Reverse Interlocking - Siemens siprotec 7SA6 User Manual

Distance protection

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Reverse Interlocking

If the distance protection 7SA6 is used as back-up protection in single-end fed

transformer feeders, the reverse interlocking function ensures a fast protection of the

busbar without endangering the selectivity for faults on the outgoing feeders.

According to Figure 6-57 the distance zones Z1 and Z2 serve as back-up stages for

faults on the outgoing lines, for example a fault in F2. For distance grading the shortest

outgoing line is to be used.

The overreach zone Z1B, whose delay time T1B must be set longer than the pickup

time Ta of the protection devices of the outgoing lines, is blocked after the pickup of

an inferior protection. The pickup signal is sent (according to Figure 6-57) via the

receive input of the distance protection.

If no signal is received this zone guarantees fast tripping of the busbar for

- faults on the busbar, such as for example in F1,

- failure of the line protection during a fault, such as for example in F2.

The reverse interlocking of the distance protection is performed by specific release or

blocking of the overreach zone Z1B. It can be realized by the blocking mode (see

Figure 6-57) or the release mode.

To avoid transient false signals after clearance of external faults, the blocking

condition of the reverse interlocking is extended by a transient blocking time (TB in

Figure 6-57).

Reverse interlocking — functional principle and grading example

Shortest Line

C53000-G1176-C133-1

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