Sel 551 - Page 56

3.18
SEL-551 Relay Instruction Manual Date Code 20110408
Relay Elements and Logic
Trip Logic
Unlatch Trip
Once Relay Word bit TRIP is asserted to logical 1, it remains asserted at
logical 1 until all the following conditions come true:
➤ Trip Duration Timer stops timing (output of the TDURD timer
goes to logical 0),
➤ SELOGIC control equation setting TR deasserts to logical 0,
and one of the following occurs:
➤ SELOGIC control equation setting ULTR asserts to logical 1,
➤ The front-panel {TARGET RESET} pushbutton is pressed,
➤ Or the TAR R (Target Reset) command is executed via the
serial port.
The front-panel {TARGET RESET} pushbutton or TAR R (Target Reset) serial
port command is primarily used during testing only (the TA R R serial port
command can also be effectively operated via Modbus
®
protocol—see
Appendix G: Modbus RTU Communications Protocol). They are used to
deassert the TRIP Relay Word bit to logical 0 if test conditions are such that
setting ULTR does not assert to logical 1 to automatically deassert the TRIP
Relay Word bit instead.
Other Applications for the Target Reset Function
Refer to the bottom of Figure 3.13. Note that the combination of the {TARGET
RESET} pushbutton and the TAR R (Target Reset) serial port command is also
available as Relay Word bit TRGTR.
Factory Settings
Example
The factory settings for the trip logic SELOGIC control equation settings are:
TR = 51P1T + 51G1T + 50P1*SH0 + LB3 (trip conditions)
ULTR = !(51P1 + 51G1) (unlatch trip conditions)
The factory setting for the Minimum Trip Duration Timer setting is:
TDURD = 9.000 cycles
See the setting sheets in Section 4: Setting the Relay for setting ranges.
Set Trip
In SELOGIC control equation setting TR = 51P1T + 51G1T + 50P1*SH0 +
LB3:
➤ Time-overcurrent elements 51P1T and 51G1T trip directly.
➤ Phase instantaneous overcurrent element 50P1 is supervised by
Relay Word bit SH0 in an ANDed condition (50P1*SH0).
Element 50P1 can only get through to trip when SH0 = logical
1 (reclosing relay is at shot = 0). After the first trip in a reclose
cycle, the shot increments from 0 to 1, SH0 = logical 0, and
element 50P1 can then not get through to trip. See Reclosing
Relay on page 3.22 for more information on reclosing relay
operation.
➤ Local bit LB3 trips directly (operates as a manual trip switch
via the front-panel). See Local Control Switches on page 3.5
and Section 6: Front-Panel Interface for more information on
local control.