Basler Electric BE1-11t - Manual (Page 111)

9424200995 Rev H 99
Neutral Current Differential (87N)
Protection
Two neutral current differential (87N) elements provide sensitive differential protection from phase-to-
ground faults in the Y-connected winding. On impedance grounded systems, ground fault levels may be
reduced below the sensitivity of the phase differential protection. The result is that ground faults within the
protected zone have to be cleared by time delayed backup overcurrent protection if sensitive differential
protection is not available.
The two, identical neutral current differential protection elements are designated 87N-1 and 87N-2.
Element logic connections are made on the BESTlogic
™Plus screen in BESTCOMSPlus® and element
operational settings are configured on the Neutral Current Differential (87N) 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, Neutral Differential (87N)
HMI Navigation Path: Settings Explorer, Protection, Settings Group x (where x = 0 to 3), Current
Protection, Neutral Differential 87N
Element Operation
The 87N element detects an imbalance between the neutral current (3I0) and ground current (IG).
CT Flip
For a legacy CT with an auxiliary CT installed, the CT Flip setting will correct the polarity of the 3I0.
Setting the CT Flip to true will introduce a 180
° phase shift internally in the 3I0 calculation.
CT Source
The CT Source setting configures the neutral current differential element to monitor the ground CT in
either CT circuit 1 or CT Circuit 2. CT circuit 1 ground terminals are designated D7 and D8 and CT Circuit
2 terminals are designated F7 and F8. For an illustration, refer to the
Terminals and Connectors chapter.
Overcorrection Coefficient
The 87N element is directionally supervised by making a comparison of two vectors, the calculated IOP
vector and the current present on the relay IG input terminals. First, the magnitude of the vector (IopMag)
is checked by the equation IopMag = 3I0 + IG to determine if it is above the user defined pickup setting.
Second, the IG quantity, as seen at the relay terminals, is used as the polarizing quantity to determine
directionality (IopDir) by the equation IopDir = IG + (OVCR * 3I0). The overcorrection coefficient (OVCR)
is used to add security to the directional element in the previous equation. For the IopDir check, OVCR is
used to offset the 3I0 measurement by the quantity determined in the Overcorrection Coefficient setting,
which at low levels of 3I0 and IG, will provide greater confidence that the directional criterion is met. The
decision to trip will be made only when IopMag is above the user defined pickup setting and IopDir is
within ±90° of the current present on the IG relay terminals.
Transient Delay
A user-defined transient delay time provides security from misoperation on false residual caused by CT
saturation during a through fault. If the transient monitor function from the phase current differential (87)
function detects CT saturation, the 87N Trip logic output is routed through a timer. The timer should be
set longer than the normal clearing time for a fault just outside the zone of protection to allow it to ride-
through until the external fault is cleared.
BE1-11t Neutral Current Differential (87N) Protection