CURRENT TRANSFORMER FACTS AND TUTORIALS

1) Bushing CTs tend to be accurate more on high currents (due to large core and less saturation) than other types.

2) At low currents, BCT’s are less accurate due to their larger exciting currents.

3) Rarely, if ever, is it necessary to determine the phase-angle error.

4) Accuracy calculations need to be made only for three-phase and single-phase to ground faults.

5) CT burden decreases as secondary current increases, because of saturation in the magnetic circuits of relays and other devices. At high saturation, the impedance approaches the dc resistance.

6) It is usually sufficiently accurate to add series burden impedance arithmetically.

7) The reactance of a tapped coil varies as the square of the coil turns, and the resistance varies approximately as the turns.

8) Impedance varies as the square of the pickup current.

9) Burden impedance are always connected in wye.

10) "Ratio correction factor” is defined as that factor by which the marked ratio of a current transformer must be multiplied to obtain the true ratio. These curves are considered standard application data.

11) The secondary-excitation-curve method of accuracy determination does not lend itself to general use except for bushing-type, or other, CT’s with completely distributed secondary leakage, for which the secondary leakage reactance is so small that it may be assumed to be zero.

12) The curve of rms terminal voltage versus rms secondary current is approximately the secondary excitation curve for the test frequency.

13) ASA Accuracy Classification:
a. Method assumes CT is supplying 20 times its rated secondary current to its burden.
b. The CT is classified on the basis of the maximum rms value of voltage that it can maintain at its secondary terminals without its ratio error exceeding a specified amount.
c. "H" stands for high internal secondary impedance.
d. "L" stands for low internal secondary impedance (bushing type).
e. 10H800 means the ratio error is l0% at 20 times rated voltage with a maximum secondary voltage of 800 and high internal secondary impedance.
f. Burden (max) - maximum specified voltage/20 x rated sec.
g. The higher the number after the letter, the better the CT.
h. A given l200/5 busing CT with 240 secondary turns is classified as l0L400: if a 120-turn completely distributed tap is used, then the applicable classification is 10L200.
i. For the same voltage and error classifications, the H transformer is better than the L for currents up to 20 times rated.

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