Alstom MiCOM P546 - 2.3.25 Delta directional comparison - selection of (I and (V threshold (Distance option only)

Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-33
AP
2.3.25 Delta directional comparison - selection of I and V threshold (Distance option only)
For best performance, it is suggested that the delta I Fwd current threshold is set at 10 to
20% In. This will ensure detection of all fault types, provided that the fault current
contribution to an earth fault at the remote end of the line will generate at least this amount of
delta. Selection of the correct Delta V Forward setting is achieved with reference to the
following table (SIR = Source to Line impedance ratio):
Lowest SIR ratio of the system
Recommended V Fwd (as a % of Vn)
 0.3
4%
 0.5
6%
 1
9%
 2
13%
 3
15%
 5
17%
10
19%
25 – 60 21%
For the reverse fault detectors, these must be set more sensitively, as they are used to
invoke the blocking and current reversal guard elements. It is suggested that all reverse
detectors are set at 66 to 80% of the setting of the forward detector, typically:
 Delta V Rev = Delta V Fwd x 0.66
 Delta I Rev = Delta I Fwd x 0.66
This setting philosophy is in-accordance with the well-proven ALSTOM Grid LFDC relay.
Deltas by their nature are present only for 2 cycles on fault inception. If any distance
elements are enabled, these will automatically allow the delta forward or reverse decisions to
“seal-in”, until such time as the fault is cleared from the system. Therefore, as a minimum,
some distance zone(s) must be enabled as fault detectors. It does not matter what time
delay is applied for the zone(s) – this can either be the typical distance delay for that Zone,
or set to maximum (10 s) if no distance tripping is required. As a minimum, Zone 3 must be
enabled, with a reverse reach such as to allow seal-in of Delta Rev, and a forward reach to
allow seal-in of Delta Fwd. The reaches applicable would be:
 Zone 3 Forward
- Set at least as long as a conventional Zone 2 (120-
150% of the protected line)
 Zone 3 Reverse
- Set at least as long as a conventional Zone 4, or
supplement by assigning Zone 4 itself if a large
reverse reach is not preferred for Zone 3.
A mho characteristic is generally advised in such starter applications, although quadrilaterals
are acceptable. As the mho starter is likely to have a large radius, applying the Load Blinder
is strongly advised.