GE 850 Instruction Manual page 531

CHAPTER 8: CONTROL
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850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
Because a relay is required on the bus tie breaker, it allows a bus-splitting operation. This is
accomplished by setting the Time Overcurrent elements in the relay on the bus tie breaker
to trip faster than the incomers, opening the bus tie before an incomer when operating
from only one source.
For Source 1 as the failed side, the Transfer (trip of Incomer 1 followed by Bus Tie closing)
from Relay 1 can be initiated by:
• Operation of the transformer 1 lockout relay (86-T).
• Operation of the Source 1 breaker auxiliary trip device (94).
• Time out of Relay 1 line voltage inverse time Undervoltage element (27) caused by low
voltage on Source 1.
A transfer initiation is blocked if:
• Any of the three breakers is not in the rack-in (connected) state.
• Incoming Breaker 2 (which is to become the new source) is presently open.
• Detection of an Overcurrent condition on bus 1, to prevent a faulted bus from being
transferred to a healthy source.
• The Line Instantaneous Undervoltage element (27) on Source 2 is operated, indicating
low voltage on the other source.
If any one of the above block conditions is present, the TRANSFER NOT READY message is
displayed by the relays.
Identical logic with all 1s and 2s interchanged applies to Relay 2 for a loss of Source 2.
Once a condition has caused the 850-1 relay on Incomer 1 (Relay 1) to initiate a transfer,
the following sequence of events will take place:
• Relay 1 trips Incomer 1 breaker (Breaker 1).
• Relay 1 issues a close signal to the 850-3 on the Bus Tie breaker (Relay 3).
• When relay 850-3 receives the close command from Relay 1, it is captured and
retained until either the Bus Tie breaker (Breaker 3) closes or the Block Transfer logic
input is received.
• Relay 850-3 is inhibited from initiating a close command to Tie Breaker by its
Synchrocheck element.
• Synchrocheck monitors the voltage on the disconnected bus, and provides the bus
decayed (residual) voltage permission-to-close when the Bus 1 voltage decays to the
pre-set level.
The three breakers are under prevent-parallel checking whenever the Transfer Scheme is
operational. If a third breaker is closed when the other two breakers are already closed, the
scheme automatically trips the breaker selected by Switch 43/10, "Selected To Trip".
Path:
Setpoints > Control > Bus Transfer
FUNCTION
Range: Disabled, Incomer 1, Incomer 2, Bus Tie
Default: Disabled
If the Transfer scheme is not required, set Transfer function to "Disabled". If not disabled,
this setpoint assigns the function of the associated circuit breaker to the relay. This
selection programs the relay to use the logic required by each of the three breakers.
Select "Incomer 1" or "Incomer 2" for this setpoint if the relay is associated with the
breaker to be used as Incomer 1 or 2 respectively. Select "Bus Tie" if the relay is
associated with the bus tie breaker.
The selection of the Incomer 1, Incomer 2 or Bus Tie setting enables the Transfer
function.
BUS TRANSFER
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