Plus Ct Withstand; Ct Size And Saturation - GE 269Plus Instruction Manual

H CURRENT TRANSFORMERS
APPENDIX HH CURRENT TRANSFORMERS
When is withstand important?
Withstand is important when the phase or ground CT has the capability of driving a large amount of current into
the interposing CTs in the relay. This typically occurs on retrofit installations when the CTs are not sized to the
burden of the relay. New electronic relays have typically low burdens (2 mΩ for the 269Plus), while the older
electromechanical relays have typically high burdens (1 Ω).
For high current ground faults, the system will be either low resistance or solidly grounded. The limiting factor
that determines the amount of ground fault current that can flow in these types of systems is the capacity of the
source. Withstand is not important for ground fault on high resistance grounded systems. On these systems, a
resistor makes the connection from source to ground at the source (generator, transformer). The resistor value
is chosen such that in the event of a ground fault, the current that flows is limited to a low value, typically 5, 10,
or 20 A.
Since the potential for very large faults exists (ground faults on high resistance grounded systems excluded),
the fault must be cleared as quickly as possible. It is therefore recommended that the time delay for short cir-
cuit and high ground faults be set to instantaneous. Then, the duration for which the 269Plus CTs subjected to
high withstand will be less than 250 ms (the 269Plus reaction time is less than 50 ms + breaker clearing time).
Care must he taken to ensure that the interrupting device is capable of interrupting the poten-
tial fault. If not, some other method of interrupting the fault should be used, and the feature in
question should be disabled (e.g. a fused contactor relies on fuses to interrupt large faults).
NOTE
The 269Plus CTs were subjected to high currents for 250 ms bursts. The CTs were capable of handling 500 A
for short bursts. The 500 A current relates to a 100 times the CT primary rating. If the time duration required is
less than 250 ms, the withstand level will increase.
How do I know how much current my CTs can output?
CT characteristics may be acquired by one of two methods.
The rating (as per ANSI/IEEE C57.13.1) for relaying class CTs may be given in a format such as these:
2.5C100, 10T200, T1OO, 10C50, or C200. The number preceding the letter represents the maximum ratio cor-
rection; no number in this position implies that the CT accuracy remains within a 10% ratio correction from 0 to
20 times rating. The letter is an indication of the CT type. A 'C' (formerly L) represents a CT with a low leakage
flux in the core where there is no appreciable effect on the ratio when used within the limits dictated by the
class and rating. The 'C' stands for calculated; the actual ratio correction should be different from the calcu-
lated ratio correction by no more than 1%. A 'C' type CT is typically a bushing, window, or bar type CT with uni-
formly distributed windings. A 'T' (formerly H) represents a CT with a high leakage flux in the core where there
is significant effect on CT performance. The 'T' stands for test; since the ratio correction is unpredictable, it is to
be determined by test. A 'T' type CT is typically primary wound with unevenly distributed windings. The subse-
quent number specifies the secondary terminal voltage that may be delivered by the full winding at 20 times
rated secondary current without exceeding the ratio correction specified by the first number of the rating.
(Example: a 10C100 can develop 100V at 20 x 5 A, therefore an appropriate external burden would be 1 Ω or
less to allow 20 times rated secondary current with less than 10% ratio correction.) Note that the voltage rating
is at the secondary terminals of the CT and the internal voltage drop across the secondary resistance must be
accounted for in the design of the CT. There are seven voltage ratings: 10, 20, 50, 100, 200, 400, and 800. If a
CT comes close to a higher rating, but does not meet or exceed it, then the CT must be rated to the lower
value.
GE Power Management
Courtesy of NationalSwitchgear.com
269Plus Motor Management Relay

269 PLUS CT WITHSTAND

H.1 269 PLUS CT WITHSTAND
H.2 CT SIZE AND SATURATION
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