only require 100 Amps, then two paralleled Honda EU6500s, or two of the new EU7000s, would be better than a 200A conventional generator when it comes to powering non-linear loads that use Switch Mode Power Supplies (SMPSs.)
 
Guy Holt, Gaffer,
ScreenLight & Grip,
www.screenlightandgrip.com
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Guy Holt

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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #24 on: December 21, 2014, 06:58:13 pm »

There was a post on Cinematography.com (available at http://www.cinematography.com/index.php?showtopic=63910) that I thought you all might find interesting. In the course of filming a scene on a football field for an independent movie, the crew was dumbfounded by some power issues that can only be explained by harmonics. The lighting set up consisted of the following: on each end of the football field the crew had rigged a 4k HMI Par and 1800W HMI MAX (M18) into scissor lifts. Two additional M18s were rigged into the announcer’s booth at center field.


From a 500 Amp diesel generator that was dropped behind the bleachers at the 50 yard line, the electricians ran out 100ft of 2/0 to a main distro box at the 50 yard line. From there they ran 150' of banded #2 AWG cable in each direction down the field towards the end zones where the lifts were.  They terminated the banded #2 runs into a 100A/120V snake bite, and from there they ran 100ft of #4 AWG to a 100A lunch box, and from the lunch box they stepped down to #6 AWG cable to power the 4k Par and a stinger to power the M18. This set-up was mirrored on either side of the field. The two additional M18s rigged into the announcers booth at center field were powered by a 50' run of #4 AWG  to a 100A Lunch box underneath the booth. Load balance was almost dead even between the legs of the generator operating single phase.


Shortly into shooting, one of the 4k Pars went down. In addition, one of the lunch boxes started buzzing loudly.  The crew replaced the bad ballast (a non-pfc Power Gems) with another and it too went down. Since they had oversized their cable runs by using #2 AWG rated for a 160A for a 70A Load (52A (4kW) + 18A (1.8kW) = 70A), they did not suspect the problem had to do with voltage drop, besides the M18s ran flawlessly. When the best boy electric checked the generator, the generator meters read 108v. The best boy was a bit flabbergasted why the voltage had dropped on the generator when they had oversized the cable to banded #2 instead of #6 and no one had touched the generator.  The answer to his questions I suspect has to do with the harmonic currents drawn by the non-pfc 4kW ballasts. My reasoning is that the harmonic currents these non-linear loads draw can have a severe adverse effect on the power waveform of even large diesel generators.


As is evident in the power quality meter reading of a non-pfc 2.5kW HMI above, the high peaked pulsed current (lower waveform) drawn by its’ smoothing capacitors is a distorted waveform that does not resemble the sinusoid of AC voltage or the current drawn by an incandescent light. As such, the current drawn includes a number of harmonic currents in addition to the 60hz fundamental (see Fast Fourier Transformation of a non-pfc 2.5kW HMI ballast below), which not only increases voltage drop but also causes voltage “flat topping.”


Voltage flat topping is a particular form of voltage drop that is caused by harmonic currents interacting with the high impedance and soft power of diesel generators. Since the smoothing capacitors of the HMI ballasts consume power only at the peak of the voltage waveform, voltage drop due to system impedance occurs only at the peak of the voltage waveform – causing the “Flat Topping” we see in the top waveform of the first power quality meter above that is characteristic of capacitive loads on generators. If the voltage waveform distortion is severe, it can cause voltage regulator sensing problems and inaccurate instrument readings in a generator’s control systems as well as ballast failure, which would explain the problems the crew experienced above.

Let’s look at the regular voltage drop component of this problem first. As you can see in the Fast Fourier Transformation of the 2.5kW HMI ballast above, non-pfc HMI ballasts draw a distorted current waveform that is rich in harmonics. The higher harmonic frequencies create what is known as "skin effect" in the cable. Skin effect is a phenomenon where the higher frequencies cause the electrons to flow toward the outer sides of a conductor. Since the flow of the electrons is no longer evenly distributed across the cross sectional diameter of the conductor, more electrons are flowing through less copper and the resistance of the conductor increases. Even though the cable was oversized in this case, this increase in resistance reduces the ability of the conductor to carry current, resulting in greater voltage drop over shorter distances.


The area of the cross sectional diameter of a conductor used by DC current (left), Low Frequency AC Current (center), High Frequency AC Currents (right).

But, voltage drop alone was not the cause of the problems the crew experienced. The other contributing factor was the voltage flat topping caused by the high impedance of their distribution system. Always remember, there are two components to the impedance of a distro: cable and generator. The skin effect caused by the harmonic currents generated by the non-pfc ballast increased the impedance of the cable. The second contributing factor was that they were operating single phase on a small generator. A 500A generator operating three-phase provides about 166A/leg. The same generator operating single phase does not provide 250A/leg, but rather the same 166A/leg but just single phase. The set up as described had about 100A on each leg. So, the generator was pretty well loaded and half that load (52A) was generating harmonic currents.

Under these circumstances one could expect much more severe voltage flat topping than that in the example of a 2.5kW HMI above. What causes flat-topped voltage? According to Ohm’s Law current reacts with impedance to cause voltage drop. For example, when encountering the high impedance of a loaded generator, a 3rd harmonic current will produce a voltage drop at a 3rd harmonic voltage. Likewise a 5th harmonic current will produce a voltage drop at a 5th harmonic voltage, etc. Each harmonic current drawn by the non-power factor corrected 4k ballast flows through the system impedance, resulting in a voltage drop at that harmonic frequency. In other words, where a distorted current waveform is made up of the fundamental plus one or more harmonic currents, each of these currents flowing into an impedance will according to Ohm’s Law, result in a voltage drop resulting in voltage harmonics appearing at the load bus and distortion of the voltage waveform.


This pattern does not appear in the voltage waveform of the grid power above because of its’ much lower impedance.  The impedance of a generator is not an easily known quantity. Depending on its’ size and design, the impedance of a generator will be 5 to 100 times that of a utility transformer and it will change as the load changes. But where they were using a fairly small generator for the harmonic load generated by the two non-pfc 4k ballasts, the internal reactance of the generator would have been sufficient to cause appreciable voltage flat-topping as well as a voltage drop.

Now for the reason the voltage output of the generator dropped even though no one touched its’ voltage regulator. The voltage output of the generator did not drop rather the flat-topped voltage caused an erroneous reading of the voltage by the meters. The flat-topped voltage manifests it self as low voltage on the generator’s meter because conventional electrical meters like those on most generators are designed to read only sinusoidal waveforms accurately – not distorted waveforms. Flat-topped voltages introduce errors into the measurement circuits of these meters, which result in low readings. Since the consequences of under measurement can be significant - overloaded cables may go undetected, bus-bars and cables may overheat, fuses and circuit breakers will trip unexpectedly - it is important to understand why meters based on "true rms" techniques should be used on power distribution systems supplying harmonic generating loads.

Most analogue meters and a large number of digital multi-meters are designed to read voltage and current quantities based on a technique known as “average reading, rms calibrated”. This technique entails taking a measurement of the average (or mean) value (0.636 × peak) and multiplying the result by the form factor (1.11 for a sine wave). The result is 0.7071 times the peak value, which is displayed as “rms”. This assumption is valid only for pure sinusoidal waveforms like the one pictured below.


To accurately measure waveforms distorted by harmonics, a meter that will measure the true rms value is required. For example, if you were to use a conventional “average reading, calibrated rms” meter to measure the current waveform below distorted by a non-linear power supply (with a peak value of 2.6A and an average of 0.55 A), its' display would give a “rms” current of 0.61A.


A meter that measures true rms will give a more accurate measurement of 1.0A for the distorted waveform above. By comparison, the reading of a conventional “average reading, calibrated rms” meter is almost 40% lower than the real value.

Now for the reason the older non-power factor corrected 4k ballasts were failing when the newer power factor corrected M18 ballasts were not. One adverse effect of flat-topped current is that it causes non-power factor corrected equipment to draw more current to maintain the power rating (watts) of the unit. This, in turn, can cause protective fuses on electrical boards of the equipment to blow. I experienced this first hand, when I first tried some years ago to operate a 4k HMI Par on a Honda ES6500 (a conventional AVR generator) with the first generation of electronic square wave ballasts - a Lightmaker.  The ballast inexplicably failed when it had never given us problems on mains power. Upon closer inspection back in our shop, we found that a protective fuse on the main board had blown. We replaced the fuse and continued to operate the ballast off of grid power without incident. But as soon as we tried to run it again on the Honda the fuse blew. Since the Lightmaker ballasts are not Power Factor Corrected (PFC), the cause of the ballast's erratic behavior was the amount of harmonic distortion it was creating in the power stream generated by the ES6500. The harmonic currents were not a problem on grid power because, given the extremely low impedance of grid distribution, they did not induce voltage distortion.  But, fed back into the power stream generated by our Honda ES6500, the same harmonic currents created voltage distortion and sufficient voltage drop from skin effect to cause sufficient increase in current to blow the protective fuses on the ballast boards. For more detailed information on the source of harmonics and how to counteract their adverse effects use this link http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorSizing%20Portable%20Generators - for a white paper I wrote on the use of portable generators in motion picture production.

Guy Holt, Gaffer, ScreenLight & Grip, www.screenlightandgrip.com
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Guy Holt

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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #25 on: May 14, 2015, 05:54:45 pm »

Just to take a pole here, I was talking with one of my friends recently who mentioned strongly disliking company switches installed at arenas and other venues. I asked him why, and his company ran into several of them at different venues where they had repeated nuisance trips stopping power without any apparent reason.

I am posting this question here even though it is the start of a separate thread (http://forums.prosoundweb.com/index.php/topic,154631.0.html) because the nuisance tripping described in the thread could have been caused by a high concentration of non-power factor corrected switch mode power supplies (SMPSs) like those commonly found in amps, power wedges, LED displays, LED lights, and HMI discharge lights. See the other thread for a full explanation and discussion.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightsandgrip.com
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Re: Non Power Factor Corrected Pro Audio Gear
« Reply #25 on: May 14, 2015, 05:54:45 pm »


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