Guard Operation - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
6.4

Guard Operation

Generally Guard Operation is not required for this relay as the Protection Data Communications is extremely
reliable. The relay's ability to detect Protection Data Signal Loss will inhibit differential operation if there is a break
in Protection Communication between relays. However, in areas where there is perceived to be a possibility of
loss of data frame synchronisation, it is possible to use over-current elements in conjunction with differential
elements to formulate a Guard feature and hence avoid any unwanted relay operation.
This can be achieved by utilising a Quick Logic equation. This example uses a 50-1 DTL element and also an
87L-1 differential element to logically AND together in an equation. The current has to be above the 50-1 setting
and there must also be a difference between local and remote current to produce an 87L-1 differential output.
These two parameters when energised will render the equation to be 'true' and hence operate a circuit-breaker.
Figure 6.4-1 Output Matrix Element Mapping
Figure 6.4-2 Quick Logic Equation
Figure 6.4-3 Output Matrix Equation / BO Mapping
The differential elements are automatically blocked if a protection signalling disturbance occurs; i.e. a discrete
guard element does not have to be set to ensure stability.
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