Ct Circuit Monitoring; Time Delays; Events And Actions - Siemens 7UT51 Instruction Manual

6
Bus Differential Protection (87B)
6.6

CT Circuit Monitoring

The differential currents of each phase are
continuously monitored on a low level. If a differential
current flows (during normal load conditions) that
corresponds to the load current of a feeder, (indicating
a missing secondary current) a fault is in the
secondary current leads. This occurrence will provide
alarm annunciation and block the differential
protection of the concerned phases.
The low-level monitor current pickup value should be
high enough to avoid a false pick up due to
transformation errors or tolerances of the current
matching when the rated CT currents are different:
1831 87 MON PU
Fault current during a fault in a CT's secondary
leads, expressed as a multiple of the rated
nominal current of the bus (Address 1301).
–
Range: 0.15 0.80 I
Default: 0.20
The low-level monitor alarm delay must be
considerably longer than the delay-before-trip time
(Address 1825) to ensure that the differential
protection is not blocked during a real fault in the
network.
1832 87 MON DLY
Time delay for fault in secondary CT leads
10 s, or ∞
–
Range: 1
Default: 2
Enter ∞ for the 87B function to never be
blocked by a CT circuit problem.
For information about the other protection-monitoring
features of the relay, see Chapter 12.
86
n
6.7

Time Delays

If an 87B pickup occurs, the 87B function will
immediately trip unless a time delay has been set in
Address 1825.
1825 87 DELAY
Time delay between pickup and trip for 87B
differential protection (in addition to the inherent
operating time of the relay).
60.00 s, or ∞
–
Range: 0.00
Default: 0.00 s
The special value ∞ will prevent any 87B trip
event from ever occurring.
If an 87B trip occurs, the contacts and LEDs
marshalled to the event will remain activated until the
87B function drops out. To ensure that the trip state
lasts long enough for the circuit breaker to operate, the
trip state can be prolonged by a reset time delay
(Address 1827).
-
1827 87T RESET
Reset delay after trip signal has disappeared (in
addition to the inherent reset time of the relay).
–
Range: 0.00 60.00 s
Default: 0.10 s
6.8

Events and Actions

This table describes the events that control and are
generated by the 87B function. They are listed in the
order that they are likely to occur, which is roughly in
numerical order by their event identification number
(shown in the "Event" column). The text in the "LCD
Abbreviation" column is the corresponding text that will
appear when viewing logs on the front panel LCD
display. In the "Oper" column, a letter "C" or "G"
indicates that the "Coming" and/or "Going" events are
logged in the Operational Log, while if in the "Fault"
column they indicate that the events are logged in the
open fault log. A letter in the "In/Out" column indicates
that the event can be controlled by a binary-signal
input ("I"), control an LED and/or signal contact ("S"),
and/or control a trip contact ("T").
Figure 6.4 on page 89 shows the logic of these events.
7UT51 v3
PRIM-2330C