10.2.1 Protection of a busbar using logic selectivity (see Figure 10-20b)

Figure 10-20b: protection of a busbar using logic selectivity

Logic selectivity is established between the incoming feeder A and outgoing feeders D1 and D2. It is assumed that selectivity with the downstream protection devices requires a time delay of 0.7 seconds on the outgoing feeders.

The duration of the logic standby is limited to 200 ms after the outgoing feeder protection time delay. This enables the incoming feeder protection to operate as back-up if a downstream protection fails.

Back-up protection is positioned at A and is activated when the logic standby order does not function normally. Its time delay is set so as to obtain time-graded selectivity with the outgoing feeders (it is included in the Sepam logic system).

Fault at :

• The protection at D1 sends a logic standby order to the protection at A.

• The circuit-breaker D1 is tripped after its time delay of 0.7 seconds.

• If the protection at D1 or the circuit-breaker D1 fails, the protection at A is activated after the logic standby time is up, i.e. after 0.7 + 0.2 = 0.9 seconds.

Fault at :

• The circuit-breaker A is tripped after its time delay of 0.1 second.

• When the logic standby signal operates abnormally (spurious activation), the back-up protection is activated after its time delay...