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Section 6
Differential protection
where any feeder bay can be connected to any of the two buses. In such cases the status of all
busbar disconnectors and all transfer disconnectors shall be given to the busbar protection.
Traditionally, the CT switching has been done in CT secondary circuits. However, with REB670
this is not the case. All necessary zone selection (that is, CT switching) is done in software.
Therefore, the CT secondary circuits are always intact and without any auxiliary relay contacts.
To provide proper zone selection (that is, busbar replica) the position information from all
relevant primary switches (that is, disconnectors and/or circuit breakers) must be given to the
IED. This is typically done by connecting two auxiliary contacts (that is, normaly open and
normaly closed aux contacts) from each primary switch to the IED binary inputs (that is,
optocouplers). In REB670 configuration one Switch Status function block shall be associated
with each primary switching device. This block is then used internally to derive the primary
object status and then pass this information to the busbar protection internal zone selection
logic.
Auxiliary contact requirements for disconnectors and circuit breakers
The position of the primary switching object is typically obtained via two auxiliary contacts of
the primary apparatus. The first auxiliary contact indicates that primary device is closed. In
protection literature it is called by different names as stated below:
•
Normally open auxiliary contact
•
"a" contact (that is, 52a)
•
"closed"
The second auxiliary contact indicates that primary device is open. In protection literature it is
called by different names as stated below:
•
Normally closed auxiliary contact
•
"b" contact (that is, 52b)
•
"open"
Typically both contacts are used to provide position indication and supervision for busbar
protection.
Minimum contact requirements
The minimum requirement for the busbar replica is the record of the disconnector position by
using just one auxiliary contact, either "normally open" or "normally closed" type. However
recording a pair of auxiliary contacts, representing the "open" and "close" position, offer
additional features which can improve the reliability of the bus replica including supervision
possibilities.
Auxiliary contact evaluation logic
Two logic schemes can be found.
• Scheme1_RADSS "If not OPEN then CLOSED"
As the name of the scheme suggests, only when the auxiliary contacts signal clean open
position ("normally open auxiliary (NO) contact input" = inactive and "normally closed auxiliary
(NC) contact input" = active), the disconnector is taken to be open. For all other signal
combinations the disconnector is considered to be closed. This scheme does not pose any
special requirements to the auxiliary contact timing. Only the disconnector NC contact must
open before the disconnector main contact is within arcing distance. The time during which
POSOPEN and POSCLOSE signal inputs disagree (that is, both binary inputs are active or
the
both are inactive) is monitored by the isolator supervision function. The maximum time
allowed before an alarm is given can be set according to the disconnector timing.
98
1MRK 505 370-UEN D
SEMOD127523-5 v3
SEMOD127523-18 v2
SEMOD127523-21 v3
SEMOD127523-24 v3
Busbar protection REB670
Application manual
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