Functions
2.4 Backup Overcurrent
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
78
2.4 Backup Overcurrent
The 7SD80 features an overcurrent protection function which can be used as either backup or emergency over-
current protection. All elements are independent of each other and can be combined as desired.
The overcurrent protection has two overcurrent elements with definite trip time and one overcurrent protection
element with inverse time delay for the phase currents and for the ground current. These elements operate di-
rectionally or non-directionally.
One additional definite-time overcurrent protection element always operates non-directionally. It features an
additional release input and can act as emergency element if the other elements are used for backup purposes.
The elements are independent of each other and can be combined in any way. Blocking by external criteria via
binary inputs is possible.
2.4.1 Operating Modes
Emergency Overcurrent Protection
The differential protection as a whole can only operate correctly if both devices receive the data of the respec-
tive other device properly. The emergency overcurrent protection in contrast requires only the local currents.
Acting as emergency overcurrent protection, it automatically replaces the differential protection as short-circuit
protection if data communication of the differential protection is faulty (emergency operation). The differential
protection is blocked in this case.
Backup Time Overcurrent Protection
If the overcurrent protection is set as backup time overcurrent protection, it will work independently of the other
protection and monitoring functions, i.e. also independently of the differential protection. The backup overcur-
rent protection can also act as the only short-circuit protection if no suitable channels for the communication
between the protection devices are available yet during the initial commissioning. It can be used as busbar pro-
tection via reverse interlocking in combination with other protection devices or as backup protection function
for protection device failure at continuing lines.
2.4.2 Non-directional Overcurrent Protection
Measured Quantities
The phase currents are fed to the device via the input transformers of the measuring input. The ground current
3I
0
is calculated from the phase currents.
Definite Time High-set Element 50-1
Each phase current is compared with the setting value 50-B2 PICKUP after numerical filtering; the ground
current is compared with 50N-B2 PICKUP. A trip command is issued after pickup of an element and expiration
of the associated time delays 50-B2 DELAY or 50N-B2 DELAY. The dropout value is about 7 % below the
pickup value, but at least 5 % of the rated current.
Figure 2-26 shows the logic diagram of the 50-1 elements. They can be blocked via binary input „>BLOCK 50-
B2“. Additionally, the ground current element can be blocked separately via the binary input „>BLOCK 50N-
B2“.
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