1
00:00:26,015 --> 00:00:32,812
In DOC protection, the fault direction is detected using the angle between current and voltage

2
00:00:33,047 --> 00:00:43,515


3
00:00:43,984 --> 00:00:50,625
In such cases, the voltage from the healthy phases is used to determine the fault direction

4
00:00:50,859 --> 00:00:59,218
So, if there is a fault in phase A, the voltage from phase BC is used to determine the fault angle

5
00:00:59,844 --> 00:01:11,250
Since a reference angle is needed to detect the fault direction, VBC(ref) must be rotated by an angle ϕ to match the current angle

6
00:01:11,563 --> 00:01:19,375
 This ϕ is typically chosen from three values: 30°, 45°, or 60°

7
00:01:19,609 --> 00:01:29,375
At this angle, it can be said that the fault has occurred directly in front of the relay. This angle is referred to as the MTA

8
00:01:30,625 --> 00:01:39,296
The range from MTA + 90° to MTA – 90° is considered the forward direction

9
00:01:39,531 --> 00:01:46,093
However, due to a measurement error of 4 degrees on both sides, a margin is considered

10
00:01:46,563 --> 00:02:00,079
Therefore, the ‘sector opening’ for this condition is set from MTA + 86° to MTA – 86, which equals 172°

11
00:02:03,000 --> 00:02:05,679
Note on Two-Phase Fault Testing

12
00:02:05,992 --> 00:02:10,679
In two-phase fault testing, to prevent the generation of residual voltage

13
00:02:10,914 --> 00:02:17,320
the angle of the voltages in each phase must be adjusted so that the residual voltage equals zero

14
00:02:17,633 --> 00:02:25,445
Therefore, if the user inputs the phase-to-phase fault voltage (for a BC fault), the relationships shown will apply:

15
00:02:25,992 --> 00:02:39,039
if the phase-to-phase fault voltage is set to 55V and the nominal voltage is 63.51V, α will be 40.9°

16
00:02:39,586 --> 00:02:47,476
In this phase-to-phase fault, to detect the fault, we use an OR condition for faults in phases B and C

17
00:02:47,866 --> 00:02:56,303
where MTA and sector opening need to be calculated separately for each phase, and the union of the two should be considered

18
00:02:56,539 --> 00:02:58,101
For Phase B:

19
00:02:58,961 --> 00:03:00,679
For Phase C:

20
00:03:06,695 --> 00:03:14,273
As shown in the diagram, the sector opening exceeds 180° by half of the overlap area

21
00:03:14,508 --> 00:03:20,445
unlike the single-phase fault where the sector opening was 180° or less

22
00:03:20,680 --> 00:03:28,882
In general, if the fault voltage is equal to the nominal value, the sector opening increases by 30° on both sides

23
00:03:29,117 --> 00:03:33,648
and this value decreases linearly with the voltage drop