BE1-951
Application
8-53
Figure 8-17. Under frequency Load Shed Bus Level Application
BE1-951 (single source sensing can also be used). These schemes are easily customized to meet the user’s
specific requirements.
Bus Level Application:
The following logic was designed to work with the preprogrammed BUS Logic Scheme described in Section
8,
Application.
Referring to Figure 8-17, if sensing voltage is above the inhibit setting, and system frequency below the 81
and 181T setting, UF_TRIP, VO6 will go high. This in turn forces VO3 (UFLO_TRP) high, closing Output
3 contact and tripping the user lockout(s) (86) devices and in turn, tripping the associated breakers. When
the “load condition” that caused the under frequency event has subsided, the system operator/dispatcher
will initiate a remote restoration procedure. A “restoration permissive” from the 951 verifies that the following
conditions are met prior to restoring load:
Conditions:
•
The under frequency trip (81and 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 (281 and 381T).
The first two conditions must be met to remove REST_BLK from the 281 and 381T over frequency elements.
Then, with sensing voltage above the inhibit setting of 281 and 381T and the system frequency above the
281 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 281 and
