IEC TS 62001-3:2026

High-voltage direct current (HVDC) systems - Guidance to the specification and design evaluation of AC filters - Part 3: Modelling aspects

IEC TS 62001-3:2026 provides in-depth consideration regarding three particularly important aspects of design, which are also mentioned elsewhere in other parts of the IEC 62001 series, which are: AC network impedance modelling, the treatment of pre‑existing harmonics in performance and rating calculations, and harmonic interaction across converters (cross-modulation).
This document concentrates on passive AC filter technology and line-commutated high-voltage direct current (HVDC) converters, but much of the content is equally relevant to VSC converter technology. Where there is a distinction, this is indicated in the text.
The scope of this document covers AC side filtering for the frequency range of interest in terms of harmonic distortion and audible frequency disturbances. It excludes filters specifically designed to be effective in the PLC and radio interference spectra.
This edition includes the following significant technical changes with respect to IEC TR 62001‑3:2016:
a) added Clause 3 on terms and definitions;
b) added new Clause 4;
c) rearranged Clause 5, Clause 6 and Clause 7;
d) updated Bibliography.

General Information

• Status: Published
• Publication Date: 28-May-2026
• ICS: 29.200 - Rectifiers. Convertors. Stabilized power supply
• Technical Committee: SC 22F - Power electronics for electrical transmission and distribution systems
• Current Stage: PPUB - Publication issued
• Start Date: 29-May-2026
• Completion Date: 26-Jun-2026

Relations

• Revises: IEC TR 62001-3:2016 - High-voltage direct current (HVDC) systems - Guidance to the specification and design evaluation of AC filters - Part 3: Modelling
• Effective Date: 05-Jun-2026

Overview

IEC TS 62001-3:2026 is a technical specification by the International Electrotechnical Commission (IEC) focusing on the specification and design evaluation of AC filters for high-voltage direct current (HVDC) systems, specifically covering modelling aspects. This document provides comprehensive guidance on AC network impedance modelling, dealing with pre-existing harmonics in system performance and rating calculations, and examining harmonic interactions across converters, including cross-modulation effects.

The guidance is primarily targeted at passive AC filter technology and line-commutated converter-based HVDC systems, but much of its content is relevant for voltage source converter (VSC) applications as well. The document addresses modelling of harmonic distortion and audible frequency disturbances, excluding filters intended for power line carrier (PLC) and radio interference frequency ranges.

Key Topics

• AC Network Impedance Modelling

  • Discusses defining, representing, and evaluating the harmonic impedance of the AC network as seen by the HVDC converter.
  • Highlights the impact of inaccurate network impedance definition on filter design and system performance.
  • Provides recommendations for responsibility and methods to ensure consistent specification of network impedance.

• Treatment of Pre-existing Harmonics

  • Explores how harmonics present in the AC network before HVDC system integration can affect filter performance and equipment rating.
  • Covers techniques for modelling, measuring, and evaluating harmonic levels to ensure compliance with performance standards.
  • Offers guidance on incremental and aggregate harmonic performance assessments.

• Harmonic Interaction Across Converters (Cross-Modulation)

  • Analyzes active converter behavior and the interaction of harmonics between AC and DC sides.
  • Details major modelling approaches to evaluate cross-modulation phenomena and their implications for AC filter design.
  • Summarizes study methods, such as time and frequency domain models, to support robust system analysis.

• Frequency Range Considerations

  • Focuses on the key frequency range associated with harmonic distortion and audible disturbances.
  • Excludes design elements related to PLC and high-frequency radio interference filtering.

• Technical Revisions

  • This edition introduces new sections: terms and definitions, an enhanced modelling overview, the rearrangement of core clauses, and an updated bibliography.

Applications

IEC TS 62001-3:2026 is intended for application in specifying, designing, and verifying AC side filters in HVDC projects. Practical uses include:

• Utilities and Consultants

  • Providing technical requirements for new HVDC projects.
  • Evaluating filter designs and ensuring optimal system integration.
  • Preparing tender documentation and assessing bids with consistent impedance modelling standards.

• System Designers and Engineers

  • Enhancing simulation accuracy for HVDC filter performance.
  • Identifying and mitigating harmonic risks associated with interconnected converter stations.
  • Ensuring correct treatment of existing network harmonics during system upgrades or expansions.

• Manufacturers and Project Developers

  • Aligning product specifications to comply with prevalent international standards.
  • Supporting equipment rating, testing, and quality assurance processes.

By following the guidelines in IEC TS 62001-3:2026, stakeholders can achieve reliable harmonic performance, reduced risk of equipment overload or instability, and extended equipment lifetime in HVDC system deployments.

Related Standards

• IEC 62001 Series:

  • IEC TR 62001-1: Overview of AC filter specification for HVDC systems.
  • IEC TS 62001-2: Harmonic performance aspects, including interference criteria and compliance verification.
  • IEC TR 62001-4: Equipment ratings, power losses, noise, and design issues for AC filters.
  • IEC TR 62001-5: Harmonic limits for HVDC systems with voltage source converters (VSC).

• Other Relevant IEC Standards:

  • IEC 61000 Series: Electromagnetic compatibility (EMC) standards.
  • IEEE 519: Recommended practices and requirements for harmonic control in electrical power systems.

These related documents provide a comprehensive framework for the specification, simulation, performance assessment, and verification of AC filter solutions in HVDC systems.

---

Keywords: IEC TS 62001-3:2026, HVDC, AC filter design, network impedance modelling, harmonic performance, cross-modulation, passive filters, VSC, system harmonics, international standards, power quality, converter interaction.

Standards Content (Sample)

IEC TS 62001-3 ®
Edition 1.0 2026-05
TECHNICAL
SPECIFICATION
High-voltage direct current (HVDC) systems - Guidance to the specification and
design evaluation of AC filters -
Part 3: Modelling aspects
ICS 29.200  ISBN 978-2-8327-1271-9

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CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 9
4 Overview of modelling . 9
5 AC network impedance modelling . 10
5.1 General . 10
5.2 Implications of inaccurate definition of network impedance . 11
5.3 Considerations for network modelling . 11
5.3.1 General . 11
5.3.2 Project life expectancy and robustness of data . 12
5.3.3 Network operating conditions . 12
5.3.4 Network impedances for performance and rating calculations . 13
5.3.5 Modelling of network components . 13
5.3.6 Representation of loads at harmonic frequencies . 15
5.4 Network harmonic impedance envelopes . 15
5.5 Methods of determining envelope characteristics . 18
5.5.1 General . 18
5.5.2 Low order harmonics . 18
5.5.3 Mid-range and higher order harmonics . 19
5.5.4 Balancing of risk and benefit . 21
5.5.5 Consideration of tolerances on harmonic bands . 21
5.5.6 Two separate envelopes for one harmonic band . 24
5.5.7 Critical envelope parameters . 24
5.5.8 Impedance envelopes for performance and rating conditions . 25
5.6 Examples of the impact of different network impedance representations . 26
5.6.1 Effect of network envelope parameters on resultant distortion . 26
5.6.2 Effect of network minimum resistance on filter rating . 29
5.7 Interharmonic impedance assessment . 30
5.8 Measurement of network harmonic impedance . 32
5.9 Conclusions . 33
6 Pre-existing harmonics . 34
6.1 General . 34
6.2 Modelling and measurement of pre-existing harmonic levels . 34
6.3 Harmonic performance evaluation, methods and discussion . 37
6.3.1 General . 37
6.3.2 "Incremental" harmonic performance evaluation . 37
6.3.3 "Aggregate" harmonic performance evaluation . 38
6.3.4 Both "incremental" and "aggregate" performance evaluation . 39
6.3.5 "Incremental" and "maximum magnification factor" harmonic
performance evaluation . 39
6.4 Calculation of total harmonic performance indices . 40
6.5 Harmonic rating evaluation . 41
6.6 Difficulties with the voltage source/worst network model for rating . 41
6.6.1 Background . 41
6.6.2 Illustration of the voltage source/worst network method . 42
6.7 Further possible calculation procedures for rating evaluation . 45
6.7.1 Using measured levels of pre-existing distortion . 45
6.7.2 Applying compatibility level voltage source at the filter busbar . 47
6.7.3 Limiting the filter bus harmonic voltage to a maximum level for filter
rating (MLFR) . 49
6.7.4 Limiting total source distortion to the defined THD . 50
6.7.5 Limiting harmonic order of pre-existing distortion . 51
6.8 Conclusions . 52
7 Harmonic interaction across converters . 52
7.1 General . 52
7.2 Indicators of where harmonic interaction is significant . 53
7.3 Interaction phenomena . 54
7.4 Impact on AC filter design . 55
7.4.1 General . 55
7.4.2 AC side third harmonic . 55
7.4.3 Direct current on the AC side . 55
7.4.4 Characteristic harmonics . 56
7.5 General overview of modelling techniques . 56
7.5.1 General . 56
7.5.2 Time domain AC-DC-AC interaction model . 58
7.5.3 Frequency domain AC-DC-AC interaction model . 59
7.5.4 Frequency domain AC-DC interaction model . 59
7.5.5 Frequency domain current source model . 60
7.6 Interaction modelling . 60
7.6.1 General . 60
7.6.2 Coupling between networks . 61
7.6.3 Driving forces . 61
7.6.4 System harmonic impedances . 62
7.7 Study methods . 62
7.7.1 Frequency domain . 62
7.7.2 Time domain . 63
7.8 Composite resonance . 63
7.9 Core saturation instability . 63
7.10 Particular considerations for back-to-back converters . 63
7.11 Issues to be considered in the design process . 64
7.11.1 General . 64
7.11.2 Fundamental frequency and load issues . 65
7.11.3 Negative phase sequence . 65
7.11.4 Pre-existing harmonic distortion . 66
7.11.5 AC network impedance . 67
7.11.6 Converter control system . 68
7.11.7 Combination with "classic" harmonic generation . 69
7.11.8 Relative magnitude of pairs of low-order harmonics . 69
7.11.9 Superposition of contributions . 70
7.12 Parallel AC lines and converter transformer saturation . 70
7.13 Possible countermeasures . 72
7.13.1 AC (or DC) filters . 72
7.13.2 DC control design . 73
7.13.3 Operating restrictions and design protections . 73
7.14 Recommendations for technical specifications . 73
7.14.1 General . 73
7.14.2 Specified design data . 74
7.14.3 Requirements regarding calculation techniques . 74
Annex A (informative) Location of worst-case network impedance . 76
Annex B (informative) Accuracy of network component modelling at harmonic
frequencies . 79
B.1 General . 79
B.2 Loads. 79
B.3 Transformers . 82
B.3.1 Transformer reactance. 82
B.3.2 Transformer resistance . 82
B.4 Transmission lines . 85
B.5 Synchronous machines . 88
B.6 Modelling of resistance in harmonic analysis software . 89
Annex C (informative) Project experience of pre-existing harmonic issues . 92
C.1 General . 92
C.2 Third harmonic overload of filters in a back-to-back system . 92
C.3 Third and fifth harmonic overload of filters in a line transmission. 93
th
C.4 Overload of a DC side 6 harmonic filter . 93
Annex D (informative) Worked examples showing impact of pre-existing distortion . 95
D.1 General . 95
D.2 Pre-existing distortions . 97
D.2.1 Example 1 – Illustration of magnification . 97
D.2.2 Example 2 – Impact of network impedance parameters . 100
Annex E (informative) Comparison of calculation methods for equipment rating,
considering pre-existing harmonics . 102
E.1 General . 102
E.2 Reference case – Converter generated harmonics only. 106
E.3 Method 1 – Source voltages behind impedance sector . 106
E.4 Method 2 – Source voltages at filter bus (see 6.7.2). 106
E.5 Method 3 – Limiting the filter bus harmonic voltage to a maximum level for
filter rating (MLFR) (see 6.7.3) . 107
E.6 Method 4 – Limiting total source distortion to the THD level (see 6.7.4) . 107
th
E.7 Method 5 – Pre-existing harmonics considered only up to the 10 , with
10 % margin on converter generation for remainder (see 6.7.5) . 110
Annex F (informative) Problems with cross modulations – Practical experience . 111
Bibliography . 114

Figure 1 – Example of a single impedance locus for harmonic orders 2 to 49 . 17
Figure 2 – Example of simple circle envelope encompassing all scatter points for
harmonic orders 2 to 49 . 17
th th
Figure 3 – Example of an impedance envelope for 7 to 13 harmonic with
associated scatter plots . 19
th th
Figure 4 – Example of an impedance envelope for 13 to 19 harmonic with
associated scatter plots . 20
th th
Figure 5 – Example of an impedance envelope for 19 to 25 harmonic with
associated scatter plots . 20
Figure 6 – Example of the need to extend the band of harmonics to allow for
resonance effects . 22
Figure 7 – Application of tolerance range in percentage of the harmonic number . 22
Figure 8 – Application of tolerance range in percentage of the harmonic number,
th th
zoomed to show 11 and 13 harmonics . 23
Figure 9 – Example showing two impedance envelopes for a particular band . 24
Figure 10 – Example of impedance envelopes under "performance" and "rating"
th th
conditions for harmonic orders 4 to 7 . 25
Figure 11 – Example of impedance envelopes "performance" and "rating" conditions
th st
for harmonic orders 25 to 31 . 26
Figure 12 – Discrete envelopes for different groups of harmonics . 27
Figure 13 – Example showing a distributed generation causing about 15 % attenuation
of ripple control signal at the PCC . 31
Figure 14 – Generic circuit model for calculation of harmonic performance or rating . 35
Figure 15 – Illustration of basic voltage quality concepts with time/location statistics
covering the whole system . 37
Figure 16 – Circuit model for calculation of incremental performance . 37
th
Figure 17 – Equivalent circuit of a network for the h harmonic . 43
Figure 18 – Typical voltage magnification factor. 43
Figure 19 – Pre-existing distortion set to measured levels (plus margin) . 45
Figure 20 – Pre-existing distortion applied directly at the filter bus . 47
Figure 21 – Harmonic voltage stress on a shunt capacitor with IEC planning levels
applied . 48
Figure 22 – Key elements of a complete AC-DC-AC harmonic interaction model . 58
Figure 23 – Equivalent circuit for evaluation of harmonic interaction with DC side
interaction frequency greater than AC side fundamental frequency . 61
th th
Figure 24 – DC side 6 harmonic voltage due to AC side 5 harmonic (fixed angle)
th
and 7 harmonic (varying angle) . 67
th th
Figure 25 – Simple circuit used to represent AC network impedance at 5 and 7
harmonics . 68
Figure A.1 – Equivalent circuit model for demonstration of worst-case resonance
between AC filters and the network . 76
Figure A.2 – Diagram indicating vectors Z , Z and Z . 77
F N H
Figure B.1 – Typical equivalent load network . 80
Figure B.2 – Relative error of equivalent load loss resistance R of using [44]
n
compared with Electra 167 [43] model . 84
Figure B.3 – Effect of temperature on transformer load loss . 85
Figure B.4 – Ratio between harmonic and fundamental frequency resistance as
calculated for balanced mode components and calculated from averages of reduced Z
matrix resistance values . 87
Figure B.5 – Ratio between harmonic and fundamental frequency resistance as
calculated for balanced mode components and calculated from averages of reduced Z
matrix resistance values, for varying earth resistivity . 87
Figure B.6 – Comparison of synchronous machine reactance between [10]
recommendation and test measurements for a salient pole hydro generator of 370 MVA . 88
Figure B.7 – Comparison of synchronous machine resistance between [10]
recommendation and test measurements for a salient pole hydro generator of 370 MVA . 88
Figure B.8 – Comparison of different approximations for resistance variations . 90
Figure B.9 – Network impedance for Araraquara substation . 91
Figure D.1 – Harmonic models for converter and for pre-existing distortion . 96
Figure D.2 – Geometrical visualisation of selecting worst-case impedance for converter
harmonics . 96
Figure D.3 – Simple filter scheme to illustrate magnification . 97
Figure D.4 – Plots illustrating magnification of various pre-existing harmonics . 100
Figure E.1 – Network impedance sector used in example . 103
Figure E.2 – Assumed filter scheme for examples of different methods of calculation . 104
Figure E.3 – IEC planning levels used for source voltages in the study . 105

Table 1 – Comparison of calculated harmonic voltage distortion between two methods
of representing network harmonic impedance . 28
Table 2 – Comparison of calculated harmonic voltage distortion considering the
variation of network impedance angle . 29
Table 3 – Comparison of calculated filter harmonic current considering the variation of
network minimum resistance and filter detuning . 30
Table 4 – Amplification factor tanΦ at different network impedance angles . 43
Table 5 – Variation of calculated filter harmonic current as a function of detuning . 48
Table 6 – Dominant frequencies in AC–DC harmonic interaction . 54
Table B.1 – Constants for resistance adjustment – Five parameter equations . 90
Table D.1 – Parameters of elements of a simplified filter scheme shown in Figure D.3 . 97
Table D.2 – Voltage and current distortion for Z = 1 Ω and varying Φ . 100
min
Table D.3 – Voltage and current distortion for Φ = ±85° and varying Z . 101
min
Table E.1 – Parameters of components of filters shown in Figure E.2 . 104
Table E.2 – Component rating calculated using different calculation methods . 106
Table E.3 – Rating calculations using method 3 – for BP1113 C1 . 107
Table E.4 – Rating calculations using method 3 – For HP24 R1 . 109
Table E.5 – Rating calculations using Method 4 – For BP1113 C1 . 110

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
High-voltage direct current (HVDC) systems -
Guidance to the specification and design evaluation of AC filters -
Part 3: Modelling aspects
FOREWORD
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shall not be held responsible for identifying any or all such patent rights.
IEC TS 62001-3 has been prepared by subcommittee 22F: Power electronics for electrical
transmission and distribution systems, of IEC technical committee 22: Power electronic systems
and equipment. It is a Technical Specification.
This first edition cancels and replaces the first edition of IEC TR 62001-3 published in 2016.
This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to
IEC TR 62001-3:2016:
a) added Clause 3 on terms and definitions;
b) added new Clause 4;
c) rearranged Clause 5, Clause 6 and Clause 7;
d) updated Bibliography.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
22F/862/DTS 22F/869/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 62001 series, published under the general title High-voltage direct
current (HVDC) systems - Guidance to the specification and design evaluation of AC filters, can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
IEC 62001 (all parts) deals with the specification and design evaluation of AC side harmonic
performance and AC side filters for HVDC schemes. It is intended to be primarily for the use of
the utilities and consultants who are responsible for issuing the specifications for new HVDC
projects and evaluating designs proposed by prospective suppliers.
The IEC TR 62001 series is structured in five parts as follows.
IEC TR 62001-1 – Overview
This part concerns specifications of AC filters for high-voltage direct current (HVDC) systems
with line-commutated converters, permissible distortion limits, harmonic generation, filter
arrangements, filter performance calculation, filter switching and reactive power management
and customer specified parameters and requirements.
IEC TS 62001-2 – Harmonic performance aspects
This part deals with telephone interference, current-based interference criteria, field
measurements and compliance verification.
IEC TS 62001-3 – Modelling aspects
This part addresses modelling of three specific aspects of design: AC network impedance
modelling, the treatment of pre-existing harmonics in performance and rating calculations, and
harmonic interaction across converters (cross-modulation).
IEC TR 62001-4 – Equipment
This part concerns steady-state and transient ratings of AC filters and their components, power
losses, audible noise, design issues and special applications, filter protection, audible noise,
seismic requirements, equipment design and test parameters.
IEC TR 62001-5 – AC side harmonics and appropriate harmonic limits for high-voltage direct
current (HVDC) systems with voltage sourced converters (VSC)
This part addresses the AC side harmonic performance of voltage sourced converters (VSC).

1 Scope
This part of IEC 62001 provides in-depth consideration regarding three particularly important
aspects of design, which are also mentioned elsewhere in other parts of the IEC 62001 series,
which are: AC network impedance modelling, the treatment of pre-existing harmonics in
performance and rating calculations, and harmonic interaction across converters (cross-
modulation).
This document concentrates on passive AC filter technology and line-commutated high-voltage
direct current (HVDC) converters, but much of the content is equally relevant to VSC converter
technology. Where there is a distinction, this is indicated in the text.
The scope of this document covers AC side filtering for the frequency range of interest in terms
of harmonic distortion and audible frequency disturbances. It excludes filters specifically
designed to be effective in the PLC and radio interference spectra.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
modelling
action of representing a physical situation to enable analysis
4 Overview of modelling
HVDC converter stations are large, complex and costly. Extensive simulations are required to
ensure that the HVDC converter station acts as intended and to meet the requirements of the
end customer. Such simulations can also be used to configure and adjust the parameters of the
HVDC converter station to optimise and enhance the operation and performance.
To ensure that the simulations are realistic, it is important the models used are appropriate to
the task in hand. For the purpose of this document, the focus on the modelling is in the
frequency domain, although where time domain studies can be more appropriate, this is
indicated. Modelling is discussed in many places throughout the various parts of the IEC 62001
series; however, this document focusses in-depth on a number of key aspects of modelling of
the HVDC converter.
Clause 5 examines the connected AC system and the appropriate methods of modelling this for
representation as a harmonic impedance.
Clause 6 considers the appropriate way to model the harmonic distortion already present in the
AC system such that effect of the connection of the HVDC converter is modelled correctly in
terms of the changes to the harmonic distortion at various points in the system and the rating
of components.
Clause 7 considers the modelling of the active behaviour of the HVDC converter itself in terms
of the harmonic inter-connection between the AC and DC sides of the converter, also referred
to as cross-modulation.
5 AC network impedance modelling
5.1 General
IEC TR 62001-1 [1] and IEC TR 62001-4 [2] discuss the important influence of network
harmonic impedance on both the performance and rating aspects of the AC filter design. For a
customer, it is one of the most difficult aspects to specify, especially if the customer is not the
owner of the network and has little direct knowledge of its composition and possible future
development. The purpose of Clause 5 is to amplify the reasons why the correct specification
of network harmonic impedance is crucial to an optimal design of AC filters and also to provide
further detailed guidance as to its assessment.
IEC TR 62001-1 [1] discusses that, normally, the customer defines the range of network
impedance to be used for filter design but that, in some cases, the customer leaves the
prospective contractors to perform this assessment.
This document reinforces a recommendation that, in the production of the technical specification
by the customer for an HVDC system, the customer rather than the prospective contractors is
responsible for the definition of the AC network impedance characteristics. This means that the
study is done only once and avoids all prospective contractors having to make their own
individual assessment of the provided data, such as system single line diagrams and associated
relevant data, details of normal and abnormal operating conditions and loading, and the effects
of future network expansion (such data are often only known to customer or utility). This would
then have the inevitable risk that each prospective contractor would assess the network
impedance in a different manner with differing results, leaving the customer to determine which
is correct or whether any of them are adequate. The customer should therefore take
responsibility for these studies, either directly or through a consultant. They can take advantage
of the longer period that is generally available before the issue of the technical specification to
prepare this information, rather than requiring the prospective contractors to individually make
the assessment during the shorter tender stage.
Specifications and design of the AC harmonic filters established at the tender stage normally
form part of the later contract. If the customer decides to postpone the detailed network
impedance study until execution of the contract, they should be aware of the following
disadvantages and manage the risks.
– Received bids are possibly not based on the same assumptions, hence can be difficult to
compare.
– Cost and space requirements of the AC filter scheme determined during the tender stage
are possibly not sufficient.
– The contractor claims change/variation orders.
– The time needed for the final design stage is prolonged.
___________
Numbers in square brackets refer to the Bibliography.
There are some instances in which the methods for determining the network harmonic

impedance described in [3] or in this document is inappropriate or require special consideration.
Such situations include the following.
– Where a proposed HVDC scheme is to be connected in parallel with an existing scheme
which is operating with adequately designed AC harmonic filters and there is a preference
for the filters to be associated with the new scheme to have identical characteristics as the
existing units, at least for the converter characteristic harmonics. In this case, any change
in the definition of network harmonic impedance from that used for design of the original
scheme will require careful consideration of the continuing viability of the existing filters and
of the combined operation of the original and new filter designs. This topic is treated in depth
in CIGRE Technical Brochure 798 [4].
– Where a proposed HVDC scheme will be connected to an AC network that is only operated
in an "islanded" mode; that is a small and well-defined network for which it can be preferable
to model the transmission lines, cables, transformers and generators, etc. explicitly rather
than to employ impedance envelopes.
5.2 Implications of inaccurate definition of network impedance
Due to the difficulties in accurately assessing the network harmonic impedance, it can be
attractive for a customer to base their specification on a simplified network definition with fairly
arbitrary parameters, probably biased towards conservative values. However, a too
conservative assessment of network impedance (e.g. an impedance having excessively high
damping angles or excessive range) can have several significant disadvantages in respect of
AC filter design:
– an increased number of different types of filters can be required to cater for network
impedance conditions that in practice cannot occur;
– an increase in switchyard space would be needed to cater for redundancy requirements as
a result of the provision of a larger number of different filter types;
– the requirement for a greater number of sharply tuned filters, the application of which can
incur excessive harmonic ratings especially when considering the effects of pre-existing
harmonic distortion;
– the need, especially at low transmitted power levels, for AC filters with a total reactive power
in excess of that which can be accepted by the AC network and therefore the requirement
for the converters to operate at either increased control angles or often the use of high
capital cost shunt reactors, both of which give rise to increased losses;
– higher initial and project lifetime operating costs.
Conversely, however, a design which is based on too narrow an assessment of network
impedance can fail to meet the required harmonic performance criteria and sometimes cannot
remain in service due to component overloading because of resonances between the AC filters
and the network which were not predicted. In such cases, the economic consequence of such
shortcomings could be more serious than those listed above due to an over-conservative
design.
It is therefore evident that efforts should be made to achieve as accurate as possible an
assessment of the network harmonic impedance.
5.3 Considerations for network modelling
5.3.1 General
IEC TR 62001-1 [1] gives references [5], [6] to various methods of deriving network harmonic
impedance.
In attempting to postulate criteria to be considered in determining network harmonic impedance,
there are very few generic rules that can be applied universally for all networks worldwide;
therefore, each network should be treated on a case-by-case basis.
The extent of the network to be modelled is also system dependent and no general rules can
be defined. One approach is to start by modelling a relatively small area of the system but
retaining sufficient to incorporate all of the contingencies required to be studied. The
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