9424200996 15-1
BE1-11m Phase Current Differential (87) Protection
15 • Phase Current Differential (87)
Protection
The phase-current differential (87) element monitors the differential current and provides differential
protection for motors. This element is available only in style MxxxxxxxPxxxxx of the BE1-11m.
Element logic connections are made on the BESTlogic™Plus screen in BESTCOMSPlus® and element
operational settings are configured on the Phase Current Differential (87) settings screen in
BESTCOMSPlus. A summary of the logic inputs and outputs and operational settings appears at the end
of this chapter.
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Current, Phase Differential (87)
HMI Navigation Path: Settings Explorer, Protection, Settings Group x (where x = 0 to 3), Current
Protection, Differential 87
Element Operation
The phase-current differential element compares the currents entering and leaving the protected motor. If
a fault is detected, the BE1-11m initiates a trip signal to isolate the motor. This action limits damage to the
motor and minimizes impact on the power system.
Mode
In Percent Differential Mode, the phase-current differential element provides three-phase percentage-
restrained differential protection with dual slopes. In Flux Balance mode, the differential CTs are
connected in a flux-balancing configuration.
CT Source (Flux Balance Mode Only)
The CT Source setting configures the phase current differential element to monitor CT circuit 1 or CT
circuit 2 on protection systems equipped with two sets of CTs. CT circuit 1 terminals are designated D1
(IA1) through D8 (IG1) and CT circuit 2 terminals are designated F1 (IA2) through F8 (IG2). For an
illustration, refer to the Terminals and Connectors chapter.
Pickup and Trip
The Pickup output occurs first, followed by the Trip output.
Pickup
The phase-current differential element asserts the Pickup output in a different manner for each mode.
Percent Differential Mode
The Pickup output becomes true when the ratio of operating current to restraint current increases above
the slope setting. The operating current is greater than the Minimum Restrained Pickup setting in any of
the three phases.
Flux Balance Mode
The Pickup output becomes true when the measured difference current in any phase exceeds the Flux
Balance Pickup level.
In BESTlogicPlus, the Pickup output can be connected to other logic elements to annunciate the
condition, control other elements in logic, and start the fault recorder (logic element FAULTTRIG).
Assertion of the Pickup output initiates a timer that begins timing to a trip. The duration of the timer is
established by the Time Delay setting.
If the pickup condition subsides before the element delay expires, the timer and Pickup output are reset,
no corrective action is taken, and the element is rearmed for any other fault conditions.
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