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Current Transformers

  1. Oct 22, 2009 #1
    Hello , I would like to ask one question. Current transformers has one of the major component in electricity but we know that per load level, VA, temperature ext accuracy of these transformers can change. CT manufacturers gives accuracy diagrams for different load levels from %5 load to full load. But what is happening when the load level is lower than %5. For example if you have 4000/5 current transformer to measure current and if you have load that can go down for sometime to 50A in primary, is it possible that you can have 0.00mA in secondary?

    Some CT manufacturers written specs as <%5 zone unstable zone, where there is no value for accuracy or there is no warranty that CT's are working probably. Is there any technical study that investigate <%5 load conditions?
     
  2. jcsd
  3. Oct 22, 2009 #2
    Yes, for conventional measuring transformers with accuracy class of 0.1, 0.2, 0.5,1,3 and 5 this limitation is %5 but for special current transformers with accuracy class of 0.2S and 0.5S this limitation is %1. Error limits for measuring current transformer cores is defined by DIN VDE 0414-1 and IEC 60185

    For Measuring core rated output: 2.5 – 5.0 – 10 – 15 – 30 VA; burden output factor = 0.8

    Classes: 0.1 – 0.2 – 0.5 – 1: valid in the range of 25 % and 100 % of the rated burden.
    Classes: 0.2 s and 0.5 s: For special applications (electrical meters that measure correctly between 50 mA and 6 A, i.e. between 1% and 120% of the rated current of 5A)
    Classes: 3 – 5: valid in the range 50% to 100% of the rated burden Label: measuring cores are identified by a combination of the rated output with the overcurrent limit factor and with the class, e.g.

    15 VA class 0.5 FS 10
    15 VA class 0.5 ext. 150%

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    Creative thinking is enjoyable, Then think about your surrounding things and other thought products. http://electrical-riddles.com
     
  4. Oct 23, 2009 #3
    Thank you for your answer. So if you have Class 0.5 current transformer for reactive power compansation, and if utility has 0.5s current tranformer for billing your compansation will not work properly and you can be charged for reactive energy.

    But I need to understand why we read 0.00mA or very less current compared to primer in current transformer when there is 50A primer current in 4000/5A current transformer. Where are we loosing this secondary current?

    As far as I understood these classes are specified in maximum burden. So my second question is if have a CT of 30VA burden and if we have load of max 0.3VA . Is there any accuracy loss also due to low burden?
     
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