P.3.98
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
increment. When testing is completed, it is recommended to reset the counters
by issuing the command COM 87L WD C at the 2AC access level. The 87L
Watchdog Monitor will also be disabled while the relay is in 87 Test Mode
(see Test Mode on page P.11.4). Undetected corruption of incoming 87L data
is the most common cause of 87L element operation without a disturbance
detection. The monitor includes two levels for counting 87L operations.
First Level Counter
The first level counts unwarranted 87L pickup operations associated with 87L
communications channel impairments. In this case, the disturbance detector
did not pick up simultaneously with the 87L element operation, preventing an
undesirable 87L trip output.
To alarm for this condition, include 87ALARM in programmable alarm output
setting OUT208 as follows:
OUT208 := NOT (SALARM OR HALARM OR 87ALARM)
After the monitor counts three such instances, it asserts Relay Word bit
87ALARM. If you prefer, you can use calibration level setting 87WDOGA to
change the alarm threshold from the default value of 3. The Group setting
EWDSEC will not affect the 87ALARM logic.
Upon receiving the alarm, attend to the channel problem and reset the
watchdog according to the following description. If you do not address the
problem and reset the alarm, the first level counter counts two (default setting)
more instances before alarming and asserting Relay Word bit 87ERR1.
Assertion of this bit inhibits 87L protection (see Figure 3.186). Address the
channel impairment that caused the alarms, and use command
COM 87LWD C to reset the first level counter alarms and reenable 87L
protection if necessary. If you prefer a count other than 5 for this second
alarm, use calibration level setting 87WDOG1 to change the 87L inhibit
threshold from the default value of 5. Figure 3.50 shows a diagram of the
Level 1 watchdog logic.
q See Figure 3.194.
Figure 3.50 Level 1 Watchdog Monitor Diagram
Second Level Counter
The second counter level counts unwarranted 87L pickup operations that the
relay prevented because no disturbance detector pickup occurred, even if those
potential 87L trips were not associated with channel activity. These instances
include unforeseen problems internal to the relay. The second level counter
alarms by asserting Relay Word bit 87ERR2, which inhibits 87L protection
after the counter counts ten such instances (see Figure 3.186). The second
counter level indicates some internal relay problem, especially if it occurs
without the first level counter alarm. First contact SEL to confirm that the
Channel
Problems
Spurious 87L
Assertions
First
Level
Counter
87WDOGA
(Calibration
Setting)
87WDOG1
(Calibration
Setting)
Relay
Word
Bits
87ALARM
87ERR1
q
87WDCT1
(Analog Quantity)
EWDSEC
(SEL
OGIC
Equation)
To Blocking
Logic
NOTE: The watchdog analog counters
(87WDCT1 and 87WDCT2) can be
added to the front panel to aid in
testing through the use of Display
Points. See Display Points on
page P.7.8.
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