9328900990 Rev L BE1-951 Application 8-57
instance, on systems with wye-connected loads, the ground units are most sensitive to this situation. On
systems with delta-connected loads, the negative-sequence units are most sensitive to this situation. It
may not be necessary to block the instantaneous units if their settings prevent them from tripping for a
switching imbalance.
To maintain proper coordination, the logic of the feeder relays (using FDR-W-IL logic) may be
interconnected with the upstream bus relay to block the equivalent ground and/or negative-sequence
function blocks in the upstream relay.
Setting Group Selection
The BE1-951 Overcurrent Protection System provides multiple settings groups for adaptive relaying. The
preprogrammed logic schemes barely tap the flexibility that is available. The following two examples
illustrate how the settings groups can be adapted for different conditions and how different setting groups
can be used to vary the system logic.
Example 1. Adapting the relay settings for different conditions:
In multifunction protection systems, the source conditions can have a major impact on sensitivity,
coordination intervals, and clearing times. Generally, the pickup and time dial settings are a compromise
between a normal condition and a worst-case condition. Contact logic from the position of the source
breakers can select which settings group is active. To achieve this, assign input D0 or D1 to a contact
sensing input. Select binary coded setting group selection (Mode 2). If D0 is set, Setting Group 0 will be
selected when the input is off (binary code 00). Group 1 will be selected when the input is on (binary code
01). Similarly, if D1 is set, Setting Group 2 will be selected when the input is on (binary coded 10).
This logic is useful in a situation where two transformers feed a single bus or two busses have a bus tie
between them. The feeder and bus relays must be coordinated so that only one source is in service (bus
tie open or one transformer out of service). However, when both sources are in service, such as when the
bus tie is closed, each bus relay sees only half of the current for a fault. This results in poor sensitivity and
slow clearing time for the bus relays.
Example 2. Adapting the logic in different setting groups:
The logic in most of the preprogrammed logic schemes can be varied in each of the different setting
groups. This is accomplished by disabling functions by setting their primary settings at zero. It's also
possible to perform more sophisticated modification of the logic in each of the different setting groups by
using the active setting group logic variables SG0, SG1, SG2, and SG3 in the BESTlogic expressions.
Output Contact Seal-In
Trip contact seal-in circuits have historically been provided with electromechanical relays. These seal-in
circuits consist of a dc coil in series with the relay trip contact and a seal-in contact in parallel with the trip
contact. The seal-in feature serves several purposes for electromechanical relays:
1. It provides the mechanical energy to drop the target.
2. It carries the dc tripping current from the induction disk contact, which may not have significant
closing torque for a low resistance connection.
3. It prevents the relay contact from dropping out until the current has been interrupted by the 52a
contacts in series with trip coil.
The first two items aren't an issue for solid-state relays, but item three is significant.
To prevent the output relay contacts from opening prematurely, a 200 millisecond hold timer can be
selected with the SG-HOLDn=1 command. Refer to Section 3, Input and Output Functions, Outputs, for
more information about this feature. If desired, seal-in logic with feedback from the breaker position logic
can be obtained by modifying the BESTlogic expression for the tripping output. To do this, set one of the
General Purpose Timers (62 or 162) for Mode 1 (Pickup/Dropout Timer). Set the timer logic so that it is
initiated by the breaker position input and set the timer for two cycles pickup and two cycles dropout.
Then AND the timer output with the tripping output and OR it into the expression for the tripping output.
The same can be done for the closing output.
Figure 8-21 illustrates the seal-in logic diagram.
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