Numerical Differential Protection Principles and Applications

Publicis Corporate Publishing

Contents

1 Introduction 8 1.1 Protection Principle 8 1.2 Numerical Differential Protection 9

2 Deflnitions 10

3 Mode of Operation 15 3.1 Introduction 15 3.2 Basic Principles 17

3.2.1 Current differential protection 17 3.2.2 Biased (stabilised) differential protection 20 3.2.3 Differential Protection with two pilot wire cores 24 3.2.4 Operating characteristics 27

3.3 Measuring Circuit for Three Phase Systems 31 3.3.1 Measurement per phase 33 3.3.2 Composite current version 33

3.4 High Impedance (HI) Differential Protection 39 3.5 Partial Differential Protection 47

4 Measuring Technique 49 4.1 Classical Analog Systems 49 4.2 Numerical Measuring Technique 52

4.2.1 Acquisition of measured values 52 4.2.2 Differential protection with instantaneous value comparison 54 4.2.3 Differential protection with phasors 55 4.2.4 Additional stabilisation with CT Saturation 60

5 Current Transformers (CTs) 64 5.1 Current Transformer Equivalent Circuits 64 5.2 Specifications for the Steady State Response of Current Transformers 66 5.3 Transient Response of the CT 68 5.4 TP Current Transformer Classes 71 5.5 Polarity of the CT 74 5.6 CT Errors 75 5.7 Dimensioning of the CT 78 5.8 Interposing CTs 86

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Contents

6 Communications 104 6.1 Transmission Channels 104

6.1.1 Pilot wires 104 6.1.2 Fibre Optic Cables 114 6.1.3 Line of sight radio links 119

6.2 Digital Protection Communication 119 6.3 Digital Communication Networks 127

7 Generator/Motor Differential Protection 129 7.1 Generator Differential Protection 130 7.2 Motor Differential Protection 144

8 Transformer Differential Protection 146 8.1 Basic Physics 146 8.2 Numerical Measured Value Processing 152 8.3 High Impedance Differential Protection 164 8.4 Devices for Transformer Differential Protection 167 8.5 Application Examples for Transformer Protection 169

9 Line Differential Protection 180 9.1 Three Core Pilot Wire (Triplet) Differential Protection 180 9.2 Two Core (Twisted Pair) Pilot Wire Differential Protection 184 9.3 Feeder Differential Protection with Digital Communication 196 9.4 Phase Comparison Protection with Digital Communication 205 9.5 Differential Protection of Feeders Including Transformers 208

9.5.1 Protection of transformer feeders 209 9.5.2 Differential protection for feeders with tee-offs (tapped lines) 210

10 Busbar Differential Protection 214 10.1 Low Impedance Busbar Differential Protection 215

10.1.1 Partially numerical busbar differential protection 7SS600 218 10.1.2 Fully-numerical busbar protection 7SS52 222

10.2 Response of the Numerical Busbar Protection in the Case of CT Saturation and the Demands Placed on CT Dimension 228

10.3 High-Impedance Busbar Protection 237

11 Relay Design 240

12 Commissioning and Maintenance 246 12.1 Commissioning 246 12.2 Maintenance 248

Literature 249 Addendum 257 Index 258

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Numerical Distance Protection Principles and Applications

Publicis Corporate Publishing

Contents

1 Introduction 11

2 Definitions 13

3 Mode of Operation 21

3.1 Fundamentals of distance protection 21 3.1.1 Concept 21 3.1.2 Relay impedance (secondary impedance) 22 3.1.3 Impedance diagram 23 3.1.4 Distance measurement 24 3.1.5 Directional fault discrimination 28 3.1.6 Starting (fault detection) 31 3.1.7 Distance zones (steps) 42 3.1.8 Zone- and timer-control 46 3.1.9 Switched and non-switched distance protection 48 3.1.10 Distance protection with signalling Channels 51 3.1.11 Power swing blocking, power swing tripping (out of step protection) . . . . 63 3.1.12 Distance protection with automatic reclosure 68 3.1.13 Distance to fault locator 75 3.1.14 Grading chart 80

3.2 Numerical distance measurement 90 3.2.1 Definition of the fault loop 90 3.2.2 Determination of the loop impedance 95 3.2.3 Numerical impedance computation 99

3.3 Numerical direction determination (polarisation) 109 3.3.1 Direction determination with fault loop voltage (seif polarisation) 109 3.3.2 Direction determination with healthy phase voltages (cross-polarisation) .111 3.3.3 Directional characteristic in the impedance plane 113 3.3.4 Selection of the cross polarisation voltage 115 3.3.5 Influence of load transfer 116 3.3.6 Implementation of voltage-memory(-ies) 119 3.3.7 Adaptive directional determination 120

3.4 Circular characteristics with numerical technology 121 3.4.1 MHO-circle 121 3.4.2 PolarisedMHO-characteristic 123 3.4.3 Load influence on polarised MHO-circles 127 3.4.4 MHO-circle with voltage memory 130

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Contents

3.5 Distance measurement, Influencing quantities 130 3.5.1 Fault resistance 130 3.5.2 Intermediate infeeds 151 3.5.3 Parallel lines 155 3.5.4 Distance protection for transformers 168 3.5.5 Non-symmetry of the line 178 3.5.6 Series-compensation 191

4 Device design 200 4.1 Intelligent electronic devices (IEDs) 200 4.2 Mechanical design 202 4.3 Relay Communications 203 4.4 Integrated functions 205 4.5 Relay terminal connections 211 4.6 Relay Operation 214

5 Application 216 5.1 General aspects 216

5.1.1 Application criteria 216 5.1.2 Shortest line length 217 5.1.3 Tripping time 218 5.1.4 Teleprotection, choice of technique 221 5.1.5 Instrument transformer requirements 223

5.2 Distance protection in the distribution System 253 5.2.1 General 253 5.2.2 Distance protection in isolated or compensated Systems 261 5.2.3 Distance protection in distribution networks with low impedance

star-point earthing 268 5.2.4 Distance protection in industrial networks 271

5.3 Distance protection in transmission networks 273 5.3.1 Generals aspects 273 5.3.2 Protection concepts 278 5.3.2.1 High-voltage overhead lines 278 5.3.2.2 EHV-line 280 5.3.2.3 1 1/2 circuit-breaker substations 283 5.3.2.4 Ring busbar 284 5.3.2.5 Double circuit line 284 5.3.2.6 Three-terminal line 285 5.3.2.7 Series-compensated lines 287

6 Protection settings 290 6.1 General aspects 291 6.2 Fault detection (3rd Zone) 291

6.2.1 Fault detection methods and setting philosohies 292 6.2.2 Security of the fault detection 293 6.2.3 Relay (Line) loadability 294

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Contents

6.2.4 Phase-selectivity 295 6.2.5 Setting of the U-I-j fault detection 297 6.2.6 Setting of the impedance fault detection 298

6.3 Setting of the distance zones 304 6.3.1 Reach (X-setting) and grading time 304 6.3.2 Are compensation (R-setting) 307 6.3.3 Specifics for the zone settings in cable networks 311 6.3.4 Adjusting the zone reach in case of large R/X-setting 314 6.3.5 Grading of distance zones with different characteristics 315 6.3.6 Setting of the power swing blocking 317

7 Calculation examples 322 7.1 Double cireuit lines in earthed Systems 322 7.2 Three terminal line (teed feeders) 335

8 Commissioning 344 8.1 Testing of the protection System 344 8.2 Test with load 346

9 Maintenance 349 9.1 Seif monitoring 349 9.2 Maintenance strategy 350

10 Bibliography 352 10.1 Technical papers 352 10.2 Books 359

11 Appendix 361 A.l Distance measurement algorithms 361

A.l.l Principle 361 A.l .2 Fourier analysis based technique 362 A.l .3 Transient behaviour 367 A.l .4 Practical application 368 A.l .5 Literature 369

A.2 Calculation with phasors and complex quantities 370 A.2.1 Definitions 370 A.2.2 Calculation with phasors and complex quantities 371

A.3 Fundamentals of symmetrical component analysis 374 A.3.1 Calculation procedure 374 A.3.2 Typical System component data 379 A.3.3 Equivalent circuits and formulas for network reduction 380

A.4 Impedances of overhead lines and cables 383 A.4.1 Single line (transposed) 383 A.4.2 Double cireuit line (transposed) 384 A.4.3 Bündle conduetor 385 A.4.4 Cable impedances 385

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Contents

A.5 Reach of back-up zones on parallel lines 387 A.5.1 Phase-to-phase faults 387 A.5.2 Phase-to-earth faults 389

A.6 Tilting of the quadrilateral top line to avoid overreach 398

Index 403

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