Programmable  Current  Source

 

 Model CT-4000 uses a state-of-the-art all solid state

 programmable current source.  The synthesized ac waveform has

 excellent purity,  with low distortion and frequency accuracy 

 better than 0.05%.  The synthesized waveform is modulated by

 the computer to a high degree of resolution so that the applied test

 current is accurate over the entire range of test levels.   Model

 CT-4000 solid state current source is not subject to ac power line

 bounce and noise (as compared to the conventional  motor driven

 variable transformer method ) and this contributes to improved

 test accuracy and repeatability.

 

 When Model CT-4000 test current is ramped to value, all critical

 system test points are monitored for over range conditions.  In

 case of misconnection or fault, the test current is immediately

 ramped to zero, and an appropriate error message is displayed. 

 This  protects the CT under test and the test equipment.  One

 ramp-up, measurement and ramp-down cycle takes about 1.0

 seconds.   This is  ample time for  high speed electronics to

 precisely read the current and phase values required for CT

 testing.

 

 Test Method 

 

 The basic test method of Model CT-4000 is to compare the CT

 under test to an internal reference who’s characteristics are

 accurately known.   The internal reference CT is a high quality

 low loss toroid operated at very low flux density.   The computer

 can evaluate the reference characteristics at any test level,  and

 correct the actual reference current  so that it appears to be a

 perfect  reference (without significant error).  The precision

 electronic comparator determines the difference between the CT

 under test secondary current and the reference secondary current.

 The comparator  difference vector is added to the corrected

 reference vector to calculate the CT under test RCF and Phase.

 

 This method  allows for accurate testing at any test point,  since

 the computer can determine the  required reference correction 

 regardless of  ratio, test level or burden of the  CT under test.

 

 Accuracy

 

 The basic  R.C.F. accuracy  of  Model CT-4000 is .018%,

 suitable for metering or relaying applications.   Accuracy at most

 ratios and test levels is better than 0.015%.  Reference accuracy 

 is approx. 0.01%.   Accuracy is best for metering class and less

 for relaying.  Measurements are certified by National Research

 Council Canada.   Detailed accuracy statements on request.

 Comparator accuracy  is .00001  + 0.003 (1.00000 –  CT

 under test ratio).    Test level accuracy is  0.5% of test level

 + 0.5% Full Scale.    Programmable measurement

 corrections (bias) can correct for small errors at any test

 point.  Bias values are stored with the test program but do

 not appear in the printouts. Traceability to international

 standards is assured by transfer standards.  Provision is

 included for on site calibration verification at any time.  

 

 Burdens

 

 Burden impedance accuracy is 1% + 10 milliohms measured

 at the burden terminals. Burdens may be passive, or a

 combination of  virtual and passive burdens.

 

 Test Fixtures

 

  Following are typical test fixtures. 

 

Test Fixture with High Current Bus Bar

Test Fixture with 4/0 Flexible Cable

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