NEUTRAL EARTHING RESISTORS or REACTORS

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NEUTRAL EARTHING RESISTORS or REACTORS 

Introduction

NEUTRAL EARTHING RESISTORS or REACTORS

    This guide describes the issues which should be considered when it is desired to install neutral earthing resistors or reactors (NER's) (including resonant reactance) on power systems having a phase to earth voltage less than 20.5kV to limit the magnitude of phase to earth fault currents, for the purpose of controlling the EPR and induced voltages on nearby telecommunication systems to below hazard levels. Nothing in this Guide shall relieve all those who own and operate electric power lines from complying with the provisions of the Electricity Regulations or any other statutory Act or Regulation.

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The guide does not describe all the conditions to be met for systems operating at higher phase to earth voltages than 20.5kV, because such systems may incorporate plant with graded insulation or may be designed for less than the basic insulation levels (BIL) needed when using an NER. The effect of insulation ageing and consequent reduction of basic insulation level has not been taken into account in this guide.

There have been many instances of telecommunication system damage due to excessive impressed current and voltage from the nearby power system (usually during earth faults on the power system). These present a hazard to telecommunication system users and staff, as well as telecommunication system plant.

The mechanisms through which this hazard or damage arises are:

• Induction (Magnetic Coupling)
• Capacitive (Electric Coupling)
• Direct Contact
• Earth Potential Rise

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Table Of Contents

1 Introduction 1

1.1 Scope 1

1.2 Background 1

1.2.1 Induction (Magnetic Coupling) 1

1.2.2 Capacitive (Electric Coupling) 1

1.2.3 Direct Contact 2

1.2.4 Earth Potential Rise (EPR) 2

1.3 Hazard to Telecommunication Networks 3

1.4 Methods of Mitigating EPR and Induction Hazard to Telecommunication Networks 3

1.4.1 Telecommunication System Isolation 3

1.4.2 Increase the Separation between the Power and Telecommunication Plant 3

1.4.3 Install Gaseous Arresters in Telecommunication Plant 3

1.4.4 Shielding of Telecommunication Conductors 4

1.4.5 Add Earth Return Conductor 4

1.4.6 Lower the Resistance of the Power Earthing System 4

1.4.7 Increase Zero Sequence Impedance using a Neutral Impedance 5

2 Definition of Terms and Drawing Symbols 6

3 Devices and Methods of Earthing 8

3.1 Overseas and NZ Practice 8

3.1.1 United Kingdom (UK) Practices 8

3.1.2 European Practices 9

3.1.3 Australian Practices 9

3.2 Solid Earthing 9

3.3 Unearthed Systems 10

3.4 Resonant Earthing (Petersen Coils, Arc Suppression Coils, Earth Fault Neutraliser Earthing) 10

3.5 Resistive/Reactive Earthing 10

3.5.1 Reduction of Earth Fault Current 11

3.6 Use of Earthing Transformers 11

4 Current Considerations 11

4.1 Source Fault Levels 11

4.2 Earth Fault Current Path 11

4.3 Effect of an NER on EPR 13

4.4 Effect of an NER on Magnetically Induced Voltages 13

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5 Voltage Considerations 15

5.1 Introduction 15

5.1.1 Lightning 15

5.1.2 Switching Surges 15

5.1.3 Electrostatic 15

5.1.4 Contact with Higher Voltage System 15

5.2 The Effect of Earthing Methods on System Voltages 16

5.2.1 Earthing the System Neutral 16

5.2.2 Effectively Earthed System 16

5.2.3 Reactance-Earthed System 17

5.2.4 Resonant-Earthed System 17

5.2.5 Resistance-Earthed System 17

5.2.6 Unearthed System 18

5.3 Transient Effects 18

5.4 Application in New Zealand 23

6.1 Introduction 24

6.2 Changes to Network Protection 24

6.3 Protection of the NER and associated Transformer 26

6.3.1 NER 26

6.3.2 Transformer Protection 26

7 Design Criteria and Device Selection 28

7.1 Introduction 28

7.2 Existing System Components 28

7.2.1 General 28

7.2.2 Transformers 28

7.2.3 Switchgear 30

7.2.4 Cables 31

7.3 Impedance Value 31

7.4 Impedance Type 31

7.4.1 Resistors 31

7.4.2 Reactors 32

7.5 Design for Reliability 33

7.6 Comparative Costs 33

8 Electrical Connections for NERs 37

9 Installation 44

9.1 Retrofit of an NER 44

9.2 New Installations incorporating NER's 44

10 Commissioning 45

11 Routine Maintenance 45

12 General AC Network Benefits of Installing NER's 45

13 The use of NERs in Mining and Industrial Applications 46

13.1 General Mining Applications 46

13.2 Underground Mining Sites 47

13.3 NER’s in other Industries 47

14 References 48

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