CHAPTER 5: SETTINGS
GROUPED ELEMENTS
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
5-219
5
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.
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 element.
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 provides 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 direct transfer trip (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),
CURR DIFF DTT
must be “Enabled.”
CURRENT DIFF KEY DTT
— This setting selects an additional protection element (besides the current differential element; for
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.