Basler BE1-CDS220, Инструкция по эксплуатации

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8-52 Application BE1-CDS220
Block Neutral and Negative Sequence Protection
The neutral and negative sequence overcurrent elements provide greater sensitivity to unbalanced faults
than the phase overcurrent elements because they can be set to pickup below balanced three phase load.
This can lead to a mis-operation during periods of load imbalance. The BE1-CDS220 provides a neutral
and 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.
Some of the preprogrammed logic schemes provide for 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. The protection engineer may wish to evaluate
this strategy based on his system, his operating practices, and his 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 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-CDS220 Current Differential System provides multiple settings groups for adaptive relaying. The
preprogrammed logic schemes barely tap the flexibility that is available. The following examples illustrate
how the setting 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 do this, assign input D0 or D1 to a contact sensing
input. Set the setting group selection mode at 2 for binary coded selection. 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 for the situation where 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 is also
possible to do 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 consisted 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 the EM relays. One is to provide mechanical
energy to drop the target. Second is to carry the dc tripping current from the induction disk contact which
may not have significant closing torque for a low resistance connection. The third is to prevent the relay
contact from dropping out until the current has been interrupted by the 52a contacts in series with the trip
coil. If the tripping contact opens before the dc current is interrupted, the contact may be damaged. The
first two of these items are not an issue for solid state relays, but the third item is an issue.