GE L90 Instruction Manual page 276

5.6 GROUPED ELEMENTS
The following settings are available for current differential protection.
• CURRENT DIFF SIGNAL SOURCE 1: This setting selects the first source for the current differential element local
operating current. If more than one source is configured, the other source currents are scaled to the CT with the maxi-
mum primary current assigned by the
source is mandatory and is assigned with the
• CURRENT DIFF SIGNAL SOURCE 2, CURRENT DIFF SIGNAL SOURCE 3, and CURRENT DIFF SIGNAL
SOURCE 4: These settings select the second, third, and fourth sources for the current differential function for applica-
tions where more than one set of CT circuitry is connected directly to L90.
• CURRENT DIFF BLOCK: This setting selects a FlexLogic™ operand to block the operation of the current differential
element.
• CURRENT DIFF PICKUP: This setting is used to select current differential pickup value.
• CURRENT DIFF CT TAP 1 and CURRENT DIFF CT TAP 2: These settings adapt the remote terminal 1 or 2 (commu-
nication channel) CT ratio to the local ratio if the CT ratios for the local and remote terminals are different. The setting
value is determined by CT
referred to as the CT primary rated current). Ratio matching must always be performed against the remote CT with the
5
maximum CT primary defined by the
See the Current differential settings example in the Application of settings chapter for additional details.
• CURRENT DIFF RESTRAINT 1 and CURRENT DIFF RESTRAINT 2: These settings select the bias characteristic for
the first and second slope, respectively.
• CURRENT DIFF BREAK PT: This setting is used to select an intersection point between the two slopes.
• CURRENT DIFF GND FUNCTION: This setting enables and disabled the 87LG neutral differential element, which may
be used to detect high-resistive faults. This element uses restrained characteristics to cope with spurious zero-
sequence current during system unbalance and signal distortions. The differential neutral current is calculated as the
vector sum of all in-zone CT input neutral currents. The restraint current is derived as the maximum of phase currents
from all terminals flowing through any individual CT, including breaker-and-a-half configurations. The 87LG neutral dif-
ferential element is blocked when the phase current at any terminal is greater than 3 pu, since the phase differential
element should operate for internal faults. To correctly derive the restraint quantity from the maximum through current
at any terminal, it is important that the 87L phase-segregated differential pickup and slope settings are equal at all ter-
minals. Refer to the Applications of settings chapter for additional details.
• CURRENT DIFF GND PICKUP: This setting specifies the pickup threshold for neutral current differential element.
• CURRENT DIFF GND RESTRAINT: This setting specifies the bias characteristic for the neutral current differential ele-
ment.
• CURRENT DIFF GND DELAY: This setting specifies the operation delay for the neutral current differential element.
Since this element is used to detect high-resistive faults where fault currents are relatively low, high-speed operation is
usually not critical. This delay will provide security against spurious neutral current during switch-off transients and
external fault clearing.
• CURRENT DIFF DTT: This setting enables and disables the sending of a DTT by the current differential element on
per single-phase basis to remote devices. To allow the L90 to restart from master-master to master-slave mode (very
important on three-terminal applications),
• CURRENT DIFF KEY DTT: This setting selects an additional protection element (besides the current differential ele-
ment; for example, distance element or breaker failure) which keys the DTT on a per three-phase basis.
For the current differential element to function properly, it is imperative that all L90 devices on the pro-
tected line have exactly identical firmware revisions. For example, revision 5.62 in only compatible with
5.62, not 5.61 or 5.63.
NOTE
5-146
CURRENT DIFF KEY DTT:
MESSAGE
Off
CURRENT DIFF
MESSAGE
TARGET: Self-reset
CURRENT DIFF
MESSAGE
EVENTS: Disabled
CURRENT DIFF SIGNAL SOURCE 1
SYSTEM SETUP
/ CT
prim_rem prim_loc
CURRENT DIFF SIGNAL SOURCE 1
CURR DIFF DTT
L90 Line Current Differential System
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
to
CURRENT DIFF SIGNAL SOURCE 4

SIGNAL SOURCES
for local and remote terminal CTs (where CT
through
CURRENT DIFF SIGNAL SOURCE 4
must be set to "Enabled".
5 SETTINGS
settings. This
 menu.
SOURCE 1
/ CT
prim_rem prim_loc
settings.
GE Multilin
is