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2.14
Backup Time Overcurrent Protection
The 7SD5 device has an integrated time overcurrent protection function. This function
may optionally be used either as back-up time delayed overcurrent protection or as
emergency overcurrent protection. Please note that this protection function is avail-
able in addition to main protection functions, such as differential and distance protec-
tion, to provide even more security.
2.14.1 General
While the general line protection 7SD5, when configured with differential protection,
can only operate properly if each device receives correctly the data of the other devic-
es, and distance protection can only operate properly if the correct measuring voltage
is applied to the device, the emergency time overcurrent protection needs only the
local currents. The emergency time overcurrent protection is automatically activated
when the data communication of the differential protection is disturbed and the mea-
suring voltage fails (emergency operation). Both the differential protection and dis-
tance protection are then blocked.
This means that emergency operation replaces the differential protection and/or the
distance protection as short-circuit protection if protection data communication fails
and the distance protection working in parallel detects a failure of the measuring volt-
ages from one of the following conditions:
• The „Voltage transformer mcb tripped" signal is received via binary input, indicating
• One of the internal monitoring functions (e.g. current sum, wire break or „Fuse-
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),
• A further overcurrent stage which has an additional enable input.
These four stages are independent from each other and are freely combinable. Block-
ing from external criteria via binary inputs is possible, as well as switch-onto-fault trip-
ping. It is also possible to release one or more of the stages in switching-onto-fault
conditions. If not all stages are required, each individual stage can be deactivated by
setting the pickup threshold to ∞.
2.14.2 Functional Description
Measured Values
The phase currents are fed to the device via the input transformers of the measuring
input. The earth current 3I
If I
be available directly as measured quantity.
If I
used, while considering the factor I4/Iph CT (address 221, see Section 2.1.2 under
margin heading „Current Connections") of Power System Data 1. If the residual
current is not connected to the fourth current input I
7SD5 Manual
C53000-G1176-C169-1
that the measured voltage signal is lost, or
Failure-Monitor") is activated, see Section 2.22.1.3.
0
is connected to the starpoint of the current transformer set, the earth current will
4
is connected to a separate earth current transformer, this measurement will be
4
2.14 Backup Time Overcurrent Protection
is either measured directly or calculated.
(address 220 I4 transformer
4
269
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