GE L90 Instruction Manual page 443

CHAPTER 5: SETTINGS
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 be equal at all terminals. See the
Application of Settings chapter for details.
— This setting specifies the pickup threshold for neutral current differential element.
CURRENT DIFF GND PICKUP
CURRENT DIFF GND RESTRAINT
— This setting specifies the operation delay for the neutral current differential element. Since this
CURRENT DIFF GND DELAY
element is used to detect high-resistive faults where fault currents are relatively low, high-speed operation is usually not
critical. This delay provides security against spurious neutral current during switch-off transients and external fault
clearing.
— This setting enables and disables the sending of a direct transfer trip (DTT) by the current differential
CURRENT DIFF DTT
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),
— This setting selects an additional protection element (besides the current differential element; for
CURRENT DIFF KEY DTT
example, distance element or breaker failure) that 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 protected line
have identical firmware revisions. For example, revision 5.62 in only compatible with 5.62, not 5.61 or 5.63.
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
— This setting specifies the bias characteristic for the neutral current differential element.
CURR DIFF DTT
must be "Enabled."
GROUPED ELEMENTS
5
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