9328900990 Rev L BE1-951 Application 8-51
Figure 8-17. Underfrequency Load Shed Bus Level Application
Conditions
• The under frequency trip (81T and 181T) is no longer present.
• The 86UF(s) is/are tripped.
• Sensing voltage is above the predetermined inhibit level.
• System Frequency is above the predetermined restore level (281T and 381T).
The first two conditions must be met to remove REST_BLK from the 281T and 381T over frequency
elements. Then, with sensing voltage above the inhibit setting of 281T and 381T and the system
frequency above the 281T and 381T setting, VO10 (REST_PER) will go high forcing VO4, UFLO_RES
high, closing Output 4 contact. As long as these conditions remain unchanged the output 4 contact will
remain closed. When the last 86UF (electrical reset lockout relay) is reset, Input 1, UF_RESTORE will go
low, blocking the 281T and 381T elements. Also, VO4 will go low, opening Output 4 contact, removing the
86UF Restore Permissive. The last 86UF to reset can initiate an optional "auto target reset circuit," thus
eliminating the need for further operator input. A one-shot, non-retriggerable timer (62), initiated by the
last lockout to reset (Input 1), is used to automatically reset the BE1-951 targets and Trip LED via "logic"
input to the Target Reset Logic. (See Figure 6-4). Fault and Event records detailing the operation are
available from the relay.
Circuit Level Application - Manual and Auto Reclosing
The following logic was designed to work with the preprogrammed OC-W-79 Logic Scheme described in
Section 8, Application.
For the Bus Application, load restoration was accomplished by resetting the lockout relays (86UFs) and
allowing the circuit reclosing relays to close the breaker and restore load. Using the Circuit Level
Application, restoration can be accomplished by an automatic or manual closure of the breaker. Both
methods are described in the following paragraphs.
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