Phase Differential - GE 469 Instruction Manual

469
Motor Management Relay

Phase Differential

PHASE
DIFFERENTIAL
NOTE
5–54
increments. The delay can be fine tuned to an application such that it still responds
very fast, but rides through normal operational disturbances. Normally, the Ground
Fault time delays are set as quick as possible, that is, 0 ms. Time may have to be
increased if nuisance tripping occurs.
Special care must be taken when the ground input is wired to the phase CTs in a
residual connection. When a motor starts, the starting current (typically 6 × FLA for
an induction motor) has an asymmetrical component. This asymmetrical current
may cause one phase to see as much as 1.6 times the normal RMS starting current.
This momentary DC component will cause each of the phase CTs to react differently
and the net current into the ground input of the 469 will not be negligible. A 20 ms
block of the ground fault elements when the motor starts enables the 469 to ride
through this momentary ground current signal.
The overreach filter removed the DC component from the asymmetrical current
present at the moment a fault occurs. This results in no overreach whatsoever,
however, the response time slows slightly (10 to 15 ms) but times still remain within
specifications.
PATH: SETPOINTS S6 CURRENT ELEM.
[ ] PHASE DIFFERENTIAL
TRIP: Off
ASSIGN TRIP RELAYS:
MESSAGE
Trip
STARTING DIFF.
MESSAGE
TRIP PICKUP: 0.10 x CT
STARTING DIFF.
MESSAGE
TRIP DELAY: 0 ms
RUNNING DIFF.
MESSAGE
TRIP PICKUP: 0.10 x CT
RUNNING DIFF.
MESSAGE
TRIP DELAY: 0 ms
These setpoints program the differential element when the differential feature is in
use. This feature consists of three instantaneous overcurrent elements for phase
differential protection. Differential protection may be considered first line protection
for phase to phase or phase to ground faults. In the event of such a fault,
differential protection may limit the damage that may occur.
Care must be taken when enabling this feature. If the interrupting device
(contactor or circuit breaker) is not rated to break potential faults, the
feature should be disabled. Alternately, the feature may be assigned to an
auxiliary relay and connected such that it trips an upstream device that is
capable of breaking the fault current. A low level differential fault can
develop into a short circuit in an instant.
A trip occurs once the magnitude of either I
differential) exceeds the Pickup Level × Differential CT Primary for a period of time
specified by the delay. Separate pickup levels and delays are provided for motor
starting and running conditions.
The Differential trip element is programmable as a fraction of the rated CT. The level
may be set more sensitive if the Differential CTs are connected in a flux balancing
configuration (3 CTs). If 6 CTs are used in a summing configuration, the values from
the two CTs on each phase during motor starting may not be equal since the CTs are
not perfectly identical (asymmetrical currents may cause the CTs on each phase to
have different outputs). To prevent nuisance tripping in this configuration, the
level may have to be set less sensitive, or the
STARTING DIFF. TRIP PICKUP
may have to be extended to ride through the problem period during
DIFF. TRIP DELAY
start. The running differential delay can then be fine tuned to an application such
that it responds very fast to sensitive (low) differential current levels.
http://www.GEindustrial.com/multilin
S6 Current Elements
PHASE DIFFERENTIAL
Range: Off, Latched, Unlatched
Range: Trip, Trip & Auxiliary2, Trip &
Aux2 & Aux3, Auxiliary2, Aux2
& Aux3, Auxiliary3
Range: 0.05 to 1.00 x CT in steps of
0.01
Range: 0 to 60000 ms in steps of 10
Range: 0.05 to 1.00 x CT in steps of
0.01
Range: 0 to 1000 ms in steps of 10
-I , I -I , or I
aIN aOUT bIN bOUT
GE Multilin
-I (phase
cIN cOUT
STARTING