Phase Overcurrent Protection - ABB REX 521 Technical Reference Manual, Standard Configurations

REX 521
9.2.3.1.
120
Protection Relay
Technical Reference Manual, Standard Configurations

Phase overcurrent protection

In the example of the incoming feeder application, the stages 3I> and 3I>> of the
overcurrent protection operate time selectively as a back-up protection of the
outgoing feeders. The 3I>>> stage is used for the short-circuit protection of the
busbar system. The set operate time of a back-up protection stage can be calculated
as follows:
Time grading +
The time grading includes (definite time):
2 X Tolerance of the + Operation time
relays' operate times
If a fault occurs in the feeder, the overcurrent protection relay of the outgoing feeder
provides a blocking signal to the overcurrent protection relay of the incoming feeder.
If the fault should occur in the busbar system, no blocking signal is issued and the
3I>>> stage of the overcurrent protection of the in-feeder provides a trip signal to
the in-feeder circuit breaker. Therefore, when no auxiliary relay is used, it is possible
to use a minimum operate time of 100 ms when a fault in the busbar system occurs.
The needed time for blocking includes:
Start time of the relay +
(40 ms)
The blocking arrangement can be extended to include the HV side overcurrent
protection of the main transformer. For example, the start signal of the high-set stage
3I>> can be routed as blocking to the high-set stage 3I>> of the HV relay.
The busbar protection and the co-operation of the relays between the different
protection levels can be arranged in many ways and they vary in different
applications. A few examples of how the described application can be changed using
some of the features of the REX 521 are described below.
The back-up protection can be made with the circuit-breaker failure protection
function of the protection relay of the outgoing feeder. A fast back-up function can
be achieved because the safety margin can be omitted when setting the CBFP time.
Furthermore, the different time settings in the outgoing feeders do not affect the
operation because each relay has a separate CBFP function. The CBFP function
requires external wiring from the protection relays of the outgoing feeders to the
relay of the in-feeder.
If an operate time of less than 100 ms is required in the busbar protection system and
the non-selective operation is allowed, one possibility is to use the instantaneous
stage 3I>>> with an instantaneous operate time. Then the start current value has to
be set to such a level that the fault most probably is in the busbar system. It is of
advantage that a fault in the busbar system does not cause serious damage due to the
Operate time of the
protection relay of the
outgoing feeder
+ Retardation time
of the CB of the relay
Input delay on the relay + Retardation time
to be blocked
(10 ms)
1MRS 751802-MUM
+ Saturation and delay
margin
+ Margin
(30 ms) (20 ms)