GE 750 Instruction Manual page 274

14.7 TRANSFER 14 S7 CONTROL
14.7 TRANSFER 14.7.1 DESCRIPTION
The Transfer scheme is intended for application to a set of three circuit breakers, two of which, (Incomers 1
and 2) connect sources of electrical energy to two busses which can be paralleled through the Bus Tie breaker.
Normal configuration of the system is with both incoming breakers closed and the bus tie breaker open. Figure
14–2: SETPOINT CONTROL (2 OF 3) on page 14–4 shows this arrangement. The equipment designations on
this drawing will be used in the discussion.
The transfer scheme is used to minimize 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 (sometimes
called 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 nor-
mal configuration, two manual and one automatic. If 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. If the sources are synchronized with synchrocheck supervision, it is
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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. In this instance,
the bus tie breaker is 'Selected To Trip' and will open. The automatic method of returning the system to normal
configuration uses the undervoltage tripped portion of the restoration scheme.
In addition to the relay required for each of the three circuit breakers, it is required to connect one contact from
a three-position switch to each relay. This switch (device 43/10) is used to select the breaker that will trip after
all breakers become closed. It is also recommended that a two-position switch (device 43/83) with three con-
tacts be connected to each relay as an 'Auto-Off' transfer scheme selector.
Because a relay is required on the bus tie breaker, it allows 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, open-
ing the bus tie before an incomer when operating from only one source.
The scheme can be used in conjunction with both non-drawout and drawout switchgear. Drawout switchgear
designs can make use of an auxiliary switch that confirms that the monitored breaker is in the 'Connected'
position, and is therefore ready for operation. Fixed breaker installations can use contacts on the associated
isolating disconnect switches (if available) for this purpose. The scheme design can be applied to:
1. Substations with no signaling from upstream equipment.
2. Substations with an upstream circuit breaker equipped with a trip signal (94).
3. Substations with a source transformer and transformer fault detection signal (86T).
4. Both 1 and 2 above.
The only differences in implementing the transfer scheme among the configurations presented above is by
connecting additional logic signals to the relay when available and by placing a jumper on one logic input if the
breaker is non-drawout and has no disconnect auxiliary switches.
Besides the setpoints and logic incorporated into the transfer scheme, the relays make use of some or all of
the logic inputs, phase instantaneous (50P-1 or 2), neutral overcurrent (50N-1 or 2), both line undervoltage ele-
ments (27-3 and 27-4), and the synchrocheck (25) features of the relays.
The following discussion is based on configuration (4) above, using drawout breakers. Schematic diagrams of
the DC connections required by an example scheme with this configuration are presented on Figure 14–11:
TRANSFER SCHEME INCOMER NO. 1 DC SCHEMATIC, Figure 14–12: TRANSFER SCHEME INCOMER
NO. 2 DC SCHEMATIC, and Figure 14–13: TRANSFER SCHEME BUS TIE BREAKER DC SCHEMATIC on
14- 750/760 Feeder Management Relay GE Power Management
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Courtesy of NationalSwitchgear.com