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Characterizing a CT via a major B-H loop?

  1. Jun 28, 2011 #1
    I contend this cannot be done!

    A popular method of modeling non-linear cores is to obtain the values for remnant flux density, saturation flux density, and the coercive force at some major hysteresis curve, and to then mathematically extrapolate a curve (a few other parameters are also needed to interface with the electrical quantities of a circuit - Ac, lm, lg, N). See "Nonlinear Transformer Model for Circuit Simulation" by Chan for the model used in LTSpice. So I figure this may be a good approach towards characterizing a CT- and I can use it in LTSpice!

    The problem with simply hooking a constant voltage source, limiting the current by some resistance, and proceeding as shown http://www.cliftonlaboratories.com/type_43_ferrite_b-h_curve.htm" [Broken] is that the value measured for the coercive force is dependent on the primary current- i.e. once we are in saturation, and a complete B-H loop is defined, current can be further increased, and Hc further increases! (Bs, Br remain constant). I have verified this in lab with a constant voltage source, limited by some large power resistors. I can source anywhere from 0-8A at 60Hz. I use a current probe amplifier (X input) and a passive 300k/2uF integrator, with the output of the capacitor on a 1x scope probe (Y input).

    I believe the error lies in CTs being defined via a constant current, rather than a constant voltage source. I suspect it is for this reason that CTs are not characterized by B-H curves, but rather by excitation curves. Unfortunately, my ability to reason falls short here, and I was hoping for a clarified explanation, or any insight into where my reasoning so far has been wrong.

    FYI: I am attempting to characterize a CTs response for very high fault currents (say, 5,000x that which would saturate it, when we are approaching the short time thermal limit- what happens there?), and would like an alternate model to the http://www2.selinc.com/techpprs/6038.pdf" [Broken]. I have extended the idea presented there for calculating the magnitudes and durations of what trends towards secondary voltage impulses at high primary levels, but would prefer an alternative approach for examination.

    Thanks a bunch.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
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