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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