GE 750 Instruction Manual page 269

CHAPTER 5: SETPOINTS
Note
NOTE
Note
NOTE
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
to "Control". An Undervoltage 3 operation signals Relay 2 to block transfer initiation
from that relay, as Source 1 is experiencing low voltage. Also, Device 27-3 is enabled
by instantaneous overcurrent to block transfer initiation. This ensures that if a fault on
the load side of Bus 1 causes a dip below the undervoltage pickup setting, transfer will
not be initiated until the voltage has risen above the voltage setting for the interval
established by the
TRANSFER DELAY THIS SOURCE
setting should be below the minimum expected normal (low) voltage, usually around
0.9 of pickup voltage. A Definite Time curve with the delay set to zero provides
instantaneous operation. The minimum operating voltage must be set to zero.
Line Undervoltage 4 (27-4) [Incomers 1 and 2 only]: Since this element is used as a
transfer scheme logic input in this application, set
"Control". An Undervoltage 4 operation initiates a transfer on loss-of-source. Typical
settings have a pickup about 0.7 to 0.8 of pickup voltage, an "Inverse Time" curve
setting, and a delay setting to provide operation in 0.7 to 1.4 seconds at 0 V. The
minimum operating voltage must be set to zero.
The pickup voltage for both Undervoltage features must not be equal, or transfer will
not take place, since both elements will pickup at the same time, causing 27-3 to block
transfer.
• Synchrocheck (25) [Incomers 1 and 2 only]: The synchrocheck function can be
selected as either Control or Alarm. It is imperative that the
setpoint be "LL&DB" (Live Line and Dead Bus) to allow initial closing of the incoming
breakers. The user establishes all other setpoints for this element.
Synchrocheck (25) [Bus Tie only]: This element is used to provide synchronism check
supervision when paralleling the busses. The Dead Source Permissive portion of this
feature is also used to measure the residual voltage on the bus that has lost source. To
ensure that transfers are supervised by the decayed voltage magnitude only, the 'in-
synchronism' elements are blocked while a transfer is in progress. The synchrocheck
function can be selected as either Control or Alarm. It is imperative that the
setpoint be either "DL|DB" (Dead Line or Dead Bus) or "DLXDB"
SOURCE PERMISSIVE
(Dead Line or Dead Bus, but not both) to allow for transfers to either incomer.
The
DEAD BUS MAX VOLTAGE
of decayed voltage above which transfers are inhibited. A normal setting for this
element is about 0.25 of pickup of nominal voltage. Because the 750/760 measures a
single phase-phase voltage, these values should be multiplied by 1/
case of a phase-ground fault on a measured phase reducing that phase voltage but
leaving the other two phases at a higher voltage. If experience shows this setpoint
causes a delay of transfer presenting problems, it is occasionally raised to a maximum
of 0.40 of pickup. The user establishes all other setpoints for this element.
The Logic Inputs for Incomers 1 and 2 and the Bus Tie relays are programmed as follows.
Note that the input number matches the wiring shown on the DC schematics. It is not
necessary that the specific inputs are programmed as shown, but field connections must
match the logic functions.
If logic inputs identified as optional on the schematics and the following table are not
required, they can be programmed to perform other functions. The Message Mode may be
programmed to Disabled or Self Reset.
setpoint. The Device 27-3 pickup
LINE UNDERVOLTAGE 4 FUNCTION
and
DEAD LINE MIN VOLTAGE
S7 CONTROL
to
DEAD SOURCE PERMISSIVE
DEAD
setpoints establish the level
to cover the
3
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