Table of Contents
Advertisement
2.8
Definite-Time Overcurrent Protection (I>, ANSI 50/51) with
Undervoltage Seal-In
The time-overcurrent protection is used as backup protection for the short-circuit protection of the protected
object. It also provides backup protection for downstream network components if faults there are not discon-
nected in time thus endangering the protected object.
The 7UM62 relay allows to choose between the input transformers of side 1 and side 2 for allocation of the
time-overcurrent protection function. This choice is made during configuration (see Section 2.4).
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 sub-
sides quickly in the event of adjacent faults (i.e. in the generator 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 that the relay drops out again, the I> stage monitors the positive-sequence component of the voltages
and uses it as an additional criterion for detecting a short-circuit. The undervoltage influencing can be disabled
and made ineffective via binary input.
2.8.1
Functional Description
I> Stage
Each phase current of side 1 or 2 (depending on configuration) is compared individually with the common
setting value I> and on overshoot signaled individually. 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 pick-up signal for a select-
able seal-in time if the value falls below a selectable threshold of the positive-sequence component of the volt-
ages after an overcurrent pickup - even if the value again falls below the overcurrent value. In this way, the
expiration 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 elapsed or if the undervoltage seal-in is blocked via a binary input,
e.g. in case of a tripping of the voltage transformer protective breaker or in case of a machine stopping, the
protective relay drops out immediately.
The seal-in logic operates separate for each phase. The first pickup starts the timer T-SEAL-IN.
The following figure shows the logic diagram of the overcurrent time protection I> with undervoltage seal-in.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
2.8 Definite-Time Overcurrent Protection (I>, ANSI 50/51) with Undervoltage Seal-In
Functions
65
Advertisement
Table of Contents
