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
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.
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.
Burden impedance accuracy is 1% + 10 milliohms measured
at the burden terminals. Burdens may be passive, or a
combination of virtual and passive burdens.
Following are typical test fixtures.
Test Fixture with High Current Bus Bar
Test Fixture with 4/0 Flexible Cable