SIST-TP CLC/TR 50609:2014

SLOVENSKI STANDARD
01-april-2014
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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
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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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
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

3 CLC/TR 50609:2014
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
DC
4.3.3 Active Power (P ) and Frequency (f) Controlling Stations . 34
DC
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

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

5 CLC/TR 50609:2014
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

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

7 CLC/TR 50609:2014
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.

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,

9 CLC/TR 50609:2014
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.

− 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.
A
C D
B
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.
11 CLC/TR 50609:2014
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.

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 General
Terminology for HVDC Transmission and is the reference for common terms and abbreviations.
Furthermore EN 62501 [2] describes electrical testing of VSC. A new proposed work item in IEC is to
develop terminology for VSC HVDC systems. In addition CIGRÉ has published a brochure 269 on basic
operational principles of VSC-HVDC [3]. Specific terms required for components and methods for
multiterminal HVDC Transmission, e.g. HVDC breakers and control modes, are described here.
The Study Group has established information exchange established with the ongoing CIGRÉ B4.52
feasibility study on DC Grids. Also in the CIGRÉ work, new terms used to describe phenomena and
components in DC grids are used. The terminology used in this report corresponds to the terminology
used in the CIGRÉ working group.
2.2 Terminology and abbreviations for HVDC Grid Systems used in this report
AC Alternating Current
DC Direct Current
ENTSO-E European Network of Transmission System Operators for Electricity
FB Full Bridge
GW Giga Watts
13 CLC/TR 50609:2014
HB Half Bridge
HSS High Speed Switch
HVAC High Voltage Alternating Current
HVDC High Voltage Direct Current
IGBT Insulated Gate Bipolar Transistor
LCC Line Commutated Converter
MMC Modular Multilevel Converter
OHL Overhead transmission line
PCC AC Point of common coupling (ac side)
PCC DC Point of common coupling (dc side)
TRV Transient Recovery Voltage
TSO Transmission System Operator
VCD• Voltage-current droop
VPD• Voltage-power droop
VPDDB• Voltage-power droop together with dead band
VSC Voltage Sourced Converter
2.3 Proposed Terminology by the Study Group
− Converter Station Controller A controller determining the voltages and currents at DC and AC
terminals of its converter station.
4.2.2, Converter Station Controller
− Dynamic Braking Device A controllable branch absorbing energy from the DC circuit.
7.2.2.2, Energy absorption capability (Dynamic Braking Device)
− HVDC Grid Controller A high level controller linked to each individual Converter Station
Controller
4.2.3, HVDC Grid Controller
3 Typical Applications of HVDC Grids
3.1 The Development of HVDC Grid Systems
Today power transmission systems are largely based on AC. AC is beneficial in most applications for a
number of reasons:
• easy to generate from mechanical energy in rotating machines or to convert back;
• easy to transform to higher voltage levels reducing power transmission losses;
• current zero crossings allow simple principles for load current switching and fault clearing.
However, there are applications where AC cannot be used or where DC technology is more economical
than AC. Examples are:
• connecting asynchronous AC systems;
• long distance Overhead Line (OHL) or cable transmission;
• power flow control in large integrated AC systems (improving system stability).
Since its introduction in the middle of the 20th century, High Voltage Direct Current (HVDC) transmission
has become a fundamental part of many electric power transmission systems worldwide. With a few
exceptions, HVDC transmission systems are built so far as point-to-point links.
The requirements for point-to-point links have been specified by the Transmission System Operators
(TSOs) involved according to the functionality needed as well as the respective AC network operating
conditions. When HVDC technology is considered for multi-terminal systems or HVDC grids, more TSOs
are involved with system planning and operation. At the same time the functional requirements and variety
of AC system conditions to be covered increases. This requires establishing efficient coordination of
design, operation, control and regulation between the connected AC and DC systems. Agreement
contracts on physical and regulatory boundaries, responsibilities and utilization of the AC/DC converter
stations as well as harmonization of technical regulations for the AC and DC systems are necessary pre-
requisites for the cooperation between the involved operators.
Multi-terminal Systems and HVDC Grids will further on be referred to in this report as “HVDC Grid
Systems”.
Similar to the evolution of AC transmission systems, HVDC Grid Systems should have the ability to grow
according to demand. It should be possible to interconnect several smaller HVDC systems into larger
integrated systems.
Some of the future HVDC Grid Systems may be financed and operated by independent HVDC Grid
System companies. To ensure the required level of interoperability, the system design and operation
requirements for the individual HVDC systems need to be harmonized in corresponding Grid Codes.
The development of HVDC Grid Systems is likely to start in small scale radial configurations, like with DC
connections between existing AC systems. The Kriegers Flak Combined Grid Solution (CGS) project in
the Baltic Sea may be amongst the first pilot projects of such future HVDC Grid Systems. If commissioned

15 CLC/TR 50609:2014
as the CGS, the offshore based HVDC Grid System will connect the expected 600 MW wind power at the
Danish area of Kriegers Flak and approx. 400 MW at the Baltic 1 and Baltic 2 German wind power plants
to the onshore AC transmission systems of Denmark and Germany and provide a connection between the
AC systems of these two countries. The commissioning period of the HVDC-based CGS is expected
between 2018 and 2020. The Kriegers Flak CGS shall be specified, commissioned and operated by the
Danish TSO Energinet.dk and the German TSO 50 Hz Transmission GmbH. The Kriegers Flak CGS
project has been granted European Union funding from the European Energy Programme for Recovery
(EEPR).
The present clause describes important system operation and design criteria for HVDC Grid Systems that
are intended to become integral part of the European electric power transmission systems. This work
gives guidance to the elaboration of the functional requirements for HVDC grid components and systems
within the European HVDC Grid Study Group. It intends also provide a contribution to the development of
a common Grid Code for European HVDC Grid Systems.
3.2 Planning Criteria for Topologies
3.2.1 General
The starting point and a governing factor when evaluating and deciding on the system topology is the
geography with the primary consumers, producers, and load flow exporters and importers. The first choice
for this system topology will be the most obvious, easiest and immediately the cheapest way to connect
the consumers and producers with each other.
The normal operational flow direction is defined from production to consumption centres. For example, in
the case of a wind power plant, bi-directional power flow may be required: In normal operation the wind
turbines produce energy that is transferred from the offshore platform to the onshore substation.
Occasionally it will also be necessary to transfer energy from the onshore substation to the offshore
platform converter when there is no wind, when the electrical equipment on the platform has to be
maintained or the wind turbines are starting up, e.g. energizing the offshore platform converter and the
offshore medium voltage (MV) and HV networks.
When choosing the system topology, the AC grid connection-point candidates are evaluated. The
evaluation comprises, but is not limited to, benchmarking of the connection-point candidates from
technical, environmental, cost-benefit and socioeconomic criteria. The benchmarking process shall also
consider if the topology and connection-point candidates are dependent on each other; e. g. alternative
topologies evolve from alternative connection-points and vice versa.
The needs and ability for the topology to expand in the future is difficult to predict. Therefore there is a risk
of “built-in” overcapacity within the selected topology to cover possible, but not necessarily provable,
future expansion. At present, the national AC grid operators apply similar, but area- and country-specific,
procedures including technical, cost-benefit and socioeconomic evaluation criteria for future grid capacity
and topology. The differences between such area- and country-specific procedures are due to different
grid design and operation practice, regulations and definition of security of supply. The common goal
should be that the AC and, in future, DC grid operators establish standard, European-level procedures.
The standard procedures should be established for decisions on the overcapacity financing model
including technical, regulatory, cost-benefit and socioeconomic evaluations of risks, benefits and
disadvantages of the topology from present and future perspectives. The future perspective should be
evaluated for mid term, i.e. for between 5 and 10 years perspective, and long term, i.e. over 10 years
perspective, from the commissioning date.

Depending on the specific topologies, there can be various constraints for system operation, as there are:
• operation conditions (grid revision plans);
• disturbances (faults and outages);
• controllability restrictions;
• limitation of maximum short circuit currents;
• load flows congestions.
Operational constraints can be present both in the DC and AC transmission systems due to limitations of
the system components. By careful selection of the system topology, adequate system capacity and
sufficient controllability and redundancy the consequences of system constraints can be minimized.
For relatively short distances and small energy flows, AC cable transmission systems are economically
competitive and advantageous to DC transmission systems. The competitive elements of DC transmission
may include the cost-benefit evaluations of the AC versus DC system components, technical and
commissioning aspects, arrangements for required ancillary services, maintenance, reliability, losses
minimization, commercial availability and experience from earlier projects etc. The evaluation shall be
performed for the (expected) lifetime period of the system. The advantages gained with a DC system
should outweigh the cost.
3.2.2 Power Transfer Requirements
The main purpose of an electrical transmission system is the exchange of energy between its sub-stations
representing energy consumption or energy production.
AC transmission systems are built for operation at high voltage levels, e.g. above 110 kV AC, whereas
distribution systems belong to medium-voltage (MV) and low-voltage (LV) levels. At present, AC
dominates the transmission systems whereas DC technology is present as part of the AC systems, for
instance, in point-to-point connectors between two AC systems. AC is also the standard for distribution
level systems.
Both HVAC and HVDC systems are utilized to transport significant amounts of power and energy over
great geographical distances. HVDC transmission has advantages if costs associated with the distances
and amounts of the power to be transmitted are lower than the costs for an appropriate HVAC
transmission. Typical break-even distances of HVAC versus HVDC transmission utilizations are in the
range of 50-110 km for cable connections, and in the range of 500-600 km for connections via overhead
transmission lines. The ranges are dependent on several technical factors such as the nominal voltage,
nominal power, losses, but also non-technical factors like rights of way. When two asynchronous HVAC
areas are interconnected, HVDC is chosen for technical reasons.
HVDC transmission is also used in parallel, with HVAC connections; this is referred to as “embedded
HVDC” systems. In addition to its main function of power transport, embedded HVDC systems are used
for stabilization and power flow control of the corresponding HVAC connections. For example, HVDC
systems in parallel with HVAC lines may be used for damping of inter area oscillations. [4], [5], [6]
Furthermore, within their design capabilities, HVDC converter stations can support the HVAC transmission
systems by providing reactive-power, AC voltage and frequency control.

17 CLC/TR 50609:2014
Until today, the main focus of HVDC transmission has been mainly on design, operation and reliability of
the AC system. The AC/DC converter stations have been interpreted as generator-like units and assigned
to the grid code requirements of the AC system operators [7]. When the HVDC Grid System expands in
terms of system dimensions and power capacity, the HVDC Grid System is expected to require ancillary
services from the adjacent AC systems. This may lead to the DC system design, operation and reliability
being based on similar procedures and criteria as applied to AC systems today. For example, similar to
the concept of HVDC Grid Systems complementing AC systems, an islanded AC system can also be used
to interconnect various HVDC links. At the same time the islanded AC system can provide connection for
local generation (or load) such as offshore wind power plants (or oil and gas platforms) [8].
3.2.3 Reliability
The reliability requirements of the HVDC grid will impact upon the redundancy and maintainability of the
individual components. Hence the reliability and security-of-supply aspects will have great influence on the
final system topology. While reliability is a well-known criterion for AC system planning, its application to
HVDC Grid Systems still needs to be defined.
a) AC System Reliability:
The expansion of an AC grid is always targeted to the agreed or predicted power and energy transmission
needs. An AC grid is developed so that safe and reliable operational management with a sufficiently high
security of supply is achieved. The security of supply has two major terms: the system adequacy and the
system security. For further use in this document, those terms are explained below.
The system adequacy is based on probabilistic analysis combining availability of transmission lines and
components, energy production and capacity as well as energy demand. The system adequacy is
measured in terms of not-delivered energy by amount and time. Periods of not-delivered energy are
defined as the time, when some consumption centres do not get all the required energy due to system
constraints, lack of energy production or lack of energy capacity. The system adequacy is evaluated for a
system in normal operation and in contingencies. The system adequacy is indexed from 0 (the lowest
grade) to 100 (the highest grade). When the
...