Current Differential Pickup; Current Differential Break Point; Ct Tap - GE L90 Instruction Manual

CURRENT DIFFERENTIAL (87L) SETTINGS
This software program is also useful for establishing test parameters. It is strongly recommended this program be
downloaded. Look for the "L90 Test Tool" on the GE Grid Solutions website, for example in the L90 support documents at
http://www.gegridsolutions.com/support/l90.htm
The differential characteristic is defined by four settings:
, and
RESTRAINT 2 CURRENT DIFF BREAK PT
a trade-off between operation on internal faults against restraint during external faults.

8.2.2 Current differential pickup

This setting establishes the sensitivity of the element to high impedance faults, and it is therefore good to choose a low
level, but this can cause maloperation for an external fault causing CT saturation. The selection of this setting is influenced
by the decision to use charging current compensation. If charging current compensation is Enabled, set pickup to a
minimum of 150% of the steady-state line charging current, to a lower limit of 10% of CT rating. If charging current
compensation is Disabled, set pickup 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, increase this setting by approximately 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.
8.2.3 Current differential restraint 1
The
CURRENT DIFF RESTRAINT 1
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 increase this to 30% if the CTs can perform quite differently during faults.
8.2.4 Current differential restraint 2
The
CURRENT DIFF RESTRAINT 2
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 increase this to 70% if the CTs can perform quite
differently during faults.
Assigning the same value to the
single slope bias characteristics.

8.2.5 Current differential break point

The
CURRENT DIFF BREAK PT
restraint 2 characteristics. Two approaches can be considered, as follows:
8
• Program the setting to 150 to 200% of the maximum emergency load current on the line, on the assumption that a
maintained current above this level is a fault
• Program the 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.

8.2.6 CT tap

If the CT ratios at the line terminals are different, the
a common base. In this case, modify the
current phasor is used as a reference to determine which differential equation is used, based on the value of local and
8-4
(breakpoint). As is typical for current-based differential elements, the settings are
setting controls the element characteristic when current is below the breakpoint, where CT
setting controls the element characteristic when current is above the breakpoint, where CT
CURRENT DIFF RESTRAINT 1(2)
setting controls the threshold where the relay changes from using the restraint 1 to the
CURRENT DIFF CT TAP 1(2)
CURRENT DIFF BREAK PT
CHAPTER 8: APPLICATION OF SETTINGS
,
CURRENT DIFF PICKUP CURRENT DIFF RESTRAINT 1
settings reverts dual slope bias characteristics into
setting must be used to correct the ratios to
and
CURRENT DIFF PICKUP
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
,
CURRENT DIFF
settings because the local