8-50 BE1-951 Application 9328900990 Rev L
Referring to Figure 8-16, when the external sudden pressure relay trip contact connected to input 3
closes, VO13 goes high, sealing the inputs of VO3 and VO4 through AND gate VO8. This allows Output
contacts 3 and 4 to transition and remain in that state until the ARSTKEY variable is asserted. The HMI
LCD display automatically goes to the Alarm Screen if VO13 is programmed as a major or minor alarm
per the automatic display priority function. In this example, VO13 is programmed to be SN-
VO13=SPR_TRIP and will display as such on the HMI when an SPR trip occurs. The trip and alarm
(pseudo target) latch will remain until the reset button on the front panel of the relay is pressed while the
Alarm Screen of the HMI, menu branch 1.3, is being displayed (reset key of the HMI is context sensitive).
Refer to Section 6, Reporting and Alarm Functions, Alarms Function for details.
Under Frequency Load Shedding with Restoration Permissive
Under frequency load shedding schemes are designed to operate when the load of a power region
outpaces generation and begins to "drag" or slow system frequency. To save the system from total
collapse, segregated blocks of load representing a percentage of the total power region load are set to
trip at various levels of declining system frequency. For example, an electric utility determines that 30% of
its load will have to be shed for a worst-case "load to generation" scenario. They decide to arrange the
load in three blocks set to trip at under frequency levels of 59.7 Hz, 59.5 Hz, and 59.3 Hz. Load
restoration is normally broken down into smaller blocks, minimizing the impact of reapplying load to the
system. Knowledge of local conditions or "restoration permissives" are normally included as part of the
system restoration process.
Historically, under frequency load shedding schemes have been applied at the bus level. With the
introduction of numeric, multifunction feeder protection relays, it has become more economical to apply
under frequency load shedding at the circuit level. Also, reliability increases as the user is no longer
depending on a single relay to sense the under frequency condition. If the BE1-951 is not available on
every circuit or user philosophy requires a bus level installation, the BE1-951 can also be applied for bus
level under frequency protection.
The BE1-951 also has an auxiliary voltage input that can be selected for the under frequency function.
This allows the user to supply one under frequency element from a transmission or high side source and
another element from a low side or distribution source. Output of the two elements is connected through
an AND gate thus requiring both under frequency elements to pickup before providing a trip output. Dual
source sensing helps to ensure operation for true system underfrequency events.
The following application tips detail examples of a "bus" and "circuit level" under frequency load shed
scheme and restoration "permissive" using dual source sensing and the programming capabilities of the
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 BE1-951 verifies that the
following conditions are met prior to restoring load:
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