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TB 533 2013 B4-52 HVDC Grid Feasibility Study

Until now most HVDC schemes have been point to point connections. A few multi-terminal schemes have been built with one extra terminal. But there have been many discussions of using HVDC for more advanced grids. The TB investigates the technical and economic feasibility to build such HVDC grids. The first question to answer is if HVDC grids offer any advantage over many point to point HVDC connections inside an AC grid. Another important question is if it will be possible to build HVDC breakers that are necessary to make the grid reliable. One more question is if one can make protections and control to the grid. These and many other challenging questions are studied in the Brochure

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TABLE OF CONTENTS

1 INTRODUCTION

2 HVAC NETWORK EXPANSION, OPERATION AND THE HVDC GRID CONCEPT

      2.1 Historical lessons

            2.1.1 Some conclusions from history

       2.2 AC Grid Operation according to ENTSO-E requirements in the context of and HVDC overlay grid

            2.2.1 Policy 1: Load-Frequency-Control and Performance

            2.2.2 Policy 2: Scheduling and Accounting

            2.2.3 Policy 3: Operational Security

            2.2.4 Policy 4: Coordinated Operational Planning

            2.2.5 Policy 5: Emergency Procedures

            2.2.6 Policy 6: Communication Infrastructure

            2.2.7 Policy 7: Data Exchanges

            2.2.8 Policy 8: Operational Training

            2.2.9 Conclusion

      2.3 Technical HVDC Grid Applications and Benefit

            2.3.1 Introduction

            2.3.2 Technical HVAC and HVDC Grid comparison

      2.4 Introduction: gradual development

            2.4.1 Interconnectors

            2.4.2 Wind Farms

            2.4.3 Others

      2.5 Building the grid in steps

            2.5.1 First steps: towards the radial HVDC Grid

            2.5.2 Development starting from an interconnector between two points in asynchronously operated

                    AC systems

            2.5.3 Development from an offshore wind farm connection

      2.6 Towards meshed grids: reliability

      2.7 Inter-operability

      2.8 References

3 AVAILABLE CONVERTER TECHNOLOGIES, VSC AND LCC COMPARISON

      3.1 Introduction

      3.2 Converter Technology for HVDC Grid - VSC

            3.2.1 Using LCC in an HVDC Grid

      3.3 References

4 MOTIVATION OF AN HVDC GRID

      4.1 Introduction

      4.2 Economical benefits

      4.3 Cost comparison of DC Grid and Point to Point HVDC schemes

            4.3.1 HVDC station cost

            4.3.2 HVDC station losses

            4.3.3 Cost of losses

            4.3.4 HVDC breaker

      4.4 Mesh Size and Economic Impact

            4.4.1 Mesh size for DC transmission

            4.4.2 Economic Consideration of Mesh Size

      4.5 Overlay DC Grid

      4.6 References

5 HVDC GRID CONFIGURATIONS 

      5.1 HVDC systems

            5.1.1 Asymmetric monopolar system with earth return

            5.1.2 Asymmetric monopole system with metallic return

            5.1.3 Symmetrical monopole grid

            5.1.4 Bipolar HVDC Grid

      5.2 Connecting monopolar converters to a bipolar HVDC Grid

      5.3 System earthing

            5.3.1 General

            5.3.2 High impedance earth

            5.3.3 Low impedance earth – Directly earthed systems

            5.3.4 Combination of bipolar and monopolar stations

            5.3.5 Recommended earthing structure

            5.3.6 Line commutated converters together with voltage source converters - earthing

6 FAULT PERFOMANCE 

      6.1 Introduction

      6.2 Fault current components

      6.3 DC fault power flow study

      6.4 Influence of VSC converters

      6.5 Influence of DC capacitors

      6.6 DC Lines and other equipment in DC Grids, and the influence on the AC grid

      6.7 References

7 PROTECTION REQUIREMENTS 

      7.1 Introduction

      7.2 Properties of DC system short circuit protections

      7.3 Differences between DC Grid protection and existing DC system protection

            7.3.1 Point-to-point VSC HVDC line protection

            7.3.2 Point-to-point LCC HVDC line protection

      7.4 AC line and busbar protection compared to DC protection

            7.4.1 Existing AC line and busbar protection means

            7.4.2 New protection means for DC protection

      7.5 Other issues for protection systems

            7.5.1 Robustness towards fault clearance

            7.5.2 Robustness towards HV architecture and evolutions

            7.5.3 Selectivity and trip order

            7.5.4 Stable for disturbances

            7.5.5 Backup

      7.6 Conclusions

      7.7 References

8 NEW COMPONENTS IN HVDC GRID - INCLUDING QUESTIONNAIRES TO MANUFACTURERS 

      8.1 HVDC Breakers for HVDC Grid

            8.1.1 Introduction

            8.1.2 Questionnaire sent to prospective HVDC breaker manufacturers

      8.2 Prototype HVDC Breaker for HVDC Grid

      8.3 DC Fault Current Limiter (FCL) in HVDC Grids

            8.3.1 Features of DC fault currents in HVDC Grids

            8.3.2 Expected performance of fault current limiting devices

            8.3.3 Current limiting methods

      8.4 DC/DC converters

            8.4.1 Background

            8.4.2 Various types of DC/DC converters

            8.4.3 Economic comparison

            8.4.4 Performance implications for DC Grids

            8.4.5 Conclusion

      8.5 DC cables survey

            8.5.1 Questionnaire contents

            8.5.2 Procedure

            8.5.3 Present day available technology

            8.5.4 Forecast of cable technology development

            8.5.5 Additional information

            8.5.6 Other general comments

      8.6 References

9 POWER FLOW CONTROL IN DC GRIDS

      9.1 Introduction

            9.1.1 Control of branch currents in a DC Grid

            9.1.2 Multiterminal HVDC Voltage Source converter operation control

      9.2 Primary and secondary controls in a DC Grid

            9.2.1 The requirements on the DC power flow control – Primary control

            9.2.2 The requirements on the DC power flow control – Secondary control

      9.3 Primary control methods

            9.3.1 Primary Control – “The Voltage Margin Control” method

            9.3.2 Primary Control – “The Voltage Droop Control” method

            9.3.3 Primary control – “DC voltage droop control with dead-band” method

            9.3.4 Primary control - “Autonomous converter control” method

      9.4 Conclusion

      9.5 Bibliography and references

10 THE REQUIREMENTS OF AN HVDC GRID - SECURITY AND RELIABILITY 

      10.1 HVDC Switchyard

11 NEEDED STANDARDIZATION 

      11.1 DC voltage

      11.2 What should be standardized at a minimum?

      11.3 Comment

12 NEW WORKING GROUPS WITHIN THE HVDC GRID AREA 

13 CONCLUSIONS

Appendix A HVDC Load flow models and calculation

Appendix B Converter Power flow during DC Grid faults

Appendix C AC/VSC HVDC power flow models

Appendix D Dynamic Simulations of DC Grids

Appendix E Transient stability model

Appendix F Technology status of LCC MTDC

Appendix G Mesh size for AC transmission

Appendix H Switching DC in an HVDC system

Appendix I Current limiting devices