BE1-951
Application
8-59
negative sequence demand function that allows monitoring and alarming to prevent load imbalances.
However, distribution systems with single-pole fault clearing and switching devices, or long single phase
laterals may have mis-operations during switching activities.
Preprogrammed logic scheme OC-W-79, OC-W-CTL, and FDR-W-IL accommodate the use of a cutoff
switch to block the ground and negative sequence 50T (used for low-set instantaneous) and the 51 (inverse
time) function blocks during switching activities. This is the most conservative approach. You may wish to
evaluate this strategy based on your system, operating practices, and setting practices. For 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 overcurrent 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, 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,
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 consists 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.
Å
It provides the mechanical energy to drop the target.
Æ
It carries the dc tripping current from the induction disk contact which may not have significant
closing torque for a low resistance connection.
Ç
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.
