7SR18 Applications Guide
Unrestricted Page 32 of 63 ©2018 Siemens Protection Devices Limited
6.2 Backup Over Current and Earth-fault Protection
The relay provides 51-n elements which can be configured as inverse definite minimum time lag (IDMTL)
elements or instantaneous / definite time lag (DTL) elements, for both phase and earth-faults. There are also 50-n
DTL elements pre-configured to DTL.
Any of these elements can be set to be in service permanently, or only when the end to end protection signal
becomes corrupted, i.e. when differential protection is no longer possible.
These elements can be set and used as guard relays for the differential protection, i.e. the differential protection
will only operate and trip when the local relay current exceeds the guard element setting(s).
The IDMTL/DTL elements can be set to grade with relays or fuses up and down-stream of the protected feeder. In
most applications, the selection of relay IDMTL characteristics will be dictated by the type of curve used on the
over current and earth-fault protection relays on the source and load side of this relay. Usually normal inverse
curves are selected for grading between relays. Extremely Inverse curves type C to IEC 60255 are often used on
H.V. transformer circuits, since this type of curve grades with L.V. fuses or moulded case circuit breakers. The
setting applied to the earth-fault elements must consider residual current caused by charging current under
normal load and under fault conditions.
6.3 Line Differential Enabled with Overcurrent Inhibited –
Overcurrent Enabled when Line Differential Inhibited
Switch-Over
The relay can be configured to switch between Line Differential Protection only and Overcurrent Protection only in
the event of Protection Communication Signalling failure i.e. have Line Differential only and Inhibit overcurrent
(back-up protection) or have Line Differential disabled and Overcurrent (back-up protection) Enabled.
This can be achieved by the use of a Quick Logic equation and some Virtual connections.
With healthy protection signalling between relays there will not be an output from the ‘Prot’n Comms Alarm’. This
is mapped to V1 (see Figure 6.3-1). Quick Logic equation E1 will utilise V1 by inverting it e.g. !V1 (NOT V1) (see
Figure 6.3-3). This means the equation E1 will be ‘True’ when the Comms is healthy and give an output from E1
to virtual connection V2 in the Output Matrix (see Figure 6.4-3). This is then connected to the Input Matrix V2
which in turn is mapped to Inhibit 51-1 & 51-2 in this example (see Figure 6.3-5).
With the protection communication signalling lost between relays there will be an output from ‘Prot’n Comms
Alarm’. This condition will make equation E1 become ‘False’ and not give an output from E1 to virtual V2 and the
Inhibit will be removed from 51-1 & 51-2 thus enabling overcurrent protection. The Line Differential Protection will
be automatically Inhibited within the software.
Figure 6.3-1 Output Matrix Protection Comms Alarm / Virtual Mapping
Figure 6.3-2 Protection Comms Alarm settings
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