2 Functions
42
7UM61 Manual
C53000-G1176-C127-3
2.6
Definite-Time Overcurrent Protection (I>, ANSI 50/51) with
Undervoltage Seal-In
The overcurrent protection is used as backup protection for the short-circuit protection
of the protected object. It also provides backup protection for downstream network
faults which may be not promptly disconnected thus endangering the protected object.
Initially, the currents are numerically filtered so that only the fundamental frequency
currents are used for the measurement. This makes the measurement insensitive to
transient conditions at the inception of a short-circuit and to asymmetrical short-circuit
currents (d.c. component).
In generators where the excitation voltage is taken from the machine terminals, the
short-circuit current subsides quickly in the event of adjacent faults (i.e. in the gener-
ator or unit transformer region) due to the absence of excitation voltage. Within a few
seconds it sinks below the pick-up value of the overcurrent time protection. To avoid
pickup dropout, the I> stage monitors the positive-sequence component of the voltag-
es and uses it as an additional criterion for detecting a short-circuit. The undervoltage
influencing can be disabled off and made ineffective via binary input.
2.6.1
Functional Description
I> Stage
Each phase current is compared individually with the
I>
common setting value and
signaled separately on overshoot. A trip signal is transmitted to the matrix as soon as
the corresponding
T I>
time delay has expired. On delivery the dropout value is set
to
±
95 % below the pickup value. For special applications, it is also possible to set a
higher value.
Undervoltage Seal-
In
The I> stage has a (disconnectable) undervoltage stage. This stage maintains the
pickup signal for a selectable seal-in time if the value falls below a selectable threshold
of the positive-sequence component of the voltages after an overcurrent pickup - even
if the value falls again below the overcurrent value. In this way, the expire of the trip
time delay and the tripping of the related breakers is also ensured in these cases. If
the voltage recovers before the seal-in time has expired or if the undervoltage seal-in
is blocked via a binary input, e.g. when the voltage transformer mcb trips or if the
machine is tripped, protection drops out immediately.
The seal-in logic operates separate for each phase. The first pickup of a phase over-
current starts the timer
T-SEAL-IN
.
The following figure shows the logic diagram of the overcurrent time protection I> with
undervoltage protection.
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