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4
Transformer Differential Protection (87T and 87HS).................................................................. 39
4.1
Method of Operation...................................................................................................................................39
4.1.1
Matching Currents ......................................................................................................................................39
4.1.2
Handling Non-Ideal Behavior......................................................................................................................41
4.2
Describing the Transformer ........................................................................................................................ 43
4.2.1
Type of Transformer ...................................................................................................................................44
4.2.2
Normal Operational Status ......................................................................................................................... 44
4.2.3
System Frequency...................................................................................................................................... 44
4.2.4
Rated Phase-to-Phase Voltage .................................................................................................................. 45
4.2.5
Rated Apparent Power ............................................................................................................................... 45
4.2.6
Phase Shift Between Windings .................................................................................................................. 45
4.3
Describing the CT Scheme......................................................................................................................... 48
4.3.1
Nominal Secondary Rating of CTs.............................................................................................................. 48
4.3.2
Nominal Primary Rating of CTs .................................................................................................................. 48
4.3.3
Three-Winding Transformers with Considerably Different Rated Powers .................................................. 48
4.3.4
Orientation of CTs.......................................................................................................................................49
4.3.5
Processing the Zero Sequence Current ..................................................................................................... 49
4.3.6
Zero-Sequence Current Elimination vs. Correction Mathematical Illustration ............................................ 50
4.3.7
Using a Ground or Neutral Current CT....................................................................................................... 54
4.4
Trip Characteristic.......................................................................................................................................55
4.4.1
Minimum 87T Trip Threshold......................................................................................................................56
4.4.2
High Set (87HS) Trip Threshold ................................................................................................................. 56
4.4.3
Lower Slope of Pickup Characteristic......................................................................................................... 57
4.4.4
Upper Slope of Pickup Characteristic......................................................................................................... 58
4.5
Through-Fault Restraint.............................................................................................................................. 58
4.6
Second-Harmonic Restraint ....................................................................................................................... 59
4.7
Higher-Harmonic Restraint ......................................................................................................................... 60
4.8
Blocking by Frequency Variance ................................................................................................................ 61
4.9
Time Delays................................................................................................................................................ 61
4.10
Events and Actions.....................................................................................................................................62
4.11
Single-Phase Transformers ........................................................................................................................ 66
5
Motor or Generator Differential Protection (87M/G)................................................................... 67
5.1
Method of Operation...................................................................................................................................67
5.2
Describing the Protected Motor or Generator............................................................................................. 67
5.2.1
Type of Protected Object ............................................................................................................................ 68
5.2.2
Normal Operational Status ......................................................................................................................... 68
5.2.3
System Frequency......................................................................................................................................68
5.2.4
Motor/Generator Information ......................................................................................................................68
5.3
Describing the CT Scheme......................................................................................................................... 69
5.3.1
CT Orientation ............................................................................................................................................ 69
5.3.2
Nominal Primary Rating of CTs .................................................................................................................. 71
5.3.3
Nominal Secondary Rating of CTs.............................................................................................................. 71
4
7UT51 v3
PRIM-2330C
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