IEC TR 62681:2014

Electromagnetic performance of high voltage direct current (HVDC) overhead transmission lines

IEC TR 62681:2014 provides general guidance on the electromagnetic environment issues of HVDC transmission lines. It concerns the major parameters adopted to describe the electromagnetic environment of a High-Voltage Direct Current (HVDC) transmission line, including electric fields, ion current, magnetic fields radio interference, and audible noise generated as a consequence of such effects. Engineers in different countries can refer to this Technical Report to ensure the safe operation of HVDC transmission lines, limit the influence on the environment within acceptable ranges, and optimize engineering costs.

General Information

• Status: Published
• Publication Date: 11-Aug-2014
• ICS: 29.240.20 - Power transmission and distribution lines
• Technical Committee: TC 115 - High Voltage Direct Current (HVDC) transmission for DC voltages above 100 kV
• Drafting Committee: WG 3 - TC 115/WG 3
• Current Stage: DELPUB - Deleted Publication
• Start Date: 25-May-2022
• Completion Date: 26-Oct-2025

Relations

• Revised: IEC TR 62681:2022 - Electromagnetic performance of high voltage direct current (HVDC) overhead transmission lines
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC TR 62681 ®
Edition 1.0 2014-08
TECHNICAL
REPORT
colour
inside
Electromagnetic performance of high voltage direct current (HVDC) overhead
transmission lines
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IEC TR 62681 ®
Edition 1.0 2014-08
TECHNICAL
REPORT
colour
inside
Electromagnetic performance of high voltage direct current (HVDC) overhead

transmission lines
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XD
ICS 29.240.20 ISBN 978-2-8322-1780-1

– 2 – IEC TR 62681:2014  IEC 2014
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Terms and definitions . 9
3 Electric field and ion current . 10
3.1 Description of the physical phenomena . 10
3.2 Calculation methods . 13
3.2.1 General . 13
3.2.2 Semi-analytic method . 14
3.2.3 Finite element method . 16
3.2.4 BPA method . 17
3.2.5 Empirical methods of EPRI . 18
3.2.6 Recent progress . 18
3.3 Experimental data . 19
3.3.1 General . 19
3.3.2 Instrumentation and measurement methods . 19
3.3.3 Experimental results for electric field and ion current . 21
3.3.4 Discussion . 21
3.4 Implication for human and natural environment . 22
3.4.1 General . 22
3.4.2 Static electric field . 23
3.4.3 Research on space charge . 23
3.4.4 Scientific review . 28
3.5 Design practice of different countries . 29
4 Magnetic field . 30
4.1 Description of physical phenomena . 30
4.2 Magnetic field of HVDC transmission lines . 31
5 Radio interference (radio noise) . 32
5.1 Description of radio interference phenomena of HVDC transmission system . 32
5.1.1 General . 32
5.1.2 Physical aspects of d.c. corona . 32
5.1.3 Mechanism of formation of a noise field of d.c. line . 33
5.1.4 Characteristics of radio interference from d.c. line . 33
5.1.5 Factors influencing the RI from d.c. line . 34
5.2 Calculation methods . 36
5.2.1 EPRI empirical formula . 36
5.2.2 IREQ empirical method . 37
5.2.3 CISPR bipolar line RI prediction formula . 38
5.2.4 Comparison of different prediction formula . 38
5.3 Experimental data . 38
5.3.1 Measurement apparatus and methods . 38
5.3.2 Experimental results for radio interference . 39
5.4 Criteria of different countries . 39
6 Audible noise . 40
6.1 Basic principles of audible noise . 40
6.2 Description of physical phenomena . 41

6.2.1 Lateral profiles . 42
6.2.2 Statistical distribution . 44
6.2.3 Influencing factors . 45
6.2.4 Effect of altitude above sea level . 47
6.2.5 Concluding remarks . 47
6.3 Calculation methods . 48
6.3.1 General . 48
6.3.2 Theoretical analysis of audible noise propagation . 48
6.3.3 Empirical formulas of audible noise . 49
6.3.4 Semi-empirical formulas of audible noise . 49
6.3.5 CEPRI (China) research results . 52
6.3.6 Concluding remarks . 52
6.4 Experimental data . 52
6.4.1 Measurement techniques and instrumentation . 52
6.4.2 Experimental results for audible noise . 53
6.5 Design practice of different countries . 53
6.5.1 General . 53
6.5.2 The effect of audible noise on people . 53
6.5.3 The audible noise level and induced complaints . 54
6.5.4 Limit values of audible noise of HVDC transmission lines in different
countries . 57
6.5.5 Recommended noise level limit . 58
6.5.6 Main conclusion . 58
Annex A (informative) Experimental results for electric field and ion current. 59
A.1 Bonneville Power Administration ±500 kV HVDC transmission line . 59
A.2 FURNAS ±600 kV HVDC transmission line . 59
A.3 Manitoba Hydro ±450 kV HVDC transmission line . 60
A.4 Hydro-Québec – New England ±450 kV HVDC transmission line . 62
A.5 IREQ test line study of ±450 kV HVDC line configuration . 63
A.6 HVTRC test line study of ±400 kV HVDC line configuration . 64
A.7 Test study in China . 66
Annex B (informative) Experimental results for radio interference . 68
B.1 Bonneville power administration’s 1 100 kV direct current test project . 68
B.1.1 General . 68
B.1.2 Lateral profile . 68
B.1.3 Influence of conductor gradient . 69
B.1.4 Percent cumulative distribution . 70
B.1.5 Influence of wind . 72
B.1.6 Spectrum . 72
B.2 Hydro-Québec institute of research . 74
B.2.1 General . 74
B.2.2 Cumulative distribution . 74
B.2.3 Spectrum . 75
B.2.4 Lateral profiles . 75
B.2.5 Cumulative distribution under different voltage . 76
B.3 d.c. line of China . 76
Annex C (informative) Experimental results for audible noise . 78
Bibliography . 81

– 4 – IEC TR 62681:2014  IEC 2014

Figure 1 – Unipolar and bipolar space charge regions of a HVDC transmission line [1] . 11
Figure 2 – Lateral profile of magnetic field on the ground of ±800 kV HVDC lines . 32
Figure 3 – The corona current and radio interference field . 33
Figure 4 – RI tolerance tests: reception quality as a function of signal-to-noise ratio . 40
Figure 5 – Attenuation of different weighting networks used in audible-noise
measurements [14] . 41
Figure 6 – Comparison of typical audible noise frequency spectra [129] . 42
Figure 7 – Lateral profiles of the AN . 43
Figure 8 – Lateral profiles of the AN from a bipolar HVDC-line equipped with 8 ×
4,6 cm (8 × 1,8 in) conductor bundles energized with ±1 050 kV [32] . 43
Figure 9 – Lateral profiles of fair-weather A-weighted sound level . 44
Figure 10 – All weather distribution of AN and RI at +15 m lateral distance of the
positive pole from the upgraded Pacific NW/SW HVDC Intertie [32] . 45
Figure 11 – Statistical distributions of fair weather Aweighted sound level measured at
27 m lateral distance from the line center during spring 1980 . 45
Figure 12 – Audible noise complaint guidelines [12] . 54
Figure 13 – Measured lateral profile of audible noise on a 330 kV AC transmission line
[149] . 55
Figure 14 – Subjective evaluation of d.c. transmission line audible noise; EPRI test
center study 1974 [31] . 55
Figure 15 – Subjective evaluation of d.c. transmission line audible noise; OSU study
1975 [31] . 56
Figure 16 – Results of the operators’ subjective evaluation of AN from HVDC lines . 57
Figure 17 – Results of subjective evaluation of AN from d.c. lines . 57
Figure A.1 – Electric field and ion current distributions for Manitoba Hydro ±450 kV
Line [37] . 61
Figure A.2 – Cumulative distribution of electric field for Manitoba Hydro ±450 kV Line
[37] 62
Figure A.3 – Cumulative distribution of ion current density for Manitoba Hydro ±450 kV
line [37] . 62
Figure A.4 – Test result for total electric field at different humidity [117] . 67
Figure A.5 – Comparison between the calculation result and test result for the total
electric field (minimum conductor height is 18 m) [117] . 67
Figure B.1 – Connection for 3-section d.c. test line [121] . 68
Figure B.2 – Typical RI lateral profile at ±600kV, 4 × 30,5 mm conductor, 11,2 m pole
spacing, 15,2 m average height [12] . 69
Figure B.3 – Simultaneous RI lateral, midspan, in clear weather and light wind for three
configurations, bipolar ±400 kV [121] . 69
Figure B.4 – RI at 834kHz as a function of bipolar line voltage 4 × 30,5 mm conductor,
11,2 m pole spacing, 15,2 m average height . 70
Figure B.5 – Percent cumulative distribution for fair weather, 2 × 46 mm, 18,3 m pole
spacing, ±600 kV . 70
Figure B.6 – Percent cumulative distribution for rain weather, 2 × 46 mm, 18,3 m pole
spacing, ±600 kV . 71
Figure B.7 – Percent cumulative distribution for fair weather, 4 × 30,5 mm, 13,2 m pole
spacing, ±600 kV . 71

Figure B.8 – Percent cumulative distribution for rain weather, 4 × 30,5 mm, 13,2 m pole
spacing, ±600 kV . 72
Figure B.9 – Radio interference frequency spectrum . 73
Figure B.10 – RI vs. frequency at ±400 kV [121] . 73
Figure B.11 – Cumulative distribution of RI measured at 15 m from the positive pole
[122] . 74
Figure B.12 – Conducted RI frequency spectrum measured with the line terminated at
one end [122] . 75
Figure B.13 – Lateral profile of RI [122] . 76
Figure B.14 – Annual cumulative distribution of RI measured at 15 m from the positive
pole [122] . 76
Figure B.15 – Comparison between calculation result and test result for RI lateral
profile [117] . 77
Figure C.1 – Examples of statistical distributions of fair weather audible noise. dB(A)
measured at 27 m from line center of a bipolar HVDC test line [14] . 79

Table 1 – Electric field and ion current limits of ±800 kV d.c. lines in China . 30
Table 2 – Electric field limits of d.c. lines in United States of America [119] . 30
Table 3 – Electric field and ion current limits of d.c. lines in Canada . 30
Table 4 – Electric field limits of d.c. lines in Brazil . 30
Table 5 – Parameters of the IREQ excitation function [120] . 38
Table 6 – Comparison of the EPRI and CISPR formula . 38
Table 7 – Parameters defining regression equation for generated acoustic power
density . 51
Table 8 – Typical sound attenuation (in decibels) provided by buildings [155] . 58
Table A.1 – BPA ±500 kV line: statistical summary of all-weather ground-level electric
field intensity and ion current density [32] . 59
Table A.2 – FURNAS ±600 kV line: statistical summary of ground-level electric field
intensity and ion current density [36] . 60
Table A.3 – Hydro-Québec–New England ±450 kV HVDC transmission line. Bath, NH;
1990-1992 (fair weather), 1992 (rain), All-season measurements of static electric E-
field in kV/m [39] . 63
Table A.4 – Hydro-Québec – New England ±450 kV HVDC Transmission Line. Bath,
NH; 1990-1992, All-season fair-weather measurements of ion concentrations in
kions/cm [39] . 63
Table A.5 – IREQ ± 450 kV test line: statistical summary of ground-level electric field
intensity and ion current density [41] . 64
Table A.6 – HVTRC ±400 kV test line: statistical summary of peak electric field and ion
currents [42] . 65
Table A.7 – Statistic results for the test data of total electric field at ground (50 %
value) [117] . 66
Table B.1 – Influence of wind on RI . 72
Table B.2 – Statistical representation of the long term RI performance of the tested
conductor bundle [122] . 75
Table B.3 – RI at 0,5 MHz at lateral 20m from positive pole (fair weather) . 77
Table C.1 – Audible Noise Levels of HVDC Lines according to [119] and [150] . 80

– 6 – IEC TR 62681:2014  IEC 2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROMAGNETIC PERFORMANCE OF HIGH VOLTAGE DIRECT
CURRENT (HVDC) OVERHEAD TRANSMISSION LINES

FOREWORD
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IEC TR 62681, which is a technical report, has been prepared by IEC technical committee 115:
High Voltage Direct Current (HVDC) transmission for d.c. voltages above 100 kV.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
115/71/DTR 115/84/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The “colour inside” logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this publication using a colour printer.

– 8 – IEC TR 62681:2014  IEC 2014
INTRODUCTION
Electric fields and magnetic fields are produced in the vicinity of an HVDC transmission line.
When the electric field at the conductor surface exceeds a critical value, known as the corona
onset gradient, positive or negative free charges leave the conductor and interact with the
surrounding air and ionization takes place in the layer of surrounding air, leading to the
formation of corona discharges. The corona discharge will not only bring out corona loss but
also produce electromagnetic environment problems.
The parameters used to describe the electromagnetic environment of an HVDC transmission
line mainly include the:
1) electric field,
2) ion current,
3) magnetic field,
4) radio interference,
5) audible noise.
To control these parameters in a reasonable and acceptable range, for years, a great deal of
theoretical and experimental research was conducted in many countries, and relevant national
standards or enterprise standards were developed. This Technical Report collects and
records the status of study and progress of electric fields, ion current, magnetic fields, radio
interference, and audible noise of HVDC transmission lines.

ELECTROMAGNETIC PERFORMANCE OF HIGH VOLTAGE DIRECT
CURRENT (HVDC) OVERHEAD TRANSMISSION LINES

1 Scope
This Technical Report provides general guidance on the electromagnetic environment issues
of HVDC transmission lines. It concerns the major parameters adopted to describe the
electromagnetic environment of a High-Voltage Direct Current (HVDC) transmission line,
including electric fields, ion current, magnetic fields radio interference, and audible noise
generated as a consequence of such effects. Engineers in different countries can refer to this
Technical Report to:
– ensure the safe operation of HVDC transmission lines,
– limit the influence on the environment within acceptable ranges, and
– optimize engineering costs.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
corona
set of partial discharges in a gas, immediately adjacent to an uninsulated or lightly insulated
conductor which creates a highly divergent field remote from other conductors
[SOURCE: IEC 60050-212:2010, 212-11-44, modified – Note 1 has been deleted.]
2.2
electric field
constituent of an electromagnetic field which is characterized by the electric field strength E
together with the electric flux density D
Note 1 to entry: In the context of HVDC transmission lines, the electric field is affected not only by the geometry
of the line and the potential of the conductor, but also by the space charge generated as a result of corona;
consequently, electric field distribution may vary non-linearly with the line potential.
[SOURCE: IEC 60050-121:1998, 121-11-67, modified – Note 1 to entry has been added.]
2.3
space-charge-free electric field
electric field due to a system of energized electrodes, excluding the effect of space charge
present in the inter-electrode space
2.4
ion current
flow of electric charge resulting from the motion of ions
2.5
magnetic field
constituent of an electromagnetic field which is characterized by the magnetic field strength H
together with the magnetic flux density B
[SOURCE: IEC 60050-121:1998, 121-11-69, modified – Note 1 has been deleted.]

– 10 – IEC TR 62681:2014  IEC 2014
2.6
radio interference
degradation of the reception of a wanted signal caused by RF disturbance
2.7
audible noise
unwanted sound with frequency range from 20 Hz to 20 kHz
[SOURCE: IEC 61973:2012, 3.1.14]
3 Electric field and ion current
3.1 Description of the physical phenomena
Electric fields are produced in the vicinity of a HVDC transmission line, with the highest
electric fields existing at the surface of the conductor. When the electric field at the conductor
surface exceeds a critical value, the air in the vicinity of the conductor becomes ionized,
forming a corona discharge. Ions of both polarities are formed, but ions of opposite polarity to
the conductor potential are attracted back towards the conductor, while ions of the same
polarity as the conductor are repelled away from the conductor. Space charges include air
ions and charged aerosols. Under the action of an electric field, space charge will move
directionally and ion current will be formed. The physical phenomena of electric field and ion
current are described in this clause.
The electric field and ion current in the vicinity of an HVDC transmission line are defined
mainly by the operating voltage and line configuration. The voltage applied to line conductors
produces an electric field distribution. Unlike High-voltage Alternating Current (HVAC)
transmission lines, the electric field produced by HVDC transmission lines does not vary with
time and, consequently, does not produce any significant currents in humans or objects
immersed in these fields.
The electric field is another aspect of the electrical environment around an overhead HVDC
transmission line. An electric field is present around any charged conductor, irrespective of
whether corona discharge is taking place. However, the space charge created by corona
discharge under d.c. conditions modifies the distribution of an electric field. The effect of
space charge on electric fields is significant.
For the same HVDC transmission lines, the corona onset gradients of positive or negative
polarities are different and the intensity and characteristics of corona discharges on positive
or negative conductors are also different. Consequently, during the design of HVDC
transmission lines, special consideration should be paid to the allowable values of the
maximum ground-level electric field and ion current density [1] .
Corona on a conductor of either positive or negative polarity produces ions of either the
positive or negative polarities in a thin layer of air surrounding each conductor [1]. However,
ions with a polarity opposite to that of the conductor are drawn to it and are neutralized on
contact. Thus, a positive conductor in corona acts as a source of positive ions and vice-versa.
For a unipolar d.c. transmission line, ions having the same polarity as the conductor voltage
fill the entire inter-electrode space between the conductors and ground. For a bipolar d.c.
transmission line, the ions generated on the conductors of each polarity are subject to an
electric field driven drift motion either towards the conductor of opposite polarity or towards
the ground plane, as shown in Figure 1. The influence of wind or the formation of charged
aerosols are not considered at this stage. Three general space charge regions are created in
this case:
—————————
Numbers in square brackets refer to the bibliography.

a) a positive unipolar region between the positive conductor and ground,
b) a negative unipolar region between the negative conductor and ground,
c) a bipolar region between the positive and negative conductors.
For practical bipolar HVDC lines, most of the ions are directed toward the opposite polarity
conductor, but a significant fraction is also directed toward the ground. The ion drift velocity is
such that it will take at least a few seconds for them to reach ground. Actually, the molecules
traveling along ion paths are not always the same ions. In fact, collisions between ions and air
molecules occur during the travel at a rate of billions per second and cause charge transfer
and reactions between ions and neutral molecules, so the ions reaching the ground are quite
different from those that were originally formed by corona near the conductor surface. The
exact chemical identity of the ions, after a few seconds, will depend on the chemical
composition and trace gases at the location.
Electric field is another component of the electrical environment around an overhead HVDC
transmission line. Electric field is caused by electrical charges, both those residing on
conductive surfaces (the transmission line conductors, the ground, and conducting objects)
and the space charges. The effect of space charge on electric field is significant.
A nonlinear interaction takes place between electric field and space charge distributions in all
three general space regions identified above in a), b), c). The nonlinearity arises because
ions flow from each conductor to ground or to the conductor of opposite polarity along the flux
lines of the electric field distribution: while at the same time, the electric field distribution is
influenced by the ionic space charge distribution. In addition to the nonlinear interaction
described above, the space charge field in the bipolar region is affected by other factors.
Mixing of ions of both polarities in the bipolar region leads firstly, to a reduction in the net
space charge density and secondly, to recombination and neutralization of ions of both
polarities.
Bipolar
region
− +
Negative Positive
region region
IEC
Figure 1 – Unipolar and bipolar space charge regions of a HVDC transmission line [1]
The corona-generated space charge, being of the same polarity as the conductor, produces a
screening effect on the conductor by lowering the electric field in the vicinity of the conductor
surface and consequently reducing the intensity of corona discharges occurring on the
conductor. In the unipolar regions, the space charge enhances the electric field at the ground
surface. The extent of electric field reduction at the conductor surface and field enhancement
at the ground surface depend on the conductor voltage as well as on the corona intensity at
the conductor surface. In the case of the bipolar region, however, the mixture of ions of
opposite polarity and ion recombination tend to reduce the screening effect on the conductor
surface. This leads to a smaller reduction in the intensity of corona activity near the
conductors than in the unipolar regions.

– 12 – IEC TR 62681:2014  IEC 2014
The electrical environment at ground level under a bipolar HVDC transmission line is,
therefore defined mainly by three quantities:
a) electric field, E,
b) ion current density, J,
c) space charge density, ρ.
The electric field produced by HVDC overhead transmission lines is a vector defined by its
components along three orthogonal axes. The space charge density is a scalar. The ion
current density is also a vector, and it is determined by the electric field and space charge
density.
Very small currents in some cases may flow through an object or person located under the
line because of exposure to the electric field and ion space charge. From the point of view of
environmental impact on persons and objects located under the line, the main consideration is
the combined exposure to the electric fields and ion currents. The scientific literature
indicates that exposure to the levels of d.c. electric field and ion current density existing under
operating HVDC transmission lines pose no risk to public health, but may cause some
induced current and annoyance effects to humans. Consequently, during designing of HVDC
transmission lines, special consideration should be paid for the allowable values of the
maximum ground-level electric field and ion current density [1].
Design of HVDC transmission lines requires the ability to predict ground-level electric field
and ion current distribution as functions of line design parameters such as the number and
diameter of sub-conductors in the bundle, height above ground of conductors and pole
spacing. Prediction methods are based on a combination of analytical techniques to calculate
the space charge fields and accurate long-term measurements under experimental as well as
operating HVDC transmission lines.
As described and illustrated in Figure 1, the ground-level electric field and ion current
environment under a bipolar HVDC transmission line can be thought primarily as a unipolar
space charge field under each pole. The bipolar space charge field between the positive and
negative conductors, however, has no significant impact on the ground-level electrical
environment. For the purpose of calculating the ground-level electric field and ion current
distributions, therefore, analytical treatment of the unipolar space charge field between each
of the positive and negative conductors and the ground plane is adequate.
Unipolar d.c. space charge fields are defined by the following equations:
ρ
∇ ⋅ E = (1)
ε
J = µρE (2)
∇ ⋅ J = 0 (3)
where
E and J are the electric field and ion current density vectors at any point in space,
ρ is the space charge density,
µ is the ionic mobility,
ε is the permittivity of free space.
The first Equation (1) is Poisson's equation, the second Equation (2) defines the relationship
between the current density and electric field vectors, and the third Equation (3) is the

continuity equation for ions. The solution of these equations, along with appropriate boundary
conditions, for the conductor-ground-plane geometry of the HVDC transmission line,
determines the ground-level electric field and ion current distributions [1].
Corona activity on conductors and the resulting space charge field are influenced, in addition
to the line voltage and geometry, by ambient weather conditions such as temperature,
pressure, humidity, precipitation and wind velocity as well as by the presence of any aerosols
and atmospheric pollution. It is difficult, if not impossible, to take all these factors into account
in any analytical treatment of space charge fields. Information on the corona onset gradients
of conductors, which is an essential input in the analytical determination of electric field and
ion current environment, is also difficult to obtain under practical operating conditions. For
these reasons, it is necessary to use analytical methods in combination with accurate long-
term measurements of ground-level electric field and ion current distributions under
experimental as well as operating HVDC transmission lines, in order to develop prediction
methods. Some of the information required in the analytical treatment, such as corona onset
gradients of conductors, can be obtained only through experimental studies. Reliable
experimental data is also essential in validating the accuracy of analytical or semi-analytical
methods for predicting the ground-level electric field and ion current distributions under HVDC
transmission lines.
NOTE 1 Industry consensus and standards have not been reached on appropriate analytical methods to capture
...

IEC TR 62681 ®
Edition 1.0 2014-08
TECHNICAL
REPORT
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inside
Electromagnetic performance of high voltage direct current (HVDC) overhead
transmission lines
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IEC TR 62681 ®
Edition 1.0 2014-08
TECHNICAL
REPORT
colour
inside
Electromagnetic performance of high voltage direct current (HVDC) overhead

transmission lines
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XD
ICS 29.240.20 ISBN 978-2-8322-1780-1

– 2 – IEC TR 62681:2014  IEC 2014

CONTENTS
FOREWORD . 6

INTRODUCTION . 8

1 Scope . 9

2 Terms and definitions . 9

3 Electric field and ion current . 10

3.1 Description of the physical phenomena . 10

3.2 Calculation methods . 13
3.2.1 General . 13
3.2.2 Semi-analytic method . 14
3.2.3 Finite element method . 16
3.2.4 BPA method . 17
3.2.5 Empirical methods of EPRI . 18
3.2.6 Recent progress . 18
3.3 Experimental data . 19
3.3.1 General . 19
3.3.2 Instrumentation and measurement methods . 19
3.3.3 Experimental results for electric field and ion current . 21
3.3.4 Discussion . 21
3.4 Implication for human and natural environment . 22
3.4.1 General . 22
3.4.2 Static electric field . 23
3.4.3 Research on space charge . 23
3.4.4 Scientific review . 28
3.5 Design practice of different countries . 29
4 Magnetic field . 30
4.1 Description of physical phenomena . 30
4.2 Magnetic field of HVDC transmission lines . 31
5 Radio interference (radio noise) . 32
5.1 Description of radio interference phenomena of HVDC transmission system . 32
5.1.1 General . 32
5.1.2 Physical aspects of d.c. corona . 32
5.1.3 Mechanism of formation of a noise field of d.c. line . 33

5.1.4 Characteristics of radio interference from d.c. line . 33
5.1.5 Factors influencing the RI from d.c. line . 34
5.2 Calculation methods . 36
5.2.1 EPRI empirical formula . 36
5.2.2 IREQ empirical method . 37
5.2.3 CISPR bipolar line RI prediction formula . 38
5.2.4 Comparison of different prediction formula . 38
5.3 Experimental data . 38
5.3.1 Measurement apparatus and methods . 38
5.3.2 Experimental results for radio interference . 39
5.4 Criteria of different countries . 39
6 Audible noise . 40
6.1 Basic principles of audible noise . 40
6.2 Description of physical phenomena . 41

6.2.1 Lateral profiles . 42

6.2.2 Statistical distribution . 44

6.2.3 Influencing factors . 45

6.2.4 Effect of altitude above sea level . 47

6.2.5 Concluding remarks . 47

6.3 Calculation methods . 48

6.3.1 General . 48

6.3.2 Theoretical analysis of audible noise propagation . 48

6.3.3 Empirical formulas of audible noise . 49

6.3.4 Semi-empirical formulas of audible noise . 49

6.3.5 CEPRI (China) research results . 52
6.3.6 Concluding remarks . 52
6.4 Experimental data . 52
6.4.1 Measurement techniques and instrumentation . 52
6.4.2 Experimental results for audible noise . 53
6.5 Design practice of different countries . 53
6.5.1 General . 53
6.5.2 The effect of audible noise on people . 53
6.5.3 The audible noise level and induced complaints . 54
6.5.4 Limit values of audible noise of HVDC transmission lines in different
countries . 57
6.5.5 Recommended noise level limit . 58
6.5.6 Main conclusion . 58
Annex A (informative) Experimental results for electric field and ion current. 59
A.1 Bonneville Power Administration ±500 kV HVDC transmission line . 59
A.2 FURNAS ±600 kV HVDC transmission line . 59
A.3 Manitoba Hydro ±450 kV HVDC transmission line . 60
A.4 Hydro-Québec – New England ±450 kV HVDC transmission line . 62
A.5 IREQ test line study of ±450 kV HVDC line configuration . 63
A.6 HVTRC test line study of ±400 kV HVDC line configuration . 64
A.7 Test study in China . 66
Annex B (informative) Experimental results for radio interference . 68
B.1 Bonneville power administration’s 1 100 kV direct current test project . 68
B.1.1 General . 68
B.1.2 Lateral profile . 68

B.1.3 Influence of conductor gradient . 69
B.1.4 Percent cumulative distribution . 70
B.1.5 Influence of wind . 72
B.1.6 Spectrum . 72
B.2 Hydro-Québec institute of research . 74
B.2.1 General . 74
B.2.2 Cumulative distribution . 74
B.2.3 Spectrum . 75
B.2.4 Lateral profiles . 75
B.2.5 Cumulative distribution under different voltage . 76
B.3 d.c. line of China . 76
Annex C (informative) Experimental results for audible noise . 78
Bibliography . 81

– 4 – IEC TR 62681:2014  IEC 2014

Figure 1 – Unipolar and bipolar space charge regions of a HVDC transmission line [1] . 11

Figure 2 – Lateral profile of magnetic field on the ground of ±800 kV HVDC lines . 32

Figure 3 – The corona current and radio interference field . 33

Figure 4 – RI tolerance tests: reception quality as a function of signal-to-noise ratio . 40

Figure 5 – Attenuation of different weighting networks used in audible-noise

measurements [14] . 41

Figure 6 – Comparison of typical audible noise frequency spectra [129] . 42

Figure 7 – Lateral profiles of the AN . 43

Figure 8 – Lateral profiles of the AN from a bipolar HVDC-line equipped with 8 ×
4,6 cm (8 × 1,8 in) conductor bundles energized with ±1 050 kV [32] . 43
Figure 9 – Lateral profiles of fair-weather A-weighted sound level . 44
Figure 10 – All weather distribution of AN and RI at +15 m lateral distance of the
positive pole from the upgraded Pacific NW/SW HVDC Intertie [32] . 45
Figure 11 – Statistical distributions of fair weather Aweighted sound level measured at
27 m lateral distance from the line center during spring 1980 . 45
Figure 12 – Audible noise complaint guidelines [12] . 54
Figure 13 – Measured lateral profile of audible noise on a 330 kV AC transmission line
[149] . 55
Figure 14 – Subjective evaluation of d.c. transmission line audible noise; EPRI test
center study 1974 [31] . 55
Figure 15 – Subjective evaluation of d.c. transmission line audible noise; OSU study
1975 [31] . 56
Figure 16 – Results of the operators’ subjective evaluation of AN from HVDC lines . 57
Figure 17 – Results of subjective evaluation of AN from d.c. lines . 57
Figure A.1 – Electric field and ion current distributions for Manitoba Hydro ±450 kV
Line [37] . 61
Figure A.2 – Cumulative distribution of electric field for Manitoba Hydro ±450 kV Line
[37] 62
Figure A.3 – Cumulative distribution of ion current density for Manitoba Hydro ±450 kV
line [37] . 62
Figure A.4 – Test result for total electric field at different humidity [117] . 67
Figure A.5 – Comparison between the calculation result and test result for the total
electric field (minimum conductor height is 18 m) [117] . 67

Figure B.1 – Connection for 3-section d.c. test line [121] . 68
Figure B.2 – Typical RI lateral profile at ±600kV, 4 × 30,5 mm conductor, 11,2 m pole
spacing, 15,2 m average height [12] . 69
Figure B.3 – Simultaneous RI lateral, midspan, in clear weather and light wind for three
configurations, bipolar ±400 kV [121] . 69
Figure B.4 – RI at 834kHz as a function of bipolar line voltage 4 × 30,5 mm conductor,
11,2 m pole spacing, 15,2 m average height . 70
Figure B.5 – Percent cumulative distribution for fair weather, 2 × 46 mm, 18,3 m pole
spacing, ±600 kV . 70
Figure B.6 – Percent cumulative distribution for rain weather, 2 × 46 mm, 18,3 m pole
spacing, ±600 kV . 71
Figure B.7 – Percent cumulative distribution for fair weather, 4 × 30,5 mm, 13,2 m pole
spacing, ±600 kV . 71

Figure B.8 – Percent cumulative distribution for rain weather, 4 × 30,5 mm, 13,2 m pole

spacing, ±600 kV . 72

Figure B.9 – Radio interference frequency spectrum . 73

Figure B.10 – RI vs. frequency at ±400 kV [121] . 73

Figure B.11 – Cumulative distribution of RI measured at 15 m from the positive pole

[122] . 74

Figure B.12 – Conducted RI frequency spectrum measured with the line terminated at

one end [122] . 75

Figure B.13 – Lateral profile of RI [122] . 76

Figure B.14 – Annual cumulative distribution of RI measured at 15 m from the positive
pole [122] . 76
Figure B.15 – Comparison between calculation result and test result for RI lateral
profile [117] . 77
Figure C.1 – Examples of statistical distributions of fair weather audible noise. dB(A)
measured at 27 m from line center of a bipolar HVDC test line [14] . 79

Table 1 – Electric field and ion current limits of ±800 kV d.c. lines in China . 30
Table 2 – Electric field limits of d.c. lines in United States of America [119] . 30
Table 3 – Electric field and ion current limits of d.c. lines in Canada . 30
Table 4 – Electric field limits of d.c. lines in Brazil . 30
Table 5 – Parameters of the IREQ excitation function [120] . 38
Table 6 – Comparison of the EPRI and CISPR formula . 38
Table 7 – Parameters defining regression equation for generated acoustic power
density . 51
Table 8 – Typical sound attenuation (in decibels) provided by buildings [155] . 58
Table A.1 – BPA ±500 kV line: statistical summary of all-weather ground-level electric
field intensity and ion current density [32] . 59
Table A.2 – FURNAS ±600 kV line: statistical summary of ground-level electric field
intensity and ion current density [36] . 60
Table A.3 – Hydro-Québec–New England ±450 kV HVDC transmission line. Bath, NH;
1990-1992 (fair weather), 1992 (rain), All-season measurements of static electric E-
field in kV/m [39] . 63
Table A.4 – Hydro-Québec – New England ±450 kV HVDC Transmission Line. Bath,
NH; 1990-1992, All-season fair-weather measurements of ion concentrations in
kions/cm [39] . 63
Table A.5 – IREQ ± 450 kV test line: statistical summary of ground-level electric field
intensity and ion current density [41] . 64
Table A.6 – HVTRC ±400 kV test line: statistical summary of peak electric field and ion
currents [42] . 65
Table A.7 – Statistic results for the test data of total electric field at ground (50 %
value) [117] . 66
Table B.1 – Influence of wind on RI . 72
Table B.2 – Statistical representation of the long term RI performance of the tested
conductor bundle [122] . 75
Table B.3 – RI at 0,5 MHz at lateral 20m from positive pole (fair weather) . 77
Table C.1 – Audible Noise Levels of HVDC Lines according to [119] and [150] . 80

– 6 – IEC TR 62681:2014  IEC 2014

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
ELECTROMAGNETIC PERFORMANCE OF HIGH VOLTAGE DIRECT

CURRENT (HVDC) OVERHEAD TRANSMISSION LINES

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 62681, which is a technical report, has been prepared by IEC technical committee 115:
High Voltage Direct Current (HVDC) transmission for d.c. voltages above 100 kV.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
115/71/DTR 115/84/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be

• reconfirmed,
• withdrawn,
• replaced by a revised edition, or

• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The “colour inside” logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this publication using a colour printer.

– 8 – IEC TR 62681:2014  IEC 2014

INTRODUCTION
Electric fields and magnetic fields are produced in the vicinity of an HVDC transmission line.

When the electric field at the conductor surface exceeds a critical value, known as the corona

onset gradient, positive or negative free charges leave the conductor and interact with the

surrounding air and ionization takes place in the layer of surrounding air, leading to the

formation of corona discharges. The corona discharge will not only bring out corona loss but

also produce electromagnetic environment problems.

The parameters used to describe the electromagnetic environment of an HVDC transmission

line mainly include the:
1) electric field,
2) ion current,
3) magnetic field,
4) radio interference,
5) audible noise.
To control these parameters in a reasonable and acceptable range, for years, a great deal of
theoretical and experimental research was conducted in many countries, and relevant national
standards or enterprise standards were developed. This Technical Report collects and
records the status of study and progress of electric fields, ion current, magnetic fields, radio
interference, and audible noise of HVDC transmission lines.

ELECTROMAGNETIC PERFORMANCE OF HIGH VOLTAGE DIRECT

CURRENT (HVDC) OVERHEAD TRANSMISSION LINES

1 Scope
This Technical Report provides general guidance on the electromagnetic environment issues
of HVDC transmission lines. It concerns the major parameters adopted to describe the

electromagnetic environment of a High-Voltage Direct Current (HVDC) transmission line,

including electric fields, ion current, magnetic fields radio interference, and audible noise
generated as a consequence of such effects. Engineers in different countries can refer to this
Technical Report to:
– ensure the safe operation of HVDC transmission lines,
– limit the influence on the environment within acceptable ranges, and
– optimize engineering costs.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
corona
set of partial discharges in a gas, immediately adjacent to an uninsulated or lightly insulated
conductor which creates a highly divergent field remote from other conductors
[SOURCE: IEC 60050-212:2010, 212-11-44, modified – Note 1 has been deleted.]
2.2
electric field
constituent of an electromagnetic field which is characterized by the electric field strength E
together with the electric flux density D
Note 1 to entry: In the context of HVDC transmission lines, the electric field is affected not only by the geometry
of the line and the potential of the conductor, but also by the space charge generated as a result of corona;
consequently, electric field distribution may vary non-linearly with the line potential.
[SOURCE: IEC 60050-121:1998, 121-11-67, modified – Note 1 to entry has been added.]

2.3
space-charge-free electric field
electric field due to a system of energized electrodes, excluding the effect of space charge
present in the inter-electrode space
2.4
ion current
flow of electric charge resulting from the motion of ions
2.5
magnetic field
constituent of an electromagnetic field which is characterized by the magnetic field strength H
together with the magnetic flux density B
[SOURCE: IEC 60050-121:1998, 121-11-69, modified – Note 1 has been deleted.]

– 10 – IEC TR 62681:2014  IEC 2014

2.6
radio interference
degradation of the reception of a wanted signal caused by RF disturbance

2.7
audible noise
unwanted sound with frequency range from 20 Hz to 20 kHz

[SOURCE: IEC 61973:2012, 3.1.14]

3 Electric field and ion current
3.1 Description of the physical phenomena
Electric fields are produced in the vicinity of a HVDC transmission line, with the highest
electric fields existing at the surface of the conductor. When the electric field at the conductor
surface exceeds a critical value, the air in the vicinity of the conductor becomes ionized,
forming a corona discharge. Ions of both polarities are formed, but ions of opposite polarity to
the conductor potential are attracted back towards the conductor, while ions of the same
polarity as the conductor are repelled away from the conductor. Space charges include air
ions and charged aerosols. Under the action of an electric field, space charge will move
directionally and ion current will be formed. The physical phenomena of electric field and ion
current are described in this clause.
The electric field and ion current in the vicinity of an HVDC transmission line are defined
mainly by the operating voltage and line configuration. The voltage applied to line conductors
produces an electric field distribution. Unlike High-voltage Alternating Current (HVAC)
transmission lines, the electric field produced by HVDC transmission lines does not vary with
time and, consequently, does not produce any significant currents in humans or objects
immersed in these fields.
The electric field is another aspect of the electrical environment around an overhead HVDC
transmission line. An electric field is present around any charged conductor, irrespective of
whether corona discharge is taking place. However, the space charge created by corona
discharge under d.c. conditions modifies the distribution of an electric field. The effect of
space charge on electric fields is significant.
For the same HVDC transmission lines, the corona onset gradients of positive or negative
polarities are different and the intensity and characteristics of corona discharges on positive
or negative conductors are also different. Consequently, during the design of HVDC
transmission lines, special consideration should be paid to the allowable values of the
maximum ground-level electric field and ion current density [1] .
Corona on a conductor of either positive or negative polarity produces ions of either the
positive or negative polarities in a thin layer of air surrounding each conductor [1]. However,
ions with a polarity opposite to that of the conductor are drawn to it and are neutralized on
contact. Thus, a positive conductor in corona acts as a source of positive ions and vice-versa.
For a unipolar d.c. transmission line, ions having the same polarity as the conductor voltage
fill the entire inter-electrode space between the conductors and ground. For a bipolar d.c.
transmission line, the ions generated on the conductors of each polarity are subject to an
electric field driven drift motion either towards the conductor of opposite polarity or towards
the ground plane, as shown in Figure 1. The influence of wind or the formation of charged
aerosols are not considered at this stage. Three general space charge regions are created in
this case:
—————————
Numbers in square brackets refer to the bibliography.

a) a positive unipolar region between the positive conductor and ground,

b) a negative unipolar region between the negative conductor and ground,

c) a bipolar region between the positive and negative conductors.

For practical bipolar HVDC lines, most of the ions are directed toward the opposite polarity

conductor, but a significant fraction is also directed toward the ground. The ion drift velocity is

such that it will take at least a few seconds for them to reach ground. Actually, the molecules

traveling along ion paths are not always the same ions. In fact, collisions between ions and air

molecules occur during the travel at a rate of billions per second and cause charge transfer

and reactions between ions and neutral molecules, so the ions reaching the ground are quite

different from those that were originally formed by corona near the conductor surface. The

exact chemical identity of the ions, after a few seconds, will depend on the chemical
composition and trace gases at the location.
Electric field is another component of the electrical environment around an overhead HVDC
transmission line. Electric field is caused by electrical charges, both those residing on
conductive surfaces (the transmission line conductors, the ground, and conducting objects)
and the space charges. The effect of space charge on electric field is significant.
A nonlinear interaction takes place between electric field and space charge distributions in all
three general space regions identified above in a), b), c). The nonlinearity arises because
ions flow from each conductor to ground or to the conductor of opposite polarity along the flux
lines of the electric field distribution: while at the same time, the electric field distribution is
influenced by the ionic space charge distribution. In addition to the nonlinear interaction
described above, the space charge field in the bipolar region is affected by other factors.
Mixing of ions of both polarities in the bipolar region leads firstly, to a reduction in the net
space charge density and secondly, to recombination and neutralization of ions of both
polarities.
Bipolar
region
− +
Negative Positive
region region
IEC
Figure 1 – Unipolar and bipolar space charge regions of a HVDC transmission line [1]
The corona-generated space charge, being of the same polarity as the conductor, produces a
screening effect on the conductor by lowering the electric field in the vicinity of the conductor
surface and consequently reducing the intensity of corona discharges occurring on the
conductor. In the unipolar regions, the space charge enhances the electric field at the ground
surface. The extent of electric field reduction at the conductor surface and field enhancement
at the ground surface depend on the conductor voltage as well as on the corona intensity at
the conductor surface. In the case of the bipolar region, however, the mixture of ions of
opposite polarity and ion recombination tend to reduce the screening effect on the conductor
surface. This leads to a smaller reduction in the intensity of corona activity near the
conductors than in the unipolar regions.

– 12 – IEC TR 62681:2014  IEC 2014

The electrical environment at ground level under a bipolar HVDC transmission line is,

therefore defined mainly by three quantities:

a) electric field, E,
b) ion current density, J,
c) space charge density, ρ.
The electric field produced by HVDC overhead transmission lines is a vector defined by its

components along three orthogonal axes. The space charge density is a scalar. The ion

current density is also a vector, and it is determined by the electric field and space charge

density.
Very small currents in some cases may flow through an object or person located under the
line because of exposure to the electric field and ion space charge. From the point of view of
environmental impact on persons and objects located under the line, the main consideration is
the combined exposure to the electric fields and ion currents. The scientific literature
indicates that exposure to the levels of d.c. electric field and ion current density existing under
operating HVDC transmission lines pose no risk to public health, but may cause some
induced current and annoyance effects to humans. Consequently, during designing of HVDC
transmission lines, special consideration should be paid for the allowable values of the
maximum ground-level electric field and ion current density [1].
Design of HVDC transmission lines requires the ability to predict ground-level electric field
and ion current distribution as functions of line design parameters such as the number and
diameter of sub-conductors in the bundle, height above ground of conductors and pole
spacing. Prediction methods are based on a combination of analytical techniques to calculate
the space charge fields and accurate long-term measurements under experimental as well as
operating HVDC transmission lines.
As described and illustrated in Figure 1, the ground-level electric field and ion current
environment under a bipolar HVDC transmission line can be thought primarily as a unipolar
space charge field under each pole. The bipolar space charge field between the positive and
negative conductors, however, has no significant impact on the ground-level electrical
environment. For the purpose of calculating the ground-level electric field and ion current
distributions, therefore, analytical treatment of the unipolar space charge field between each
of the positive and negative conductors and the ground plane is adequate.
Unipolar d.c. space charge fields are defined by the following equations:
ρ
∇ ⋅ E = (1)
ε
J = µρE (2)
∇ ⋅ J = 0 (3)
where
E and J are the electric field and ion current density vectors at any point in space,
ρ is the space charge density,
µ is the ionic mobility,
ε is the permittivity of free space.
The first Equation (1) is Poisson's equation, the second Equation (2) defines the relationship
between the current density and electric field vectors, and the third Equation (3) is the

continuity equation for ions. The solution of these equations, along with appropriate boundary

conditions, for the conductor-ground-plane geometry of the HVDC transmission line,

determines the ground-level electric field and ion current distributions [1].

Corona activity on conductors and the resulting space charge field are influenced, in addition

to the line voltage and geometry, by ambient weather conditions such as temperature,

pressure, humidity, precipitation and wind velocity as well as by the presence of any aerosols

and atmospheric pollution. It is difficult, if not impossible, to take all these factors into account

in any analytical treatment of space charge fields. Information on the corona onset gradients

of conductors, which is an essential input in the analytical determination of electric field and
ion current environment, is also difficult to obtain under practical operating conditions. For

these reasons, it is necessary to use analytical methods in combination with accurate long-

term measurements of ground-level electric field and ion current distributions under
experimental as well as operating HVDC transmission lines, in order to develop prediction
methods. Some of the information required in the analytical treatment, such as corona onset
gradients of co
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