2.6 Definite-Time Overcurrent Protection (I>, ANSI 50/51) with Undervoltage Seal-In
43
7UM61 Manual
C53000-G1176-C127-3
Figure 2-4 Logic Diagram of the Overcurrent Stage I> with Undervoltage Seal-In
2.6.2 Setting Notes
General Overcurrent protection is only effective and available if address 112 O/C PROT. I>
is set to Enabled during configuration. If the function is not needed it is set to
Disabled.
Overcurrent Stage
I>
Address 1201 O/C I> is used to switch the definite time-overcurrent stage I> ON and
OFF, or to block only the trip command (Block relay). For setting the I> overcurrent
stage it is the maximum anticipated load current which is most relevant. Pickup due to
overload should never occur since the protection may trip if short command times are
set. For this reason, a setting between 20 % and 30 % over the expected peak load is
recommended for generators, and a setting of about 40 % for transformers and
motors.
The trip time delay (parameter 1203 T I>) must be coordinated with the time grading
of the network in order to ensure that the protective equipment closest to the corre-
sponding fault location trips first (selectivity).
The selected time is only an additional time delay and does not include the operating
time (measuring time, dropout time). The delay can be set to ∞. After pickup the
element will then not trip. Pickup, however, will be signaled. If the I> stage is not re-
quired at all, address 1201 O/C I> is set to OFF. This prevents tripping and the gen-
eration of a pickup message.
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