Measurements; Protection; 1.Overcurrent Protection - ABB REX 521 Technical Reference Manual, Standard Configurations

1MRS 751802-MUM
11.3.2.
11.3.3.
11.3.3.1.
Protection Relay
Technical Reference Manual, Standard Configurations

Measurements

Following measurements are supported in this application example:
• True RMS measurement of phase currents
• True RMS measurement of phase-to-phase voltages
• System frequency measurement

Protection

All the necessary settings for the protection functions can be set via the Relay
Setting Tool or the local HMI. The settings can be found in
Main menu\Protection\.
Overcurrent protection
Overcurrent protection of the bus sectionalizer is used for disconnecting the faulty
sector of the busbar from the healthy sector to minimize disturbances in distribution
of electrical energy.
In the example of the bus sectionalizer application, the stage 3I> of the overcurrent
protection of the bus sectionalizer and incoming feeder operate time selectively as a
back-up protection of the outgoing feeders. The stage 3I>> is used for blocking of
the incoming feeders. The stage 3I>>> is used for short-circuit protection of the
busbar system.
If a fault should occur in the busbar system, the stage 3I>> of the bus sectionalizer
provides blocking signal to the 3I>> stage of the incoming feeders. Also, the 3I>>>
stage of the overcurrent protection of the bus sectionalizer provides a trip signal to
the bus sectionalizer circuit breaker.
When a fault is limited to one busbar sector, the blocking from the 3I>> stage of the
bus-sectionalizer overcurrent protection disappears and the 3I>> stage of the
overcurrent protection of the incoming feeder provides a trip signal to the circuit
breaker of the incoming feeder.
If a fault occurs in the feeder, the overcurrent protection relay of the outgoing feeder
provides a blocking signal to the overcurrent protection relays of the bus
sectionalizer and incoming feeders. Therefore, it is possible to use a minimum
operate time of 100 ms for the overcurrent protection 3I>>> stage of the bus
sectionalizer and for the 3I>> stage of the incoming feeders when a fault in the
busbar system occurs.
The needed time for blocking includes:
Start time of the relay +
(40 ms)
The set operate time of a back-up protection stage 3I> of the bus sectionalizer can
be calculated as follows:
Time grading +
Input delay on the relay + Retardation time
to be blocked
(10 ms)
Operate time of the
protection relay of the
outgoing feeder
REX 521
+ Margin
(30 ms) (20 ms)
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