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Introduction - GE UR Series Instruction Manual

Line differential relay.
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9 APPLICATION OF SETTINGS
9.2 CURRENT DIFFERENTIAL (87L) SETTINGS
A program is available from the GE Power Management website that is quite helpful in selecting settings
for the specific application. Checking the performance of selected element settings with respect to known
NOTE
power system fault parameters makes it relatively simple to choose the optimum settings for the applica-
tion.
This program is also very useful for establishing test parameters. It is strongly recommended this program
be downloaded.
The differential characteristic is primarily defined by four settings: CURRENT DIFF PICKUP, CURRENT DIFF RESTRAINT
1, CURRENT DIFF RESTRAINT 2 and CURRENT DIFF BREAK PT (Breakpoint). As is typical for current-based differential
elements, the settings are a trade-off between operation on internal faults against restraint during external faults.
This setting established the sensitivity of the element to high impedance faults, and it is therefore desirable to choose a low
level, but this can cause a maloperation for an external fault causing CT saturation. The selection of this setting is influ-
enced by the decision to use charging current compensation. If charging current compensation is Enabled, pickup should
be set to a minimum of 150% of the steady-state line charging current, to a lower limit of 10% of CT rating. If charging cur-
rent compensation is Disabled, pickup should be set to a minimum of 250% of the steady-state line charging current to a
lower limit of 10% of CT rating.
If the CT at one terminal can saturate while the CTs at other terminals do not, this setting should be increased by approxi-
mately 20 to 50% (depending on how heavily saturated the one CT is while the other CTs are not saturated) of CT rating to
prevent operation on a close-in external fault.
This setting controls the element characteristic when current is below the breakpoint, where CT errors and saturation
effects are not expected to be significant. The setting is used to provide sensitivity to high impedance internal faults, or
when system configuration limits the fault current to low values. A setting of 10 to 20% is appropriate in most cases, but this
should be raised to 30% if the CTs can perform quite differently during faults.
This setting controls the element characteristic when current is above the breakpoint, where CT errors and saturation
effects are expected to be significant. The setting is used to provide security against high current external faults. A setting
of 30 to 40% is appropriate in most cases, but this should be raised to 50% if the CTs can perform quite differently during
faults.
Note: settings RESTRAINT 1 and RESTRAINT 2 at the same value reverts dual slope bias characteristics into single slope
bias characteristics.
This setting controls the threshold where the relay changes from using the Restraint 1 to the Restraint 2 characteristics,
and is very important. Two approaches can be considered
1.
Setting at 150 to 200% of the maximum emergency load current on the line, on the assumption that a maintained cur-
rent above this level is a fault
2.
Setting below the current level where CT saturation and spurious transient differential currents can be expected.
The first approach gives comparatively more security and less sensitivity; the second approach provides less security for
more sensitivity.
GE Power Management
9.2 CURRENT DIFFERENTIAL (87L) SETTINGS
L90 Line Differential Relay

9.2.1 INTRODUCTION

9.2.2 CURRENT DIFF PICKUP
9.2.3 CURRENT DIFF RESTRAINT 1
9.2.4 CURRENT DIFF RESTRAINT 2
9.2.5 CURRENT DIFF BREAK PT
9
9-3

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L90

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