D2843-01.vsd
02-08-99
Restraint
Current
Operating
Current
Restrained
Element
Fund
I
op
Minimum
Pickup
Slope
87ND Restrained Pickup
Tap
Adjustment
Fundamental
Phasor
Fund
per unit
Ground
Phasor
87ND
Restrained
Trip
TM
Settable
Delay
Figure 4-17. 87 Neutral Differential Protection Functional Block Diagram
The characteristic of the Restrained Element function is the same as that for the phase differential shown
in Figure 4-10 with the exception that maximum restraint is always used. This comparator has a slope
setting and a minimum pickup setting. The slope setting is the ratio of operate current to restraint current.
The minimum pickup setting determines the minimum sensitivity. If the ratio of operate current to restraint
current is above the slope setting and the operate current is above the minimum pickup setting, the
87NDPU (87 neutral element picked up) logic output is set.
A timer provides security from misoperation on false residual caused by CT saturation during a through
fault. If the transient monitor function from the 87 phase differential function detects CT saturation, the
87NDT trip logic output is routed through the 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.
If the target is enabled for this function, the target reporting function will record an 87ND target when the
87NDT logic output is TRUE and the fault recording function
trip
logic expression is TRUE. See Section 6,
Reporting and Alarm Functions, Fault Reporting,
for more details on the target reporting function.
BESTlogic Settings for Neutral Differential
BESTlogic settings are made from the
BESTlogic Function Element
screen in BESTCOMS. Figure 4-18
illustrates the BESTCOMS screen used to select BESTlogic settings for the 87ND function. To open the
BESTlogic Function Element
screen, select
Percentage Differential
from the
Screens
pull-down menu.
Then select the
87ND/187ND
tab. Select the appropriate
BESTlogic
button for 87ND or 187ND.
Alternately, these settings can be made using the SL-87ND and SL-187ND ASCII commands.
Figure 4-18. BESTlogic Function Element Screen, Neutral (87ND)
4-20
BE1-CDS240 Protection and Control
9365200990 Rev F
Summary of Contents for BE1-CDS240
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Page 40: ...ii BE1 CDS240 Quick Start 9365200990 Rev F This page intentionally left blank ...
Page 152: ...ii BE1 CDS240 Metering 9365200990 Rev F This page intentionally left blank ...
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Page 292: ...ii BE1 CDS240 Human Machine Interface 9365200990 Rev F This page intentionally left blank ...
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Page 349: ...Figure 12 5 Horizontal Rack Mount Front View 9365200990 Rev F BE1 CDS240 Installation 12 5 ...
Page 361: ...Figure 12 17 Typical DC Connection Diagrams 9365200990 Rev F BE1 CDS240 Installation 12 17 ...
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