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Actually this calculation does not mean that all windings will necessarily carry these
load currents continuously.
other windings In proportion to their voltage ratings.
the relay tap setting so that the relay will not operate for any external fault.
3. Select CT ratios so that the secondary current corresponding to maximum
secondary thermal rating
bus. for example. the CT ratio should be selected so that the CT thermal rating will not be ex-
ceeded by the maximum load current in either breaker.
currents can be properly matched by
times lowest current).
For Wye connected CT's
Tap Current
For Delta connected CT's
Tap Current
Where N is the number of CT secondary turns.
4.
Check the matching of relay currents to relay taps to keep the mismatch error as
Calculate the percent of mismatch as follows: on two winding transformers, determine the ratio of
the two relay currents and the tap values
by the smaller
For
ations of currents or taps.
If taps cannot be selected to
necessary to choose a different CT ratio on one
relay currents and relay taps.
Check to see that the sum of the relay currents that
the terminals of'the power transfoner is less
during which a fault current flows in the relay can be definitely limited to
hiqher current can be
(Amperes)'
Also check that the sum of the multiples of tap current on an internal or
exceed
CURRENT TRANSFORMER RATIO ERROR
.
CT ratio error
the instantaneous unit being set at its normal setting which is
pickup is raised above this value, the 20 percent figure must be reduced as described under the
heading CHARACTERISTICS.
The calculations listed below are for the worst fault condition, as far as CT
which is an internal ground fault between the CT and the transfoner winding with none of the fault Current
through the neutral of the protected
Courtesy of NationalSwitchgear.com
100% Transformer KVA
(Line KY)
This is only
amperes).
In the case where a transformer is connected to
N
100%
N
ii the percent of mismatched.
transformers, the percent of mismatch error should be checked for all combin-
this percentage error within allowable limits, it will be
in accordance with the relation:
seconds
be less than 20 percent
convenient
of calculating the currents in
This is the requirement for selecting
Also select CT ratios so that the relay
of the relay taps.
(Highest current not more than 3
The differences
The mismatch normally should not exceed 5:.
more lines to obtain
be applied to the relay for a fault at
220 amperes
8 times relay rated tap Current. This
times tap rating. If the instantaneous
S
does not exceed
ring
as possible.
these ratios, divided
better match between
for 1 second.
If the period
shorter time,
fault do
is concerned
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