Siemens 6MD85, Manual

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Direction Determination
Description
General
Every phase has a separate direction-measuring element. If the threshold value in a phase is exceeded, the
direction determination is started for this phase. If there are multiphase short circuits, all measuring elements
involved perform direction determination independently. If one of the determined directions matches the set
direction, the stage picks up (see descriptions of the stage logic).
The direction is determined by calculating the phase angle between the short-circuit current and a reference
voltage.
Measurands for Direction Determining
The directional measuring element uses the short-circuit current of the phase concerned and the cross-polar-
ized phase-to-phase voltage (as the reference voltage) to determine the direction. This means that the direc-
tion can still be determined unambiguously and correctly, even if the short-circuit voltages collapse
completely when a 1-phase or 2-phase fault occurs (close-up fault).
The phase-to-phase voltages are calculated when phase-to-ground voltages are connected.
The cross-polarized voltage (reference voltage) is vertical in relation to the short-circuit voltages for 1-phase-
to-ground faults (
Figure 7-97 , left). For 2-phase short circuits, the position of the reference voltages changes
up to 30 o , depending on the extent to which the short-circuit voltages collapse ( Figure 7-97 , right).
[dwdocp02-240611-01.tif, 1, en_US]
Figure 7-97 Cross-Polarized Voltages for Direction Determination
The following table shows how measurands are assigned for direction-determination purposes in the event of
different types of fault.
Table 7-2 Measurands for Direction Determining
Threshold-
Value
Exceeding Measuring Element
A B C Ground
Current Voltage Current Voltage Current Voltage Current Voltage
A I
A
V
BC
– – – – – –
B – – I
B
V
CA
– – – –
C – – – – I
C
V
AB
– –
Gnd – – – – – – I
r
V
0
A, Gnd – V
BC
– – – – I
r
V
0
B, Gnd – – I
B
V
CA
– – I
r
V
0
C, Gnd – – – – I
C
V
AB
I
r
V
0
A, B I
A
V
BC
I
B
V
CA
– – – –
7.6.7
7.6.7.1
Protection and Automation Functions
7.6 Directional Overcurrent Protection, Phases
694 SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017