IEC TS 62001-2:2026

High-voltage direct current (HVDC) systems - Guidance to the specification and design evaluation of AC filters - Part 2: Harmonic performance aspects

IEC TS 62001-2:2026 gives a guidance to the specification and design evaluation of AC filters of high-voltage direct current HVDC) systems, specifically on harmonic performance aspects. This document focusses on three specific areas of interest. Clause 4 discusses telephone interference related to the operation of HVDC transmission, including the derivation of appropriate harmonic limits. Clause 5 deals with all aspects of current-based harmonic performance criteria and their application. Clause 6 is concerned with field measurement and verification of compliance with specified harmonic limits.
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 62001‑2:2016:
a) added Clause 3 on terms and definitions;
b) split old Clause 3 to form new Clause 4 and Clause 5;
c) extensive updating of text to reflect progress in time;
d) transferred most of IEC TR 62001‑3:2016, Annex C, to Clause 6.

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 62001-2:2016 - High-voltage direct current (HVDC) systems - Guidance to the specification and design evaluation of AC filters - Part 2: Performance
• Effective Date: 05-Jun-2026

Overview

IEC TS 62001-2:2026 is an International Electrotechnical Commission (IEC) Technical Specification that provides essential guidance for the specification and design evaluation of AC filters in high-voltage direct current (HVDC) systems, focusing especially on harmonic performance aspects. The document is highly relevant to both line-commutated converters and voltage source converter (VSC) HVDC technology, with particular emphasis on passive AC filter solutions.

Harmonic distortion in HVDC systems can affect not only system efficiency and equipment life but also cause external disturbances such as audible noise and telephone interference. IEC TS 62001-2:2026 addresses practical engineering considerations, supporting utilities, consultants, and engineers in ensuring that harmonic emissions remain within acceptable limits and that compliance is verified effectively.

Key Topics

This standard covers three primary areas vital for the design and operation of HVDC AC side filters:

• Telephone Interference:
  Evaluates the risks and provides methods for determining the necessity and derivation of harmonic limits to minimize interference with nearby telephone communications. Criteria such as Telephone Interference Factor (TIF) and IT (the product of RMS current and TIF) are discussed.
• Current-based Harmonic Criteria:
  Outlines guidelines for setting and applying current-based performance criteria to control harmonics. This includes practical methods to assess harmonic current flow, define appropriate limits, and align specifications with project requirements.
• Field Measurement & Compliance Verification:
  Provides direction on planning and executing measurements to verify that actual system performance aligns with specified harmonic limits. This section is crucial for confirming performance both at commissioning and during ongoing operation.

The document also highlights scenarios where AC filter design must consider specific system configurations, exposure conditions, and local telecommunication practices.

Applications

IEC TS 62001-2:2026 is a valuable reference for:

• HVDC Project Specification:
  Assists utilities, consultants, and EPC contractors in drafting robust specifications for new HVDC projects, ensuring harmonic performance is addressed proactively.
• Filter Design and Evaluation:
  Supports manufacturers and engineers in selecting and verifying passive AC filter solutions to control harmonic distortion and meet performance requirements.
• Mitigation of External Disturbances:
  Offers guidance to telecommunications providers and system planners in identifying and addressing potential electromagnetic interference from HVDC transmission lines.
• Operational Compliance:
  Facilitates ongoing compliance monitoring, including periodic field measurements and reporting, to ensure sustained power quality and adherence to regulatory limits.

The technical guidance provided by this document is relevant to a wide range of HVDC configurations, especially where maintaining power quality and avoiding interference with adjacent infrastructure is a priority.

Related Standards

IEC TS 62001-2:2026 is part of the broader IEC 62001 series, which addresses various aspects of AC filter specification and performance in HVDC systems. Related and referenced standards include:

• IEC TR 62001-1:
  Overview of AC filter specifications, permissible distortion, and performance calculation.
• IEC TS 62001-3:
  Modelling aspects pertinent to AC network impedance, existing harmonics, and harmonic interactions.
• IEC TR 62001-4:
  Equipment considerations, including ratings, losses, audible noise, and seismic requirements.
• IEC TR 62001-5:
  Harmonic limits and performance for VSC-HVDC systems.
• IEC 61000 Series:
  International standards for electromagnetic compatibility (EMC) and harmonic limits.
• IEEE Std 368 and IEEE 519:
  Industry benchmarks for harmonic control and telephone interference criteria.

Practical Value

Conformance with IEC TS 62001-2:2026 ensures that HVDC projects incorporate industry best practices for harmonic performance, mitigating technical and regulatory risks associated with harmonic emissions and external interference. By clearly understanding and applying the guidance within this IEC standard, organizations can enhance grid reliability, protect adjacent infrastructure, and uphold power quality in increasingly complex transmission environments.

Keywords: HVDC systems, IEC 62001-2, AC filters, harmonic performance, telephone interference, harmonic limits, power quality, design evaluation, field measurement, compliance verification, passive filter technology, IEC standards.

Standards Content (Sample)

IEC TS 62001-2 ®
Edition 1.0 2026-05
TECHNICAL
SPECIFICATION
High-voltage direct current (HVDC) systems - Guidance to the specification and
design evaluation of AC filters -
Part 2: Harmonic performance aspects
ICS 29.200  ISBN 978-2-8327-1261-0

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CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Telephone interference . 7
4.1 Overview of telephone interference issues . 7
4.2 Determining the necessity for telephone interference limits . 9
4.3 Defining appropriate telephone interference limits . 10
4.3.1 General . 10
4.3.2 Mechanisms of interference . 11
4.3.3 Noise performance coordination levels . 12
5 Current-based interference criteria . 14
5.1 Overview . 14
5.2 Derivation of parameters . 14
5.3 Illustrative values of current-based limits . 17
5.4 Determination of IT limits for a specific project . 19
5.4.1 General . 19
5.4.2 Identification of the decisive transmission lines . 19
5.4.3 Inductive coordination study . 23
5.4.4 Pre-existing harmonics and future growth . 23
5.5 Recommendations for technical specifications . 25
5.6 Consequences of harmonic current limit for filter design . 27
5.7 Telephone infrastructure mitigation options . 27
5.8 Experience and examples . 28
5.8.1 General . 28
5.8.2 Review of design requirements . 29
5.8.3 Measured current levels of schemes in service . 29
5.8.4 Example of actual telephone interference problems . 31
5.9 Conclusions . 32
6 Field measurements and verification . 33
6.1 Overview . 33
6.2 Equipment and subsystem tests . 33
6.2.1 General . 33
6.2.2 Fundamental frequency impedance and unbalance measurement . 33
6.2.3 Frequency response curve . 34
6.3 System tests . 34
6.4 Measuring equipment . 34
6.4.1 Overview . 34
6.4.2 AC filter energization . 37
6.4.3 Verification of the reactive power controller . 37
6.4.4 Verification of the specified reactive power interchange . 37
6.4.5 Verification of the harmonic performance . 37
6.4.6 Verification of audible noise . 41
6.5 In-service measurements . 41
6.5.1 General . 41
6.5.2 Measurement parameters . 41
6.5.3 Categories of measurements . 41
Annex A (informative) Voltage and current distortion – Telephone interference . 43
A.1 Voltage distortion limits for HV and EHV networks . 43
A.1.1 General . 43
A.1.2 Recommended limits for HV or EHV networks. 44
A.2 Harmonic current in generators . 46
A.3 Causes of telephone interference . 46
A.4 Definition of telephone interference parameters . 48
A.5 Discussion . 52
A.6 Coupling mechanism from power-line current to telephone disturbance
voltage . 53
Annex B (informative) Example of induced noise calculation with Dubanton equations . 54
B.1 General . 54
B.2 Residual IT . 54
B.3 Balanced IT . 55
Annex C (informative) Illustration of the benefit of including a TIF requirement in the
technical specification . 56
Annex D (informative) Specification of IT limits dependent on network impedance . 58
Annex E (informative) The impact of AC network harmonic impedance and voltage
level on the filter design necessary to fulfil an IT criterion . 62
E.1 General . 62
E.2 Assumptions and pre-conditions . 63
E.3 Harmonic impedance of AC network . 65
E.4 Filter design . 68
E.5 Explanation of the difference in impact of relative and absolute performance
criteria on required filter Mvar . 69
Annex F (informative) List of typical HVDC projects . 71
Bibliography . 73

Figure 1 – Conversion factor from positive sequence current at the sending end to
positive sequence current at the receiving end, and input impedance of a 230 kV line,
124 km long, 1 000 Ω-m . 21
Figure 2 – Conversion factor from positive sequence current to residual current, and
input impedance of a 230 kV line, 124 km long, 1 000 Ω-m . 22
Figure 3 – Simple circuit for calculation of harmonic performance taking into account
pre-existing harmonics . 24
Figure 4 – Simplified diagram of system in Table 6 . 30
Figure 5 – Simplified diagram of system in Table 7 . 31
Figure 6 – Locations for voltage measurements for power quality assessment . 36
Figure 7 – Converter variables for harmonic performance tests . 38
Figure 8 – Example of measurements made during a ramp of the converters . 40
Figure A.1 – Contributions of harmonic voltages at different voltage levels in a simple
network . 43
Figure A.2 – C-message and psophometric weighting factors . 47
Figure A.3 – Flow-chart describing the basic telephone interference mechanism . 53
Figure D.1 – Simplification of the detailed network used for telephone interference
simulation . 58
Figure D.2 – Induced voltage in telephone circuit vs. network impedance, for unitary
current injected . 59
Figure D.3 – IT limits as defined for different network impedances . 60
Figure E.1 – Converter harmonics un-weighted (A) and IT weighted (kA) on 240 kV
base . 64
Figure E.2 – Converter Mvar absorption versus load . 65
Figure E.3 – Impedance sector diagram and RL-equivalent circuit . 66
Figure E.4 – Simplified converter/system topology . 66
Figure E.5 – Simplified circuit including overhead transmission line . 67

Table 1 – Performance thresholds for metallic noise . 13
Table 2 – Performance thresholds for longitudinal noise . 13
Table 3 – Performance thresholds for balance . 13
Table 4 – Illustrative maximum telephone line length to achieve the North American
recommended longitudinal ng level, as a function of balanced IT level, earth resistivity
and separation distance . 17
Table 5 – Illustrative maximum telephone line length to achieve the North American
recommended longitudinal ng level as a function of residual IT level, earth resistivity
and separation distance . 18
Table 6 – Measured 95 % values of THFF and I of a 600 MW scheme (3 phases) . 30
pe
Table 7 – Measured 95 % values of THFF and I of a 300 MW scheme (3 phases) . 31
pe
Table A.1 – Voltage distortion limits from IEEE 519 . 44
Table A.2 – Compatibility levels for harmonic voltages (in percent of the nominal
voltage) in LV and MV power systems . 45
Table A.3 – Indicative values of planning levels for harmonic voltages in HV and EHV
power systems . 45
Table E.1 – Required total amount of installed filter Mvars to meet an IT limit of 25 kA
for 600 MW transmission . 63
Table F.1 – Some HVDC schemes – Specified telephone interference criteria . 71

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
High-voltage direct current (HVDC) systems -
Guidance to the specification and design evaluation of AC filters -
Part 2: Harmonic performance aspects

FOREWORD
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shall not be held responsible for identifying any or all such patent rights.
IEC TS 62001-2 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 IEC TR 62001-2 published in 2016. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to
IEC TR 62001-2:2016:
a) added Clause 3 on terms and definitions;
b) split old Clause 3 to form new Clause 4 and Clause 5;
c) extensive updating of text to reflect progress in time;
d) transferred most of IEC TR 62001-3:2016, Annex C, to Clause 6.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
22F/863/DTS 22F/871/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.
Harmonic performance is discussed in several places throughout the various parts of the
IEC 62001 series and is also the topic of the IEC 61000 series.
The IEC 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 gives a guidance to the specification and design evaluation of AC filters
of high-voltage direct current HVDC) systems, specifically on harmonic performance aspects.
This document focusses on three specific areas of interest. Clause 4 discusses telephone
interference related to the operation of HVDC transmission, including the derivation of
appropriate harmonic limits. Clause 5 deals with all aspects of current-based harmonic
performance criteria and their application. Clause 6 is concerned with field measurement and
verification of compliance with specified harmonic limits.
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
harmonic performance
level of monitored parameter compared to limit value
4 Telephone interference
4.1 Overview of telephone interference issues
Interference with nearby telephone systems is a potential issue both for the AC transmission
lines associated with an HVDC converter, and for the DC transmission lines. Much of the
material in Clause 4 could apply to both; however, the main focus is on the AC side. Telephone
interference from DC transmission lines is comprehensively described in CIGRE TB811 [1] .
___________
Numbers in square brackets refer to the Bibliography.
Permissible distortion limits and performance measures for limiting telephone interference, such
as the telephone interference factor (TIF), the product of the RMS current (A) and the TIF (IT)
– the definitions of these criteria are shown in 5.2 and Clause A.4 – are discussed in detail and
summarized in IEC TR 62001-1 [2]. Where these measures are applied with strict limits,
particularly current-based criteria such as IT, they can be a decisive or limiting factor for filter
design. Thus, these measures can directly affect the costs of filters and the concomitant effects
of larger filters (extra station space, shunt reactors to compensate excess reactive power
produced by the filters, etc.). On the other hand, a few HVDC projects have experienced high
levels of telephone interference that caused problems during commissioning and early
operation. Reference [2] considers basic interference criteria, defines telephone interference
limits and discusses consequences of the telephone interference for filter design.
Because these criteria, based on psophometric or C-message weighting of harmonics, are
specific to evaluation of noise induced on telephone circuits electromagnetically coupled to AC
lines, they should only be specified where significant coupling between AC transmission lines
and analogue telephone circuits can be reasonably anticipated. This document provides
guidance for discriminating those situations where risk of telephone interference exists.
However, there are many factors that affect the potential for telephone interference and it is not
possible to provide definitive, quantitative guidelines. One of the most elusive factors is the
propagation of harmonic currents through the AC system. Experience has shown that significant
harmonic HVDC-created current flow can exist in lines that are remote from the converter
station and beyond transformations to other transmission voltage levels. A full inductive
coordination study, which involves the calculation of harmonic current flow in the system in
order to determine the problematic transmission lines and the assessment of their actual
coupling with the adjacent telephone lines, is the only mean to assess the interference potential
with any certainty.
The specification of telephone interference should also take into account local particularities,
as discussed in 4.2.
A valuable paper produced by the Joint Task Force 02 of WG14.03/CC.02 [3] gives a very
complete description of the inductive coordination process and the main parameters affecting
telephone interference. The IT limits are based on experience from the Finnish telephone
system, while making use of some approximations for the network characteristics. This
document will focus on North American practice for IT limits, although the principles and
calculation methods are applicable worldwide and will indicate the important system parameters
that should be defined in a technical specification.
In systems where telephone interference potential can be judged to exist, proper specification
of harmonic current- and voltage-based performance criteria are of great importance to protect
the interests of the HVDC system owner. If not sufficiently addressed by the specifications, and
should telephone interference problems arise, the consequences to the HVDC owner can be
severe. Resolution of telephone interference after the HVDC system is placed into service can
be highly expensive and time consuming. Post-commissioning resolution of telephone
interference is complicated by the fact that the interference directly affects parties other than
the HVDC owner, i.e. the telephone system operator and its subscriber customers. It is possible
that the HVDC system can be forced to cease operation by legal or regulatory action until the
HVDC filtering system is redesigned and modified or telephone system mitigation measures are
applied. When the whole process of inductive coordination is done correctly, it is much easier
to face a problem at the initiation of the project.
If not used with consideration, the requirements, and equally important how to evaluate them,
can lead to an unduly complex and costly design. Clause 4 attempts to clarify many aspects of
the subject, presenting the theory, assessing practical experience and providing guidelines.
4.2 Determining the necessity for telephone interference limits
While voltage distortion control is a common concern for any electrical network, telephone
interference is highly project dependent. Interference can occur when harmonic currents flow
in an AC transmission line which runs parallel to telephone lines. The harmonic currents induce
a disturbing voltage in the telephone lines which is proportional to the length of exposure and
the per unit mutual impedance between the two circuits. Subclause 4.2 specifically deals with
harmonic limits related to telephone interference such as IT, TIF, I and THFF. These criteria
eq
aim to control the interference induced in cable wire telephone lines transmitting signals in the
(vocal) audible frequency band, i.e. approximately between 200 Hz and 3 500 Hz.
There is no easy way to give quantitative guidance as to the conditions where telephone
interference has the potential to be of significance for a project, or where specific telephone-
interference oriented specifications are necessary to protect the buyer. Qualitative guidelines
are provided below. If there is concern that a project can have susceptibility to one or more of
these factors, an inductive coordination study is desirable to guide the development of
specifications.
Conditions known to increase the susceptibility to AC-side telephone interference are the
following.
– Long sections of exposure between AC lines carrying converter harmonic currents and
telephone lines.
– Close proximity of AC transmission lines and parallel telephone lines.
– Even moderate separation distances and longitudinal exposures if combined with very high
earth resistivity.
– Single-wire telephone lines.
NOTE Even shielded twisted-pair telephone circuits do not provide complete balance of the induced votage in
each conductor, and such circuits are by no means exempt from potential interference issues.
– Radial transmission line(s) to the converter station, where all converter harmonics are
forced into the one single-circuit or double-circuit line.
– AC transmission systems having a hybrid overhead/underground design, with overhead
lines interspersed with underground cable sections.
– AC transmission systems with a large number of capacitor banks in electrical proximity to
the converter station, causing numerous resonances in the AC network. Analysis is
complicated in these systems because all combinations and permutations of capacitor bank
status should be considered.
Converter harmonic currents are not limited to the AC lines terminating at the converter station.
Harmonic currents can penetrate several tiers into the transmission network and can cross over
transformers to other voltage levels. This can be problematic when lower-voltage transmission
lines are more closely coupled to telephone circuits. There is a general tendency for harmonic
currents to diminish for tiers remote from the converter station, but this general trend can be
offset by resonance conditions to produce greater harmonic current levels at second and higher
tier lines than on first-tier lines connected to the converter station.
The following conditions can be assumed to indicate non-existence of telephone interference
issues at vocal frequency, and thus no need for psophometric or C-message weighted
specifications:
– all exposed telephone circuits are fibre optic cable;
– multiplex systems (time or frequency multiplexing):
– no telephone circuits exposed.
Worldwide experience of HVDC has shown that, in numerous schemes, telephone interference
limits have not been specified, yet no problems have been experienced. Indeed, telephone
systems are very similar from country to country but others parameters affecting the potential
for interference can be quite different. In North America for instance, telephone interference is
a big concern because of the structure of the telephone and transmission systems in rural areas
favouring long exposures and close proximity. There is also powerful legal protection for
consumers and utilities with a risk of serious economical consequences for an HVDC project
causing excessive telephone interference. On the other hand, in China for example, most
telephone lines are generally remote from HV transmission lines. Huge HVDC infrastructure
projects can have a "national interest" dimension which means that, in terms of the overall effect
on society, it is more important to build them quickly and economically, and to address possible
telephone noise problems as a separate issue.
Nearly all homes and small businesses in North America and many other parts of the world are
still connected to the phone network by the same pair of twisted copper wires that have been
in use for decades. Given the continued hurdles to fibre deployment and the increasingly high
transmission speeds available over the existing copper network, it is likely that copper will
continue to be the industry's standard for many years to come. This is especially true in rural
areas due to the economics of installing fibre optic cabling or coaxial cabling through low density
areas. However, in many countries, the cellular phone digital technologies are tending to
leapfrog analogue landline telephone system. Furthermore, telecom operators' tariffs in these
countries are guiding people to use mobile phones only.
Past experience within the utility and the telephone company with telephone interference from
existing facilities would be the best reference since it is likely to reflect the particular situation
where the new HVDC project will have to operate.
4.3 Defining appropriate telephone interference limits
4.3.1 General
IEC TR 62001-1 [2] gives general recommendations for determination of limits without detailed
studies due to possible short time schedule, lack of computational tool, lack of telephone system
data or if no serious interference problems are expected because of harmonic distortion. It
TM
refers to IEEE Std 368 [4] which gives a table suggesting range of limits applicable to HV
and EHV transmission lines, with a clear warning that telephone interference should be carefully
studied on a case-by-case basis. The table of values is merely illustrative and its derivation is
TM
not given. Older versions (1993 and earlier) of IEEE Std 519 -2022 [5] and CAN/CSA-C22.3
no. 3-98 [6] have copied this table with no apparent verification of its validity. On the other hand,
experience shows that some HVDC schemes with a specified IT emanating from a converter
bus of between 25 000 A and 50 000 A function with no problems of telephone interference.
Applying these previous limits without any study is therefore not recommended.
If it has been established that there is a significant risk of telephone interference related to a
particular HVDC project, a detailed study is required to assess the limits for the AC filter
performance specifications. Subclause 4.3 gives a general description of the procedure to
calculate the influence of a given transmission line on adjacent telephone lines. The method
presented is based on the North American practice because interference problems appear to
be more acute in that part of the world, and focuses on telephone cable systems, but the same
basic principles apply to other systems with different susceptibility levels. Tables of illustrative
values of coupling are provided.
It is also necessary to assess the harmonic current flow in transmission lines adjacent to the
HVDC project in order to identify the ones that should be considered for the telephone
interference requirement of the HVDC project, and their possible level of interference.
Recommendations are given on the required information about the AC system that should be
included in a specification to achieve an adequate AC filter design.
4.3.2 Mechanisms of interference
Harmonic currents flowing in a transmission line induce harmonic voltages and currents in
nearby installations. This voltage can be measured between one end of the telephone conductor
and ground, with the remote end grounded, and is called the longitudinal voltage. The
longitudinal voltage induced in any parallel conductor can be calculated as shown in
Formula (1):
k
U ()IZ× (1)
gm∑ jn
n jn
j=1
where
n is the harmonic number;
j is the conductor number;
k is the number of conductors on the transmission line;
U
g
n
is the longitudinal voltage at harmonic n;
I is the phasor current in conductor j at harmonic n;
jn
Z
m
jn
is the mutual impedance between conductor j and telephone line at harmonic n,
including the screening effect of the ground wires and any other nearby grounded
conductors.
In Formula (1), the harmonic currents flowing in the transmission line are calculated by the
HVDC converter contractor according to the method defined in the technical specifications.
The mutual impedance depends mainly on earth resistivity, separation between transmission
and telephone lines, transmission line configuration and frequency. Inductive coordination
studies require the calculation of mutual impedances for a large number of exposures between
AC transmission lines and telephone lines. The calculation usually includes the effect of
screening conductors like shield wires or any other extended conductive installation close by.
This calculation is generally done by computer programs such as PSCAD/EMTDC, EMTP,
CORRIDOR, MathCAD, MATLAB, Python, CDEGS . However, for simple cases, Dubanton
equations [7] can be used with satisfactory results for a typical range of values of exposures.
In addition, the calculation of coupling for irregular exposures involves breaking down the
exposures into a series of parallel sections and adding these together to obtain the total
coupling [8], [9]. Some computer programs have the capacity to calculate mutual impedances
for irregular exposures (Crinoline toolbox in EMTP, CDEGS).
Modern telephone lines use shielded cables to transmit the voice signal to each customer via a
twisted conductor pair. The shield supports the same harmonic induced voltages as the
conductor pair but allows current flow through its grounded ends which cancels out part of the
induced voltage in the conductor pair and is very effective at higher frequencies to reduce the
longitudinal voltage. The resulting interference voltage is the difference between both conductor
longitudinal voltages, which is called the metallic or transverse voltage, and is the voltage which
appears across a telephone receiver.
NOTE The terms "common mode" for longitudinal and "differential mode" for transverse are also used.
___________
PSCAD/EMTDC, EMTP, CORRIDOR, MathCAD, MATLAB, Python, CDEGS are examples of suitable products
available commercially. This information is given for the convenience of users of this document and does not
constitute an endorsement by IEC of these products.
=
The ratio between metallic and longitudinal voltage is called the balance of the circuit and is
frequency dependent. The metallic voltage is then weighted to reflect the frequency response
of the ear and the telephone system. The C-message weighting is used in North America while
psophometric weighting is used in Europe. Other parts of the world adopt one or other of these
methods.
The total effective noise will be calculated by the root of the squares of these weighted
components for each harmonic to be considered. The total weighted metallic noise voltage is
given by Formula (2):
nh=  jk= 
  (2)
V I× Zh K× B× C
∑∑
h jn jn n n n
 
n 11j
 
where
h is the maximum order of harmonic to be considered;
C is the C-message or psophometric weighting of harmonic n;
n
K is the telephone circuit shielding factor at harmonic n;
n
B is the telephone circuit balance at harmonic n.
n
The telephone circuit shielding and balance factors are generally provided by the telephone
companies. In practice, the shielding improves with frequency while the balance gets worse as
frequency increases. The combined factor is almost constant over the frequency range of
interest.
TM
IEEE Std 1124-2003 [10] provides a great deal of information about the calculation of mutual
impedances and the characteristics of the different parameters relevant to an inductive
coordination study. The methodology of inductive coordination for a DC transmission line is
basically the same as for an AC line. Useful information on the management of electromagnetic
interference by power systems on telecommunication systems can also be found in [11].
Influence of voltage and current distortion on telephone interference level is considered in
Annex A.
4.3.3 Noise performance coordination levels
ITU-T EMC-1.6 [12] used in Europe and elsewhere states that the psophometric voltage
measured across a resistance of 600 Ω at one end of the line with the remote end terminated
with its characteristic impedance should not exceed 0,5 mV.
The North American standards ([13], [14]) recommend limiting the noise contribution on the
customer loop to 20 dBrnC. The telephone circ
...