E.1.1 Mm3 Ct Withstand - GE MM3 Instruction Manual

APPENDIX E
APPENDIX E CT ISOLATIONE.1 CT ISOLATION
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, while the older electromechanical relays
have typically high burdens (e.g. 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.
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
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 100 V at 20 × 5A, 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.
The curve below represents a typical excitation curve for a CT. The Y-axis represents secondary exciting volt-
age; the X-axis represents the secondary exciting current. When the CT secondary exciting voltage level is
picked off the graph, the corresponding secondary exciting current is the amount of current required to excite
the core of the CT. With respect to the ideal CT that conforms perfectly to its ratio, the exciting current could be
considered loss.
GE Power Management MM3 Motor Manager 3
E.1 CT ISOLATION

E.1.1 MM3 CT WITHSTAND

E.1.2 CT SIZE AND SATURATION
E
E-1