Technical Guidelines for Radial HVDC Networks

This Technical Report applies to HVDC Systems having more than two converter stations connected to a common DC network, also referred to as HVDC Grid Systems. Serving the near term applications, this report describes radial HVDC network structures as well as pure VSC based solutions. Both grounded and ungrounded DC circuits are considered. Based on typical requirements applied to state of the art HVDC converter stations today this report addresses aspects that are specifically related to the design and operation of converter stations and DC circuits in HVDC Grid Systems. The requirements from the AC systems as known today are included. Secondary effects associated with changing the AC systems, e.g. the replacement of rotating machines by power electronic devices, are not within the scope of the present report. The report summarizes applications and concepts of HVDC Grid Systems with the purpose of preparing the ground for standardization of such systems. The interface requirements and functional specifications given in this document are intended to support the specification and purchase of multi-vendor multiterminal HVDC Grid Systems.

Technischer Leitfaden für radiale HGÜ-Netze

Directives techniques pour les réseaux HVDC radiaux

Tehnične smernice za enosmerna, radialno napajana VN omrežja

General Information

Status
Published
Publication Date
11-Mar-2014
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
10-Mar-2014
Due Date
15-May-2014
Completion Date
12-Mar-2014

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SLOVENSKI STANDARD
SIST-TP CLC/TR 50609:2014
01-april-2014
7HKQLþQHVPHUQLFH]DHQRVPHUQDUDGLDOQRQDSDMDQD91RPUHåMD
Technical Guidelines for Radial HVDC Networks
Ta slovenski standard je istoveten z: CLC/TR 50609:2014
ICS:
29.240.01 2PUHåMD]DSUHQRVLQ Power transmission and
GLVWULEXFLMRHOHNWULþQHHQHUJLMH distribution networks in
QDVSORãQR general
SIST-TP CLC/TR 50609:2014 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CLC/TR 50609:2014
TECHNICAL REPORT
CLC/TR 50609
RAPPORT TECHNIQUE
February 2014
TECHNISCHER BERICHT
ICS 29.240.01
English version
Technical Guidelines for Radial HVDC Networks
Directives techniques pour les réseaux Technischer Leitfaden für radiale HGÜ-
HVDC radiaux Netze
This Technical Report was approved by CENELEC on 2013-12-09.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,

the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany,

Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland,

Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2014 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. CLC/TR 50609:2014 E
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Contents Page

Foreword ...................................................................................................................................................... 7

0 Introduction .................................................................................................................................. 8

0.1 The European HVDC Grid Study Group ....................................................................................... 8

0.2 Technology ..................................................................................................................................... 9

0.2.1 Converters .............................................................................................................................. 9

0.2.2 DC Circuit ............................................................................................................................... 9

0.2.3 Technological Focus of the European HVDC Grid Study Group .................................... 10

1 Scope ........................................................................................................................................... 12

2 Terminology and abbreviations ................................................................................................ 12

2.1 General .......................................................................................................................................... 12

2.2 Terminology and abbreviations for HVDC Grid Systems used in this report ........................ 12

2.3 Proposed Terminology by the Study Group .............................................................................. 13

3 Typical Applications of HVDC Grids ........................................................................................ 14

3.1 The Development of HVDC Grid Systems.................................................................................. 14

3.2 Planning Criteria for Topologies ................................................................................................. 15

3.2.1 General .................................................................................................................................. 15

3.2.2 Power Transfer Requirements ............................................................................................ 16

3.2.3 Reliability .............................................................................................................................. 17

3.2.4 Losses ................................................................................................................................... 19

3.2.5 Future Expansions............................................................................................................... 21

3.3 Technical Requirements .............................................................................................................. 21

3.3.1 General .................................................................................................................................. 21

3.3.2 Converter Functionality ...................................................................................................... 22

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3.3.3 Start/stop Behaviour of Individual Converter Stations .................................................... 23

3.3.4 Network Behaviour during Faults ...................................................................................... 24

3.3.5 DC-AC Interface Requirements .......................................................................................... 25

3.3.6 The Role of Communication ............................................................................................... 26

3.4 Typical Applications – Relevant Topologies ............................................................................. 27

3.4.1 General .................................................................................................................................. 27

3.4.2 Radial Topology ................................................................................................................... 27

3.4.3 Meshed Topology ................................................................................................................ 29

3.4.4 HVDC Grid Systems Connecting Offshore Wind Power Plants ...................................... 29

3.4.5 Connection of a wind power plant to an existing HVDC VSC link .................................. 30

4 Principles of DC Load Flow....................................................................................................... 31

4.1 General .......................................................................................................................................... 31

4.2 Structure of Load Flow Controls ................................................................................................ 31

4.2.1 General .................................................................................................................................. 31

4.2.2 Converter Station Controller .............................................................................................. 31

4.2.3 HVDC Grid Controller .......................................................................................................... 32

4.3 Converter Station Control Functions ......................................................................................... 34

4.3.1 General .................................................................................................................................. 34

4.3.2 DC Voltage (U ) Stations ................................................................................................... 34

4.3.3 Active Power (P ) and Frequency (f) Controlling Stations ............................................ 34

4.4 Paralleling Transmission Systems ............................................................................................. 35

4.4.1 General .................................................................................................................................. 35

4.4.2 Paralleling on AC and DC side ........................................................................................... 35

4.4.3 Paralleling on the AC side .................................................................................................. 35

4.4.4 Steady-State Loadflow in Hybrid AC/DC Networks .......................................................... 36

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4.5 Load Flow Control ........................................................................................................................ 38

4.5.1 DC Voltage Operating Range .............................................................................................. 38

4.5.2 Static and Dynamic System Stability ................................................................................. 39

4.5.3 Step response ...................................................................................................................... 39

4.6 HVDC Grid Control Concepts ...................................................................................................... 40

4.6.1 General .................................................................................................................................. 40

4.6.2 Voltage-Power Droop Together with Dead Band .............................................................. 45

4.6.3 Voltage-Current Droop ........................................................................................................ 48

4.6.4 Voltage-Power Droop — Control of the HVDC Grid Voltage ........................................... 54

4.7 Benchmark Simulations of Control Concepts ........................................................................... 57

4.7.1 Case Study ........................................................................................................................... 57

4.7.2 Results .................................................................................................................................. 58

4.7.3 Conclusions ......................................................................................................................... 60

4.7.4 Interoperability ..................................................................................................................... 61

5 Short-Circuit Currents and Earthing ........................................................................................ 61

5.1 General .......................................................................................................................................... 61

5.2 Calculation of Short-Circuit Currents in HVDC Grid Systems ................................................. 61

5.3 Network Topologies and their Influence on Short-Circuit Currents ....................................... 63

5.3.1 Influence of DC Network Structure .................................................................................... 63

5.3.2 Influence of Line Discharge ................................................................................................ 66

5.3.3 Influence of Capacitors ....................................................................................................... 67

5.3.4 Contribution of Converter Stations.................................................................................... 69

5.3.5 Methods of Earthing ............................................................................................................ 72

5.4 Secondary Conditions for Calculating the Maximum/Minimum Short-Circuit Current ......... 73

5.5 Calculation of the Total Short-Circuit Current (Super Position Method) ................................ 74

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5.6 Reduction of Short-Circuit Currents .......................................................................................... 75

6 Principles of HVDC Grid Protection ......................................................................................... 76

6.1 General .......................................................................................................................................... 76

6.2 HVDC Grid System ....................................................................................................................... 77

6.3 AC/DC Converter .......................................................................................................................... 78

6.3.1 General .................................................................................................................................. 78

6.3.2 DC System ............................................................................................................................ 79

6.3.3 HVDC Switchyard................................................................................................................. 80

6.3.4 HVDC System without Fast Dynamic Isolation ................................................................. 80

6.3.5 HVDC System with Fast Dynamic Isolation ...................................................................... 80

6.4 DC Protection ................................................................................................................................ 81

6.4.1 General .................................................................................................................................. 81

6.4.2 DC Converter Protections ................................................................................................... 81

6.4.3 Protective Shut Down of a Converter ................................................................................ 83

6.4.4 DC System Protections ....................................................................................................... 84

6.4.5 DC Equipment Protections ................................................................................................. 84

6.5 Clearance of Earth Faults ............................................................................................................ 84

6.5.1 Clearance of a DC Pole-to-Earth Fault ............................................................................... 84

6.5.2 Clearance of a Pole-to Pole Short Circuit ......................................................................... 85

6.5.3 Clearance of a Converter side AC Phase-to-Earth Fault ................................................. 85

7 Functional Specifications .......................................................................................................... 85

7.1 General .......................................................................................................................................... 85

7.2 AC/DC Converter Stations ........................................................................................................... 86

7.2.1 DC System Characteristics ................................................................................................. 86

7.2.2 Operational Modes............................................................................................................... 89

7.2.3 Testing and Commissioning ............................................................................................... 95

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7.3 HVDC breaker ............................................................................................................................... 96

7.3.1 System Requirements ......................................................................................................... 96

7.3.2 System Functions ................................................................................................................ 96

7.3.3 Interfaces and Overall Architecture ................................................................................... 96

7.3.4 Service Requirements ......................................................................................................... 96

7.3.5 Technical System Requirements ....................................................................................... 96

Annex A (informative) HVDC – Grid Control Study ................................................................................ 99

Annex B (informative) Fault Behaviour of Full Bridge Type MMC ...................................................... 106

B.1 Introduction ............................................................................................................................ 106

B.2 Test Results ............................................................................................................................ 106

B.3 DC to DC Terminal Faults ...................................................................................................... 106

B.4 DC Terminal to Ground Faults .............................................................................................. 107

B.5 Conclusion .............................................................................................................................. 107

Bibliography ............................................................................................................................................. 119

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Foreword

This document (CLC/TR 50609:2014) has been prepared by CLC/TC 8X “System aspects of electrical

energy supply”.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent

rights.

This document has been prepared under a mandate given to CENELEC by the European Commission

and the European Free Trade Association.

This document was already sent out within CLC/TC 8X for comments and the comments received were

discussed within CLC/TC 8X/WG 06 and were incorporated in the current document as far as appropriate.

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0 Introduction
0.1 The European HVDC Grid Study Group

Existing power systems in Europe have been developing for more than 100 years to transmit the power,

generated mainly by fossil and nuclear power plants to the loads. Climate change, limited fossil resources

and concerns over the security of nuclear power are drivers for an increased utilization of renewable

sources, such as wind and solar, to realize a sustainable energy supply. According to the geological

conditions, the location of large scale renewable energy sources is different to the location of existing

conventional power plants and imposes new challenges for the electric power transmission networks,

such as extended transmission capacity requirements over long distances, load flow control and system

stability. The excellent bulk power long distance transmission capabilities, low transmission losses and

precise power flow control make High Voltage Direct Current (HVDC) the key transmission technology for

mastering these challenges, in particular for connection of offshore wind power plants to the onshore

transmission systems.

While the power system reinforcement is already underway by a number of new point-to-point HVDC

interconnections, the advantages offered by multiterminal HVDC systems and HVDC grids become more

and more attractive. Examples are grid access projects connecting various wind power plants or

combining wind plants with point-to-point transmission, e.g. in the North and Baltic Seas. Multiterminal

projects are already in execution and there is planning for pan-European HVDC grids. In this document,

multiterminal HVDC systems and HVDC grids are referred to as HVDC Grid Systems.

To become reality, HVDC Grid Systems need, in addition to the necessary political framework for cross

country system design, construction and operation, competitive supply chains of equipment capable of

operating together as an integrated system. This marks a significant change in the HVDC technology

market. While today - with very few exceptions – a HVDC transmission system has been provided by a

single manufacturer, future HVDC Grid Systems will be built step by step composed of converters and

HVDC substations supplied by different manufacturers. Interoperability will thus become a fundamental

requirement for future HVDC technology.

Common understanding of basic operating and design principles of HVDC Grid Systems is seen as a first

step towards multi vendor systems, as it will help the development for the next round of European

multiterminal projects. Furthermore, it will prepare the ground for more detailed standardization work.

Based on an initiative by the DKE German Commission for Electrical, Electronic and Information

Technologies, the European HVDC Grid Study Group has been founded in September 2010 to develop

“Technical Guidelines for first HVDC Grids”. The Study Group has the following objectives:

• to describe basic principles of HVDC grids with the focus on near term applications;

• to develop functional specifications of the main equipment and HVDC grid controllers;

• to develop “New Work Item Proposals” to be offered to CENELEC for starting standardization work.

CIGRÉ SC B4, CENELEC TC8x and ENTSO-E and “Friends of the Supergrid” are involved at an

informative level with the results of the work.

Members affiliated with the following companies and organizations have been actively contributing to the

results of the Study Group achieved so far: 50 Hz Transmission, ABB, ALSTOM, Amprion, DKE,

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TransnetBW, Energinet.dk, ETH Zurich, National Grid, Nexans, Prysmian, SEK, Siemens, TenneT and TU

Darmstadt.

As a starting point the Study Group has been investigating typical applications and performance

requirements of HVDC Grid Systems. This information helps elaborating the basic principles of HVDC

networks, which are described in the following clauses:
• Clause 3, Typical Applications of HVDC Grids;
• Clause 4, Principles of DC Load Flow;
• Clause 5, Short-Circuit Currents and Earthing;
• Clause 6, Principles of HVDC Grid Protection.

From the technical principles described, functional specifications for the main equipment of HVDC

networks are derived and summarized in Clause 7.
0.2 Technology
0.2.1 Converters

HVDC transmission started more than 60 years ago. Today, the installed HVDC transmission capacity

exceeds 200 GW worldwide. The vast majority of the existing HVDC links are based on so-called Line-

Commutated-Converters (LCC). LCC today are built from Thyristors. The power exchange of such

converters is determined by controlling the point–on-wave of valve turn-on, while the turn-off occurs due to

the natural zero crossing of valve current forced by the AC network voltage. That is why LCC rely on

relatively strong AC systems to provide conversion from AC to DC and vice versa.

With so-called Voltage Sourced Converters (VSC), a different type of converters has been introduced to

HVDC transmission slightly more than a decade ago. VSCs today utilize Insulated Gate Bipolar

Transistors (IGBT) as the main switching elements. IGBTs have controlled turn-on as well as turn-off

capability making the VSCs capable of operating under weak AC system conditions or supplying power

systems where there is no other voltage source, also referred to as passive networks.

The evolution of VSC transmission was started with so-called Two-Level converters at the end of the

1990s and has commenced to Three Level Converters and further to Modular Multilevel Converters

(MMC) which have made their break-through in the mid to late 2000s. All MMC type converters apply the

same principle of connecting a number of identical converter building blocks in series. However, at the

present time there are basically two types of such building blocks: referred to as Half-Bridge (HB) and Full-

Bridge (FB) modules.

Other converter equipment which have been proposed for HVDC Grid applications, such as DC/DC

converters, load flow controllers, etc. are not discussed in this document.
0.2.2 DC Circuit

Similar to AC networks, HVDC transmission systems can be distinguished by their network topologies as

radial and/or meshed networks and with respect to earthing in effectively grounded and isolated systems.

Both aspects influence the design criteria and the behaviour of the HVDC system.
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− Radial and Meshed Topologies:

In radial systems, there is not more than one connection between two arbitrary nodes of the network. The

DC voltages of the converter stations connected to each end of a line solely determine the power flow

through that line, for example in Figure 1-1, station C is radially connected with station D.

In meshed systems, at least two converter stations have more than one connecting path. Without any

additional measures the current through a line will then be determined by the DC voltages of the converter

stations as well as the resistances of the parallel connections. In Figure 1-1, the DC circuit connecting

stations A, B and C forms a meshed system while C and D is a radial connection. A HVDC Grid System

having a meshed topology can be operated as a radial system if parallel connections are opened by

disconnectors or breakers.
C D

Figure 1-1 — Example of an HVDC Grid System having a meshed and radial structure

− Earthing:

DC circuits can be effectively grounded if one DC pole is connected to earth through a low ohmic branch.

Such systems are also referred to as asymmetrical Monopoles or just “Monopoles”. Two Monopoles of

opposite DC voltage polarity are often combined into so-called bipolar systems or just “Bipoles”.

Isolated DC circuits do not have a low ohmic connection to ground on the DC side. These configurations

are also referred to as “Symmetrical Monopoles”.
0.2.3 Technological Focus of the European HVDC Grid Study Group

Various technologies are available for building HVDC Grid Systems. Some of them have already been

used in commercial projects; others are in the demonstration phase or are in an early stage of discussion.

This applies to the converter technology as well as the topologies of connecting them into a HVDC Grid

System.
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Serving the near term applications, the Study Group decided to focus its scope of work on radial HVDC

network structures as well as pure VSC based solutions. Both grounded and ungrounded DC circuits are

considered.

The integration of HVDC Grid Systems is seen as an important part of developing future electric power

systems. The Study Group bases its work on typical requirements applied to state of the art HVDC

converter stations today and investigates aspects that are specifically related to the design and operation

of converter stations and DC circuits. The requirements from the AC systems as known today are

included. Secondary effects associated with changing the AC systems, e.g. the replacement of rotating

machines by power electronic devices, are not within the scope of the Study Group.

The Study Group report summarizes the selected results of work and gives recommendations for the next

steps towards preparing the ground for standardization of HVDC multiterminal systems and HVDC Grid

Systems.

The interface requirements and functional specifications given in this document are intended to support

the specification and purchase of multi vendor multiterminal HVDC Grid Systems.
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1 Scope

This Technical Report applies to HVDC Systems having more than two converter stations connected to a

common DC network, also referred to as HVDC Grid Systems. Serving the near term applications, this

report describes radial HVDC network structures as well as pure VSC based solutions. Both grounded and

ungrounded DC circuits are considered.

Based on typical requirements applied to state of the art HVDC converter stations today this report

addresses aspects that are specifically related to the design and operation of converter stations and DC

circuits in HVDC Grid Systems. The requirements from the AC systems as known today are included.

Secondary effects associated with changing the AC systems, e.g. the replacement of rotating machines by

power electronic devices, are not within the scope of the present report.

The report summarizes applications and concepts of HVDC Grid Systems with the purpose of preparing

the ground for standardization of such systems.

The interface requirements and functional specifications given in this document are intended to support

the specification and purchase of multi-vendor multiterminal HVDC Grid Systems.
2 Terminology and abbreviations
2.1 General

In the work undertaken here it has been identified that a common list of terminology and abbreviations

used specifically to describe HVDC Grids should be established.

The International Electrotechnical Commission (IEC) standard EN 60633 [1] describes G

...

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