Protective Elements
• ④·
»Mho Offs.Imp.Angle«
= 270°
Alternatively:
According to the IEEE Std. C37.102-2006 recommendation and in case of lacking of connected
system data the MHO circle may be constructed with a positive impedance reach of 1.5 times the transformer
impedance and an offset impedance reach of 2 times the generator transient reactance. The system impedance
angle is 90° in this case, which leads to a positive impedance angle of 90° and an offset impedance angle of 270°.
The distance between the blinders and the origin is generally derived from the impedance swing position where the
angular separation between the system and the generator exceeds the stability limit. If no stability study is available,
the separation angle is typically set to 120°.
In the diagram above, the angle of
Z
S
is shown quite far away from 90° (i. e. the R-part of
Z
S
is quite large),
because otherwise, it would be difficult to show all involved data. However, for the calculation of the blinder
distances, we are going to neglect the small displacement on the R-axis, so that we set: |
Z
S
| ≈
X
S
Therefore the distance of the blinders can be calculated as follows:
Blinder B = ½(
X'
d
+
X
T
+
X
S
)⋅tan(90° − ½δ
S
)
δ
S
is the pole slip angle at the stability limit (i. e. where an unstable power swing occurs).
Blinder A = ½(
X'
d
+
X
T
+
X
S
)⋅tan(90° − ½δ
O
)
δ
O
is the pole slip angle where the »Operate« signal is issued.
Via the trip delay time an additional delay between the »Operate« and the »Trip« can be defined.
For Out of Step detection with the generator running in motoring mode, the blinder values will be used vice versa,
which has to be considered when calculating the above blinder settings.
Timer Settings
The minimum dwell time (measured between entering the MHO circle and crossing the first blinder) is used to
differentiate power swing events from other transient events like faults:
• δ
S
= pole slip angle at stability limit (as above)
• δ
S
= pole slip angle when the impedance reaches the MHO circle.
This angle is usually 90° if the Out of Step characteristic is configured according to the available system
data as shown in the diagram above.
•
f
S,max
= maximum slip frequency
The maximum dwell time of the impedance within the MHO circle:
• δ
MHO
= pole slip angle range which is covered by the MHO circle.
This angle range is usually 270°−90° = 180° if the Out of Step characteristic is configured according to the
available system data as shown in the diagram above
•
f
S,min
= minimum slip frequency
807 MCDGV4 DOK-HB-MCDGV4-2E
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