Multilin SR469 - Phase Differential

4. SETPOINT PROGRAMMING S6 CURRENT ELEMENTS
4-45
4.7.7 PHASE DIFFERENTIAL
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FUNCTION:
The setpoints here may be used to program the Differential element if 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.
Note: 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 con-
nected such that it trips an upstream device that is capable of breaking the fault current. A low level differential fault can de-
velop into a short circuit in an instant.
The Differential Trip element will function as follows: Once the magnitude of either Ia
IN
-Ia
OUT
, Ib
IN
-Ib
OUT
, or Ic
IN
-Ic
OUT
(phase differential)
exceeds the Pickup Level x Differential CT Primary for a period of time specified by the Delay, a trip will occur. 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, during motor starting, the values
from the two CTs on each phase may not be equal as 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 starting differential level may have to be set
less sensitive, or the starting differential time delay may have to be extended to ride through the problem period during 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.