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TB 619 2015 HVDC CONNECTION OF OFFSHORE WIND POWER PLANTS

The first wave of HVDC connected offshore wind power plants (WPPs) has been commissioned and many more are planned in the North Sea, along with other sites around the world. VSC-based HVDC has become the preferred solution for large offshore WPPs, with cable distances typically above 100 km (including both offshore cable and on shore cable to the converter terminal) to the AC grid connection point. This is largely due to several technology advantages offered by VSCs, when compared to other HVAC or HVDC options, resulting in a more economically attractive transmission solution. In addition, a number of HVDC submarine cable connections for power exchange between countries are being planned and the possibility of connecting WPPs to these interconnections, and to future HVDC grids, are being seriously considered. The issues associated with expanding a WPP and HVDC connections with equipment from multiple vendors are subjects which need to be developed further, but are outside the scope of this brochure. Compliance with Grid Codes (GCs), which define the performance during normal and abnormal operating conditions, is another subject area in need of further development. Existing GCs are however written for AC connected WPPs, and for an offshore WPP these conditions typically apply only at the AC grid connection point. This raises the possibility of optimizing the overall WPP and the HVDC converter, with potential economic and maintenance benefits. However, if the HVDC connection and the WPP are provided by different vendors, such optimization cannot be done properly unless concerns about IP rights and operation benefits are clearly laid out and understood by all stakeholders involved. Guidelines and recommendation for point to point and multi terminal HVDC connection of offshore WPPs are therefore highly needed and of mutual interest for the HVDC and WTG industries in order to be able to provide the best possible solutions for all stakeholders.

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TABLE OF CONTENTS

GLOSSARY OF ABBREVIATIONS AND SPECIAL TERMS
EXECUTIVE SUMMARY
1 INTRODUCTION
      1.1 Background
      1.2 Technical Brochure (TB) Scope
2 VSC-HVDC CONFIGURATIONS
      2.1 Introduction
      2.2 Background
      2.3 General VSC-HVDC Design Considerations
      2.4 VSC HVDC Configurations
            2.4.1 Point-to-Point Connection
            2.4.2 Multi-Infeed Connection
            2.4.3 Emerging Configurations - Multi-Terminal VSC-HVDC
            2.4.4 Future Configurations - VSC-HVDC Grid
      2.5 Basic Offshore WPP Configuration
      2.6 Basic Configuration of Point-to-Point VSC HVDC for Offshore WPP
      2.7 Balance of the Plant
      2.8 References
3 HVDC CONNECTED OFFSHORE WPPS: WTG SELECTION AND CONCEPTUAL DESIGN LAYOUTS
      3.1 Introduction
      3.2 Unique Aspects of WTGs for Offshore Applications
            3.2.1 Factors affecting availability of Offshore WTGs
            3.2.2 Offshore WTG Design Targets
      3.3 WTG Technologies
            3.3.1 Type 3: Doubly-fed Induction Generator
            3.3.2 Type 4 Full Scale Converter Connections
            3.3.3 Foundations of Offshore WTG’s
            3.3.4 Future Trends in Offshore WTG Technology
      3.4 Connection Between VSC HVDC Converter Station and WPP Collector Substations
      3.5 WPP Collector Station Transformers
      3.6 Design of the WPP Internal Cable Collector Network
            3.6.1 Cable Technology
            3.6.2 Substation Placement
            3.6.3 Cable Routing and Cable Sizing
            3.6.4 Emergency Stand-by Power
      3.7 Design of the WPP HVDC Export Cable Connection
      3.8 Bibliography
4 FUNCTIONAL REQUIREMENTS FOR HVDC AND OFFSHORE WIND POWER PLANT
      4.1 Introduction
      4.2 System Under Analysis
      4.3 Control and Protection Functions Required
            4.3.1 Functions required
            4.3.2 Considered Situations
      4.4 Control and Protection Integration
           4.4.1 Control Integration
            4.4.2 Protections Integration
      4.5 Other issues
            4.5.1 Resonance and Harmonics
            4.5.2 Start Up-Shut Down Sequence
            4.5.3 Controller Instability and Miscoordination
5 INTERCONNECTION REQUIREMENTS
      5.1 Interface Connection Points
      5.2 Plant Capacity
      5.3 Offshore Transmission Ownership
            5.3.1 Great Britain
            5.3.2 Germany
      5.4 Voltage and Reactive Power
      5.5 Active Power and Frequency Control
      5.6 Protection and Fault Ride Through
      5.7 Power Quality
      5.8 Signal/Communication/Control Points
      5.9 Model and Data Provision
      5.10 Commissioning, Compliance Testing and Operational Sequence
      5.11 Grid Codes Examples
            5.11.1 Germany – TenneT TSO GmbH [6]
            5.11.2 Great Britain - National Grid Electricity Transmission plc. (National Grid) [7]
            5.11.3 European Network Transmission System Operator for Electricity: ENTSO-e, Network Code
            5.11.4 Offshore Requirements Summary and Recommendation
      5.12 References
6 STUDY REQUIREMENTS, DATA, MODELING, AND RECOMMENDED SIMULATIONS
      6.1 Introduction
      6.2 Steady State Calculations
            6.2.1 Steady State Power Flow
            6.2.2 Short Circuit Calculation
      6.3 Quasi Steady-state Load Flow Models (Long Term Stability)
      6.4 Short-term Stability (Dynamic RMS and/or EMT Modelling)
            6.4.1 Simulation during voltage dip after fault inception
            6.4.2 Simulation of power recovery and system stability after the disturbance
            6.4.3 Simulation of stability during changes in normal operation
      6.5 Time and Frequency Domain Modelling Above Fundamental Frequencies
            6.5.1 Insulation Coordination Study
            6.5.2 High Frequency Time Domain Modelling
            6.5.3 Frequency Domain Modelling
      6.6 Harmonic Analysis, Flicker and Sub-synchronous Interactions
      6.7 Study to be made at each stage of the project
            6.7.1 Developing Stage (Feasibility Study)
            6.7.2 Concept Design Stage
            6.7.3 Design Stage (Basic and Detailed)
            6.7.4 Documentation Stage
      6.8 References
7 OVERVIEW OF PLANNED/DECIDED OR BUILT HVDC CONNECTED WPP PROJECTS
      7.1 Offshore Applications
      7.2 BorWin1
      7.3 DolWin1
      7.4 BorWin2
      7.5 HelWin1
      7.6 SylWin1
      7.7 DolWin2
      7.8 HelWin2
      7.9 DolWin3
      7.10 BorWin3
8 GUIDELINES, RECOMMENDATIONS, AND CONCLUSIONS
      8.1 Introduction
      8.2 Design Guidelines and Tradeoffs
            8.2.1 Reliability and Availability
            8.2.2 Overload Capability
            8.2.3 Control of Real Power
            8.2.4 Grid Codes
            8.2.5 Expandability
            8.2.6 Conventional vs. Specialized WTG Designs
            8.2.7 Off-Shore Grid Protection
            8.2.8 Auxiliary Power and Start-Up
            8.2.9 Control Interaction
            8.2.10 Study Requirements, Data, Modelling, and Simulation Studies
            8.2.11 Need for Standards
            8.2.12 Recommendations for High Level Control Strategy
      8.3 Conclusions
      8.4 Outlook
      8.5 Reference