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P746/EN AP/G31 Application Notes
(AP) 6-
12
MiCOM P746
Example:
Infeed CT Contribution* PC%
2500 / 11300 – ID>2 ID>2
Infeed 1 2500 / 5 2500 / 11300
2500
=
11300
2000 / 11300 – ID>2 ID>2
Infeed 2 2000 / 1 2000 / 11300
2000
=
11300
2000 / 11300 – ID>2 ID>2
Infeed 3 2000 / 1 2000 / 11300
2000
=
11300
1200 / 11300 – ID>2 ID>2
Infeed 4 1200 / 1 1200 / 11300
1200
=
11300
1200 / 11300 – ID>2 ID>2
Infeed 5 1200 / 1 1200 / 11300
1200
=
11300
1200 / 11300 – ID>2 ID>2
Infeed 6 1200 / 1 1200 / 11300
1200
=
11300
1200 / 11300 – ID>2 ID>2
Infeed 7 1200 / 1 1200 / 11300
1200
=
11300
Σ
In = 2500 + (2 × 2000) + (4 × 1200) = 11300
We assume the infeeds will contribute to the ID>2
fault in proportion to their CT pimary current (worst
situation)
then PC% = ID>2 /
Σ
In of Infeed
CTs
AP
2.2.1.2 87BB Settings (Compensated Earthed Network Schemes)
2.2.1.2.1 Sub-station features
Only 4 values have to be known:
1. Maximum load current in a feeder
2. Minimum phase to phase fault current (Ph-Ph min.) in a bus
3. Maximum single phase steady state faulty current (Ph-N Max.) in a bus
4. Number of independent bars
5. Maximum number of infeeds
2.2.1.2.2 Differential Busbar Protection
9 values have to be chosen:
1. ID>1, usual recommendation is 1,2 × (Ph-N Max.) with a minimum of 2% of the
biggest CT primary winding and less than 80% of the minimum load,
2. Slope k1 (ID>1), usual recommendation is 10%.
3. ID>1 Alarm Timer (from 0 to 600 s) shall be greater than the longest Busbar protection
time
4. Slope k2 usual recommendation is 50 / 65%.
5. ID>2 recommendation is: Lower than 0.8 × (Ph-Ph min) and higher than 1.2 × Iload
Max and if possible, equal to 6 × (ID>1).
6. Slope kCZ usual recommendation is 30%.
7. IDCZ>2 usual recommendation is: Lower than 0.8 × (Ph-Ph min) and higher than
1.2 × Iload Max and if possible equal to 6 × (ID>1).