8-34 BE1-951 Application 9328900990 Rev L
breaker, a bus breaker-failure lockout relay, or other lockout relays that trip the transformer high side such
as the 86T transformer differential lockout relay.
Provision for external breaker failure initiation (BFI) is accommodated by IN3 of the BACKUP relay. The
150T function blocks provide fault detector supervision of IN3. The BACKUP logic scheme uses the
pickup outputs of the 150TP/N/Q function blocks to drive the initiate input of the breaker failure function
block. A maximum time delay setting for the 150TP/N/Q function blocks is needed to ensure that they
don't trip and target. The breaker failure function block is also initiated by a protective trip (VO11). Keep in
mind, if you are tripping for a bus fault via a lockout relay, the additional time delay of the lockout relay
should be added to your breaker failure time delay setting.
The breaker failure function block has an independent fast dropout phase current detector that senses a
breaker opening and stops timing. An open breaker is detected when the current drops below 10 percent
of the nominal CT input (1 A or 5 A) for the relay.
A time delay setting of zero disables the BF function block. This permits the traditional radial system
backup scheme of coordinated relays tripping different breakers.
Contingency Operation - Backup Protection for BACKUP Relay Out of Service
When the BACKUP relay is out of service, full high-speed bus fault protection and overload protection are
provided by the BUS relay. Breaker failure protection is not provided during this double-contingency
situation.
Contingency Operation - Backup Protection for Feeder Relay Out of Service
OUT3 of each feeder relay should be wired to an auxiliary transfer relay (83/Fn) with one normally open
and one normally closed contact. Under normal conditions, OUT3 of a feeder relay is closed and the 83
auxiliary relay is picked up. When a relay is in test mode and out of service or withdrawn from its case,
the 83 auxiliary relay will drop out.
The normally open contact (NO in shelf state) of the 83/Fn auxiliary relay is wired to IN2 of the BACKUP
relay to signal the BUS and BACKUP relays to change to Setting Group 1. When Setting Group 1 is
active, the BUS relay 50T and 51 overcurrent function blocks trip an auxiliary tripping relay (94/BU) via
OUT5.
In Setting Group 1, the BACKUP relay 51 time settings must coordinate with the BUS relay 51 time dial
settings. Since the feeder relays provide a blocking signal to the BUS relay upon pickup of the 51 function
blocks, it isn't necessary for the 51 time dial settings of the BUS relay to coordinate with the feeder relays
in Setting Group 1. Therefore, the 51 time dial settings of the BUS relay can be reduced in Setting Group
1 to provide the necessary coordination interval between the BUS relay and the BACKUP relay for this
contingency. This minimizes the time delay that needs to be added to the BACKUP relay time dial
settings and provides a greater opportunity to keep the setting below the transformer damage curve.
The tripping output of the 94/BU auxiliary relay and the normally closed contacts (form B) of the 83/Fn
auxiliary relay are wired in series with the feeder breaker trip coil. This allows the 94/BU relay to trip the
feeder breaker when the feeder relay is out of service.
When the BUS and BACKUP relays are in feeder relay backup mode, relay responses to the various
faults are summarized in the following paragraphs:
• A feeder relay detecting a fault will send a blocking signal to the BUS relay to prevent it from
issuing a high-speed trip. The 51 functions of the BUS and BACKUP relays are set to coordinate
with each other and the feeder relays.
• An out of service feeder relay on a feeder with a fault will not send a blocking signal to the BUS
relay so the BUS relay will trip the feeder breaker via the 94 and 83 relay contacts. Fault clearing
occurs after the 2 to 4 cycle coordination interval set on the BUS relay 50T functions or after the
BUS relay 51 time if the fault is further out. For this reason, the BACKUP relay 51 functions must
be set to coordinate with the BUS relay in this setting group.
• A bus fault will cause the BUS relay to trip the feeder breaker with the relay out of service
because no blocking signal will be sent by any of the feeder relays. Since this won't clear the
fault, the BACKUP relay will clear the fault with its 18 to 20-cycle coordination interval.
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