Minimum Contact Requirements; Auxiliary Contact Evaluation Logic - ABB REB670 Applications Manual

Section 6
Differential protection
6.1.3.6
6.1.3.7
108

Minimum contact requirements

The minimum requirement for the busbar replica is the record of the disconnector position
by using just one auxiliary contact, either NO or NC 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 the OPEN and CLOSED signal inputs disagree (that is, both binary
inputs are active or 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.
Scheme2_INX "Closed or open if clear indication available otherwise last
position saved"
As the name of the scheme suggests, only when the auxiliary contacts signal clean OPEN
or clean CLOSED position disconnector is considered to be open respectively closed.
However this poses the stringent requirements on the auxiliary contacts that the CLOSED
signal must become active a certain time (>150 ms) before current starts flowing for
example, through arcing. Otherwise this current will not be taken into account in the
busbar protection and this can result in a maloperation. Therefore, good timing of two
auxiliary contacts is definitely required.
The time during which the OPEN and CLOSED signals disagree (that is, both binary
inputs are active or both are inactive) is monitored by the isolator supervision function for
both of the above two schemes. The maximum time allowed before an alarm is given can
be set according to the disconnector timing.
Table 12 and the following two figures summarize the properties of these two schemes.
1MRK 505 337-UUS -
Application manual