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Thread: Ground bonding on mixed Wye /delta connection - fault protection

  1. #1
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    Ground bonding on mixed Wye /delta connection - fault protection

    Greetings to all.
    I am doing a study and experimentation for a problem that has emerged since the wide use of switch mode power supplies for large audio amplifiers (10kw+) has become commonplace in my field. These power supplies are very non-linear, and under some circumstances can cause very large neutral currents in Y120/208 stage power distributions.
    During conservative operation on 120v (wide crest factor) the neutral currents are manageable, but still excessive. When these amplifiers are driven very hard and limiting kicks in, the crest factor narrows and the neutral currents can rise very quickly to large values. A quick rough calculation that does not take into account the efficiency of an amplifier suggests (and testing confirms) that a 10KW amplifier can ask for 80 amps or so peak on 120v.
    Clearly this is an untenable situation with 20 or 30 10kw amplifiers wailing away. So I am looking into operating the amplifiers on a delta connection. Most of these amplifiers are voltage sensing and auto range when they see the voltage to which they are connected. Some have "world" power supplies that can operate on single-ended (120 North American or 240 European with neutral), split phase (120/240), 3-phase wye (208 North American) or 3-phase delta with a variety of world power formats.
    Some do not. My area of experimentation is with those that will only operate on 120 with neutral and ground, 240 with neutral and ground, 240 with two hot legs and ground, and in a pinch, on "single phase" 208 (two legs of North American Y120/208 with ground.

    In these power distributions, there is also the need for 120v circuits for "trivial" loads (no device exceeds more than 10 amps or so) for mixing consoles, processing equipment, stage power for guitar amps, etc.

    I have determined that a particular amplifer I am working with that is designed for single ended 120v or 240v and split phase 240v, will also run satisfactorily on 208v "single phase" (two legs of 208 and ground).

    It is common practice in stagecraft to mix Y and delta connected loads on Y120/208 (3 legs, neutral, ground) sources. For example, the electronics run on 120v (single leg, neutral, ground) and other equipment such as 3-phase hoists to suspend loudspeakers (3 legs and ground) run on a delta connection to a Y secondary source.

    Nothing suffers from this mixed connection.

    A quick note about how these amplifiers are used. In a perfect world, with normal music, there is a wide differential between the average power output level and the peak power output level, as much as 10db, i.e. 1000w vs. 10,000 watts. The amplifiers are generally capable of short term bursts of 200ms or so at the high power level and they spend most of their time at the lower power value. The joule storage in the amplifiers generally take care of these peaks. When operating in this profile, the average current draw is actually very low. 10-20A or so per amplifier.

    However, some types of modern music contain passages with sustained low-frequency notes that become long-term RMS in character rather than peaking. That's when these amplifiers draw BUCKETS of current as the joule storage cannot be replenished and the switch-mode power supplies have to ramp up to supply the output stages.

    Some of these systems are VERY large. 200-400kw or more systems are not unusual, and there has recently been a major tour go out with a system that approaches, and can exceed, a megawatt of peak power. In these very large systems, we are generally talking about amplifiers that operate from a 208v delta connection.

    My question is about medium to large systems that have amplifiers that do not operate from 208 delta, and operate from Y120/208 with neutral. There can be astonishing neutral currents in the power distributions during peak power output and especially long-term maximum power output. Worse, the character of the current draw is a pulse width modulated power supply in each amplifier that is running at a 180khz to 250khz switching frequency. There is all manner of garbage on the system neutrals. The power supplies are free-running and do not utilize a master clock, so their current draw is not synchronized and the spectral content of the garbage on the neutral is very complex.

    What I want to do to largely eliminate or control neutral problems, given that mixed wye and delta connections are commonplace on a wye facility source transformer or generator (generator being a whole additional issue), by running amplifiers that are not designed to run on 3-phase, but which will operate satisfactorily on 2 legs of 208 on individual connections to 2 legs of a delta connection to a wye transformer in rotation (i.e. amp #1 on X-Y, amp #2 on Y-Z, amp #3 on Z-X) in arrays of 3 or 6 or 9 amplifiers while still using a the wye connection for trivial 120v loads.

    It is my perception that since the neutral of the wye secondary of the facility source transformer or a wye connected generator is bonded to ground at that source, that the essential function of the grounding of the delta connection is satisfied, as if there were a zig-zag or delta-wye grounding transformer for ground fault protection.

    The amplifiers all have substantial chassis ground connections, and the power supplies float within the chassis, since they can accept either hot/neutral/ground or hot/hot/ground by design.

    Is my perception that ground fault protection for both Wye and Delta connected equipment exists by virtue of the neutral of the wye source secondary being bonded to earth correct?

    Whew!

    Thanks for considering my question. I read and watch your forum and facebook page religiously.

    Greg Carttar

  2. #2
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    There are no wye or delta connected loads, there are line to line loads and line to neutral loads. The combination wye delta you mention is more correctly called a Multi Wire Branch Circuit.

    You are correct that the neutral to ground bond is to clear a fault. What's important is that that bond only be made in one place.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  3. #3
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    Correct terminology noted. Thanks.

    So to recap...significant individual and unsymmetrical non-linear line-to-line loads, mixed with significant or insignificant individual and unsymmetrical line-to-neutral linear and non-linear loads is not an issue, either to the loads or to the system as a whole, given a source transformer of sufficient capacity.
    Unsymmetrical is used in the sense that a battery of amplifiers or a bunch of small loads are not all doing the same thing individually and hitting the power distro with varying current draw values.

    A valid ground fault protection path exists for both the line-to-line and line-to-neutral when the wye neutral is correctly bonded to ground.

    Thanks for the reply and I'm open to further comments or clarification for sure.

    Greg Carttar

  4. #4
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    Quote Originally Posted by Greg Carttar View Post

    Is my perception that ground fault protection for both Wye and Delta connected equipment exists by virtue of the neutral of the wye source secondary being bonded to earth correct?


    Quote Originally Posted by Greg Carttar View Post
    A valid ground fault protection path exists for both the line-to-line and line-to-neutral when the wye neutral is correctly bonded to ground.
    Take note that the earth plays no part of fault clearing at the voltages you are talking about, bonding the non-current-carrying parts of the system to the system neutral is what clears a fault, see 250.4(A) and particularly 250.4(A)(5)

    Roger
    Last edited by roger; 07-09-17 at 03:30 PM.
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  5. #5
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    I think you meant to refer to bonding the non-current-carrying parts of the system, not the non-conductive parts.

    Sent from my XT1585 using Tapatalk

  6. #6
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    I don't have time for much thought on this at the moment, but you might look into how data centers deal with similar problems. On the one hand, the supplies are generally smaller (<2kw) and cleaner-by-design, but there are lots of them and a there will be a mix of line-line and line-neutral loads.

    I also take issue with manufacturers that sell poorly-behaved products, like some of these large amps. That's a separate rant.

    Which amps are you using/studying? Being able to look at their docs could be interesting.

  7. #7
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    Quote Originally Posted by GoldDigger View Post
    I think you meant to refer to bonding the non-current-carrying parts of the system, not the non-conductive parts.

    Sent from my XT1585 using Tapatalk
    Ooooops yes I did, thanks. I went back and changed it.

    Roger
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  8. #8
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    The neutral/earth bond is assumed, and I incorrectly used the term "earth" to refer to the already bonded neutral/earth.

    Yes, non-current carrying conductor, which in these systems in a misnomer....the system neutral has LOTS of current on it under some momentary conditions, that is the core issue and the reason for wanting to run the amplifiers on line-to-line instead of line-to-neutral.

    There are several brands of Class D amplifiers that have this effect on the neutral. The brands that will accomodate line-to-line-to-line do not.

    Is there anything nowadays that does NOT have a switching power supply? It's nuts.

    A Class D audio amplifier is really nothing more than a PWM motor speed controller with a fancy modulator and a low-pass filter on the output.

    Greg Carttar

  9. #9
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    Quote Originally Posted by Greg Carttar View Post

    Yes, non-current carrying conductor, which in these systems in a misnomer....the system neutral has LOTS of current on it under some momentary conditions,
    As it should and will have.

    Quote Originally Posted by Greg Carttar View Post
    that is the core issue and the reason for wanting to run the amplifiers on line-to-line instead of line-to-neutral.
    Assuming these installations are not in dwelling units you should spend some time in article 647.

    Roger
    Moderator

  10. #10
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    These sound systems are not installations. These are traveling systems, and the lighting, rigging (dozens of CM 1-ton chain hoists), and video wall systems that accompany them are in and out in one day. A big tour will have perhaps 20 semi-trailers or more.

    No time to debug power issues.

    It's not unusual for a major venue to have multiple 400-600A Y120/208 drops for production power.

    This has been very helpful, and thanks very much for the clarifications of proper connection terminology.

    I'll stop back by when I've completed my experiments.

    Greg Carttar
    3rd St. R & D Production Services
    Last edited by ActionDave; 07-09-17 at 11:56 PM. Reason: too much personal information

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