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Connecting two zero sequence CT in parallel

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    Connecting two zero sequence CT in parallel

    Hello friends
    I have a strange case that I would like to share with you and take your advice
    I have a MV switchgear divided in two sections (A&B) and a Tie breaker between them
    each section has a feeding breaker
    The strange side that both feeders of the switchgear will be connected to one breaker (one breaker supplying power to two breakers) ... actually this is the provided design and it is out of my scope.
    a zero sequence CT shall be connected on the outgoing cables (6 cables) of the source breaker for earth fault protection , but as i mentioned before the 6 cables will be divided, each three cables will go to breaker
    the cables size is 630 mm2 and the provided CT is not enough to put 6 cable inside (the CT inner diameter is 200mm), so can two CTs be used ( one CT for each circuit) and connect the CTs secondary together then to the protection relay ??!

    Click image for larger version

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    Please advice and do not hesitate to ask if the subject is unclear

    #2
    170921 EDT

    Theoretically yes.

    You can parallel ideal current sources to a finite impedance, but not series them. With practical devices and instrumentation will the function work well, and will you get the answer you want?

    You can series ideal voltage sources, but you can not parallel them.

    Another question to ask is: if you parallel measurement current transformers do you get the instantaneous or average current value for your intended purpose?

    .

    Comment


      #3
      The other minor point is that the paralleled CTs will detect only a ground fault current in the total circuit. Not likely to happen, but equal but opposite fault currents in the two branches would not show up on the combined CT output.
      This is not a practical problem, nor is it even a technical quibble if the specification goal is only to detect zero sequence current on the three wire source circuit.

      FWIW (which is not a lot unless you are touching the terminals), your CT placement would not detect a ground fault in the splitter wiring connecting the single input run to the two output runs.

      Comment


        #4
        imo use 2 relays or another ct (summed 1 per phase)

        Comment


          #5
          Why not just get a ZSCT with a big enough window? Seems like a lot of trouble.
          __________________________________________________ ____________________________
          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


            #6
            http://m.energy.siemens.com/us/pool/...chtopics50.pdf

            When there are too many cables for onezero-sequence ground CT, multiple zero-sequence CTs can be used. It is essentialthat each set of three-phase cables and thecorresponding ground cables pass througha single, zero-sequence CT. For example,with three cables per phase, two three-phase sets of cable and the correspondingground cables could pass through one
            CT, and the remaining three-phase set
            of cables and the corresponding groundcables would pass through a second CT.This assures that all of the currents arebalanced, and that the continuous current-carrying capability of the zero-sequenceground CT is not exceeded.

            Comment


              #7
              Originally posted by Ingenieur View Post
              imo use 2 relays or another ct (summed 1 per phase)
              I think that's what I would do.
              Si hoc legere scis nimium eruditionis habes.

              Comment


                #8
                After thinking about it he likely has 2 0 seq ct's already
                that would give him better resolution than 3 phase ct since the ratio could be much smaller
                may be moot if unlimited gf (vs ngr system)
                both will saturate on thecway to tripping

                a bunch of ways to skin this cat

                Comment


                  #9
                  Just a question......
                  What does the protection relay trip?
                  Si hoc legere scis nimium eruditionis habes.

                  Comment


                    #10
                    Originally posted by Besoeker View Post
                    Just a question......
                    What does the protection relay trip?
                    good question
                    I assume a shunt or uv on cb1?
                    likely shunt
                    people don't like resetting uv's after a power outage

                    Comment


                      #11
                      Originally posted by Ingenieur View Post
                      good question
                      I assume a shunt or uv on cb1?
                      Possibly so.
                      But with two CTs and two relays why not protect the circuits independently?
                      Si hoc legere scis nimium eruditionis habes.

                      Comment


                        #12
                        Originally posted by Besoeker View Post
                        Possibly so.
                        But with two CTs and two relays why not protect the circuits independently?
                        fed from a common cb
                        opening cba or b won't clear a fault between them and cb1

                        Comment


                          #13
                          Originally posted by Ingenieur View Post
                          fed from a common cb
                          opening cba or b won't clear a fault between them and cb1
                          But depending on the location of the fault, the two CT system will not even necessarily detect a fault where you propose.

                          Comment


                            #14
                            Originally posted by GoldDigger View Post
                            But depending on the location of the fault, the two CT system will not even necessarily detect a fault where you propose.
                            not talking about detecting but clearing

                            not sure why the relay anyways
                            ngr?
                            inst only cb?
                            lower trip than afforded by cb?
                            leakage?

                            a gf will trip the cb if the system is grounded
                            should be in the inst range and I doubt the relay would be faster

                            Comment


                              #15
                              Originally posted by gar View Post
                              170921 EDT

                              Theoretically yes.

                              You can parallel ideal current sources to a finite impedance, but not series them. With practical devices and instrumentation will the function work well, and will you get the answer you want?

                              You can series ideal voltage sources, but you can not parallel them.

                              Another question to ask is: if you parallel measurement current transformers do you get the instantaneous or average current value for your intended purpose?

                              .
                              The most thing i care about that the gf will work properly or not, the CT shall trip CB1 only,if any unbalance current in one of the two circuits happened, will it trip CB1?

                              Comment

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