The following ABB PDF document describes the sense, the potentials and the use of directional protection and directional zone selectivity functions, 'D' and 'SdZ D' respectively:
ABB's PR123/P and the PR333/P relay units carry out excludable directional protection ('D') against short-circuit with adjustable fixed time active both with self-powering and with auxiliary supply. This protection functionality is very similar to protection 'S' with fixed time, with the capacity to recognize the current direction during the fault period as well.
The 'D' makes it possible to determine whether the fault is on the supply side or load side of the circuit-breaker, and then to obtain selectivity.
'SdZ D' function is useful in ring and grid type systems where, in additional to the zone, it's essential to define the direction of the power flow that supplies the fault. This function is available exclusively on PR123/P and PR333/P relays, and it can be only set to 'on' when zone selectivity S and G are set to off and there is an auxiliary power supply (at 24 V DC).
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Alternatively, go to http://search.abb.com/library/Download.aspx?DocumentID=1SDC007401G0201&LanguageCode=en&DocumentPartId=&Action=Launch