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    Faulty Transformer

    Hello All. We recently moved into a new commercial faculty. We are trying to figure out our high no load power consumption. Our first bill showed a consumption of 3400kWh while we were moving. During the move we did not use any equipment other than computers, lights and AC for a 1500SQFT office.

    We did some digging and found that our Delta-Delta 480V to 240V transformer was putting out a lot of heat. We shut off all the loads on the secondary and measured input current on the 480V primary.

    Primary input current (No load)
    L1 = 8A
    L2 = 8.4A
    L3 = 11.5A

    Is this typical input current for a unloaded 150kVA transformer?

    I just did some more detailed measurements with equipment running and it appears the primary input current is unbalanced. However the voltages are close.

    Primary input current (Loaded)
    L1 = 29.2A
    L2 = 11.4A
    L3 = 34.5A

    Primary phase to phase voltages (Loaded)
    L1-L2 = 494V
    L2-L3 = 491V
    L1-L3 = 491V

    Primary phase to neutral voltages (Loaded)
    L1 = 284V
    L2 = 285V
    L3 = 283V

    Secondary phase to phase voltages (Loaded)
    L1-L2 = 247V
    L2-L3 = 244V
    L1-L3 = 243V


    Secondary phase to neutral voltages (Loaded)
    L1 = 122V
    L2 = 213V
    L3 = 122V

    I called the utility out and they claim the power outside of the building is fine. My electrician thinks the utility is sending us unbalanced power causing the transformer to heat up.

    Any ideas? Any help would be appreciated.


    Click image for larger version

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    #2
    190717-1799 EDT

    RV6:

    My electrician thinks the utility is sending us unbalanced power causing the transformer to heat up.
    I don't think this electrician knows what he is talking about. What would be the definition of unbalanced power? Does he mean unbalanced voltage? You don't have that. Your voltage balance on both primary and secondary is very good. I do think "L2 = 213V" is a typo, and should read 123. Other measurements do not support a reading of 213.

    Whether you have neutrals or not you can imagine you do. On the primary this would be 277 V nominal line to neutral.

    Each phase can supply 1/3 of the kVA rating. Thus, full load phase current is 50,000/277 = 180 A. A no load line input current of 10 A is peanuts.

    If this were a resistive load it would be about 3000 W.

    But it is mostly inductive. But suppose it was all power loss, then you have a continuous 10,000 W load or 10 kW.

    Over a 30 day period this would be 320*10 = 320 kWH. This is 1/10 the 3400 you indicated was the bill.

    Further your transformer is not itself using 320 kWH because much of the noload current is not resistive.

    Someone else can give you a realistic estimate of actual losses in the transformer.

    .

    Comment


      #3
      Originally posted by gar View Post
      190717-1799 EDT

      I do think "L2 = 213V" is a typo, and should read 123. Other measurements do not support a reading of 213.
      The secondary is a center tapped 240/120V delta. The nominal voltage for L2 would be 208V, so 213 looks okay.
      Just because you can, doesn't mean you should.

      Comment


        #4
        Originally posted by RV6 View Post
        We did some digging and found that our Delta-Delta 480V to 240V transformer was putting out a lot of heat. We shut off all the loads on the secondary and measured input current on the 480V primary.
        There is no way to know that the transformer is putting out too much heat without using a thermometer.
        For a transformer, like your, I wold expect that it feel like a space heater.
        Just because you can, doesn't mean you should.

        Comment


          #5
          Originally posted by jim dungar View Post
          There is no way to know that the transformer is putting out too much heat without using a thermometer.
          For a transformer, like your, I wold expect that it feel like a space heater.
          Thanks for the replies!

          The core of the transformer measures around 250F no load. Certain parts of the case are uncomfortable to touch.

          Comment


            #6
            Originally posted by RV6 View Post
            The core of the transformer measures around 250F no load. Certain parts of the case are uncomfortable to touch.
            If you can touch it at all, it is not overheating.
            The center, internal areas, of the transformer is rated for 150C over a 40C ambient.
            Just because you can, doesn't mean you should.

            Comment


              #7
              190717-2107 EDT

              Sorry I did not see the center tapped winding. Yes the voltages as displayed in the post are good.

              .

              Comment


                #8
                Remember, Watts = Amps x Volts x Power Factor (x1.732 for 3 phase). The current readings are relatively meaningless because the Power Factor on an unloaded transformer is likely very low, maybe even as low as .3 or less. That means that the WATTS, what you are actually billed for, will be low as well.
                __________________________________________________ ____________________________
                Many people are shocked when they discover I am not a good electrician...

                I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

                Comment


                  #9
                  190718-1307 EDT

                  Jaref's ballpark for PF = 0.3 is certainly a good estimating value with no other knowledge available.

                  I checked PF on two small transformers using a Kill-A-Watt EZ:

                  1. Stancor P8662 nominal 120 V in, and 24 V at 2 A out (VA out = 48).

                  No load input at 123 V
                  --- 0.04 A, 3.3 W, 5.8 VA, 0.59 PF

                  2. Signal Transformer A41-175-24 nominal 120 V in, and 24 V at 7.3 A out (VA out = 175).

                  No load input at 123 V
                  --- 0.23 A, 4.5 W, 28.8 VA, 0.15 PF


                  Big difference. Transformer design will determine how these values fall out

                  Comment


                    #10
                    Originally posted by jim dungar View Post
                    There is no way to know that the transformer is putting out too much heat without using a thermometer.
                    For a transformer, like your, I wold expect that it feel like a space heater.
                    No there isn't, but the heat lost comes from true power losses/resistance and not from reactive current. The more reactive current you have however the more current flows through conductors and the more resistance losses you will have.
                    I live for today, I'm just a day behind.

                    Comment


                      #11
                      190718-1943 EDT

                      When you measure the real power input to an unloaded transformer, then that power defines the transformer temperature rise over ambient for whatever cooling conditions exist.

                      This noload power measurement includes all losses from the reactive current I^2*R, and core losses at noload.

                      .

                      Comment


                        #12
                        Thank you all for the help. They sent out another electrician yesterday to do some measurements. The electrician is going to call the transformer manufacture and see what the transformer losses should be. I might have to look into buying or renting a power analyzer.


                        We are considering going with a smaller transformer or putting a disconnect on the larger transformer and running small transformer for the lights and office.


                        Meanwhile I will check what the voltage tap is set at on the primary and measure the 480V line voltage during the off hours. Setting it for a higher voltage should reduce the losses.


                        I will post with what we come up with.

                        Thanks again for the help.

                        Comment


                          #13
                          Originally posted by RV6 View Post
                          Thank you all for the help. They sent out another electrician yesterday to do some measurements. The electrician is going to call the transformer manufacture and see what the transformer losses should be. I might have to look into buying or renting a power analyzer.


                          We are considering going with a smaller transformer or putting a disconnect on the larger transformer and running small transformer for the lights and office.


                          Meanwhile I will check what the voltage tap is set at on the primary and measure the 480V line voltage during the off hours. Setting it for a higher voltage should reduce the losses.


                          I will post with what we come up with.

                          Thanks again for the help.
                          Voltage may drop once you have some significant loads - if you are going to have any.
                          I live for today, I'm just a day behind.

                          Comment


                            #14
                            Originally posted by RV6 View Post
                            Thank you all for the help. They sent out another electrician yesterday to do some measurements. The electrician is going to call the transformer manufacture and see what the transformer losses should be. I might have to look into buying or renting a power analyzer.


                            We are considering going with a smaller transformer or putting a disconnect on the larger transformer and running small transformer for the lights and office.


                            Meanwhile I will check what the voltage tap is set at on the primary and measure the 480V line voltage during the off hours. Setting it for a higher voltage should reduce the losses.


                            I will post with what we come up with.

                            Thanks again for the help.
                            Why don't you just have the transformer tested?

                            Comment


                              #15
                              7.2.1.1 Transformers, Dry Type, Air-Cooled, Low-Voltage, Small
                              Page 28
                              ANSI/NETA MTS-2019
                              NOTE: This category consists of power transformers with windings rated 600 volts or less and
                              sizes equal to or less than 167 kVA single-phase or 500 kVA three-phase.

                              A. Visual and Mechanical Inspection
                              1. Inspect physical and mechanical condition.
                              2. Inspect anchorage, alignment, and grounding.
                              3. Prior to cleaning the unit, perform as-found tests, if required.
                              4. Clean the unit.
                              5. Inspect bolted electrical connections for high resistance using one or more of the following
                              methods:
                              1. Use a of low-resistance ohmmeter in accordance with Section 7.2.1.1.B.1.
                              2. Verify tightness of accessible bolted electrical connections by calibrated torquewrench
                              method in accordance with manufacturer’s published data or Table 100.12.
                              3. Perform a thermographic survey in accordance with Section 9.
                              6. Perform as-left tests.
                              7. Verify that as-left tap connections are as specified.

                              B. Electrical
                              1. Perform resistance measurements through bolted connections with a low-resistance
                              ohmmeter in accordance with Section 7.2.1.1.A.5.1.
                              2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply
                              voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s
                              published data, use Table 100.5. Calculate the dielectric absorption ratio or polarization
                              index.
                              *3. Perform turns-ratio tests at the designated tap position.

                              C. Test Values – Visual and Mechanical
                              1. Compare bolted connection resistance values to values of similar connections. Investigate
                              values which deviate from those of similar bolted connections by more than 50 percent of
                              the lowest value. (7.2.1.1.A.5.1)
                              2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the
                              absence of manufacturer’s published data, use Table 100.12. (7.2.1.1.A.5.2)
                              3. Results of the thermographic survey shall be in accordance with Section 9. (7.2.1.1.A.5.3)
                              4. Tap connections are left as found unless otherwise specified. (7.2.1.1.A.7)

                              D. Test Values – Electrical
                              1. Compare bolted electrical connection resistances to values of similar connections.
                              Investigate values which deviate from those of similar bolted connections by more than 50
                              percent of the lowest value.
                              2. Minimum insulation-resistance values of transformer insulation should be in accordance
                              with manufacturer’s published data. In the absence of manufacturer’s published data, use
                              Table 100.5. Values of insulation resistance less than this table or manufacturer’s
                              recommendations should be investigated. The dielectric absorption ratio or polarization
                              index shall be compared to previously obtained results and should not be less than 1.0.
                              3. Turns-ratio test results should not deviate more than one-half percent from either the
                              adjacent coils or the calculated ratio.

                              Comment

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