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Grounding Riddle No.32 – Voltage transformer grounding

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  • #799
    Mahesh

      Is it necessary to connect star point of Primary of VT (Y-Y) to earth in a resistance grounded system?
      Is it necessary to connect star point of Primary of VT (Y-Y) to earth in a resistance grounded system?

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    • #2664
      GLAVIZH

        Primary shoudn’t be grounded at all . In case of three separate single phase VT to form a three phase VT or Shell type VT , also secondary shouldn’t be grounded . Only in case of core type VT , secondary is allowed and to say it’s better to be grounded.

        #2666
        Hamid

          For similar discussion you can refer to Transformer Riddle No.1 .

          #2689
          Mahesh

            Thanks Glavizh & Hamid for your recommendations. The 3 VTs are single phase and connected in wye-earthed / wye earthed configuration. If the primary side & secondary side VT star point earthing is remove as per your recommendation, what are the consequences on the protection & metering fuctions provided by Numerial Protection relay 50/51, 50N/51N, 46, 50BF, 27D, 27R, 27, 59, 59N , 47, 81H/81L , 25 and V, A, kW, kWH.

            #2690
            Hamid

              In three phase core type measuring transformer just removing of primary grounding is recommanded. I think it will not cause any relay malfunctions when you use three seperated VT.

              #2691
              Mahesh

                In my opinion the VT primary impedance is very very high (I do not have actual VT primary impedance value) as compared to the grounding reststor value 1500 Ohm. Hence the VT star point will provide path for very negligible earth fault current. Hence what is the reason to recommed removal of VT primary star point earthing? Kindly elaborate.

                #2693
                Hamid

                  In three single voltage transformer with independent magnetizing flux path, regarding protection requirments it is not important. However sharing of ground fault current path with primary power supply in VT neutral point can to damage the VT when its primary side is grounded.

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