GE 750 Instruction Manual page 205

S7 Control
NOTE
GE Multilin
The transfer scheme minimizes the effect of outages on one of the incoming
supplies by opening the incoming breaker connected to that supply, and then re-
energizing the dead bus by closing the bus tie breaker to transfer the dead bus to
the live source. To protect against damage to motors connected to the dead bus, the
bus tie breaker is not allowed to close, after a transfer has been initiated, until the
decaying (residual) voltage on the bus has been reduced to a safe level.
After the lost source has been reestablished, the scheme provides three methods to
restore the system to normal configuration. Two methods are manual and one is
automatic.
• Manual Method 1, when the sources cannot be synchronized:
The bus tie breaker must be manually opened before the open incomer can be
manually closed. In this procedure the incomer will only be allowed to close if
the incoming source (Line VT) voltage is above a live threshold and the load
(Bus VT) voltage is below a dead threshold value.
• 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.
The scheme will then automatically open a breaker that had been previously
selected to trip if all three breakers become closed. If the bus tie breaker is
'Selected To Trip', it will be tripped by the system and will open.
• Automatic method:
The automatic method of returning the system to normal configuration uses the
undervoltage tripped
restoration will work only if the relay has issued a trip signal during the tripping
process of the faulty incomer. The required trip signal is not issued if the Line
Undervoltage 4 Element (27-4) is set for Alarm or Control.
If Undervoltage Restoration
Undervoltage 4 (27-4) must be set to Trip, instead of Control or Alarm, and
the feature Block Trip on Double Loss will not function properly.
In addition to the relay required for each of the three circuit breakers, the system
requires two manual operated control switches or equivalent devices.
1. Device 43/10: Select to Trip Control Switch.
This is a three-position switch with at least three contacts, one for each relay,
which obey the following table:
Contact No.
1: Incomer 1
1
2
3
It is required to connect one contact to each relay. This switch selects the
breaker that will trip in the eventuality that all three breakers become closed, to
prevent the two incoming systems to remain connected in parallel. The contacts
can be either normally-open or normally-closed, depending on the asserted
logic of the corresponding logic input used for this purpose.
2. Device 43/83: Auto-Off.
This is an optional three-position switch or equivalent device, with at least three
contacts (one for each relay) that obeys the following table:
Contact No.
1: Off
1: Incomer 1
2: Incomer 2
3: Tie Breaker
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portion of the restoration
is used to restore
Position
2: Incomer 2
X
X
Position
2: Auto
X
X
X
750/760
Feeder Management Relay
scheme. Undervoltage
the system, Line
3: Incomer 3
X
5–109