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Circulating currents in banks of transformers with star-delta connection

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    Circulating currents in banks of transformers with star-delta connection

    Firts of all i want to apologize for the gramatical errors, i'm not a native english speaker, but i really need help with this topic.
    I'm an intern in the electrical company of my country, at the present time the company is focusing its efforts to reduce tha technical and non technical loses in the distribution system, i was assigned the task of look into the behavior of star-delta connection transformers banks of high consumers, my job is basically use a amperimetric clamp and measure the currents of MV side, LV side and circulating currents, also power factor, impedancy of the bank and position of the tap's selector. The electrical system in my country has been left behind for decades making it a non-regulated system until now, i often find customers with circulating currents in the LV side, 8 times higher than the most high line current, note that almost all clients have unbalanced loads and power factor up to 0.9, what makes this to happen? if you see how much current enters in the primary and how much is out trought the secondary you can see heavy losses, what makes de banks to have this kind of behavior? the currents between phases shouldn't be line currents/√3 ? an unbalanced bank in delta will have phase currents higher than tha line currents in an effort to "balance" the loads? I know that for a real analysis we need the angles of the currents, but with the information avaliable, what conclusions can we obtain?

    This is a short list of the costumers that I've inspected, the blue was de only one with a "theoretical" behavior, the others seems to have a erratical demeanor.

    Client Primary Voltage Secundary Voltage Total KVA of the bank Impedancy's % Primary Current A Primary Current B Primary Current C Secundary Current A Secundary Current B Secundary Current C Neutral Current AB Current BC Current CA Current
    FARAJ JESUS 13,8 120/240 225 2,32 0,4 1 1,7 62,1 28,1 47,2 15,9 28,8 58 61,3
    CANAHUATI MOURRA ISSA ABDALA 13,8 240 225 2,39 0,2 4,1 5,5 94,9 185 74,8 27,9 104,3 153,2 89,1
    MINISTERIO EDUC CTHA 13,8 240 225 1,7 1,1 0,1 1,1 58,9 65,2 8 6 40,9 2,8 38,7
    ZIPPERS DE HONDURAS S A 13,8 120/240 225 1,8 1,7 2,9 2 11,2 15,9 0,3 10,7 84,9 105,3 89,7
    COMERCIAL MADERERA NORIEGA 13,8 240/480 501 2,8 2,7 5,8 5,4 48,7 76 67,7 0,9 51,4 94,8 54
    AGUA DE CHOLOMA 13,8 240/480 300 1,84 8,2 11,8 3,7 205 211 208 7,3 149,9 181,3 69
    CORDOBA RODOLFO 13,8 240/480 225 NV 7,5 7,3 7,2 22,1 10,1 15,8 34,3 230 215 225
    EXA S A 34,5 240 55 NV 0,3 0,5 0,3 66 91 27,8 23,6 63,1 44,3 36,1
    INVERSIONES MONSOL 34,5 240 225 NV 1,25 0,3 0,6 24,2 20,6 22 2,7 86,1 88,7 99,4
    BABUN SIKAFFY ROBERTO RAMIRO 34,5 240 250 NV 1,4 1,7 0,1 76,8 73,7 63,1 9 2,3 65 74,1
    PRODUCTOS MINERALES VENTURA 34,5 240 300 2,3 0,5 12,6 1,6 241 233 140,1 11 231 75,4 73
    COMPAÑÍA AZUCARERA CHUMBAGUA 34,5 480 501 3,1 1,4 2,4 1,4 3,3 0,2 1,6 *** 53,6 50,9 55,2
    COOPROLAVE 34,5 240 225 2,18 0,3 0,4 0,3 19,6 29 16,9 3,3 21,3 28,9 34,9
    MOLINA ROSALES JOSE ANTONIO 34,5 240 300 2,28 y2,23 3,4 3,9 3 456 436 422 *** 252 286 253
    FUNDACION ESCUELA MHOTIVO 13,8 120/240 501 2 2,2 5,1 0,8 208 233 90,3 7,8 94,6 9,3 96,1
    CHAVEZ M JUAN CARLOS 13,8 240/480 501 2,2 2,5 4,2 1,4 131,8 134,8 128,3 3,3 25,1 58,3 71,6
    BENEFICIO MENDEZ SA 13,8 120/240 300 1,79 0,2 0,1 0,1 7,8 7,6 7,8 0,3 2,5 5,3 2,4
    INAMI 13,8 120/241 225 2 0,1 0,1 0,1 4,3 2,8 0,1 1,3 4,3 0,2 0,1

    Click image for larger version

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    #2
    Diego,

    Lots of theory you will need to review with your senior supervising engineers.

    I'm not sure what circulating currents you are concerned about.

    To determine losses, calculate the kVA on the primary and then kVA on the secondary and compare them (divide one by the other). For older transformers, 97% efficiency is pretty normal, especially if there are harmonics it will be likely lower.
    Ron

    Comment


      #3
      Diego...

      Your presentation lacks consistency! Can you provide parameters obtained from Open-Circuit and Closed-Circuit tests?

      Regards, Phil Corso

      Comment


        #4
        Star delta seems like an odd configuration. What are their applications and why are they used?

        (I assume this is on topic enough)
        Ethan Brush - East West Electric. NY, WA. MA

        "You can't generalize"

        Comment


          #5
          Originally posted by DiegoZ View Post
          This is a short list of the costumers that I've inspected,
          Not sure about the ethics of listing out clients names on a forum.
          Maybe the mods could comment on this for future reference.
          Si hoc legere scis nimium eruditionis habes.

          Comment


            #6
            Originally posted by Besoeker View Post
            Not sure about the ethics of listing out clients names on a forum.
            Maybe the mods could comment on this for future reference.
            Dont worry, I used fake names for the clients.

            Comment


              #7
              Originally posted by electrofelon View Post
              Star delta seems like an odd configuration. What are their applications and why are they used?

              (I assume this is on topic enough)
              There are many uses for star delta configuration in my country, like water pumps, fans for poultry farms , motors for coffee factories etc.

              Comment


                #8
                Originally posted by ron View Post
                Diego,

                Lots of theory you will need to review with your senior supervising engineers.

                I'm not sure what circulating currents you are concerned about.

                To determine losses, calculate the kVA on the primary and then kVA on the secondary and compare them (divide one by the other). For older transformers, 97% efficiency is pretty normal, especially if there are harmonics it will be likely lower.

                Click image for larger version

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ID:	2379800 here you can see a good example of my problem, this is the "Cordoba Rodolfo" case, here we have a 3*75kva star-delta bank with a transformation relation of 13.8kV to 480/240 v , this bank is used to feed a poultry farm, if you see the images 3 and 4 you can see the currents in one secondary phase is just 22.1 A, meanwhile in the "bridges" that connects the invidivual transformers into a delta connection, (images 1 and 2) the currents are up to 215 A , it is important to remark that the other currents in rest of the bridges are similar (more or less 200A) and in the secondary you only have currents about 20A, i dont know the angles of the currents nor the voltajes, this is why i can calculate the real efficiency, if a use only apparent power the effiency of this case in specific is only 7.22% , and this is not the only case where the efficiency is below than 50%.

                Comment


                  #9
                  Originally posted by ron View Post
                  Diego,

                  Lots of theory you will need to review with your senior supervising engineers.

                  I'm not sure what circulating currents you are concerned about.

                  To determine losses, calculate the kVA on the primary and then kVA on the secondary and compare them (divide one by the other). For older transformers, 97% efficiency is pretty normal, especially if there are harmonics it will be likely lower.
                  Originally posted by Phil Corso View Post
                  Diego...

                  Your presentation lacks consistency! Can you provide parameters obtained from Open-Circuit and Closed-Circuit tests?

                  Regards, Phil Corso
                  Sadly we cant cut the service of the clients, and my supervisor told me to measure the currents in normal operation conditions for every client, so i only have the information on the transformer data plate, the currents that i measured, and the voltages of the distribution net.

                  Comment

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