IEC TR 63411:2025
(Main)Grid connection of offshore wind via VSC-HVDC systems
Grid connection of offshore wind via VSC-HVDC systems
IEC TR 63411:2025 discusses the challenges of connecting offshore wind farms via VSC-HVDC, key technical issues and emerging technologies. The potential solutions include new technologies, methods and practices to provide more flexibility and improve the efficiency of power systems. The primary objective of this document is to provide a comprehensive overview of challenges, potential solutions, and emerging technologies for grid integration of large-scale offshore wind farms via VSC-HVDC. It is expected that this document can also provide guidance for further standardization on relevant issues. The purpose of this document is not intended to hinder any further development of state-of-art technologies in this field. This Technical report is not an exhaustive document in itself to specify any scope of work or similar, between a purchaser and a supplier, for any contractual delivery of a HVDC project/equipment. It is expected that this document is used for pre-study and then to make studies, specification for delivery of specific HVDC project, as applicable.
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IEC TR 63411 ®
Edition 1.0 2025-01
TECHNICAL
REPORT
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inside
Grid connection of offshore wind via VSC-HVDC systems
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IEC TR 63411 ®
Edition 1.0 2025-01
TECHNICAL
REPORT
colour
inside
Grid connection of offshore wind via VSC-HVDC systems
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.180; 29.020 ISBN 978-2-8327-0144-7
– 2 – IEC TR 63411:2025 © IEC 2025
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Practices and challenges . 10
4.1 Practices . 10
4.1.1 General . 10
4.1.2 Projects in North Sea, Germany. 10
4.1.3 Nan’ao project, China . 11
4.1.4 Hybrid Interconnector project, Belgium and Denmark . 12
4.2 Challenges . 12
5 Optimal planning . 13
5.1 General . 13
5.2 The core and key issues . 14
5.2.1 Planning process of offshore wind integration via VSC-HVDC . 14
5.2.2 Designing of offshore wind integration via VSC-HVDC . 15
5.3 Summary . 17
6 Coordinated voltage control . 17
6.1 General . 17
6.2 The core and key issues . 18
6.2.1 Coordinated voltage control . 18
6.2.2 Reactive power regulation replacement strategy . 20
6.3 Summary . 21
7 Coordinated active power control . 21
7.1 General . 21
7.2 The core and key issues . 21
7.2.1 Active power . 21
7.2.2 Coordinated active power control . 22
7.2.3 Frequency control . 25
7.3 Summary . 27
8 Fault response and coordinated control . 27
8.1 General . 27
8.2 The core and key issues . 29
8.2.1 Fault response in different kind of faults . 29
8.2.2 Duty segregation in fault response and coordinated control . 31
8.3 Fault control . 32
8.3.1 Optional fault ride-through technical solutions . 32
8.3.2 Technical requirements in coordinated control . 33
8.4 Summary . 33
9 Multi-frequency oscillation . 35
9.1 Problem statement . 35
9.2 The core and key issues . 35
9.2.1 General . 35
9.2.2 Impedance-based stability analysis method . 35
9.2.3 Impedance scanning analysis method . 38
9.2.4 Passivity analysis method . 39
9.2.5 Modal analysis method . 40
9.2.6 Electro-magnetic-transient simulation . 41
9.3 Multi-frequency oscillation suppression technology . 41
9.3.1 General . 41
9.3.2 Controller parameter optimization . 41
9.3.3 Control structure optimization and active damping control . 42
9.3.4 Passive filters . 42
9.3.5 Operational scenarios . 43
9.4 Summary . 44
10 Control and protection function verification . 45
10.1 Problem statement . 45
10.2 The key issues . 45
10.2.1 The C&P verification system . 45
10.2.2 Functions of C&P verification system . 46
10.3 The key verification and evaluation items . 48
10.3.1 General . 48
10.3.2 Verification for multi-frequency oscillation control . 48
10.3.3 Verification for AC fault ride through control . 48
10.3.4 Verification for energy-consuming control and protection device . 48
10.3.5 Verification for black start control . 49
10.4 Summary . 49
11 Testing and commissioning. 49
11.1 General . 49
11.2 Special scheme of testing and commissioning . 49
11.2.1 Process of testing and commissioning . 49
11.2.2 Joint commissioning . 51
11.3 The core and key issues . 51
11.3.1 The process and sequence of commissioning . 51
11.3.2 Offshore converter station test . 53
11.3.3 Dynamic braking system test . 54
11.3.4 Transmission commissioning . 55
11.3.5 Interaction commissioning . 56
11.4 Summary . 57
12 Black start . 58
12.1 General . 58
12.2 Black start process . 59
12.3 The core and key issues . 60
12.3.1 Black-start electric source configuration . 60
12.3.2 Establish the voltage and frequency of the offshore AC system . 60
12.3.3 Maintain the steady-state power balance of the offshore AC system . 62
12.3.4 Transient impact during the black-start process . 62
12.3.5 VSC-HVDC control . 63
12.3.6 Requirements for auxiliary equipment and secondary systems . 63
12.4 Summary . 63
13 Emerging innovative solution . 64
13.1 General . 64
13.2 The core and key issues . 64
– 4 – IEC TR 63411:2025 © IEC 2025
13.2.1 Offshore wind power integrated with DRU-HVDC transmission . 64
13.2.2 DC collection network for large offshore wind farm . 69
13.3 Summary . 71
14 Conclusion . 72
Figure 1 – Schematic structure of the Nan’ao VSC-MTDC project . 11
Figure 2 – Overall process of transmission planning of offshore wind farm . 14
Figure 3 – Topology of parallel MTDC . 15
Figure 4 – Topology diagram of offshore wind farm cluster connected via VSC-HVDC . 19
Figure 5 – Two-level voltage coordinated control . 20
Figure 6 – Illustration of ancillary frequency control for wind turbine . 22
Figure 7 – Frequency control structure with communication . 23
Figure 8 – Frequency control structure without communication . 23
Figure 9 – Frequency droop control schematic diagram . 24
Figure 10 – Technical frame for frequency Control . 25
Figure 11 – Schematic diagram for inertia control . 27
Figure 12 – Faults in offshore wind power collection system . 30
Figure 13 – Faults in receiving-end bulk power system . 31
Figure 14 – Mathematical model of impedance method . 35
Figure 15 – Typical control strategy of voltage controlled VSC-HVDC . 36
Figure 16 – Block diagram of wind power converter . 37
Figure 17 – Comparison of impedance characteristic curve . 38
Figure 18 – Effect of control parameters on impedance characteristics . 42
Figure 19 – Control diagram of virtual arm resistance method . 42
Figure 20 – Potential parallel-connected damping harmonic filters for VSC . 43
Figure 21 – A typical serial-connected damping filters. 43
Figure 22 – Control and protection system life cycle diagram . 45
Figure 23 – The schematic diagram of simulation system . 46
Figure 24 – Test process for the grid-connection scheme . 50
Figure 25 – Commissioning process of the VSC-HVDC project . 52
Figure 26 – Offshore converter station test on land . 54
Figure 27 – Schematic diagram of DC braking chopper . 55
Figure 28 – Black start process of offshore wind farm grid-connected via VSC-HVDC . 60
Figure 29 – The grid forming mode with the WT speed as the control objective . 61
Figure 30 – Pitch angle control strategy of wind turbine generator with the rated turbine
speed as control objective . 62
Figure 31 – Parallel-connected hybrid DRU-VSC-HVDC transmission system . 65
Figure 32 – Improved topology of the auxiliary VSC . 65
Figure 33 – Series-connected hybrid DRU-VSC-HVDC transmission system . 66
Figure 34 – Start-up of series-connected hybrid DRU-VSC-HVDC transmission system . 66
Figure 35 – DRU-HVDC transmission with self-synchronizing wind turbines . 67
Figure 36 – Self-synchronizing control of the wind turbine . 68
Figure 37 – Multi-phase-shift transformer configurations . 68
Figure 38 – Passive harmonics filters for DRU-HVDC transmission systems . 69
Figure 39 – DC1 . 69
Figure 40 – DC2 . 70
Figure 41 – DC3 . 71
Table 1 – VSC-HVDC-based offshore wind projects in Germany . 10
Table 2 – Summary of possible fault response and control . 28
Table 3 – Summary of power maintenance or shutdown after the clearance of faults in
a point-to-point VSC-HVDC system . 29
Table 4 – Summary of duty segregation for VSC-HVDC systems . 34
Table 5 – Summary of technical requirement for VSC-HVDC systems . 34
Table 6 – Comparison of different analysis methods . 44
Table 7 – C&P system Functions . 47
– 6 – IEC TR 63411:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GRID CONNECTION OF OFFSHORE WIND VIA VSC-HVDC SYSTEMS
FOREWORD
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IEC TR 63411 has been prepared by subcommittee SC 8A: Grid integration of renewable
energy generation, of IEC technical committee TC 8: Systems aspects of electrical energy
supply. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
8A/177/DTR 8A/184/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
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The committee has decided that the contents of this document will remain unchanged until the
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specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
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– 8 – IEC TR 63411:2025 © IEC 2025
INTRODUCTION
New technical solutions to meet the engineering development of grid integration of offshore
wind generation via HVDC are the content of this document. The new solutions include new
technologies, methods and practices to provide more flexibility and improve the efficiency of
power systems, constantly balancing generation and load.
The development of offshore wind is progressing steadily, and VSC-HVDC systems are
commonly adopted to link long distance offshore wind farms with the onshore bulk power grid.
According to this fact, the purpose of this Technical Report (TR) specifically focuses on the
planning, interaction and coordinated control between offshore wind farms and VSC-HVDC
systems.
For various stakeholders, including transmission system operators, offshore wind farm owners,
research institutes and so on, this Technical Report is to collect information from regulatory
contents including relevant issues in different countries and regions, and work out a TR for
offshore wind farm Integration via DC Technology, which mainly addresses the technology
development tendency, best practices, and the future standardization activities.
The aim of this document is to draft a strategic, but nevertheless technically oriented and
referenced TR, which represents the core and key issues of offshore wind integration via
VSC-HVDC systems. Offshore wind farm developers and owners, transmission system
operators have a common understanding about the key issues based on practices and
challenges between offshore wind farms and VSC-HVDC systems.
GRID CONNECTION OF OFFSHORE WIND VIA VSC-HVDC SYSTEMS
1 Scope
The voltage source converter based on high-voltage direct current (VSC-HVDC) transmission
technology has attracted increasing attention because of its advantages such as flexible control,
supply to passive systems, and black start capability, which has been widely used in offshore
wind farm integration. Although offshore wind farms generate electricity just like any other
power plants on a system-wide level, such offshore wind generation has quite distinctive
characteristics to be considered in terms of capacity optimization, voltage and power control,
fault response, multi-frequency oscillation, power DC collection, etc., when compared to
conventional generation integration via HVDC. Understanding these distinctive characteristics
and their interaction with the other parts of the power system is the basis for integrating large-
scale offshore wind farms via VSC-HVDC.
This document discusses the challenges of connecting offshore wind farms via VSC-HVDC, key
technical issues and emerging technologies. The potential solutions include new technologies,
methods and practices to provide more flexibility and improve the efficiency of power systems.
The primary objective of this document is to provide a comprehensive overview of challenges,
potential solutions, and emerging technologies for grid integration of large-scale offshore wind
farms via VSC-HVDC. It is expected that this document can also provide guidance for further
standardization on relevant issues. The purpose of this document is not intended to hinder any
further development of state-of-art technologies in this field.
This Technical report is not an exhaustive document in itself to specify any scope of work or
similar, between a purchaser and a supplier, for any contractual delivery of a HVDC
project/equipment. It is expected that this document is used for pre-study and then to make
studies, specification for delivery of specific HVDC project, as applicable.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC TR 62001-5, High-voltage direct current (HVDC) systems – Guidance to the specification
and design evaluation of AC filters – Part 5: AC side harmonics and appropriate harmonic limits
for HVDC systems with voltage sourced converters (VSC)
IEC 62747, Terminology for voltage-sourced converters (VSC) for high-voltage direct current
(HVDC) systems
IEC 62934, Grid integration of renewable energy generation – Terms and definitions
IEC TR 63401-1, Dynamic characteristics of inverter-based resources in bulk power systems –
Part 1: Interconnecting inverter-based resources to low short circuit ratio AC networks
– 10 – IEC TR 63411:2025 © IEC 2025
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TR 62001-5,
IEC 62747, IEC 62934, IEC TR 63401-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
4 Practices and challenges
4.1 Practices
4.1.1 General
The number of projects using HVDC to integrate renewable energy (RE) is growing, mainly due
to the significant acceleration of offshore wind development. These projects are, to date, mainly
based on point-to-point HVDC systems.
4.1.2 Projects in North Sea, Germany
BorWin1 is the world’s first HVDC system connecting an offshore wind farm. It connects the
BARD Offshore 1 wind farm and other offshore wind farms near Borkum, Germany to the
European power grid by VSC-HVDC. The project started in 2007 and was put into operation in
2009.
The BARD Offshore 1 wind farm includes eighty 5 MW wind turbines located in the North Sea
that are 130 km away from the coast. The rated capacity of the converter station is 400 MW,
the DC voltage level is ±150 kV, and the total length of the DC connection is 200 km, including
125 km of submarine cables and 75 km of underground cables.
Since the BorWin1 project, Germany has developed a series of offshore wind farm projects
based on VSC-HVDC systems in the North Sea, as shown in Table 1. The VSC-HVDC
transmissions of these systems, except that of BorWin1, are based on modular multi-level
technology, and the highest voltage level reaches ±320 kV.
Table 1 – VSC-HVDC-based offshore wind projects in Germany
Project Transmission Capability Length Operation Time
BorWin1 ±150 kV,400 MW 200 km 2010
BorWin2 ±300 kV,800 MW 200 km 2015
BorWin3 ±320 kV,900 MW 160 km 2019
DolWin1 ±320 kV,800 MW 165 km 2015
DolWin2 ±320 kV,900 MW 135 km 2016
DolWin3 ±320 kV,900 MW 160 km 2018
HelWin1 ±250 kV,576 MW 130 km 2015
HelWin2 ±320 kV,690 MW 130 km 2015
SylWin1 ±320 kV,864 MW 205 km 2015
DolWin6 ±320 kV,900 MW 90 km 2023
DolWin5 ±320 kV,900 MW 130 km 2024
These projects have strongly supported the development of VSC-HVDC-based offshore wind
farm grid connections. The renewable power generation farms and VSC-HVDC systems achieve
power conversion and transmission based on the fast controllability of power electronic
converters. Under operating conditions, in case some oscillation phenomenon occurs when the
two systems are jointly operated, the relevant suppression measures for these phenomena
focus on designing additional damping control systems for the generation units and improving
the control strategy of the VSC-HVDC converters.
4.1.3 Nan’ao project, China
Nan'ao is located in the eastern sea of Guangdong Province in China. This project is the world’s
first multi-terminal HVDC system for offshore wind farms. It involves a three-terminal
VSC-HVDC transmission system including two sending terminals and one receiving terminal.
The sending end converter stations are the Qing'ao Station and Jinniu Station, and the receiving
end converter station is the Sucheng Station. The capacities are 50 MVA, 100 MVA, and
200 MVA, respectively, and the DC voltage level is ±160 kV. The Tayu converter station was
planned, but hasn’t been constructed yet.
Modular multilevel converter (MMC) technology was used in this project. This project was
formally put into operation at the end of 2013 and is the world's first demonstration
multi-terminal VSC-HVDC project. Figure 1 shows the schematic diagram of this multi-terminal
VSC-HVDC demonstration project. The Nan’ao multi-terminal VSC-HVDC has three main
operation modes: an AC feeder and DC feeder in parallel, DC feeder only, and STATCOM.
However, the lack of DC circuit breakers poses risks to the operation of the DC-feeder-only
mode.
Before the project was put into operation, the power grid in the Nan’ao area was very weak,
and the fluctuations of the grid-connected wind power system had a large impact on the local
power grid. After this project was put into operation, the flexible control capabilities of
VSC-HVDC were used to provide not only support for the wind power integration, but also
effective support for the stability of the local power grid.
Figure 1 – Schematic structure of the Nan’ao VSC-MTDC project
– 12 – IEC TR 63411:2025 © IEC 2025
4.1.4 Hybrid Interconnector project, Belgium and Denmark
European Electricity Transmission System operators Elia (Belgium) and Energinet (Denmark)
have announced in February 2021 to set up a working group to examine the feasibility of a
subsea cable between Belgium and Denmark that links the high-voltage grids of both countries
over a distance of more than 600 km. A 'hybrid' design, which means that it brings wind energy
from offshore wind farms to onshore power grids and can also be used as an interconnector
between different electricity grids, is being examined. On the Danish side, the interconnector
connects to a new 'energy island' to be built 80 km off the Danish coast and to which a large
10 GW wind farm is eventually connected.
When the interconnector between Belgium and Denmark is completed, the cable run through
the territorial waters of four countries: Belgium, the Netherlands, Germany and Denmark. This
gives Belgium direct integration to the renewable bulk generation in order to decarbonizes its
energy-intensive industry and achieve the European climate targets. The start of the feasibility
study follows the political cooperation agreement signed by the Belgian and Danish Energy
Ministers. The project dovetails with the implementation of the European Green Deal, which
aims for Europe to become the first carbon-neutral continent by 2050 by increasing the current
capacity of offshore wind from 25 GW to 300 GW.
It is the first time Belgium is connected to a new electricity market that is further away than
neighbouring countries. Hybrid technology also enables Belgium to gain direct integration to
large wind farms in the far northern part of the North Sea where the meteorological conditions
are different from those off the coast of Belgium. This provides greater security of supply and
helps the Belgian energy-intensive industry to continue decarbonising.
4.2 Challenges
When large-scale offshore wind farms are connected to the VSC-HVDC system over long-
distance transmission, the operation characteristics of offshore wind farms and the VSC-HVDC
system affect each other. The adaptability of offshore wind farms connected to the VSC-HVDC
system becomes an important issue affecting grid operation. In particular, the complex
interaction between offshore wind farms and the power grid brings new challenges to the
offshore wind farms and VSC-HVDC system.
1) Planning optimization. The planning of offshore wind farms and its integration is significantly
important considering the relatively high investment of offshore wind farms and the
VSC-HVDC system. When the capacity of offshore wind farms is larger than the VSC-HVDC
capacity, wind curtailment is inevitable. Also, if the VSC-HVDC capacity is larger than
offshore, the overall investment becomes excessive.
2) Voltage control. At present, the study on hierarchical voltage control for aggregated wind
farms focuses on the AC grid. As the islanded aggregated offshore wind farms connected
via VSC-HVDC are decoupled from the AC network, the optimization target and mode of
which is quite different from the traditional connection. New voltage control strategies
combined with its characteristics are developed.
3) Active power control. As the utilization of offshore wind farm in the power grid increases,
synchronous power generation units are gradually decommissioned, causing the inertia and
active power reserve capacity of the entire system to be reduced. This brings challenges to
the frequency stability of the power grid. Developments of enhanced frequency regulation
methods are required.
4) Fault control. When large-scale offshore wind farms are connected to a VSC-HVDC system,
the operational characteristics of offshore wind farms and VSC-HVDC grid interact with each
other. Various methods of fault ride-through inevitably have a certain impact on the offshore
wind turbine, and even have impacts on the regulations and standards for offshore wind
farms with interconnection to power system via DC.
5) Multi-frequency oscillation. In a VSC-HVDC-based large-scale offshore wind farm
integration system, a large number of power electronic converters, DC lines, and smoothing
reactors are involved. The resonant frequencies usually occur within the system. In case
the system undergo system oscillation, the overall safety and stability of the system are
threatened. Therefore, sub- and super-synchronous oscillation problems caused by the
interaction of the VSC-HVDC and wind power control are considered when planning and
operating VSC-HVDC-based large-scale offshore wind farm systems.
6) Function verification. It is the mainstream practice of offshore wind farms via VSC-HVDC
transmission system to build the simulation verification system with actual control and
protection devices and to verify the effectiveness of various functions of HVDC control and
protection system and offshore wind together. Compared with the conventional VSC-HVDC
projects or wind power projects connected to the grid using AC transmission mode, the
control and protection system simulation verification of wind power projects connected to
the grid using VSC-HVDC transmission has different simulation models, different
composition for verification system and different characteristics, such as different charging
modes of offshore converter station, different control modes of VSC-HVDC, different AC
fault ride through characteristics of onshore converter station, etc.
7) Testing and commissioning. The system commissioning is the final on-site test for
VSC-HVDC projects with integration of offshore wind farms, which aims to ensure that the
whole system are safely and reliably operated, as well as meet the grid connection
requirements. By comparison with the system commissioning of regular VSC-HVDC
projects, the system commissioning procedure and method of the VSC-HVDC projects with
integration of offshore wind farms are different, due to the special operating condition and
the existence of onshore dynamic braking system, the offshore wind farms and converter
stations.
8) Black start. For the normal situation, the HVDC converter establishes reference voltage, but
in some special situation a number of offshore wind turbines can black start the offshore
wind farm island as an alternative. It is necessary to configure the type and capacity of black
start power supply according to the actual situation, so as to at least meet the black start of
the first or first batch of offshore wind turbines in offshore wind farms.
9) Emerging innovative solution. With the rapid development of DC technology, offshore wind
power developer hope to make full use of the advantages of DC technology or from a
cost-saving perspective to explore the possibility of applying various DC transmission
technologies to offshore wind power integration.
5 Optimal planning
5.1 General
Multiple issues are considered before a wind farm is connected to the grid, including the
capacity and commissioning time of the wind farm, the selection of connection point, and the
design of the transmission system.
Firstly, the capability of the target power system to accommodate the wind power is evaluated.
The power system usually has enough flexibility for load-balance and ramping under any
possible scenario, especially when the wind power ramps heavily and reversely to the load. The
evaluation is conducted by investigations on the features of the wind resource, and
comprehensive simulations on the operation of the wind farm and receiving-end power system.
In terms of transmission, VSC-HVDC technology offers a valuable option for the integration of
offshore wind farm. VSC-HVDC technology has higher transmission capability and lower cost
for long distance undersea transmission. It also provides dynamic reactive power/voltage
support to the AC grid, which helps to reduce voltage fluctuations caused by wind power and
improve the system stability under fault conditions. The design of the VSC-HVDC system for a
specific wind farm is also optimized for better performance and less investment cost.
– 14 – IEC TR 63411:2025 © IEC 2025
Clause 5 mainly focuses on two issues in the planning stage of an offshore wind farm. The first
is the transmission planning of the wind farm, and the second is the optimal design of the VSC-
HVDC transmission system.
5.2 The core and key issues
5.2.1 Planning process of offshore wind integration via VSC-HVDC
In order to better accommodate intermittent wind power and maximize the cost-benefit ratio of
wind farm construction, the following process is usually used in the planning stage of an
offshore wind farm:
1) Introduce the basic conditions of the wind farm. Estimate the total capacity of the wind farm
based on the area of the site and the wind resource. Collect the wind data of the site and
evaluate the power generation profile.
2) Investigate the electricity demand and grid structure of power systems in line with the
planned wind farm. Evaluate the appropriate region to consume the wind power and
determine the optimal transmission route based on the capacity of the wind farm and the
characteristics
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