Delta Directional Element Setting Guidelines; Delta Thresholds; Distance Protection Worked Example - GE MiCOM P40 Technical Manual

P446SV
6.10

DELTA DIRECTIONAL ELEMENT SETTING GUIDELINES

For the Delta directional element, the relay characteristic angle (RCA) settings must be based on the average
source + line impedance angle for a fault anywhere internal or external to the line. Typically, the Dir Char Angle is
set to 60°, as it is not essential for this setting to be precise. When a fault occurs, the delta current will never be
close to the characteristic boundary, so an approximate setting can be applied.
6.10.1

DELTA THRESHOLDS

For best performance, set the Dir. I Fwd current threshold at 10 to 20% In. This ensures detection of all fault types
if the fault current contribution to an earth fault at the remote end of the line generates at least this amount of
delta. To select the correct Delta V Forward setting, refer to the following table which compares SIR (Source to Line
impedance ratio) with recommended Delta V Forward setting (DV Fwd).
Lowest SIR ratio of the system
>= 0.3
>= 0.5
>= 1
>= 2
>= 3
>= 5
>= 10
25 – 60
The reverse fault detectors must be set more sensitively, as they are used to invoke the blocking and current
reversal guard elements. We suggest that all reverse detectors are set at 66 to 80% of the setting of the forward
detector, typically:
● Dir. V Rev = Dir. V Fwd x 0.66
Dir. I Rev = Dir. I Fwd x 0.66
●
Due to the implementation method, Deltas 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 in the DISTANCE
SETUP column 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 'Disabled' in the SCHEME LOGIC column, 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 Dir. Rev, and a
forward reach to allow seal-in of Dir. Fwd.
The applicable reaches 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 if a large
●
reverse reach is not preferred for Zone 3.
We generally advise a Mho characteristic in such starter applications, although quadrilaterals are acceptable. As
the Mho starter is likely to have a large radius, we strongly advise applying the Load Blinder.
6.11

DISTANCE PROTECTION WORKED EXAMPLE

This section presents a worked example of how to set the Distance protection. For this case study, we assume that
Zone 1 Extension is not used, and that only three zones are required for basic Distance protection (Zones 1 and 2
Forward Directional, and Zone 3 Forward Offset). The following settings are derived:
Line Impedance
●
● Residual Compensation
P446SV-TM-EN-1
Recommended DV Fwd (as a % of Vn)
4%
6%
9%
13%
15%
17%
19%
21%
Chapter 7 - Distance Protection
157