7SR18 Applications Guide
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Contents
Document Release History ................................................................................................................................. 2
Software Revision History .................................................................................................................................. 2
Section 1: Common Functions ............................................................................................................................ 7
1.1 Multiple Settings Groups .................................................................................................................... 7
1.2 Binary Inputs ..................................................................................................................................... 8
1.2.1 Alarm and Tripping Inputs ..................................................................................................... 8
1.2.2 The Effects of Capacitance Current ...................................................................................... 9
1.2.3 AC Rejection ........................................................................................................................ 9
1.2.4 Use of Binary Inputs in control and tripping circuits .............................................................. 10
1.3 Binary Outputs ................................................................................................................................. 12
1.4 LEDs ............................................................................................................................................... 12
Section 2: Protection Functions ........................................................................................................................ 13
2.1 Time delayed over-current (51/51G/51N) .......................................................................................... 13
2.1.1 Selection of Over-current Characteristics ............................................................................ 14
2.1.2 Reset Delay ....................................................................................................................... 15
2.2 Cold Load Settings (51c) .................................................................................................................. 15
2.3 Instantaneous Over-current (50/50G/50N) ........................................................................................ 16
2.3.1 Blocked Over-current Protection Schemes .......................................................................... 16
2.4 Negative Phase Sequence Over-current (46NPS) ............................................................................. 17
2.5 Undercurrent (37) ............................................................................................................................ 18
2.6 Thermal Overload (49) ..................................................................................................................... 18
Section 3: CT Requirements ............................................................................................................................ 19
3.1 CT Requirements for Differential, Over-current and Earth-fault Protection .......................................... 19
3.1.1 Over-current Protection CTs ............................................................................................... 19
3.1.2 Earth-fault Protection CTs................................................................................................... 19
Section 4: Current Transformer Requirements .................................................................................................. 20
4.1 Current Transformer Ratio Selection ................................................................................................ 20
4.2 Current Transformer Class/Rating .................................................................................................... 20
4.3 CT Formulae ................................................................................................................................... 22
Section 5: Determining Current Transformer Requirements ............................................................................... 24
5.1 Step 1 - Determine the Bias Break Point Setting ............................................................................... 24
5.2 Step 2 - Determine the Fault Level and X/R Ratio of a Through-fault ................................................. 24
5.3 Step 3 – Estimate of Total Resistance of the CT Secondary Circuit ................................................... 25
5.3.1 Example CT Requirement - Solidly Earthed 10 km 132 kV Feeder ....................................... 26
5.3.2 Fault Level and X/R for a Phase Through-fault .................................................................... 28
Section 6: Relay Functions & Settings .............................................................................................................. 30
6.1 Current Differential Protection .......................................................................................................... 30
6.2 Backup Over Current and Earth-fault Protection................................................................................ 32
6.3 Line Differential Enabled with Overcurrent Inhibited – Overcurrent Enabled when Line Differential
Inhibited Switch-Over ............................................................................................................................... 32
6.4 Guard Operation .............................................................................................................................. 34
6.5 Protection Signalling ........................................................................................................................ 35
6.6 Inter-tripping .................................................................................................................................... 35
6.6.1 External Inter-trip example using separate CB Fail device.................................................... 35
6.6.2 Inter-trip example using Fascia Function Keys to operate CBs. ............................................ 37
Section 7: Differential Protection Settings For Feeder Circuits ........................................................................... 40
7.1 Capacitive Charging Current – Cable and Hybrid Feeders ................................................................. 40
7.2 Plain Poly Phase Cable Feeders ...................................................................................................... 41
7.3 Single Phase Cable Feeders ............................................................................................................ 41
7.4 Overhead Line Feeder ..................................................................................................................... 41
7.5 Earth Fault Sensitivity ...................................................................................................................... 42
7.5.1 Solid or Effective Neutral Earthing....................................................................................... 42
7.5.2 High Impedance and Resistance Earthed Neutrals .............................................................. 42
7.5.3 Isolated (unearthed) and Reactance Earthing ...................................................................... 47
Section 8: Control Functions ............................................................................................................................ 48
8.1 Auto-reclose Applications Guide ....................................................................................................... 48
8.1.1 Basic Scheme .................................................................................................................... 49
8.1.2 Basic Scheme with Remote Deadtime delay ....................................................................... 54
8.1.3 Guarded Scheme ............................................................................................................... 56
Section 9: Supervision Functions ...................................................................................................................... 57
9.1 Circuit-Breaker Fail (50BF) ............................................................................................................... 57
9.1.1 Settings Guidelines ............................................................................................................ 57
9.2 Current Transformer Supervision...................................................................................................... 59
9.3 Trip/Close Circuit Supervision (74T/CCS) ......................................................................................... 60
9.3.1 Trip Circuit Supervision Connections ................................................................................... 60
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