DNVGL-RP-0046 Qualification procedure for offshore high ...

RECOMMENDED PRACTICE

DNV GL AS

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DNVGL-RP-0046:2014-08

Qualification procedure for offshore high-voltage direct current (HVDC) technologies

© DNV GL AS 2014-08

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This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document, and is believedto reflect the best of contemporary technology. The use of this document by others than DNV GL is at the user's sole risk. DNV GL does not accept any liabilityor responsibility for loss or damages resulting from any use of this document.

FOREWORDThe recommended practices lay down sound engineering practice and guidance.

Recommended practice – DNVGL-RP-0046:2014-08

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C

hanges –

curr

entCHANGES – CURRENT

General

On 12 September 2013, DNV and GL merged to form DNV GL Group. On 25 November 2013 Det Norske

Veritas AS became the 100% shareholder of Germanischer Lloyd SE, the parent company of the GL Group,

and on 27 November 2013 Det Norske Veritas AS, company registration number 945 748 931, changed its

name to DNV GL AS. For further information, see www.dnvgl.com. Any reference in this document to “Det

Norske Veritas AS”, “Det Norske Veritas”, “DNV”, “GL”, “Germanischer Lloyd SE”, “GL Group” or any other

legal entity name or trading name presently owned by the DNV GL Group shall therefore also be considered

a reference to “DNV GL AS”.

This is a new document.

ACKNOWLEDGEMENT

DNV GL and STRI would like to express its gratitude to the eleven participants which have contributed with

financial support and even more importantly actively contribution throughout the project execution in terms

of sharing knowledge and experience.

The following organizations (in alphabetic order) have participated in the project:

ABB, Alstom Grid, Elia, Dong Energy, Europacable, Scottish Power, Statkraft, Statnett, Statoil, Svenska

Kraftnät and Vattenfall.


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C

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ntsCONTENTS

CHANGES – CURRENT .................................................................................................. 3

Sec.1 Introduction.................................................................................................. 6

1.1 General...................................................................................................6

1.2 Objective................................................................................................6

1.3 Scope .....................................................................................................6

1.4 Structure of the Recommended Practice ................................................6

1.5 Case examples and risk assessment.......................................................6

1.6 Relationship to other codes and standards.............................................7

1.7 Definitions and abbreviations.................................................................7

1.8 References .............................................................................................8

Sec.2 Introduction to offshore HVDC transmission technologies ............................ 9

2.1 HVDC in comparison with AC transmission ...........................................10

2.2 Application of offshore HVDC ...............................................................10

2.3 Challenges with offshore HVDC ...........................................................11

Sec.3 Introduction to technology qualification ..................................................... 12

3.1 Motivation for technology qualification ................................................12

3.2 Roles in technology qualification..........................................................12

3.3 The six-step technology qualification process ......................................12

3.4 Use of technology qualification in different project phases ..................13

3.5 Input to the technology qualification process ......................................14

3.6 Results from the technology qualification process ...............................14

3.7 Qualification of complex systems .........................................................14

Sec.4 TQ Step 1: Qualification basis...................................................................... 15

4.1 Technology description ........................................................................154.1.1 General ......................................................................................154.1.2 Specific offshore HVDC transmission technology description...............15

4.2 Requirement specification....................................................................184.2.1 General ......................................................................................184.2.2 Specific offshore HVDC transmission technology requirement

specification ................................................................................18

Sec.5 TQ Step 2: Technology assessment ............................................................. 20

5.1 Technology decomposition ..................................................................205.1.1 Technology decomposition for offshore HVDC system........................20

5.2 Technology categorisation....................................................................225.2.1 Categorisation of offshore HVDC technologies ..................................22

Sec.6 TQ Step 3: Threat assessment..................................................................... 24

6.1 Definition of probability classes and consequence classes ...................24

6.2 Definition of risk categories .................................................................25

6.3 Failure mode identification and risk ranking methodologies ................26

Sec.7 TQ STEP 4: Qualification plan ...................................................................... 27

7.1 Qualification strategy...........................................................................27

7.2 Selection of qualification methods........................................................27

7.3 Qualification methods for offshore HVDC technologies.........................27

7.4 Description of qualification activities ...................................................28


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C

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ntsSec.8 TQ Step 5: Execution of qualification plan ................................................... 30

8.1 Execution of the qualification activities ................................................30

8.2 Data collection and documentation ......................................................30

8.3 Traceability and transparency of data ..................................................30

Sec.9 TQ Step 6: Performance assessment ........................................................... 31

App. A Qualification of an offshore point-to-point HVDC link.................................. 32

A.1 Qualification basis (Step 1 in the TQ process) ..................................... 32

A.2 Technology assessment (Step 2 of the TQ process)............................. 35

A.3 Threat assessment (Step 3 of the TQ process) .................................... 35

A.4 Qualification plan (Step 4 of the TQ process) ...................................... 38

A.5 Execution of qualification plan and performance assessment (Step 5 and 6 of the TQ process)......................................................... 38

App. B Qualification of a multi-terminal offshore HVDC system.............................. 39

B.1 Qualification basis (Step 1 of the TQ process)..................................... 39

B.2 Technology assessment (Step 2 of the TQ process)............................. 40

B.3 Threat assessment (Step 3 of the TQ process) .................................... 42

B.4 Qualification plan (Step 4 of the TQ process) ...................................... 44

B.5 Execution of qualification plan and performance assessment (Step 5 and 6 of the TQ process)......................................................... 44

App. C Standardization work and other initiatives.................................................. 45


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SECTION 1 INTRODUCTION

1.1 GeneralAs offshore wind farms are being built farther from the coast and more offshore oil and gas installations are

electrified from shore, there will be an increasing need for long distance underwater power transmission. For power transmission, high voltage direct current (HVDC) avoids the large charging currents causing

energy losses in 50/60 Hz AC cable systems. HVDC therefore allows power transmission through cables over

longer distances and higher capacities compared to what is feasible when using AC transmission. Hence, HVDC will often be the preferred solution for long distance power transmission.

A number of point-to-point HVDC systems with subsea power cables and the AC/DC converters placed

onshore are in operation. For HVDC systems with one or more of the AC/DC converters placed offshore there

are few reference projects and limited operational experience. Since offshore HVDC technologies are still

immature there is a lack of relevant standards, guidelines and recommendations for stakeholders to rely

on. The immature nature of offshore HVDC technologies causes uncertainties and increased risk exposure

for involved stakeholders. Hence project development and execution of projects becomes unnecessarily

complicated and time consuming for all involved parties.

DNV GL’s technology qualification methodology provides a systematic way to manage the uncertainties

related to implementation of new technology in cases where fitness for purpose cannot solely be relied on

by demonstrating compliance with relevant standards, guidelines and recommendations. The procedure

makes it possible to identify and analyse the risks associated with the new technology, and provide evidence

that it is suitable for its intended use. It can therefore play an important role in increasing the confidence

in new offshore HVDC technologies and facilitating a faster, more efficient and more reliable deployment of

offshore HVDC transmission.

This recommended practice (RP) specifies a procedure for technology qualification that can be used to prove

that offshore HVDC transmission technologies are suitable for their intended use.

1.2 ObjectiveThe objective of this RP is to provide a systematic method for qualification of offshore HVDC transmission

technologies. The RP gives guidance on how to utilise the generic qualification procedure for new

technology, presented in DNV-RP-A203, on offshore HVDC transmission technologies.

1.3 ScopeThe scope of the RP is applicable for offshore HVDC systems, subsystems and components that can be

defined as new offshore HVDC transmission technologies or concepts. Interfaces between offshore HVDC

transmission technologies and offshore installation and auxiliaries already covered by existing offshore

standards are covered by the procedure.

1.4 Structure of the Recommended PracticeThis document is structured into three parts:

— Introductory part (Sec.1 to Sec.3) where the principles of qualification are presented and the

qualification process is introduced. Also a brief introduction to offshore HVDC technologies is given.

— Main body (Sec.4 to Sec.9) which describes the qualification process for offshore HVDC technologies.

— Appendices that include additional and supplemental information as well as examples for illustration and

clarification purposes.

1.5 Case examples and risk assessmentTo improve the reader’s understanding of the recommended practice, the technology qualification process

has been demonstrated on the following two selected case examples:

— A hypothetical point-to-point HVDC link with one offshore converter station, connecting an offshore wind

farm to the AC onshore grid.


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— A hypothetical multi-terminal HVDC system connecting two offshore wind farms and one offshore oil and

gas platform to the AC onshore grid.

The qualification examples are included in App.A and App.B.

As the objective of the examples is only to illustrate the qualification process, the qualification performed

in the examples is less comprehensive than it would be for a real technology qualification.

1.6 Relationship to other codes and standardsGeneric qualification procedures for new technology are covered by DNV-RP-A203 /1/, whereas DNV-DSS-

401 /2/ describes the services offered by DNV GL based on DNV-RP-A203. While these procedures cover a

generic approach, the present document provides a more specific qualification procedure on how to utilize

DNV RP-A203 for qualification of offshore HVDC transmission technologies.

DNV-OS-J201 /5/ covers offshore AC substations, including safety requirements, structural design,

electrical design, fire and explosion protection, access and transfer, emergency response, construction,

inspection and maintenance.

DNV-OS-D201 /11/ covers offshore electrical systems.

Existing IEC and IEEE standards for HVDC and electrical systems are to some extent applicable also for

offshore HVDC.

1.7 Definitions and abbreviations

Table 1-1 Definitions

TechnologyQualification Plan

The qualification activities specified with the purpose of generating qualification evidence and the logical dependencies between the individual pieces of qualification evidence.

Technology Qualification program

The framework in which the Technology Qualification Process is executed as detailed in Sec.3.

Verification Confirmation by examination and provision of objective evidence that specified requirements have been fulfilled (ISO 8402:1994).

Reliability The ability of an item to perform a required function under given conditions for a given time interval or at a specified condition. In quantitative terms, it is one (1) minus the failure probability.

Technology qualification Technology qualification is the process of providing the evidence that the technology will function within specified limits with an acceptable level of confidence.

Offshore HVDC system A HVDC system where at least one converter is placed offshore on an offshore structure.

Offshore installation A collective term to cover any structure, buoyant or non-buoyant, designed and built for installation at a particular offshore location.

Offshore substation A collective term for high voltage AC (transformer) and high voltage DC (converter) platforms as well as associated accommodation platforms located offshore

Milestone A point in the Technology Qualification Process that signifies an agreed stage has been achieved which may be used to trigger other events such as recognition, reward and further investment

Decision Gate A point in time where a decision is taken on whether or not to continue a technology development process or a project development.


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1.8 References

Table 1-2 Abbreviations

DNV GL DNV GL AS

TQ Technology qualification

HVDC High-voltage direct current

AC Alternating current

VSC Voltage source converter

RP Recommended practice

MTBF Mean time between failure –the mean time between two consecutive failures

MTTR Mean time to repair – the mean time before the item is repaired.

FAT Factory acceptance test

FME(C)A Failure modes, effect (and criticality) analysis

FT Fault tree

TRL Technology readiness level

DG Decision gate

/1/ Det Norske Veritas, 2013. Recommended practice DNV-RP-A203 Qualification Procedures for New Technology, Høvik, Norway

/2/ Det Norske Veritas, 2012, Service Specification DNV-DSS-401 Technology Qualification Management, Høvik, Norway

/3/ Det Norske Veritas, 2011. Technical report prepared for Statnett, Assessment of Standards for Offshore Grids and Statnett's future role, Høvik, Norway

/4/ CIGRE, 2011, Technical Brochure, TB 483 Guidelines for the Design and Construction of AC Offshore Substations for Wind Power Plants, Paris

/5/ Det Norske Veritas, 2013. Offshore Standard, DNV-OS-J201, Offshore substations for wind farms. Copenhagen, Denmark

/6/ CIGRE, 2005 Technical Brochure, TB 269 VSC Transmission prepared by WG B4.37, Paris

/7/ IEC, 2011, Technical Report, IEC/TR 62543 – High-voltage direct current (HVDC) power transmission using voltage sourced converters (VSC), Switzerland

/8/ CENELEC, 2013, Technical Report, TR 50609 - Technical Guidelines for first HVDC Grid

/9/ CIGRE B4, The CIGRE B4 DC Grid Test System, B4-57 / B4-58, T. K. Vrana, Y. Yang, D. Jovcic, S. Dennetière, J. Jardini, H. Saad; available at: http://b4.cigre.org/Publications/Documents-related-to-the-development-of-HVDC-Grids

/10/ IEC 62501:2009 - Voltage sourced converter (VSC) valves for high-voltage direct current (HVDC) power transmission - Electrical testing

/11/ Det Norske Veritas, 2011, DNV-OS-D201, Electrical Installations, Høvik, Norway

/12/ CIGRE, 2012, Technical Brochure, TB 496 Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500 kV, prepared by WG B1.32, Paris

/13/ CIGRE Technical Brochure 346 Protocol for reporting the Operational Performance of HVDC Transmission Systems


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SECTION 2 INTRODUCTION TO OFFSHORE HVDC TRANSMISSION

TECHNOLOGIES

This section briefly introduces the HVDC technology and the potential technical and economic benefits which

form the incentives to exploit HVDC for offshore systems.

Electric power is generally generated as AC at power generating plants, which may be located far from load

centres. This power, traditionally, is transmitted to the load centres on three-phase, AC transmission lines.

In order to transfer the electric power from the generating plants in an efficient way with low power loss,

the voltage level is increased. The voltage is increased by step-up transformers at the sending end and the

power is transferred to the consumption centres through high voltage cables or overhead lines.

High-voltage direct current (HVDC) transmission systems are based on direct current, as opposed to the

more common high-voltage AC transmission systems. The HVDC system forms an asynchronous link

between the sending and receiving end, where AC power is rectified to DC, transported through DC lines,

and then inverted to AC again.

A simplified outline of an HVDC system is given in Figure 2-1.

Figure 2-1 HVDC system outline drawing

The conversion of electric power from AC to DC and vice versa is taking place in converter stations at each

end of the DC link. The converter station comprises power electronic converters for inverting and

rectification of the power, and normally also components such as converter transformers, smoothing

reactors, harmonic filters etc. The transmission lines, which may generally consist of power cables or

overhead lines, transfer the electric power in the form of DC voltage and current from the sending end to

the receiving end.

In an HVDC system, the converters and cables or overhead lines can be arranged into a number of

configurations. Conventional configurations are divided into monopolar and bipolar HVDC links, which again

can be subdivided further.

In a monopolar link, each terminal has one converter (i.e. only one three-phase bridge). The terminals can

be connected either in a symmetric configuration, with two power lines or cables operated at equal and

opposing polarity with the mid-point of the DC-side grounded known as the symmetric monopole, or in an

asymmetric configuration, with one line or cable operated at a high potential and with return through either

a low-voltage metallic conductor or earth/sea electrodes. The arrangement with earth electrodes implies a

continuous earth current, and is hence often not acceptable.

These possible arrangements of monopolar HVDC links are illustrated in Figure 2-2 and Figure 2-3. The

symmetrical monopole is the predominant scheme for offshore HVDC systems.

Figure 2-2 Example of an asymmetric monopolar HVDC link configuration with earth return

Figure 2-3 Example of symmetric monopolar HVDC link configuration


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In a bipolar HVDC link, each terminal has two converters that are operated at opposing polarity from a

common neutral reference. The corresponding poles at the different terminals are connected by cables or power lines as shown in the figure below. The neutrals at each terminal must also be connected to provide

a return path in case of imbalance between the two poles. This connection can be provided either by a

metallic conductor or ground/sea electrodes. In normal operation the neutral connection will only carry a minor current, but it may also be used as return conductor in case of fault at one pole. An advantage of this

topology is the possibility of operating the DC link in monopolar configuration (with reduced capacity) in

case of failure of one of the poles. With monopolar configuration, a single failure would cause complete interruption of power flow.

A possible arrangement of a bipolar HVDC link is given in Figure 2-4.

Figure 2-4 Example of a bipolar HVDC link with neutral connected to earth

Similar to AC networks, HVDC transmission systems can be designed as radial and/or meshed networks. In a meshed HVDC transmission system, converter stations are interconnected through a minimum of two

transmission lines, allowing power to be transferred via more than one path between stations.

2.1 HVDC in comparison with AC transmissionSeveral factors influence the selection between AC or HVDC solutions, such as cost, reliability, and technical limitations.

The voltage conversion is straightforward in an AC system, and is based on the conventional and well-

proven AC transformer technology. HVDC systems are based on a complex conversion process between AC

and DC voltages. Conversion solutions suitable for offshore HVDC systems are still emerging, with relatively limited operational experience especially regarding higher ratings on power capacity and voltage insulation levels.

AC transmission distance is limited by the inductance and capacitance of the transmission lines. For cable transmission, which is the only option of offshore applications, the transfer capacity is decreased in the AC

case due to the charging current of the cables. The charging current is linearly dependent on the length of

the cable and the system voltage. On the other hand, HVDC transmission provides high power-transfer capacity over long distances, both in the case of overhead lines and cables. AC transformers combine large

power capacities and high voltage insulation levels, with relatively low losses and low maintenance

requirements while the use of DC converters implies higher losses. Losses in DC cables are lower compared to AC and there is a break even distance where the AC losses combined exceeds the DC losses combined.

Furthermore, HVDC solutions provide a flexible and controllable power flow and the possibility of

interconnecting asynchronous power systems, which is not possible directly through AC.

An offshore power transmission system, interconnecting different power systems, integrating large scale

offshore wind power plants and offshore oil-and-gas installations, has been proposed as a means to achieve

greenhouse gas reductions. Traditional AC may provide economically beneficial solutions to transmit electrical power in many cases. However, with higher power capability requirements and increased distance

to shore, HVDC will in many cases be the preferred solution.

2.2 Application of offshore HVDC There is in principle three applications of offshore HVDC technologies:

— grid connection of offshore wind to shore (power to shore)

— grid connection of offshore oil and gas installations (power from shore)

— connection of offshore nodes including multi-terminal HVDC systems.


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2.3 Challenges with offshore HVDC

With respect to technology qualification, HVDC transmission technologies which are standardised and

deemed state of the art onshore are not necessarily new technologies. But, if they are to be utilised offshore

they will by definition encounter a new application area within which the technology is unproven due to a

lack of field history. The technology is then classified as novel. This implies that equipment and or system

reliability and availability in accordance with applicable performance requirements given a different

environment may introduce new risks which should be addressed. Technology qualification of the novel

technology will assist in verifying that it is fit for service.

Some of the main challenges for offshore HVDC systems lie in the lack of widely accepted standards for

offshore HVDC systems and components, environmental conditions offshore, accessibility, marine operation

limitations, and all necessary interfaces with for example offshore installations, wind farms, oil and gas

installations and land based installations.

Offshore HVDC transmission systems will typically face environmental conditions which are different, and

potentially more extreme to some of its components or subsystem, than those encountered on land.

Examples are mechanical loading from vibrations and accelerations, temperature and humidity and salt

pollution. Environmental conditions including weather and sea conditions will have a further impact on

equipment storage and transportation and other marine operations required throughout the lifecycle of the

equipment which may not be encountered with land based installations.

The volume and weight of HVDC equipment to be installed offshore should, compared to offshore oil and

gas installations, not be a challenge in itself. However, e.g. centre of gravity and large volumes might

introduce challenges for the structure of the installation. Furthermore platform design may set constraints

on solutions and physical dimensions of components. In combination with an increasing demand for higher

transfer capacities, dimensional restraints can constitute a major challenge.

Given the remote location, and possible difficulties associated with maintenance and repair, achieving a

sufficiently reliable system may constitute a significant challenge. In the long term, future development of

meshed offshore HVDC systems, being the source of multiple novelties related both to component

development and control and operation issues, is expected to be a major challenge. Furthermore, the ability

to extend the HVDC systems into larger and/or meshed systems places requirements on the interoperability

between HVDC systems from different suppliers.


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SECTION 3 INTRODUCTION TO TECHNOLOGY QUALIFICATION

Technology qualification is a systematic method to manage the uncertainties related to implementation of

new technology. The objective of the method is to provide evidence that the technology will function within

specific limits with an acceptable level of confidence. This section presents the motivation for using

Technology Qualification and gives an overview of DNV GL’s technology qualification method.

3.1 Motivation for technology qualificationImplementation of new technology or technology with limited experience introduces uncertainties that

imply risks for technology developers, financiers and end-users. As new technologies are usually only partly

covered by existing standards, guidelines or recommendations, it can be difficult for the involved

stakeholders to achieve a common understanding on whether or not a new technology is fit for purpose,

and thereby to build the confidence necessary for deploying the new technology.

DNV’s technology qualification method, described in DNV-RP-A203 /1/, provides a systematic way to

manage the uncertainties related to deployment of new technologies or technologies with limited

experience. The method is particularly valuable in cases where fitness for purpose cannot be relied on solely

by demonstrating compliance with relevant standards, guidelines and recommendations. The method

makes it possible to identify and analyse the risks associated with the new technology, and provide evidence

that it is suitable for its intended use. Technology qualification can facilitate the deployment of new

technology by reducing the risk for all stakeholders, including developers, manufacturers, financers and

end-users.

3.2 Roles in technology qualificationTechnology qualification can be applied by both technology developers and purchasers to assess the robustness of new technology. A technology developer may either use TQ internally to monitor the

technology development process, or to demonstrate the maturity of the technology to potential investors

or buyers. A purchaser will typically apply technology qualification to assess one or several alternative technologies considered for a development project. A TQ initiated by a purchaser will typically be conducted in cooperation with the technology supplier, to gain access to the information required for evaluating the

technology. In cases where strict regulations on public procurements exist, it is important to ensure separation of the technology qualification of technologies and the procurement itself.

In any of the above mentioned cases, one or more third parties may be involved to facilitate the qualification process, to provide independent judgment, or for performing analysis and tests of the technology.

3.3 The six-step technology qualification processDNV GL’s method for Technology Qualification is structured in a six-step process presented below:

Technology qualification basis

— Establishing a basis for the qualification by defining the technology to be qualified, its functions, its

intended use, its operating environment, as well as the expectations to the technology and qualification targets.

Technology assessment

— Decomposing the technology into elements, categorizing the various elements by degree of novelty

based on industry experience.

Threat assessment

— Assessing threats by identifying potential failure modes and failure mechanisms and estimating

probabilities and consequences associated with each failure mode.

Establishing the qualification plan

— Developing a plan comprising the qualification activities necessary to address the identified risks.


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Execution of the qualification plan

— Executing the activities specified in the technology qualification plan, collecting evidence through documented experience, numerical analyses and tests.

Performance assessment

— Assessing whether the evidence produced meets the requirements of the technology qualification basis.

Each step in the process is described in detail for offshore HVDC technologies in Sec.4 to Sec.9.

The flowchart in Figure 3-1 illustrates the process flow of the TQ process.

Figure 3-1 The six step technology qualification process

The output of each step in the process is used as input to the next step. The feedback loops indicates that

it might be necessary to modify the original design or qualification requirements to incorporate newly

identified threats or knowledge about the technology obtained from the qualification activities.

If the conclusion in the last step/performance assessment is that the technology has met all the

requirements set in the qualification basis, the technology qualification has been successful. This may either mean that the technology is qualified and fit for purpose, or it can have reached some intermediate milestone in the development of the technology. This will depend on what type of requirements that were

stated in the qualification basis.

3.4 Use of technology qualification in different project phasesDevelopment of new technologies usually follows a stepwise process, starting with a business idea, proceeding through a number of preliminary development stages, before it can be considered fit for purpose

and deployed. An example of a typical stepwise development process is illustrated in Figure 3-2.

Figure 3-2 Example of a technology development process


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To proceed to the next development phase, the technology usually has to pass decision gates (DG) where

the continuation of the development process will be decided. The outcome will typically depend on the

technology developer’s ability to provide the evidence that the technology has reached a certain level of

maturity and robustness. By applying technology qualification in the development process, the decision

makers will have a better basis for decisions on whether or not to continue or deploy a new technology.

3.5 Input to the technology qualification processThe input to the technology qualification should comprise all information required to assess the novel

elements of the technology to be qualified, including information on its intended usage and operating

environment, as well as requirements on performance. This information shall be collected and structured in

the first step of the technology qualification process, the qualification basis. The extent and the accuracy of

this information will vary depending on the development stage of the technology considered.

3.6 Results from the technology qualification processThe results from the technology qualification process are the conclusions on whether the new technology

meets the requirements defined in the qualification basis or not. These conclusions must be supported by

evidence and documentation of all activities performed during the qualification process. This documentation

shall provide sufficient transparency and traceability to allow independent assessment of the conclusions.

3.7 Qualification of complex systemsQualification of a complex system like an entire HVDC link will normally require the different subsystems

making up the overall system to be qualified separately before the combined system can be evaluated with

respect to fitness for purpose. The requirements of each subsystem shall be consistent with the

requirements specified for the overall system. The qualification of the combined system shall focus

particularly on the interaction between the respective subsystems.

Typically, many of the subsystems will be well-known and covered by existing standards; hence it may

seem unnecessary to perform a full qualification for these. It is, however, recommended to perform at least

a simplified assessment for all subsystems, as the novel elements may have unforeseen effects on the

presumably well-known elements.


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SECTION 4 TQ STEP 1: QUALIFICATION BASIS

The purpose of the qualification basis is to define the expectations of the technology. This is done by

specifying the technology to be qualified and the (functional) requirements that the technology will be

tested against. The qualification basis consist of a technology description and requirements specification

All information that is expected to have relevance for the expectations should be provided in the

qualification basis. The qualification basis will usually have to be updated as new information is gained

during the qualification process. Output of the qualification basis is a technology description and

requirement specification as complete as possible depending on the maturity of the technology to be

qualified.

The qualification basis will, as for the full TQ process, be developed in close collaboration between

manufacturers, client and potentially a third party.

Reference is made to DNV-RP-A203 /1/ section 6.

4.1 Technology description

4.1.1 GeneralThe technology description shall describe the technology to an extent and at a level of details appropriate

to the claim to be proven through the qualification. The information to be provided will depend on the type

of technology, and at what development stage the technology has reached. The technology description

should also make it possible to identify possible threats represented by the new technology and to create a

qualification plan to assess these threats.

The technology description should include:

— purpose for which the technology is intended.

— system description of the technology to be qualified, including system boundaries and boundary

conditions

— relevant standards and industry practices

— functional/ operational limitations

— interfaces to other systems

— main principles for technology life cycle

— operating environment.

4.1.2 Specific offshore HVDC transmission technology descriptionOffshore HVDC systems can be described using the principles used to describe onshore HVDC systems and

offshore AC electrical systems (see e.g. DNV-OS-J201). The fact that parts of the systems will be placed

offshore will require extra attention on non-electrical properties like vibrations, and corrosion compared to

onshore systems.

The technology description shall contain but not be limited to the information provided in the block diagram

below (Figure 4-1) and in the following sections.


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Figure 4-1 Illustration of some of the key elements to be included in a technology description of offshore HVDC transmission technologies

4.1.2.1 System description

Intention with the system description is to give an overview of the technology, its main interfaces and its

operational limits. Reference is made to DNV-OS-J201 /5/. The block diagram in Figure 4-2 indicates

important elements to be used for describing the electrical system.


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Figure 4-2 Illustration of some of the key elements to be included in the electrical system description

4.1.2.2 Standards and industry practices

A large part of equipment and components used in offshore HVDC transmission is covered by already

existing standards and regulations. Compliance with these standards will usually ensure safe and reliable

operation for the equipment within the defined scope. Hence, additional qualification activities will usually

not be required for these elements. However, it will still be necessary to verify that the standard ensures

sufficient performance and reliability, that the standards are valid for the specific application, and that the

equipment does comply with the standards. This documentation should be included in the qualification

basis.

4.1.2.3 Interfaces

Although many parts of an offshore HVDC system will be covered by already existing standards, the

interfaces between the different elements (that are each covered by their respective standards) will often

not be covered by these. These interfaces are a key issue and will hence have to be identified in the

Qualification Basis and covered by the technology qualification process. One example of such interfaces

would be between the offshore installation (covered by DNV-OS-J201) and electrical components (to a large

extent covered by e.g. IEC standards).

4.1.2.4 Limitations

Range of maximum and minimum operating conditions and characteristics of the offshore HVDC

transmission technologies should be provided. Description on how compliance will be gained with the

requirements specified in [4.2] to be included.

4.1.2.5 Life cycle principles

It should be described how the life cycle principles comply with the requirements specification ([4.2]).

Illustrations of elements to be included for description of the life cycle principles is included in Figure 4-1.

4.1.2.6 Environment

Reference is made to DNV-OS-J201/5/ section 5 and DNV-OS-D201/11/ section 3. Important aspects to

cover in the technology description are e.g. outdoor and indoor environmental conditions and vibrations.


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4.2 Requirement specification

4.2.1 GeneralThe requirement specification, shall as for the technology description, describe relevant requirements and

performance for the technology at a level of details appropriate to the claim to be proven through the

qualification. The information to be provided will depend on the type of technology, and at what

development stage the technology has reached. This will typically depend on the customer’s needs, as well

as other stakeholders requirements, such as authorities, compliance with rules and regulations etc. It is of

crucial importance that the requirements, and the impact of setting them, are thoroughly discussed,

understood, and agreed by all project participants.

The requirements should be quantifiable, to make it easier to test against the criteria. However, at early

stages of development it may not be possible to quantify the expectations of the technology. In such cases,

it may be possible to formulate temporary qualitative requirements

Typical requirements to be included in the requirement specification are:

— functionality and performance requirements including reliability, availability and maintainability

requirements (RAM)

— regulatory and statutory requirements

— safety, health and environmental requirements (SHE)

— life cycle requirements.

4.2.2 Specific offshore HVDC transmission technology requirement specificationThe requirements for offshore HVDC technology are highly dependent on type of technology to be qualified,

e.g. a different set of requirements would be necessary for a complete offshore HVDC system compared to

an offshore converter. Also the area of which the technology is to be deployed as well as stakeholders’

priorities influence the type of information required.

There are no industry wide accepted specifications of requirements for offshore HVDC technology. To a large

extent the different stakeholders involved will have to agree upon which requirements should be specified

from case to case.

Likely there will be similar requirements to e.g. availability of systems and components compared to what

is used onshore, while safety, health, and environment (SHE) requirements will have to be in accordance

with existing rules and regulations from the offshore and maritime industry depending on type of installation

and manning. Maintainability and reliability requirements will be specific to each case and dependent on

e.g. accessibility of the offshore platform. More stringent requirements on maintainability and reliability for

offshore HVDC technologies could be expected in order to achieve satisfactory availability of components

and systems.

The requirement specification for offshore HVDC transmission technology should contain but not be limited

to the information provided in Figure 4-3.


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Figure 4-3 Illustration of some of the key elements to be included in a requirement specification of offshore HVDC transmission technologies


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SECTION 5 TQ STEP 2: TECHNOLOGY ASSESSMENT

The purpose of technology assessment is to identify novel elements of the technology. This is achieved by

dividing the technology into manageable pieces, allowing for assessment of novelty and identification of

related key challenges and uncertainties. Within this context, novelties may be related to issues regarding

technical solutions, application, environment, etc.

Input to the technology assessment comes from the qualification basis, containing amongst others

descriptions of the technology, functional requirements and intended use. The output is a list of identified

novel elements classified based on their degree of novelty.

The technology assessment consists of two main steps, technology decomposition in which the technology

is broken down into manageable elements and the following technology categorisation where degree of

novelty of each element is assessed.


5.1 Technology decomposition

To be able to assess the novel elements of a technology, the technology is decomposed into pieces that are

manageable for categorization in terms of novelty. Breakdown shall be performed by decomposing the

technology applying at least one of the following views: functions, sub-systems and components and

operations in all project phases.

5.1.1 Technology decomposition for offshore HVDC systemFor decomposition of offshore HVDC technologies approaches utilising functions or sub-systems is

considered to be appropriate. In the first approach, the system is divided into a number of main functions,

corresponding to the fulfilment of the intended purpose of the technology. These are further decomposed

into sub-functions, required for satisfactory implementation by supporting or adding up to the main

function. At the appropriate level, sub-functions are delegated to hardware and software components.

Functions should be defined in a way that provides answers to key questions like: when is it started/

stopped, what are the characteristic modes of operation, what is transported and from where, what are the

performance requirements etc. In the analysis of complex systems, it is recommended to utilize a

hierarchical structure, allowing for linking the expectations on the technology to functions and sub-

functions. Generally, technology decomposition of an offshore HVDC installation may require larger effort

than for a corresponding onshore system. The reason for this is the greater complexity of support systems,

arrangements required for access, resulting in possibly higher requirements on the level of redundancy.

Figure 5-1 and Figure 5-2 illustrate examples of technology decomposition related to an offshore point-to-

point HVDC system. An alternative approach of decomposing a HVDC system may be through following the

path of power transport, as presented in Figure 5-3. Application of such alternative approach in parallel to

a function or component oriented analysis may facilitate identification of interactions between subsystems

or components. It is recommended to decompose offshore HVDC systems into subsystems and components

as a minimum.

In Figure 5-1, the decomposition is presented from a system oriented approach, where an offshore HVDC

scheme is divided into sub-systems and components. Special care should be taken in order to capture how

and the level at which the main functions depend on the interfaces and interactions between the

components.

A different approach is illustrated by Figure 5-2, presenting decomposition of an installation/maintenance/

decommissioning activity, seen from a functional perspective. This decomposition method provides detailed

information specific to the activities outside normal operation, which may prove valuable in the technology

qualification.


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Figure 5-1 Example of technology breakdown of offshore HVDC system considering some of the key sub-systems and components

Figure 5-2 Example of breakdown of an installation activity


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Figure 5-3 Example of decomposition of a HVDC system based on the path of power flow

5.2 Technology categorisationNew technology typically evolves from existing proven technologies and thus normally only some elements

of the technology to be qualified are in fact novel. These elements are of interest as novelty is associated

with uncertainty. In order to identify where uncertainty is greatest, elements of technology identified

through technology decomposition are categorised based on novelty. Both the novelty of the technology

itself and its application area affect the uncertainty associated with the technology, and should be assessed.

Categorisation is performed according to Table 5-1, giving three levels related to application area and

Degree of novelty of the technology itself, respectively. Application area, classified as Known, Limited

Knowledge or New, refers to the experience of the operating condition or the purpose of application. A

different environment or different application corresponds to an increased uncertainty. If there is no

experience in the industry for a particular application of the technology, this would correspond to “New”,

while “Known” would represent the situation if there is sufficiently documented knowledge for the use of

the technology for similar conditions and applications.

The technology itself is assessed by degree of novelty of the technology, using the terms Proven, Limited

field history and New or unproven. Any change in existing technology (parts, functions, processes,

subsystems, architecture, interfaces etc), will lead to increased uncertainty, and degree of novelty will

change from Proven towards Limited field history or New or unproven.

Elements falling into category 1 represent proven technology with no new technical uncertainties where

proven methods for qualification, testing, etc can be used to document performance margins. Elements in

category 2 to 4 are defined as new technologies, having an increasing degree of technical uncertainty.

Elements falling into these categories shall be taken forward to the next step of technology qualification for

further assessment.

It should be noted that technology categorization does not consider the consequences of failure, only the

uncertainty. However, combination of uncertainties with associated consequences may be used to

determine the technology criticality, allowing for prioritising qualification activities.

5.2.1 Categorisation of offshore HVDC technologiesThe approach for categorisation of technologies related to offshore HVDC does not differ from what is

utilised in other applications.

The main uncertainties of offshore HVDC systems are associated with the application area, i.e. the use of

HVDC components offshore. Due to the fact that converter stations of existing HVDC installations are with

only few exceptions located onshore, application area could from one point of view be regarded as new for

in principle all systems and components. One exception is submarine DC-cables, a field where there is

significantly more documented service experience available. However, components that are already used

in the same application onshore and does not experience conditions different from such operation should

typically not be classified as belonging to New. Thus, application area of components operating in controlled

Table 5-1 Categorisation of technology

Application

Area

Degree of novelty of technology

ProvenLimited

experienceNew or

unproven

Known 1 2 3

Limited experience

2 3 4

New 3 4 4


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climatic environment and electrical conditions similar as onshore, can in many cases be considered as

Known. HVDC equipment experiencing the outdoor offshore climate or significant mechanical stresses at

transport, installation or operation, will on the other hand directly be considered as belonging to New. This

also applies to equipment exposed to different electrical conditions due to the offshore application. In

contrast, the application area of components of the platform structure and its auxiliary systems can in many

cases be referred to as Known, as existing proven solutions often may be reused. The difference in

utilisation of platforms may result in considering the application of some structures as New, where the high

voltages and currents in the HVDC system may have impact.

Looking at the degree of novelty of technology elements, this could range from Proven to New or unproven

for components of both the primary HVDC system and the platform with its auxiliary systems. Utilisation of

HVDC technology frequently applied in onshore applications would typically result in Proven, while systems

with e.g. increased voltage rating would correspond to New or unproven. Furthermore, interfaces may be

found between the HVDC components and the platform, resulting in interactions between sub-systems or

components which are unproven from a technology perspective.


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SECTION 6 TQ STEP 3: THREAT ASSESSMENT

The objective of this step is to identify all relevant failure modes of concern for the elements defined as new

technology in the technology assessment and, for each, assess the associated risks.

The inputs to the failure mode identification are the qualification basis (Sec.4) and the list of the new

technology elements identified in the technology assessment. The output is a failure mode register

containing all identified failure modes of concern and their associated risk. Note that it is impossible to

develop an adequate qualification plan unless the potential failure modes have been identified and are

understood.


6.1 Definition of probability classes and consequence classesThe probability classes should be developed to capture the span in failure rates for all the identified failure

modes. For systems consisting of a variety of different components, the categories may range from failures

expected to occur several times a year to failures not expected in several thousand years, (like failures in

steel structures). Typically two or three classes are defined between the extremes very high and very low.

Table 6-1 shows an example of failure probability classes. The classes must be chosen in each individual

case using expert judgment and previous experience.

Similar to the probability classes, consequence classes must be chosen in each individual case using expert

judgement and previous experience. An example of consequence classes is shown in Table 6-2. The relation

between energy not supplied and expenses depends on type of load or generation connected to the HVDC

system. Generally expenses corresponding to energy not supplied for offshore oil and gas installation is

higher than for an offshore wind application. The example in Table 6-2 is more relevant for offshore wind

(power to shore) than for e.g. grid connection of offshore oil and gas installations (power from shore).

Table 6-1 Example of failure probability classes

No. Name DescriptionIndicative Annual

Failure Rate

1 Very Low Failure is not expected < 10-3

2 Low Failure would normally not occur during design life of the equipment 10-3 - 10-2

3 Medium Failure can be expected to occur during the design life of the equipment 10-2 - 10-1

4 High Failure occurs several times during the lifetime of the equipment 10-1 - 1

5 Very high Failure occurs several times per year of the equipment >1


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The number of defined consequence categories may vary depending on the application. In some cases

several types of consequences may be combined into one single category. Regardless how the categories

are defined, the consequences in different categories given the same score should represent similar levels

of severity. For a particular event with more than one type of impact, the type of impact giving the highest

class shall be governing in the selection of a single consequence class.

6.2 Definition of risk categories

The risk of a failure mode is the product of combined probability and consequence. To ensure an orderly

and cost-efficient execution of the qualification activities, the identified failure modes shall be ranked and

prioritised according to their associated risk. This is done by using a risk matrix, like the example in Table

6-3.

As for the probability class and consequence class tables, also the risk matrix must be customised to the

particular case. The risk matrix will typically classify the threats according to the following categories:

Table 6-2 Example of failure consequence classes for offshore wind applications

No. Name

Impact on:

Safety Environment ExpensesEnergy not supplied1) Reputation

1 Very Low Superficial injuryNo or light effect on environment

< 5000 EUR < 100 MWhlocal public

awareness but no public concern

2 LowSlight injury, nolost work days

Minor environmental

effect

5 000 -50 000 EUR

100 - 1000 MWhlocal public

concern

3 MediumSlight injury, few lost work days

Considerable environmental

effect

50 000-500 000 EUR

1 - 10 GWhregional public/slight national

media attention

4 High

Major injury, long term absence/

Permanent disability

Major environmental

effect0.5 -5 MEUR 10 - 100 GWh

National impact and public concern

5 Very high FatalityMassive

environmental damage

> 5 MEUR > 100 GWh

Extensive negative attention

in international media

1)CIGRE Technical Brochure 346 /13/ defines e.g. Equivalent Forced Outage Duration and Equivalent Forced Outage Hours which can also be used as a consequence class.

Table 6-3 Example of a risk matrix

Consequence

Probability

1Very low

2Low

3Medium

4High

5Very high

5 Very High Low risk Medium risk High risk High risk High risk

4 High Low risk Medium risk Medium risk High risk High risk

3 Medium Low risk Low risk Medium risk Medium risk High risk

2 Low Low risk Low risk Low risk Medium risk Medium risk

1 Very Low Low risk Low risk Low risk Low risk Low risk

Table 6-4 Risk categories

Term Definition

Low risk Threat will not be included in the further qualification activities.

Medium risk To be decided on an individual basis whether the threat will be subject to further qualification or not. Risk reducing measure should be considered.

High risk Risk reducing measure shall be implemented.


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The reason for not including a threat in the further qualification may either be because the associated risk

is sufficiently low, or because it is considered acceptable to leave the handling of the risk to a vendor without

the oversight of the qualification team. In the latter case the risk associated with the initial threat may not

be acceptable itself, although it is deemed acceptable to leave it to the vendor to handle it.

Risk acceptance involves a qualified balancing of benefits with risks. Stakeholders who may agree on the

degree of risk involved may disagree on its acceptability. Hence, acceptable risk is a subjective measure.

6.3 Failure mode identification and risk ranking methodologies There are several techniques for identifying and analysing failure modes that are commonly used in the

industry. The selection of method should take into consideration the complexity and maturity of the concept

being considered. Various methods for risk analysis can be used for the Failure mode identification and risk

ranking (FMIRR) step. Table 6-5 lists some of the advantages and disadvantages with different methods.

Of the methods listed above, FMECA is the most commonly used method for risk identification. It is intuitive,

widely known, and can be used for most kind of systems. Nevertheless, the method has its shortcomings,

particularly for systems where combinations of several failure modes are important.

It is recommended that an FMECA is always performed as part of a threat assessment, to come up with an

initial list of failure modes. Following, one or several of the other methods may be applied, either for

identifying additional threats, or for analysing the probabilities or consequences of already identified failure

modes or combinations of failure modes.

It is recommended to perform the identification and ranking of the failure modes through interdisciplinary

workshops, comprising experts within all fields of relevance for the technology to be qualified. The

participants in workshop must have the necessary competence to understand the technology, failure

modes, failure mechanisms and consequences of failure.

A good facilitator is a prerequisite for a successful threat assessment workshop. If conflicting interests are

anticipated, it may be desirable to have a third party facilitating the workshop.

Table 6-5 Advantages and disadvantages with different risk analyses methods

Method Advantages Challenges and disadvantages

Failure mode, effect and criticality analysis (FMECA)

Highly systematic as well as simple to apply

Investigating ONE failure mode at a time may not identify critical combinations of failures

Hazard and Operability study (HAZOP) Highly systematic tool which enables identification of the most inconceivable incidents

Resource consumingRequires detailed information for producing useful results.Experienced facilitator required

Fault Tree Analysis (FTA) Thorough investigation of (already) identified incident

Not applicable for identifying (new) incidents.Time consuming to set upNot suitable for accurately modelling all types of systems

Structured what-if checklist (SWIFT) Applicable even if detailed design information is not available

Experienced facilitator essential, as well as good checklists

Operational Problem Analysis (OPERA) Emphasis on the product interfaces Emphasis on technical problems and human error without going into details about causes


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SECTION 7 TQ STEP 4: QUALIFICATION PLAN

The objective of this step is to select qualification activities to adequately address potential threats identified

during the preceding steps and to establish a plan for executing the activities in a structured manner.

The inputs for the qualification plan are the threats identified in the risk assessment and the qualification

basis.

The development of the qualification plan comprises the following main steps:

— high-level planning to develop the overall qualification strategy

— analysis of the identified threats and selection of qualification methods

— description of the qualification activities and procedures.

The qualification activities usually constitute the bulk of the costs associated with a technology qualification.

An appropriate and well-structured plan is therefore of key importance to ensure an orderly and cost-

efficient execution of the qualification activities.


7.1 Qualification strategyAs a first step in developing the qualification plan, an overall qualification strategy should be made. The

strategy shall specify the evidence required to meet the objective of the qualification, and outline how the

evidence should be provided in order of priority.

The identified threats should be prioritized according to their associated risk. This implies that one shall first

address any threats that may cause major changes to the design or ultimately termination of the

qualification due to the technology’s failure to meet the defined requirements. An overall milestone plan is

recommended to reflect the chosen strategy towards the goals defined for the qualification.

7.2 Selection of qualification methodsAppropriate qualification methods shall be selected to address the identified threats and to provide evidence

for evaluating whether the requirements specified in the qualification basis have been met.

The choice of qualification method will depend on the technology to be qualified, the type of threat in

question and its associated risk level.

A quick cost-benefit evaluation should be conducted for each planned activity, to ensure that the resources

are spent in the best way and at the same time provide sufficient evidence within the available time.

The following methods can be used, separately or in combination, to provide qualification evidence:

— collection and evaluation of previous documented experience with similar technology and operating

conditions

— review and expert evaluation of documentation available for the new technology

— modelling of the technology and the operating environment followed by analysis by numerical or

analytical methods

— laboratory tests, prototype test, pilot test, or full-scale testing when possible.

One qualification activity may address several threats, or several qualification activities may be needed to

address a single threat.

7.3 Qualification methods for offshore HVDC technologiesThe qualification methods to be chosen for a particular system depend on the threats to be investigated.

For an offshore HVDC system one may expect many of the same threats as for offshore systems and

electrical systems in general. In addition, there may be particular issues caused by the specific features of

an offshore HVDC system.


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Offshore application – typical issues and qualification methods

For any technical system situated offshore, threats which are of particular relevance would be those caused

by for example:

— wave motions

— vibrations and shocks

— corrosion

— ingress of water and salt.

In addition remote location and possibly reduced accessibility, which may result in more serious

consequences of an incident, shall be taken into account.

Typical qualification methods for evaluating impact from wave motions or other met-ocean conditions are

FEM analysis such as ultimate-, fatigue- or accidental-limit-state analysis. Model tests in wave pools may

also be used to verify wave impact on the platform.

For corrosion issues, review of material data, identification of possible corrosion causing contaminants, as

well as use of e.g. FEM software for calculating electrochemical potential and current distributions will be

typical methods. Also experimental methods, such as accelerated life testing may be used to evaluate issues

related to corrosion or fatigue, or for verification of models.

Electrical systems – typical issues and qualification methods

For electrical systems, typically issues related to e.g. current carrying capacity, insulation level and

coordination, short circuit capability etc, may be evaluated by the qualification activities. For electrical

power systems based on power electronics, issues related to harmonics and corresponding resonances,

electromagnetic interference and control may be of particular interest. These issues may typically be

assessed by use of simulation software for system and network studies or by means of physical testing of

components and systems in test laboratories. For high voltage systems in offshore environment, particular

qualification activities may be required to address performance or solutions in areas such as contamination

and moisture effects on high-voltage components, containment of leaking oil or gas, handling of fires,

possibilities to perform maintenance and repair and personal safety issues. This may be investigated e.g.

by laboratory tests, review of documentation and comparison with requirements set by applicable

standards.

In addition to evaluating the failure modes and performance of the respective components and sub-

systems, also the performance of the systems combined, including its interfaces, should be evaluated. This

can be performed by e.g. Monte Carlo simulation of the overall system for various operating modes and

failure states.

Many of the components and subsystems making up an offshore HVDC system will be covered by existing

standards. Compliance with standards can be accepted as qualification evidence, provided it can be verified

that the relevant parts of the standard is valid for the particular application and operating conditions and that compliance implies fulfilment of the requirements in the qualification basis. This should be documented

for traceability.

7.4 Description of qualification activitiesThe technology qualification plan shall give the direction for the qualification process and facilitate a

structured execution of the qualification activities. However, as the results from one qualification activity

may require alterations either to the technology or to the subsequent qualification activities, it is usually not advisable to make a detailed plan for the entire qualification in advance. A better strategy is to start

with the overall milestone plan, and develop a detailed plan for reaching the next milestone every time a

milestone is met.

The milestone plan should comprise the following information for each milestone:

— the evidence to be obtained at the milestone

— the threat/requirement each piece of evidence relates to

— the reasoning that relates the evidence to the threats/requirements.

— suggested qualification activities to obtain the desired evidence


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— success criteria for the evidence in meeting the milestone.

Then, for the activities required for reaching the next milestone, the following details should be included in

the qualification plan:

— detailed description of the activities to be performed

— schedule and deadline

— responsible person

— estimated costs

— documentation requirements

— any activities to verify that the qualifications have been performed according to plan.

The qualification plan will hence be a living document that will be updated several times during the

qualification. Nevertheless, it is important that the documentation from all activities is kept in its original

form, to ensure traceability.


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SECTION 8 TQ STEP 5: EXECUTION OF QUALIFICATION PLAN

The objective of this step is to conduct the qualification activities according to the qualification plan and to

document the execution and the obtained results.


8.1 Execution of the qualification activitiesBefore an activity is started, the instructions for execution should be reviewed and understood by all

involved participants.

If the qualification activities are performed by external parties, a participant from the qualification team

shall be involved to be able to take decisions in case of unexpected events.

After an activity is completed, the documentation should be reviewed to assess whether it has been

performed according to specifications and if the acceptance criteria have been met.

8.2 Data collection and documentationThe results from the qualification activities will be the basis for evaluating whether the performance

requirements are achieved or not. The documentation requirements should be specified before the

execution of the qualification activity, and should be understood by all involved participants.

8.3 Traceability and transparency of dataIn order to ensure traceability, the data shall be organized in such a manner that there is a clear link

between the steps of the technology qualification process, from the technology qualification basis to

performance assessment. It shall be possible to trace the threats that have been identified, how they have

been addressed in the qualification activities, what evidence has been obtained, and how that evidence

meets requirements in the qualification basis. This provides opportunity for independent review of the

qualification conclusions and will enable reuse of evidence in future projects, e.g. qualification of refined

versions of the technology or other technology based on elements of this technology.


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SECTION 9 TQ STEP 6: PERFORMANCE ASSESSMENT

The objective of the performance assessment is to evaluate whether the requirements set in the

qualification basis have been met, based on the evidence obtained through the qualification activities.


Key steps of the Performance Assessment are:

— Review the identified threats from the risk assessment and the performance requirements for the

technology from the qualification basis.

— Confirm that the qualification activities have been performed according to specifications and that the

results are reliable.

— Compare the results from the qualification activities against the stated acceptance criteria and evaluate

whether the available evidence are sufficient for concluding that the performance requirements are met

or not.

In case of failure of the technology to satisfy the requirements; clearly identify the reasons for failure and

consider the following options:

— Modify the design to fulfil the stated requirements. A new iteration of the qualification must be

performed to confirm that the new design fulfils the requirements, and that the new design doesn’t

introduce new problems. The new iteration should focus on the recent design updates; however it should

be comprehensive enough to capture any undesirable effects resulting from the updates.

— Modify the requirements to match the capabilities of the technology. This will inevitably mean that the

technology is qualified against less ambitious performance targets. Nevertheless, this may be the best

solution if the technology is not likely to meet the original requirement with reasonable alterations to

the design. Lowered requirements will usually not require a new iteration of the qualification process if

fulfilment of the new requirements is already documented through previous qualification activities.

— Abort the qualification. This may be the solution if serious problems are discovered for the technology

that cannot be resolved by modifications to the original design.

In case of success; make an end report to document the qualification process and the obtained results,

documentation requirements are given in DNV-RP-A203 section 2.3.1 /1/.


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APPENDIX A QUALIFICATION OF AN OFFSHORE POINT-TO-POINT

HVDC LINK

This case example demonstrates the technology qualification procedure applied on a hypothetical HVDC link

for connecting an offshore wind farm to the onshore grid. The example covers the first four steps in the TQ

procedure, namely the qualification basis, the technology assessment, the risk assessment and the first part

of the qualification plan.

As the objective of the example is only to demonstrate the TQ process, a complete qualification of the entire

system has not been performed. Instead, the case example starts by presenting the system to be qualified

and a suggested approach for qualification based on decomposing the system into subsystems (as described

in section [3.7]). The example then demonstrates the different steps in the qualification for one selected

subsystem.

A.1 Qualification basis (Step 1 in the TQ process)The following section presents the Qualification Basis for the case example, including the technology

description and the requirements that the technology will be qualified against.

A.1.1 Technology descriptionThe system to be qualified is a 500 MW HVDC link, connecting a wind farm situated 200 km from shore to

the onshore AC transmission grid. The system has a symmetric monopolar configuration and is based on

modular multilevel VSC converter topology. A principle sketch of the system is given in the figure below.

Figure A-1 System overview

The system to be qualified comprises the offshore converter station, the power transmission cables and the

onshore converter station, which each can be further decomposed. Below is given a brief description of

these three main elements, whereas a further decomposition is indicated only for the offshore converter

station.

Offshore converter station

The offshore converter station connects the offshore AC substations from the wind farm, transforms the

power to the transmission voltage and converts it to DC, before the power is transferred to shore via the

transmission cables.

An overview of the subsystems making up the converter station is given below.

Table A-1 Decomposition into subsystems for the offshore converter station

Converter system Internal grounding Water supply and sanitation systems

Transformer system External lightning protection Topside structure

AC Switchgear Ventilation system Jacket primary structure

DC Switchgear Central cooling system Jacket secondary structure

Control and protection systems Firefighting and protection Piling system

Communication systems Accommodation Service crane

Auxiliary power system Safety systems Helideck


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The main circuit of the converter station comprises the transformer system, the converter system, the high-

voltage switchgear, plus the cables, busbars etc. that connect the systems. The transformer system

comprises two three-phase transformers for transforming the voltage from 150 kV to 360 kV. The power is

converted to DC by the converter system comprising a reactor and a converter module for each of the three

phase units. The gas-insulated switchgear ensures prompt disconnection of the relevant parts of the system

in case of a fault or during maintenance. The different subsystems are connected by power cables, bushings

and busbars, which in this context are considered elements of the respective subsystems.

In addition to the systems above, the converter station comprises centralized systems for control and

protection, communication, grounding, auxiliary power, etc. to enable safe and controlled operation of the

converter station. Furthermore, the platform comprises accommodation, water supply and sanitary systems

to accommodate workers in periods when the platform will be manned. A helideck and a service crane on

the top deck facilitate transport of personnel and equipment.

The platform structure supporting the converter station consists of the topside and the substructure.The

topside is a welded steel structure covered by steel plates to protect the equipment from water and salt

ingress. The substructure is a four-legged steel jacket structure which is pinned to the sea floor by steel

piles. J-tubes are attached to the jacket structure for guiding the high voltage AC and DC cables onto the

platform.

Transmission cable

The transmission cable system consists of one pair of HVDC cables operated in a symmetric monopolar

configuration. Each cable has a length of 220 km, divided between 200 km submarine cable and 20 km land

cable.

Onshore converter station

The onshore converter station converts DC back to AC and feeds the power into the onshore grid. The

onshore converter station comprises a similar electrical system as the one on the offshore platform, but is

normally operating in inverter mode. The converters modules and reactors are situated inside a building

while the transformers and switchgear are placed outdoor.

A.1.2 Requirements specificationThe HVDC scheme shall be able to transfer 500 MW of electric power from the wind farm to the onshore

grid with specified availability. Further, the system shall fulfil grid codes at the AC grid connection point.

The operational life should be at least 30 years.

Guidance note:

The system requirements should first be defined for the combined system, and then derived for the different subsystems based on

the original requirements. This should be accomplished by a functional analysis. A description of applicable techniques is given in to

DNV-RP-A203 /1/. For the combined system to be qualified, all subsystems must demonstrate satisfactory performance separately

and as a combined system.

---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

A.1.3 Qualification basis for the selected subsystemThe following section presents the qualification basis for the selected subsystem, the offshore transformer

system. This system was chosen for illustrative purposes since it represents a fairly simple system and at

the same time having a limited track record offshore.

A.1.3.1 Technology description of the transformer system

The transformer system comprises two three-phase transformers, which are run in parallel during normal

operation, sharing the total load. Each of the transformers has rated capacity equal to the wind farm

installed capacity, to ensure the converter to be able to run at full capacity even in case of failure at one

transformer. Each transformer is connected to gas-insulated switchgear on both the primary and secondary

side. Over-voltage stresses are limited by surge arresters installed on each unit. Each transformer has

attached sensors and relays for condition monitoring that are connected to central monitoring and control

systems by means of fibre optic cables. Each transformer is installed in its own fire-resistant room with its

own air-ventilation system. The transformers are mounted to the floor by means of bolts and vibration-

preventive rubber pads.


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Important interfaces with adjacent subsystems are included in the table below.

A.1.3.2 Requirements specification for the transformer system

General requirements for the transformer system in the case example are given in Table A-3.

Guidance note:

As the objective of the case example is primarily to illustrate the principles for the qualification process, the list of requirements has

been made very brief.

Other examples of possible requirements for the transformer system could be:

— load losses (possibly at specified harmonic content)

— ability to withstand specified mechanical, thermal or electrical stress for certain time intervals

— ability to operate in specified faulty conditions (error in connected equipment etc.)

— maximum noise and vibration levels

— electromagnetic compatibility (EMC) requirements

— accuracy of measuring equipment

— maintenance requirements.

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Table A-2 Interfaces between the transformer system and adjacent subsystems

Interfacing system Interface at Interface

Switchgear Primary terminal Power cables

Switchgear Secondary terminal Power cables

Topside structure Transformer tank bottomBolts and vibration-preventive rubber pads

Central cooling system Transformer cooling system Heat exchanger

Control and protection system Sensors, alarm and tripping relays Fibre-optic cables

Ventilation system Transformer exterior Ventilation air

Internal grounding Transformer neutral Grounding cable

Table A-3 Performance requirements for the transformer system

Rated power (per transformer): 600 MVA

Rated voltage: 150/360 kV

Input voltage tolerance: +/- 15%

Service life: 30 years


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A.2 Technology assessment (Step 2 of the TQ process)

To highlight any novel elements in the transformer system, the different elements were classified according

to novelty with regard to design and application area. The main results are given in the figure below.

Figure A-2 Results from the technology assessment for the transformer system

Figure A-2 shows that all elements of the transformer systems were classified as proven with regard to

technology maturity. All the components are well known and have been widely used in power transmission

for decades. The same type of transformer system has also been used in several VSC HVDC systems with

a good track record. Nevertheless, due to limited offshore experience for this kind of transformers, the

entire transformer system has been classified as “yellow”, and should be investigated further in the threat

assessment. The offshore environment can be expected to have implication for the performance and

reliability of the system, e.g. due to vibrations, moisture and salinity, and possibly also due to limited

accessibility for maintenance.

It can be argued that not all the elements of the transformer system are likely to be affected very much by

the offshore conditions, hence classifying all elements as “yellow” may be regarded somewhat conservative.

A conservative approach is however recommended in cases where the potential effects of a new application

or environment are uncertain.

In cases when there is doubt whether a new application will have implications for the performance or

reliability of an element, it is generally recommended to include this element in the Threat Assessment. If,

however, none of the threats identified in the Threat Assessment can be directly linked to the new

application, the new application is assumed to have limited significance and should not be considered

further for this element.

A.3 Threat assessment (Step 3 of the TQ process)According to the TQ procedure, a thorough threat assessment should be performed to identify all possible

threats associated with the technology and its application. This assessment should take into account both

the technology itself and its interaction with the interfacing systems. The risk for each threat should be

estimated by analysing the associated criticality and likelihood.

However, as this case example is only for demonstrating the TQ process and due to limitations in available

data, only a simplified FMECA was performed, focusing solely on a few selected failure modes.

The probability and consequence classes used in the threat assessment are given in the tables below. For

simplicity, the probability and consequence tables given in section Sec.6 were used.

Application

Technology maturity

Proven Limited experience New or unproven

Proven

Limited experience

Oil valvesTransformer tankBushingsInsulation/cooling liquidLiquid circulation systemHeat exchangerTransformer active part Tap-changerConservator and breatherTemperature and oil level indicatorsBuchholz relayOnline condition monitor

New application


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Note:

The tables are only examples, and are not valid on a general basis. As the acceptable risks may vary from case to case, these tables

should be customised for every project.

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Table A-4 Probability classes for the treat assessment of the transformer system

Failure probability classes

No. Name DescriptionIndicative Annual

Failure Rate

1 Very Low Failure is not expected < 10-3

2 Low Failure would normally not occur during design life of the equipment 10-3 - 10-2

3 Medium Failure can be expected to occur during the design life of the equipment 10-2 - 10-1

4 High Failure occurs several times during the lifetime of the equipment 10-1 - 1

5 Very high Failure occurs several times per year of the equipment >1

Table A-5 Consequence classes for threat assessment of the transformer system

(This example is more relevant for offshore wind (power to shore) than for grid connection of offshore oil and gas installations (power from shore).

Failure consequence classes

No. Name

Impact on:

Safety Environment ExpensesEnergy not supplied1 Reputation

1 Very Low Superficial injury No or light effect on environment

< 5000 EUR < 100 MWhlocal public

awareness but no public concern

2 LowSlight injury, no lost work days

Minor environmental

effect

5 000 - 50 000 EUR

100 - 1000 MWhlocal public

concern

3 MediumSlight injury, few lost work days

Considerable environmental

effect

50 000 -500 000 EUR

1 - 10 GWhregional public/slight national

media attention

4 High

Major injury, long term absence/

Permanent disability

Major environmental

effect0.5 - 5 MEUR 10 - 100 GWh

National impact and public concern

5 Very high FatalityMassive

environmental damage

> 5 MEUR > 100 GWh

Extensive negative

attention in international

media1CIGRE Technical Brochure 346 defines e.g. Equivalent Forced Outage Duration and Equivalent Forced Outage Hours which can also be used as a consequence class.

Table A-6 Risk matrix for the threat assessment of the transformer system

Risk matrix

Consequence

Probability

1Very low

2Low

3Medium

4High

5Very high

5 Very High Low risk Medium risk High risk High risk High risk

4 High Low risk Medium risk Medium risk High risk High risk

3 Medium Low risk Low risk Medium risk Medium risk High risk

2 Low Low risk Low risk Low risk Medium risk Medium risk

1 Very Low Low risk Low risk Low risk Low risk Low risk


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The results from the simplified threat assessment are given below:

Figure A-3 Summary from a simplified threat assessment for two selected failure modes for the transformer

system

Figure A-3 shows the evaluation of two selected failure modes. “Dielectric failure” here refers to all types

of short-circuit or open-circuit states in the transformer windings or between the transformer winding and

other parts of the transformer. As shown in the table, each failure mode can have several possible failure

mechanisms and several possible consequences. Each combination of failure mechanism and possible

outcome is given an associated consequence and probability score, as well as a risk level referring to the

predefined risk matrix.

The table shows that the combinations with outcome “transformer fire or explosion” have all been assigned

the score “low risk”, while the combinations with the outcome “transformer breakdown” have been assigned

“medium risk”. This implies that further investigation should be considered for all the listed failure

mechanisms, but that that investigation of fire or explosion risks in particular is regarded unnecessary.

The low probability scores associated with the outcome “fire or explosion” are due to the fact that this will

usually be avoided by protective systems. A transformer fire or explosion would require multiple systems

to fail, which is associated with a very low probability. A transformer breakdown caused by the same

mechanisms, however, is regarded less improbable. Although several of the elements in the table have been

classified as «medium risk», this doesn’t necessarily mean that extensive qualification efforts will be

required for all of them. According to section 6, risk mitigating measures should be considered for all

“medium risk” threats, but these may still be accepted at an individual basis.

The main reason for the relatively high risk scores in Figure A-3 is the high consequence of a transformer

breakdown at an offshore converter station. According to the risk matrix in Figure A-2, even a relatively low

probability score of 2 results in “medium risk” for this type of failure. As it may not be worthwhile (or even

possible) to reduce the probability of these threats to a probability score of 1, it may be necessary to accept

some “medium risk” in this case. For these threats, the best one can do may be to make sure the

consequence of a possible transformer breakdown is kept as low as possible. This can be done e.g. by

making sure to have adequate repair procedures for the case of a transformer failure to reduce unnecessary

downtime etc.

For illustration purposes only the two mechanisms “Mechanical stress during transport or installation” and

“Vibrations due to wave motions” were selected for further investigations. For these threats, the associated

probabilities were regarded uncertain, and to be on the safe side it was decided to collect more evidence

through qualification activities.

Cons. Prob. Risk

1.01 Transformer breakdown 4 2 Med

1.02 Transformer fire or explosion 5 1 Low













Active part

(Transformer

core and

windings, incl.

Insulation)

Transform

electrical power at

150 kV from the

primary side to

electrical power at

360 kV on the

secondary side

Offshore application,

vibrations or shocks

during transport and

operation

Dielectric failure

caused by

misalignment or

fatigue of the

windings or winding

insulation

Dielectric failure

caused by intensive

ageing of the winding

insulation

Dielectric stress due

to lightning or

switching transients

Furan analysis

DP analysis

Vibtations due to

wave motions

Manufacturing failure Factory acceptance

tests

Thermal stress due to

malfunction of cooling

system

Gas-in-oil analysis

Furan analysis

DP analysis

Manufacturing failure Factory acceptance

tests

Mechanical stress

during transport or

installation

Commissioning tests

Mechanical stress

due to overcurrents

caused by fault

currents or inrush

currents

ConsequenceRisk Ranking

ID Component Function New aspectFailure mode /

Risk

Failure mechanism

or causeDetection


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A.4 Qualification plan (Step 4 of the TQ process)

A simplified milestone plan for investigating the highlighted failure modes is given below:

The following approach is suggested to address the mechanical stress on the transformer active part during

transport and installation:

— Perform expert review of the planned routes for transport, procedures and equipment to be utilized for

transport and lifting etc. to estimate expected and possible accidental acceleration forces on the

equipment.

— Assess the equipment’s capabilities to withstand the foreseen forces based on documented experience

and design documentation.

— Ultimate-limit-state analysis by using FEM software can be performed, to obtain more accurate

knowledge about the mechanical stresses.

— If the first steps indicate that the risk of failure is unacceptable, changes to the transport or installation

procedures or to the transformer design or assembly should be considered.

The following approach is suggested to investigate the mechanical stresses on transformer active part

resulting from vibrations due to wave motions:

— Investigate expected vibration conditions on the platform based on analyses performed during the

platform design or experience from similar offshore constructions in comparable wave conditions.

— Calculate the resulting forces and possible resonance in the transformer active part by means of

appropriate software tools.

— Assess the equipment’s capabilities to withstand the foreseen forces based on documented experience

and design documentation.

A.5 Execution of qualification plan and performance assessment (Step 5 and 6 of the TQ process)As the purpose of this case example is only to illustrate the qualification process, and no qualification has

been performed for a real system, the last two steps were not included. For a description of the two last

steps, see section 8 and 9, or DNV-RP-A203 /1/.


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APPENDIX B QUALIFICATION OF A MULTI-TERMINAL OFFSHORE

HVDC SYSTEM

Similarly as for the case example addressing a point-to-point HVDC link, the procedure for technology

qualification is demonstrated by its application to a multi-terminal offshore HVDC system. As the objective

of the example is only to demonstrate the technology qualification process, a complete qualification of the

entire system has not been performed. Instead, a general example system, described on a system level,

has been utilized as a basis for further assessment of selected subsystems in more detail. Further, since

general offshore aspects related to e.g. impact from met-ocean conditions have been addressed in the first

case example, the present case example is focused on challenges related to meshed multi-terminal

configurations. In this context, an HVDC grid is considered to be meshed if there is more than one

connection between at least one pair of converters in the grid.

It should be emphasized that multi-terminal offshore HVDC grids are not expected to be realized in a near

future. Further such grids will probably evolve from interconnection and integration of already existing

point-to-point links rather than construction of new meshed structures.

The case example covers the first four steps in the technology qualification procedure: the Qualification

Basis, the Technology Assessment, the Risk Assessment, and the first part of the Qualification Plan.

B.1 Qualification basis (Step 1 of the TQ process)The following section presents the qualification basis for the example case, including the technology

description and the requirements that the technology will be qualified against.

B.1.1 Technology descriptionThe fictitious four terminal example system layout selected as basis for evaluation is presented in Figure B-

1. It consists of four AC and DC buses to which four AC/DC converters are connected. The DC buses are

interconnected by DC cables, creating a meshed structure. The AC buses belong to one out of four different

grids (A-D), where A is intended to represent an onshore AC grid; B an offshore load like e.g. an aggregated

oil and gas production facility; and C and D offshore wind power plants. Further, the HVDC system is based

on VSC.

The selected example grid could possibly have developed from one original point-to-point connection

between buses A and B, intended to provide power from shore. At subsequent design of the offshore wind

power plants providing power at bus C and D, it was decided to create a meshed (ring) structure, increasing

redundancy of power transfer to/from shore without the need of an additional onshore converter station.

For the present illustration, offshore HVDC converter stations and submarine cables can, in principle, be

assumed to be similar to what has been discussed previously for the point-to-point case example. Details

regarding these components are thus not given here. However, in a complete technology qualification

process, the technology description should contain information on subsystems and components relevant for

the evaluation.

From the system operation perspective, single line diagrams and ratings, conditions at interfaces to

adjacent systems, standards, regulations and grid codes typically define the system and conditions for operation.

Thus, in the present example, focusing on system aspects of the HVDC grid, system boundaries are defined

as the AC buses on the high voltage side of converter transformers. At these boundaries, the HVDC system has to fulfil requirements set by grid codes and other regulations.

Examples of requirements on the HVDC system could be:

— level of support of reactive power,

— limitations on harmonic currents or voltages,

— ability to provide certain level of active power,

— ability to transfer power from one node to another, and

— system availability

— system losses.


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Due to the fact that possibilities to fulfil requirements will be strongly dependent on the overall system

availability, the technology description should contain a description of maintenance activities required to

obtain a sufficient reliability over time.

All standards relevant for the qualification of the technology should be gathered and studied to identify

standardized requirements and parts of the technology that falls outside the conventional standards. At

present there are only a few related to VSC technology available, but several others are in the process of

being developed. Selected standards and ongoing standardization activities are presented in App.C.

Figure B-1 Layout of multi-terminal HVDC example system

B.1.2 Requirements specificationThe requirement specification of an offshore multi-terminal HVDC system would contain a large set of

requirements, comprising requirements on functionality and performance, fulfilment of regulations, SHE

aspects etc.

On a power transmission system level, main requirements could typically be on functionality and

performance, e.g. ability to transfer power between specific buses with a specific reliability level, and

regulatory aspects such as grid codes. From these main requirements, a number of requirements necessary

for assessment of subsystems may be derived. A typical requirement is that the system fulfils an operating

criterion, such as the N-1 criterion, which implies that the failure of any single equipment should not result

in disconnection of load.

Thus, for the present example, focusing on electrical power system level, three main requirements are

identified for the HVDC system:

— The HVDC system should be able to provide sufficient power to the offshore loads in system B.

— The HVDC system should be able to transfer the produced power from the offshore wind power plants

in systems A and B.

— The HVDC system should fulfil grid codes specified by AC systems A-D.

Availability requirements associated with the above capabilities would typically also be different. It might,

for example, be considered to be more important to provide power to system B than to transfer power

between the other systems, which should be reflected in allowed number of outages and total down time.

B.2 Technology assessment (Step 2 of the TQ process)In order to identify novel elements of the technology, it should first be decomposed into manageable pieces,

allowing for assessment of novelty and uncertainties related to each of these. It is important to note that

technology components such as the master controller are also subsystems even though they may appear

to be overarching aspects of the system.

B.2.1 DecompositionConsidering the multi-terminal HVDC system in Figure B-1, this can be decomposed into four main sub-

systems: the converter stations, cables, overhead lines and control centre (which could be located in a

converter station). For the purpose of the present example, the converter station has been further


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decomposed into sub-systems, with special focus on the control and communication, see Figure B-2.

Implementation of a DC Fault Current Breaker has been treated separately and may include its own specific

DC components as well as hardware and software related to control and protection.

A more thorough decomposition of the converter station can be found in case 1.

Figure B-2 Example of decomposition of multi-terminal system. It should be noted that this decomposition only includes a part of all systems and components of a multi-terminal system

B.2.2 Categorisation To identify novel elements of the technology, the different sub-systems in Figure B-2 were categorized

according to novelty with respect to technology maturity and application area. For simplicity, in the present

case “application area” refers solely to the use in a multi-terminal HVDC grid. It should be emphasized that

the placement of components in the table are for illustration purposes only and might not reflect their actual

status. The resulting categorisation is shown in Table B-1.

Table B-1 Example of categorization of sub-systems in multi-terminal grid

Impact of multi-terminal

Technology maturity

Proven Limited experience New or unproven

Proven Cables AC componentsDC componentsIntra-station communication

Limited experience

Control centreVSC converterConverter control

New application

Inter-station communicationDC Fault Current BreakerEmergency controlPower flow control


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As indicated in Table B-1, cables, AC and DC components and intra-station communication are considered

to be Proven with respect to technology maturity. All these components are well known and have been

widely used in power transmission applications for decades. The same types of components have also been

used in several VSC HVDC systems with a good track record. Moreover, the same components are not

foreseen to experience any major impacts from application in a multi-terminal grid, and thus they are

categorised as Proven in this respect.

The technology maturity of VSC converters depends on, amongst others, voltage and power ratings, design,

and control strategies. In this example, technology maturity of converters have been categorized as Limited

experience since the converter ratings are above any previously installed VSC ratings. Furthermore, it can

also be argued that there is limited service experience if converters are based on multi-level VSC converter

technology. On the categorization dimension defined by the impact of applying the technology in a multi-

terminal system, VSC converters are considered having Limited experience, since: on one hand it is not

foreseen much difference in operational impact compared to a point-to-point system, while on the other

hand, control and protection aspects may place new requirements on the converter compared to a point-

to-point system. For the present example, focusing on system aspects of a multi-terminal application,

components of communication and control systems are categorized as being of different maturity levels.

Control strategies for emergency and power flow are considered as New with respect to technology maturity

as well as application area due to the fact that an HVDC grid is a new application, and hence there are no

conventional solutions that can be directly applied. Based on service experience from previous HVDC

applications, communication systems are considered to be Proven technology, experiencing different level

of impact from the use in a multi-terminal grid. Inter-station communication, assigned a high level of

uncertainty (New application), and is for example believed to be more critical when applied in a multi-

terminal HVDC grid compared to a point-to-point link. Such communication is also closely connected to

foreseen challenges related to interoperability of equipment from different vendors increasing the

uncertainty.

In this case example, the inter-station communication and power flow control, categorized as new

technologies, are selected and taken to the next step of the TQ process.

B.3 Threat assessment (Step 3 of the TQ process)A simplified threat assessment was performed for the two systems selected in the previous section, the

inter-station communication and the power flow control.

As in the first case example, the probability and consequence tables given in Sec.6 were used for the threat

assessment. As previously stated, however, these are not valid on a general basis and should be customised

for every project.

The main results from the threat assessment are presented in Table B-2.


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In Table B-2, failures resulting in no/very low consequences have been omitted. This could be situations

where e.g. redundant systems fully take over the functionality of the faulted component without any

interruptions, or when redundant systems are out of operation but necessary information exchange can be

handled via manual operations.

It should be noted that probabilities presented in Table B-2 are preliminary assessments which are based

on assumptions. By further break down of failure mechanisms and associated causes, and performing a full

risk analysis, improved estimates may be obtained. As Table B-2 illustrates, combinations of multiple

Table B-2 Example of threat assessment of the multi-terminal system

Notes:1 Failing communication from one converter station to the others/control centre, with redundant systems out of operation, is assumed to lead to an automatic reduction in power transfer capacity of the affected station. Other stations are without impact and can remain in normal operation 2 Overloading requires that backup system and over-current protection are out of operation.3 Failure in determination of correct power flow, with redundant systems out of operation, results in insufficient power transfer the station, including tripping.

Cons. Prob. Risk

1 Communication

1.1 Inter station

communication

Transfer information

between computer

systems in converter

stations

Meshed HVDC grid,

reliability and

redundancy

Automatic

communication

not possible

Broken fibre or wire Reduced capacity of station A 1 4 2 Med

1.2 Reduced capacity of station B 1 3 2 Med

1.2 Reduced capacity of station C or D 1 2 2 Low

1.3 Failing electro-optical

converter

Reduced capacity of station A 1 4 2 Med



1.5 HF filters and coupler failure Reduced capacity of station A 1 4 2 Med



1.8 A/D -D/A converter and

amplifier failureReduced capacity of station A 1 4 2 Med



1.11 Radio link failure Reduced capacity of station A 1 4 2 Med



1.14 Power supply failure Reduced capacity of station A 1 4 2 Med



1.17 Computer and I/O failure Reduced capacity of station A 1 4 2 Med



1.20 Software failure Reduced capacity of station A 1 4 2 Med



2 Control

2.1 Power flow control Allow for coordinated

control of power flow

in the HVDC grid

Meshed HVDC grid,

control

Non-optimised

power flow

Inter-station communication

faluire

Grid codes not fulfilled 2 4 Med

2.2 Over-loadning of cables2 5 1 Med

2.3 Insufficent power transfer to B3 5 3 High

2.4 Insufficent power transfer from C&D3 2 3 Med

2.5 Computer failure Grid codes not fulfilled 2 4 Med




2.9 Software failure Grid codes not fulfilled 2 4 Med




2.13 Lack of input data (from

measurements or regulations)

Grid codes not fulfilled 2 4 Med




2.17 Human factor Grid codes not fulfilled 2 4 Med



Insufficent power transfer from C&D3 2 3 Med

Consequence

Risk Ranking

ID Component Function New aspectFailure mode /

Risk

Failure mechanism

or cause


DNV GL AS

failures or abnormal conditions may lead to severe consequences. Hence, methodologies suitable for

analysis of failure combinations, such as fault and event tree analyses, should be applied as part of the

qualification activities to investigate the selected failure modes. The use of experts and expert opinions is

a vital part of such methodology.

B.4 Qualification plan (Step 4 of the TQ process)The objective of this step is to select and prepare a structured plan for execution of qualification activities

to adequately address failure modes and uncertainties classified as potential threats in the preceding step.

As a first action, an overall qualification strategy should be established, specifying what evidence is required

to meet the objective of the qualification and outlining how the evidence should be provided. Identified

threats should be prioritized according to their associated risk, allowing for first addressing threats having

the greatest expected impact.

For the present case example, qualification activities addressing threats towards the two sub-systems inter-

station communication and power flow control have been chosen for further investigation.

According to the threat assessment in Table B-2, the greatest risks associated with both inter-station communication failure and power flow control failure would be lack of power at bus B, directly affecting main

qualification requirements on system capacity and availability. For the communication such consequence

would result from a situation where the primary system fails, at the same time as redundant systems are not in operation and there is no defined or implemented solution for handling of communication loss. For the power flow control system, a failure has to occur when redundant systems are out of operation.

According the TQ procedure, threats associated with a “high risk” should first be eliminated, and thereafter threats characterised as having “medium” risk should be addressed.

Consequently, as a first step, it is decided to prioritize qualification activities in detail assessing probabilities

of failure mechanisms/causes listed in Table B-2 which are relevant for power availability at station B. The aim of these activities should be to derive refined estimates of probability for each mechanism, allowing for

determination of the total availability of the respective sub-systems. The outcome should allow for

identification of need for improvement of e.g. specific hardware solutions, reducing the associated risk to “medium” or “low”.

A list of possible activities addressing failure probabilities of primary causes is given below:

— Expert assessment of available documentation describing: system design including component design

and installation; solutions for implementation of redundancy; maintenance requirements and policies;

and spare policies.

— Collection and expert assessment of service experience from installations of similar systems in similar

environments. Information could be expected to be found from documentation of existing AC and HVDC

installations, oil and gas industry, IT and telecom, etc.

— Verification through testing. In order to verify the level of redundancy, models of the communication

system and power flow control should be developed and used for testing of overall system performance

when one or several sub-systems fail. In order to make the activity cost effective, the test system would

probably be implemented as a combination of software simulation and hardware. Test procedures

should be carefully planned, allowing for catching situations where total system performance is relying

on redundant subsystems.

In order to asses if the HVDC system can meet the stated requirements on power transfer capacity at all times, power systems studies should be performed. The analysis should include limitations and

requirements from interfacing AC systems, which may influence the transfer capacity of the HVDC system.

Such studies should involve contingency analysis, to identify a list of critical contingencies which will define the transfer capacity of the HVDC system at different operating conditions.

B.5 Execution of qualification plan and performance assessment (Step 5 and 6 of the TQ process)As the purpose of this case example is only to illustrate the qualification process, and no qualification has

been performed for a real system, the last two steps were not included. For a description of the two last

steps, see section 8 and 9, or DNV-RP-A203 /1/.


DNV GL AS

APPENDIX C STANDARDIZATION WORK AND OTHER INITIATIVES

There is a number of on-going and finalized standardization work and initiatives in the industry which is

important for the use of the RP. Below is a list of a few selected activities as of October 2013.

Cigré

— SC B4 - HVDC and Power Electronics

— B4-52, B4-55, B4-56, B4-57, B4-58, B4-59, B4-60

— SC B1 - Insulated Cables

— B1.27, B1.32, B1-34, B1-35, B1.38, B1.40, B1.43.

EC DG Energy

— Working group for offshore/onshore grid development.

NSCOGI

— WG 1 Offshore Transmission Technology.

ENTSO-E

— Regional Group North Sea (RG NS).

IEC/CENLEC

— TC 115 High Voltage Direct Current (HVDC) transmission for DC voltages above 100 kV

— CLC/SR 115 High Voltage Direct Current (HVDC) Transmission for DC voltages above 100 kV

(Provisional)

— IEC 62747 – Terminology for voltage sourced converters (VSC) for HVDC systems

— IEC 62751-2 - Determination of power losses in high-voltage direct current (HVDC) converter stations.

German commission for electrical, electronic & information technologies

— Technical guidelines for first HVDC grids - A European study group.

DNV GL

— DNV-OS-J201 Offshore Substations for Wind Farms.

Recom

mended p

ractic

e –

DN

VG

L-R

P-0

046:2

014-0

8 P

age 4

6

DN

V G

L A

S

Identified relevant standards for offshore HVDC /3/

Figure C-1 Selected standards for offshore HVDC identified in the report “Assessment of Standards for Offshore Grids and Statnett's Future Role” /3/

Standard Topic Source Component Application Area Voltage

CIGRE 086 1994 Overvoltages on HVDC cables. Final Report Cigre Cables Onshore/Offshore Electrical 400kV

CIGRE 097 1995 System tests for HVDC installations Cigre General Onshore/Offshore Testing/Commisioning N/A

CIGRE 215 2002 HVDC converter stations for voltages above +/- 600 kV Cigre Converter Onshore/Offshore Electrical <800kV

CIGRE 370 2009 Integration of large Scale Wind Generation using HVDC and Power Electronics Cigre General Onshore/Offshore Electrical N/A

CIGRE WG B4.52 HVDC Grid Feasibi l ity Study Cigre General Onshore/Offshore Feasibil ity N/A

DNV-OS J201 Offshore substations for wind farms DNV General Offshore Safety <15kV

DNV-OS-D201 Electrical installations, offshore standard DNV General Offshore Electrical <15kV

DNV-RP-B401 DNV Recommended Practice, Cathodic Protection Design DNV Surface protection Offshore Corrosion Protection NA

EN 50336 Bushings for transformers and reactor cable boxes not exceeding 36 kV Cenelec Bushings Onshore/Offshore Electrical <36kV

EN 50522:2010 Earthing of power installations exceeding 1 kV a.c. Cenelec General Onshore Electrical >1kV

EN 61936-1:2010 Power instal lations exceeding 1 kV a.c. - Part 1: Common rules ) superseds HD637 S1 Cenelec Earthing, bonding Onshore Electrical >1kV

EN 61936-1:2010 Power instal lations exceeding 1 kV a.c. - Part 1: Common rules ) superseds HD637 S1 Cenelec General Onshore Electrical >1kV

FSS Code International Code for Fire Safety Systems IMO Fire protection Offshore Fire NA

IEC 60071-1 Insulation co-ordination – Definitions, principles and rules IEC Switchgear Onshore/Offshore Electrical <800kV

IEC 60071-2 Insulation co-ordination – Application guide IEC Switchgear Onshore/Offshore Electrical <800kV

IEC 60076 Power Transformers IEC Transformer Onshore/Offshore Electrical N/A

IEC 60168:

Tests on Indoor and Outdoor Post Insulators of Ceramic Material or Glass for Systems with

Nominal Voltages Greater Than 1 000 V IEC Bushings Onshore/Offshore Testing/Commisioning >1kV

IEC 60633 Terminology for high-voltage direct current (HVDC) transmission IEC General Onshore/Offshore Terminology >100kV

IEC 61378-2 Converter transformers - Part 2: Transformers for HVDC applications IEC Transformer Onshore/Offshore Electrical N/A

IEC 61378-3 Converter transformers - Part 3: Application guide IEC Transformer Onshore/Offshore Electrical N/A

IEC 61803 Determination of power losses in high-voltage direct current (HVDC) converter stations IEC Converter Onshore/Offshore Losses <800kV

IEC 61892 Mobile and fixed offshore units – Electrical instal lations IEC General Offshore Electrical <35kV

IEC 61975 High-voltage direct current (HVDC) instal lations - System tests IEC General Onshore/Offshore Testing/Commisioning >100kV

IEC 62001

High-voltage direct current (HVDC) systems - Guidebook to the specification and design

evaluation of A.C. fi lters IEC Reactive component Onshore/Offshore Electrical <800kV

IEC 62199 Bushings for D.C. application IEC Bushings Onshore/Offshore Electrical >1kV

IEC 62271 High-voltage switchgear and controlgear IEC Switchgear Onshore/Offshore Electrical <245kV

IEC 62501

Voltage sourced converter (VSC) valves for high-voltage direct current (HVDC) power

transmission - Electrical testing IEC Converter Onshore/Offshore Testing/Commisioning <800kV

IEC 62544 Active fi lters in HVDC applications IEC Reactive component Onshore/Offshore Electrical <800kV

IEC/TR 62543

High-voltage direct current (HVDC) power transmission using voltage sourced converters

(VSC) IEC General Onshore/Offshore Electrical <800kV

IEEE Std 525-2007 IEEE Guide for the Design and Instal lation of Cable Systems in Substations IEEE Cables Onshore/Offshore Electrical <35

IEEE Std.1240-2000 IEEE Guide for the Evaluation of the Reliabil ity of HVDC Converter Stations IEEE Converter Onshore/Offshore Reliability N/A

IEEE Std.C37.122-2010 IEEE Standard for High Voltage Gas-Insulated Substations Rated Above 52 kV IEEE General Onshore/Offshore Electrical >52kV

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