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  • #16
    Originally posted by jflorey2 View Post

    Amen there. I've seen an awful lot of problems caused by cheap PWM supplies putting large nonlinear loads on power distribution systems, especially on three phase power. Fortunately, modern power-factor requirements for medium to large switchmode supplies are starting to ameliorate that problem.
    You need to mitigate more then just the PF. For that matter back in the day before I really understood about THD I use to design and help install PF correcting capacitor systems. What I did not know at the time was that if you don't take the harmonics into consideration just adding capacitors without filters can cause a worse problem for the power distribution system including hurting the PF.

    My job is to get engineers to understand the pitfalls of high THD when they design their distribution systems. Depending on where the harmonic generating loads exist, a harmonic filter or transformer should be used. Based on what my company has done for some pretty big Casino companies they all now install filters on the distribution panel that feeds their slot machines. Those puppies are really affected by harmonics.

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    • #17
      Originally posted by SunEagle View Post

      My job is to get engineers to understand the pitfalls of high THD when they design their distribution systems. Depending on where the
      harmonic generating loads exist, a harmonic filter or transformer should be used.
      While aware of the PF problem, I know very little about fixing it. I suppose those are gigantic 180 HZ notch filters? Bruce Roe

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      • #18
        Originally posted by SunEagle View Post
        You need to mitigate more then just the PF. For that matter back in the day before I really understood about THD I use to design and help install PF correcting capacitor systems. What I did not know at the time was that if you don't take the harmonics into consideration just adding capacitors without filters can cause a worse problem for the power distribution system including hurting the PF.
        Absolutely. When I think of bad power factor I think of two big issues:
        1) Reactive power, caused by big inductors (motors mostly) shifting the impedance of the load from resistive to reactive
        2) Peak rectification, caused by cheap switching supplies with big rectifiers + filter caps on their front end. (Often called harmonic distortion due to the very non-sine-wave characteristic of the current draw due to peak rectification.)

        1) got all the attention for a long time because pumps, fans and industrial motors were one of the biggest consumers of power on the grid. Then computers and solid state electronics came along, and suddenly everyone was peak-rectifying - first with massive transformers and bridge rectifiers (which had their own problems) and then eventually with AC input rectification and switchmode supplies. It caught a lot of people by surprise; not only did it cause problems with grid transformers, it even affected distribution wiring. Suddenly the return on a three-phase system wasn't just a "just in case of imbalance" conductor; it started carrying serious current. Indeed, with peak rectification, you could even get cases where the neutral carried more current than any of the three phases, which was very non-inutitive for people used to dealing with the traditional problems of bad power factor.

        The advent of power factor requirements for larger power supplies has, fortunately, started to ameliorate the problem.
        Last edited by jflorey2; 12-31-2016, 01:35 PM.

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        • #19
          You pretty much got it. While a bad PF can add to a commercial or industrial electric bill (low PF penalties) it is pretty much ignored until their distribution transformers run out of space due to all of the losses. To improve the PF, capacitors were added. The problem with installing PF capacitors is that they increase the distortion of the 11th harmonic. That only added to the issue of the 5th and 7th which are usually generated by 3 phase electronic motor drives.

          Harmonics caught a lot of people by surprise starting with IBM when they added all of those personal computers in one of their offices. Those computers increased the "triplian" harmonics (3rd, 9th, 15th) which also caused the neutral conductor to carrier 2 to 3 times the current is should have. There is a simple way to eliminate those harmonics.

          IBM was lucky that they had a contractor working for them at the time who identified and resolved those harmonics.

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          • #20
            Originally posted by J.P.M. View Post

            Understood. I bet we do have friends of friends. Long, boring story, I once was at a crossroads of changing careers to music. It never was much more than a hobby/interest - percussion, mostly drums - or to serious engineering....
            "Q: What do you call someone who likes to hang out with musicians?




            A: A drummer. "


            Highlight to see the answer.



            SunnyBoy 3000 US, 18 BP Solar 175B panels.

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            • #21
              Originally posted by inetdog View Post

              "Q: What do you call someone who likes to hang out with musicians?




              A: A drummer. "


              Highlight to see the answer.


              That would then be someone with no musical talent who hangs out with musicians.

              Onward.

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