2 Functions
208
7SA522 Manual
C53000-G1176-C155-3
2.11 Overcurrent protection
The 7SA522 features a time overcurrent protection function which can be used as
either a back-up or an emergency overcurrent protection. All elements may be config-
ured independently of each other and combined according to the user's requirements.
2.11.1 General
Whereas the distance protection can only function correctly if the measured voltage
signals are available to the device, the emergency overcurrent protection
only requires
the currents. The emergency overcurrent function is automatically activated when the
measured voltage signal is lost, e.g. due to a short circuit or interruption of the voltage
transformer secondary circuits (emergency operation). The emergency operation
therefore replaces the distance protection as short circuit protection if loss of the mea-
sured voltage signal is recognized by one of the following conditions:
• Pickup of the internal measured voltage monitoring (“Fuse–Failure–Monitor”, refer
to Subsection 2.19.1) or
• If the signal “Failure: Feeder VT (MCB tripped)” is received via binary input, indicat-
ing that the measured voltage signal is lost.
If one of these conditions arise, the distance protection is immediately blocked and the
emergency operation is activated.
If the overcurrent protection is set as a back-up overcurrent protection
, it will work in-
dependently of other protection and monitoring functions, i.e. also independently of
the distance protection. The back-up overcurrent protection could for instance be used
as the only short-circuit protection if the voltage transformers are not yet available
when the feeder is initially commissioned.
For the overcurrent protection there are in total four stages for the phase currents and
four stages for the earth currents as follows:
• Two overcurrent stages with a definite time characteristic (O/C with DT),
• One overcurrent stage with inverse time characteristic (IDMT),
• One further overcurrent stage which is preferably used as a stub protection, but
which can be applied as an additional normal definite time delayed stage.
These four stages are independent of each other and are freely combinable. Blocking
by external criteria via binary input is possible as well as rapid (non delayed) tripping
(e.g. by an external automatic reclose device). During energization of the protected
feeder onto a dead fault it is also possible to release any stage, or also several, for
non-delayed tripping. If some stages are not needed, those not needed can be deac-
tivated by setting the pickup value to ∞.
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