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Thread: non Linear loads

  1. #11
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    Quote Originally Posted by junkhound View Post
    ...
    OP should check on the type power supplying the new computers...
    With luck, they haven't been purchased yet and there's time to specify that power-factor correction will be implemented.

  2. #12
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    Quote Originally Posted by junkhound View Post
    There is probably a delta-Y somewhere in the system.

    Circa 1992, company installed about 200 new computers in the offices. Not PFC corrected inputs, straight rectifiers in those older power supplies.

    About a week later the poco transformer on the roof smoked due to all the triplett harmonics.

    OP should check on the type power supplying the new computers, they likely have a PFC circuit in the front end, so no problem. Easy to check, slap on a CT and look at the current waveform.
    OP has single phase service supplied by single coil with a center tap as far as we know.

    Unless you want to call the transformer(s) back at the POCO substation part of "the system".

  3. #13
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    Quote Originally Posted by Dennis Alwon View Post
    A friend called and asked some questions about a job he may do.

    A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.
    I wouldn't even worry about harmonics in this installation.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  4. #14
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    Jul 2013
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    Consider the 'remote building' and the 40 kW of computer loads into the discussion,
    plus the discussions in other threads about how the poco prorate their distribution transformers (eg. feeding 5 different 200 A residential panels with a single 25 kVA transformer) (BTW, the OP's 800W computers are 'BIG' computers)

    Say no PFC on computers, a guess at 0.75 PF. Guess at least another 2 kW load besides the computers, plus another 15 kW just for the air conditioner just to keep the computer room cool enough.

    Better at least tell the poco what the loads are before smoking something ?

  5. #15
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    Quote Originally Posted by kwired View Post
    I won't claim to be an expert on harmonics, but we are always told they are additive in the neutral. So something to chew on a little before someone that knows harmonics better chimes in - for single phase multiwire applications the effects would not likely be as bad as they are for three phase multiwire applications as the additive effects of the harmonic currents is only coming from two ungrounded conductors instead of three. What I don't know is if the harmonics are still additive when there is 180 degree phase angle or if it is only additive because of the 120 degree angles of a wye system.
    They are only "additive on the neutral", when the harmonic multiplier of the fundamental frequency is divisible by the number of phases. So your fifth harmonics are not additive on the neutral, but your even harmonics in split phase, and your multiple of 3 harmonics in 3-phase, are.

  6. #16
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    Whether harmonics are additive or cancel in the neutral, harmonics are present in the phases and they could cause maloperation of the very equipment producing them. Also, POCO may fine you if there is a harmonics penalty clause. So it is prudent to measure and ensure harmonics distortion is within limit

  7. #17
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    Quote Originally Posted by Dennis Alwon View Post
    A friend called and asked some questions about a job he may do.

    A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.

    The question is if you have a load on the neutral of 1600 watts for one circuit (2 machines), and 1600 watts on the neutral for the other phase, then is load on the neutral 1600 watts. If I think about the service as a multiwire branch circuit then the neutral load would be "zero" but I don't think it works that way.

    Secondly, once the neutral is calculated and the non-linear loads are more than 50% then how do we size that neutral ?

    Thirdly, do we need to even worry about this on single phase since the code mentions harmonics for a 3 phase wye and not single phase.
    I assume you mean a 120-0-120V system? I don't often deal with single phase non-linear loads on a system like that but here are my thoughts for what they're worth.

    If the computers are all the same and are distributed equally on each half with a common neutral back to the supply then you might reasonably assume that, for the non-linear loads on each half, their input current waveforms would the same magnitude but in antiphase and cancel even if they are not sinusoidal.

    The three phase case is different. If you have phase to neutral non-linear loads you get third order harmonics (three times the supply frequency) so they end up being in phase with other so add arithmetically in the neutral. That's why oversized neutrals are (sometimes) specified.
    Si hoc legere scis nimium eruditionis habes.

  8. #18
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    Quote Originally Posted by Dennis Alwon View Post
    A friend called and asked some questions about a job he may do.

    A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.

    The question is if you have a load on the neutral of 1600 watts for one circuit (2 machines), and 1600 watts on the neutral for the other phase, then is load on the neutral 1600 watts. If I think about the service as a multiwire branch circuit then the neutral load would be "zero" but I don't think it works that way.

    Secondly, once the neutral is calculated and the non-linear loads are more than 50% then how do we size that neutral ?

    Thirdly, do we need to even worry about this on single phase since the code mentions harmonics for a 3 phase Wye and not single phase.
    Be careful with MWBC.'s. Your must verify you are not using 2 phases of a 208/120V 3 Phase system. The neutrals to currents NOT cancel each other out like they do with a 120/240V Single Phase system.

    208/120V 3 PH - using 2 of the 3 phases (3W MWBC)
    1PH Nc= Highest current of the 2 phases

    240/120V 1PH - Using both of the phases (3W MWBC)
    1PH Nc= Sum of the two phases / 2 (in a balanced system it cancels each other out to 0 Amps.

    See Mike Holt's Video's on his home page. " Calculating MWBC circuits."

  9. #19
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    Quote Originally Posted by Unbridled View Post
    Be careful with MWBC.'s. Your must verify you are not using 2 phases of a 208/120V 3 Phase system. The neutrals to currents NOT cancel each other out like they do with a 120/240V Single Phase system.
    Dennis does state that it's single phase and I asked for clarification on that point.
    Si hoc legere scis nimium eruditionis habes.

  10. #20
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    Quote Originally Posted by Besoeker View Post
    Dennis does state that it's single phase and I asked for clarification on that point.
    It's a safe bet that it's single phase.

    99% of single family residences only get a single phase service here. It takes a lot of money and cajoling to get a utility to run 3 phase out to a residential service, often including you having to pay the full cost for them to run the third wire from the nearest 3 phase connection point, which can be miles away.
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