Advertisement
GEK-100637
Figure 2 shows an application where an instantaneous overcurrent relay may he used to prevent
attempted interruptions of fault currents above breaker rating. For some system conditions, heavy
close-in line faults such as Fl might exceed the interrupting rating of the line breaker (3). The
application of an instantaneous overcurrent relay set near the line brcaker rating permits
operation of the bus tie breaker (BT) to relieve the duty on the line breaker for these critical
faults before operation by the line breaker. The SBC231 is particularly well suited to this type of
application since it has less than ten percent transient overreach and operating times of
approximately one-half cycle (60 hertz basis) at l.5X pick up. (See Figure 4.) It is usually
possible to coordinate this high-set overcurrent protection right up to the interruption limits of the
line breaker without introducing any delay in the line relaying that would affect other less critical
faults such as at F2.
Figure 3 shows a similar application for protection of a circuit switcher at a transformer tap on a
transmission line. The SBC231 overcurrent relay can be set to operate just below the interrupting
capacity of the circuit switcher to avoid possible Interrupter failure for faults at the high side of the
transformer at Fl. The SBC would be connected to block the operation of the circuit switcher
until the fault is cleared at line terminals A and B.
Applications of the SBC231 as a breaker-failure relay would be accomplished on a per-breaker
basis. On this basis the current inputs to a particular SBC relay come from the current
transformers that measure the current in the associated breaker. The SBC DC supply is enabled
by the protective relays associated with the zone of protection, and the output of the SBC is
connected to a definite timer, such as a Type SAM timing relay. The timer is set with sufficient
delay to permit interruption of the associated breaker when operated by the primary relays. (See
Figure 5.) If the breaker fails to interrupt successfully within this time, the timer will time out and
initiate tripping of all breakers necessary to clear the fault. A detailed discussion of breaker
failure applications for various bus configurations is included in GEK-100636.
The SBC231 is also well suited to applications such as pilot and distance relay line protection
schemes requiring high-speed overcurrent detectors.
CONFIGURATIONS
Output Studs
The relay is delivered with a Normally Open Contact at Studs 15 and 16; it can be changed to a
Normally Closed Contact by connecting the Fast-On terminal from H15A on the backplane to
terminal block stud 15. (See Figure 7.)
The SBC23I is delivered with a Power Supply Alarm contact at studs 7 and 8. (See Figure 7.) This
Power Supply Alarm contact will close if the battery voltage drops below 19 VDC. To configure
the relay to have an Output Contact at Studs 7 and 8, move relay K4 from position K4B to
position K4A on the backplane.
Input Currents
All SBC231 models are delivered in the configuration that accommodates two phase-current
inputs and one ground current input. However, moving connector PM on the backplane to cover
positions 3 through 8 will enable the relay to monitor three phase currents. (See Figures 7 and
When connector PL4 is in position 1 through 6, and the external connections are as shown in
Figure 1, the current at Studs 1, 2, and 3,4 is monitored by the phase-current pickup and the
current at studs 5 and 6 is monitored by the ground-current pickup.
*
The output relays on the SBC have a pickup time of 6
-
lOms. and a dropout time of 3
-
5 ms.
-4-
Advertisement
