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    UPS not accepting power from new generator

    I recently bought and installed a stand-by generator for my company's IT dept. The gen. is a 15kw generator by generac. Out put voltage from gen. is 125 volts line to neutral and 250 volts line to line. Freq is 63 hz with no load. Problem is when I shut down main power, generator will start correctly, the automatic transfer switch will transfer power from main to gen power correctly, but the UPS system does not seem to accept the power from the generator. Ups will go from battery mode to online mode and back to battery mode over and over. Has anyone had this problem or have any ideas to help that does not invole having to purchase a high dollar isolation transformer? Thanks for any help.

    #2
    Re: UPS not accepting power from new generator

    What size and type is the UPS?
    John
    Data Center Technical Operations Manager
    Connecticut Unlimited Electrical Contractor E-1
    Associate Member I.A.E.I
    The World Needs More Electricians & Fewer Electrical Installers

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      #3
      Re: UPS not accepting power from new generator

      The ups is a 3-18 kva Axxium pro made by Best power.

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        #4
        Re: UPS not accepting power from new generator

        Is that a 18kVA UPS served by a 15kW generator? Is there other load on the gen in addition to the UPS? How much load does the UPS have on it? Are the input settings on the UPS adjustable (what does the UPS consider poor or loss of power?) before it tries to protect the load and go to battery?
        Is there an input filter on the UPS?
        Do you have a harmonic meter?
        Ron

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          #5
          Re: UPS not accepting power from new generator

          It sounds like this is what is happening:
          </font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">You turn off the main power, so the UPS goes into battery mode.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">The generator starts up and becomes ready to take load.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">The UPS sees the power as being available, and transfers to on-line mode.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">The generator receives a nearly full load all at once, and goes through its loading transient. Frequency and voltage will dip, until the governor and voltage regulator have time to react and to correct them back to normal frequency and voltage.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">The frequency dip or the voltage dip or both are severe enough for the UPS to see a loss of its AC power source. It then transfers back to battery.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">That unloads the generator, which (after another short transient) will return to normal frequency and voltage.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">The UPS will then see that the AC source is available, and go back to online mode.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif"></font>
          • <font size="2" face="Verdana, Helvetica, sans-serif">And around the circle we go again.</font>
          <font size="2" face="Verdana, Helvetica, sans-serif">It sounds like you need to look into the reaction times of the governor and voltage regulator, and see if you can reduce the dips associated with generator loading to within the UPS requirements for input power.
          Charles E. Beck, P.E., Seattle
          Comments based on 2017 NEC unless otherwise noted.

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            #6
            Re: UPS not accepting power from new generator

            Here's the manual for the Axxium Pro.

            When I did a PDF search on the word generator, page 17 shows a Generator Sensitivity settings menu. Page 21 shows a input signal feature that tells the UPS to adjust its thresholds when "On-Generator".

            Bear in mind, after an on-battery event, the battery recharge current contributes to a higher input load.
            John
            Data Center Technical Operations Manager
            Connecticut Unlimited Electrical Contractor E-1
            Associate Member I.A.E.I
            The World Needs More Electricians & Fewer Electrical Installers

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              #7
              Re: UPS not accepting power from new generator

              I've run into this before, with the Best units. The Best UPS can be adjusted for the input voltage and frequency, this can be done with a control panel or software. I have called Best Tech Support before and they are very good and can assist.
              Moderator-Washington State
              Ancora Imparo

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                #8
                Re: UPS not accepting power from new generator

                Quite a common problem, mostly caused by poor planning/engineering or a lack of it.

                1 The generator needs to have twice or more output of the UPS input as a rule of thumb. So if you have a 18KVA UPS, you need at least a 36 KVA generator

                2. The generator needs to be able to communicate with the UPS, so both the generator and UPS must have the capibility to do this. By that I mean the upper end UPS units like Best have an input signal from the generator that tells it it is being put on generator. If the UPS has the option, the Rectifiers will walk-in and gradually step in its load rather than slam the generator with full load.

                What is happening is the UPS is dragging down the generator, when that happens the voltage and frequency sag, and the UPS trips, and the process starts over again.

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                  #9
                  Re: UPS not accepting power from new generator

                  Thanks to all for the help. Generator is too small. The people at Best Power suggested a generator 3.5-5 times bigger than the UPS. Once again, thanks for the help all.

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                    #10
                    Re: UPS not accepting power from new generator

                    Well 3.5 to 5 times is a bit of overkill, but gauranteed to work. 2 to 3 times is the norm.

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                      #11
                      Re: UPS not accepting power from new generator

                      well, i'm sure the people who sell the generators are going to try to solve your problem with selling you something more expensive, that might not be your only alternative. try putting a time delay on the restart of high startup current, non-essential equipment (ie HVACs) and stagger them by 1 minute or so each, that might take care of the dips enough to get a solid feed going to your UPS.

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                        #12
                        Re: UPS not accepting power from new generator

                        never mind... just looked at the size of your generator and realized you probabbly dont have anything like that.

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                          #13
                          Re: UPS not accepting power from new generator

                          Not to take anything away from all of the comments posted so far, but it might be far easier than all this. Look for any laser printers, copiers, etc. on the load side. They may have been there all along and you never noticed them since they would only cause a rattling type harmonic noise in the UPS every time the rectifiers fired. But connect a generator and it knocks it to it's knees. The generator controller sees the frequency spikes and gets really unhappy.

                          Just a thought. Your mileage may vary.

                          Good luck.
                          Brad Darnell
                          Master Electrician -Electrical Contractor
                          IN-NJ-WI-CT-LA-OH-KY-TX-FL-NV-GA-TN-MS-OK-IL-NC-AL-KS

                          "The Code doesn't say what you think it says" - Charles E. Beck, P.E.

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                            #14
                            Re: UPS not accepting power from new generator

                            After the fact comment,

                            The Generac 15KW has stepper motor throttle control system which without any load tends to wobble engine speed (and therefore frequency output)due to incremental step of stepper motor. This smooths out with a little load on the generator.

                            Inverters that are picky about frequency tend to reject the wobbling frequency. Sometimes a little load (couple hundred watts)on generator is all that it take to make it acceptable to the inverter.

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