GE Multilin 850, Instruction Manual

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8–74 850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
BUS TRANSFER CHAPTER 8: CONTROL
• Manual Method 2 - when the sources are synchronized with Synchrocheck
supervision: It is possible to manually close the open incomer to parallel all three
breakers (setpoints for Synchrocheck associated with the Synchrocheck function
must be set). The scheme then automatically opens a breaker that has been
previously selected to trip if all three breakers become closed. If the Bus Tie Breaker is
“Selected To Trip,” it is tripped by the system and opens.
• Automatic Method : The automatic method of returning the system to normal
configuration uses the Undervoltage Restoration scheme. Setpoints for the
Undervoltage Restoration must be set.
Besides the setpoints and logic incorporated into the Transfer Scheme, the relays make
use of:
•
Phase Instantaneous Overcurrent elements (Device 50P)
• Neutral Instantaneous Overcurrent elements (Device 50N)
• Line Undervoltage elements (Device 27)
• The Synchrocheck (Device 25) features of the Bus Tie relay
• Trip and Close Output Relays
• Contact Inputs.
Optionally
• Breaker (breaker status open, closed, disconnected)
• Breaker Control (manual/remote close or open)
• The Synchrocheck (Device 25) features of the Incomer 1 and Incomer 2 relays
• Undervoltage Restoration scheme (automatic restoration of the system to normal
configuration).
Instantaneous Phase (50P), and Neutral Overcurrent (50N) as fault detectors, can be used
as inputs to the Transfer Scheme logic in order to block a transfer while a fault is present
on the load side of the breaker. These elements block a transfer while a fault, which can
cause a severe voltage dip, is present on the load side of the breaker. This fault is cleared
by Time Overcurrent protection on the incomer or an upstream breaker. The 50P element
is set above the maximum current caused by either the bus motor contribution to an
upstream fault, or the maximum current during low voltage conditions. The 50N element is
be set to detect arcing ground faults, but allow permitted unbalances.
Fault detectors are required for Incomer 1 and Incomer 2 relays only.
Line Instantaneous Undervoltage is required to block Transfer initiation from the other
relay, as the other source is experiencing low voltage. In addition, Line Instantaneous
Undervoltage 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 is not initiated until the voltage has risen above the voltage setting for the
interval established by the Transfer Delay This Source setpoint. The Pickup setting is below
the minimum expected normal (low) voltage, usually around 0.9 of the 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 Time Undervoltage is required to initiate 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.
Line Instantaneous Undervoltage and Line Time Undervoltage are required for Incomer 1
and Incomer 2 relays only.
Synchrocheck (25) is required to supervise the initial closing of the incoming breakers, to
provide synchronism-check supervision when paralleling the busses, or to measure the
residual voltage on the bus that has lost source.