GE STD15C Instructions Manual page 6

GEK-45307
For Delta-connected CTs
Tap Current
=
LnL
N
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 low as possible.
Calculate the percentage of mismatch as follows: on two—winding transformers,
determine the ratio of the two relay currents and the tap values selected. The
differences between these ratios, divided by the smaller ratio, is the percent
of mismatch. The mismatch normally should not exceed 5%.
For three-winding transformers, the percent of mismatch error should be checked
for all combinations of currents or taps.
If taps cannot be selected to keep this percentage error within allowable
limits,
it will be necessary to choose a different CT ratio on one or more
lines, to obtain a better match between relay currents and relay taps.
5.
Check to see that the sum of the relay currents that will be applied to the
relay for a fault at the terminals of the power transformer is less than 220
amperes RMS for 1 second. If the period during which a fault current flows in
the relay can be definitely limited to a shorter time, a higher current can be
accommodated in accordance with the relation:
(Amperes)? x seconds
48,400
Also check that the suni of the multiples of tap current on an
internal or
external fault do not exceed 150.
CURRENT TRANSFORMER RATIO ERROR
The CT ratio error must be less than 20% at 8 times relay rated tap current. This
is based on the instantaneous unit being set at its normal setting, which is 8 times
tap rating.
If the instantaneous unit pickup is raised above this value, the 20%
figure must be reduced, as described in the CHARACTERISTICS section.
The calculations listed below
are for the worst—fault condition, as far as CT
performance is concerned, which is an internal ground fault between the CT and the
transformer winding, with none of the fault current supplied through the neutral of
the protected transformer.
1. Determine the burden on each CT, using the following expressions:
a. For Wye-connected CTs
=
+
Ne+2f
+
2R Ohms (Equation 1)
b.
For Delta-connected CTs
Z
=
2B
+
Ne
+
2f
+
2R Ohms
(Equation 2)
1000
6