IEC TS 63529:2026

DC side harmonics & filtering in HVDC transmission systems

IEC TS 63529:2026, which is a Technical Specification, is intended to inform and guide the harmonic design of the DC side of HVDC projects. It considers all aspects of AC current and voltage occurring on the DC circuit and also the interaction with adjacent systems.
The switching action in HVDC converters results in a wide spectrum of harmonics. These range from the fundamental frequency to the radio frequency range. Traditionally the specification of DC side harmonic performance has been limited to the frequency range of interest for induced audible noise on nearby telephone systems. Often a limit of 50th harmonic has been applied, corresponding to 2 500 Hz or 3 000 Hz on 50 Hz and 60 Hz systems respectively. Occasionally the range has been extended to 5 000 Hz. This frequency range has corresponded to the spectrum of characteristic harmonic generation from thyristor line commutated converters (LCC).
The introduction of HVDC voltage sourced converters (VSC) has meant that although the magnitude of DC side harmonic generation from these converters is generally lower, the generated spectrum of interest extends to higher frequencies.
The scope of this document therefore covers the frequency range up to approximately 5 000 Hz. Higher frequencies are mentioned only when relevant. The scope excludes the much higher frequency ranges appropriate to PLC communication and the radio interference spectra.

General Information

• Status: Published
• Publication Date: 13-Apr-2026
• ICS: 29.200 - Rectifiers. Convertors. Stabilized power supply
• ICS: 29.240.01 - Power transmission and distribution networks in general
• Technical Committee: TC 115 - High Voltage Direct Current (HVDC) transmission for DC voltages above 100 kV
• Drafting Committee: WG 7 - TC 115/WG 7
• Current Stage: PPUB - Publication issued
• Start Date: 14-Apr-2026
• Completion Date: 17-Apr-2026

Overview

IEC TS 63529:2026 - DC Side Harmonics & Filtering in HVDC Transmission Systems is a technical specification developed by the International Electrotechnical Commission (IEC). This document provides essential guidance for the design and management of harmonics and filtering on the DC side of high voltage direct current (HVDC) transmission systems. The specification addresses both line commutated converters (LCC) and voltage sourced converters (VSC), highlights the sources and impacts of harmonic generation, and outlines robust approaches to modeling, mitigating, and specifying harmonic performance and filter requirements.

Harmonics produced by switching actions within HVDC converters can span a wide frequency range, influencing not only power system equipment but also nearby telecommunication infrastructure. The technical specification’s scope primarily covers harmonics up to 5,000 Hz, emphasizing their interactions, control, and mitigation within HVDC projects while considering interface impacts with adjacent AC systems.

Key Topics

IEC TS 63529:2026 addresses multiple critical areas, including:

• Harmonic Generation: Describes DC side harmonic sources associated with both LCC and VSC HVDC technologies and explains cross-modulation from the AC side.
• Modeling and Analysis: Provides methodologies for harmonic studies, including approaches for time and frequency domain analysis, harmonic current calculations, and DC/AC system modeling.
• Telecommunication Interference: Outlines assessment and mitigation of inductive coupling between DC lines and telecommunication cables, addressing the risk of induced voltages and noise.
• Filter Design: Details design and rating considerations for DC filters, different filter topologies, configuration, and operational modes to effectively manage DC side harmonics.
• Harmonic Performance Limits: Specifies procedures and criteria for setting harmonic and induced noise limits based on international standards and field practices.
• Mitigation Measures: Discusses strategies for minimizing harmonic impacts, including filter deployment, optimized equipment selection, and interface actions with telecommunication lines.
• Operational and Safety Considerations: Considers performance during abnormal conditions, requirements for protection, reliability, loss management, and safety aspects linked to harmonic currents.

Applications

IEC TS 63529:2026 is highly relevant for the following applications:

• HVDC System Design and Upgrades: Provides engineers and designers with harmonics analysis and mitigation strategies crucial for reliable and compliant HVDC infrastructure.
• Filter Specification and Procurement: Assists procurement professionals and equipment manufacturers in setting technical requirements for DC filters in alignment with global best practices.
• Telecommunications Protection: Guides utilities and consultants in assessing and protecting nearby telecommunication and signaling assets from HVDC-induced disturbances.
• Regulatory Compliance: Supports stakeholders in meeting international performance standards and licensing criteria, especially concerning electromagnetic interference and public safety.
• Hybrid and Modern HVDC Installations: Informs the adaptation of harmonic control approaches for LCC, VSC, and hybrid converter topologies, as well as cable and overhead line systems.
• Grid Integration and Expansion: Ensures effective planning and integration of HVDC links into existing AC networks and evolving multi-terminal HVDC grids.

Related Standards

IEC TS 63529:2026 is complemented by several related international standards and guidelines, including:

• IEC 61000 Series: Electromagnetic compatibility (EMC) standards for harmonics and system disturbances
• IEC 61803: Guidance on the installation and operation of HVDC transmission systems
• IEC 60071 Series: Insulation coordination for high-voltage systems
• IEEE and ITU-T Recommendations: Noise and induced voltage limits for telecommunication interference
• IEC 60725: Reference impedances for harmonic emission testing
• IEC 60283: Testing requirements for power cables and accessories

By following IEC TS 63529:2026 and associated standards, engineers and project teams can ensure HVDC transmission systems are harmonically robust, operationally safe, and compliant with international performance and compatibility requirements. This enhances system longevity, supports reliable energy transmission, and safeguards adjacent critical infrastructure.

Standards Content (Sample)

IEC TS 63529 ®
Edition 1.0 2026-04
TECHNICAL
SPECIFICATION
DC side harmonics & filtering in HVDC transmission systems

ICS 29.200; 29.240.01 ISBN 978-2-8327-1168-2

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CONTENTS
FOREWORD . 13
INTRODUCTION . 15
0.1 Introduction . 15
0.2 Continuing relevance . 15
1 Scope . 16
2 Normative references . 16
3 Terms, definitions, symbols and abbreviated terms . 16
3.1 General terms . 17
3.2 Specific terms . 18
3.3 Symbols . 21
3.4 Abbreviated terms . 21
4 Overview of this document . 22
4.1 General . 22
4.2 Experience . 22
4.3 Literature . 22
4.4 Harmonic generation . 23
4.5 Modelling . 23
4.6 Inductive coupling to telecommunication systems . 23
4.7 Mitigation measures . 23
4.8 Elimination or reduction of DC filters . 24
4.9 DC side low-order resonance . 24
4.10 Various other issues . 24
4.11 Specification and design . 24
4.12 Illustrative example . 24
4.13 Reference material . 24
5 DC side harmonic generation – LCC technology . 25
5.1 General . 25
5.2 DC side harmonic generation – basic concepts . 25
5.2.1 Idealized assumptions . 25
5.2.2 Influence of non-idealities . 26
5.3 Stray capacitance and the three-pulse model . 28
5.4 Influence of non-idealities . 28
5.5 Harmonic cross-modulation from AC side . 28
6 DC side harmonic generation – VSC technology . 30
6.1 General . 30
6.2 Harmonic generation from VSC using switch type valves . 31
6.2.1 Overview . 31
6.2.2 Two-level converter with carrier-based PWM . 31
6.2.3 Two-level converter with optimized PWM (OPWM) . 34
6.3 Harmonic generation from multilevel VSC . 36
6.3.1 Overview . 36
6.3.2 Harmonic generation from a CTL converter. 37
6.3.3 Harmonic generation from MMC type converters . 38
6.4 Harmonic cross-modulation from AC side . 41
6.5 Influence of non-idealities . 41
6.6 Ground mode harmonics . 42
6.7 Converter impedance . 43
6.7.1 Overview . 43
6.7.2 Passive impedance . 44
6.7.3 Active impedance . 44
7 Modelling and analysis . 45
7.1 General . 45
7.2 Frequency domain, time domain and other techniques . 45
7.3 HVDC converter modelling for LCC technology . 46
7.4 HVDC converter modelling for VSC technology . 47
7.5 DC line modelling . 48
7.6 HVDC cable modelling . 49
7.7 DC switchyard equipment modelling . 49
7.8 Detuning due to frequency and component deviations . 50
7.8.1 Overview . 50
7.8.2 Individual consideration of filter component detuning and AC frequency
variation . 51
7.8.3 Equivalent frequency detuning . 52
7.9 Harmonic current calculation . 53
7.10 Frequency dependent modelling . 54
7.11 Detailed aspects to be considered in studies for LCC . 54
7.12 Transfer of disturbances between AC systems . 55
8 Inductive coupling to telecommunication lines . 56
8.1 General . 56
8.2 Analytical method . 57
8.3 Analysis of harmonic currents in the DC system . 58
8.4 Mutual impedance between power lines and telephone lines . 58
8.4.1 Overview . 58
8.4.2 Calculations of mutual impedance . 58
8.4.3 Variation of mutual impedance with frequency, resistivity and separation . 63
8.4.4 Influence of earth resistivity . 66
8.4.5 Crossings and angled exposures . 67
8.5 Induced voltages on telephone lines . 67
8.5.1 Overview . 67
8.5.2 Longitudinal induced voltages . 68
8.5.3 HVDC transmission line shield wires . 68
8.5.4 Telephone circuit shielding factor (K ) . 69
n
8.5.5 Telephone circuit balance factor (B ) . 70
n
8.5.6 Psophometric weighting . 70
8.5.7 Calculation of differential (metallic mode) weighted induced voltage . 71
8.6 Equivalent disturbing current method (I ) . 73
eq
8.6.1 Methodology . 73
8.6.2 Determination of H factor . 74
n
8.6.3 Differences between a 50 Hz and a 60 Hz system . 78
8.7 Modal analysis . 78
8.7.1 General . 78
8.7.2 Modal analysis of HVDC conductor currents . 78
8.7.3 Modal analysis of mutual impedance . 79
8.7.4 Modal analysis of coupling between HVDC and communication lines . 81
8.8 Induced voltage in a test line . 84
8.9 Electrode lines . 84
8.10 Inductive coupling from HVDC underground cables . 85
8.11 Frequency range . 86
8.12 Digital telecommunications systems . 87
8.13 Railway signal interference . 87
9 Harmonic performance limits . 88
9.1 General . 88
9.2 Procedure to determine limits . 89
9.3 Standard limits for induced noise . 90
9.3.1 Overview . 90
9.3.2 IEEE and Canadian standards . 90
9.3.3 ITU-T standard . 92
9.3.4 Comparison between standards . 92
9.3.5 Allowance for other noise sources . 92
9.4 Derivation of equivalent disturbing current (I ) limit . 93
eq
9.5 Rapid estimation of permissible disturbing current limit . 94
9.5.1 Overview . 94
9.5.2 Using IEEE recommendations . 95
9.5.3 Using ITU-T recommendations. 96
9.6 Considerations for definition of limits . 97
9.6.1 Overview . 97
9.6.2 Variation of performance limits along the transmission corridor. 97
9.6.3 Variation of harmonic limits with time duration . 99
9.6.4 Unbalanced bipolar operation . 99
9.6.5 Consideration of pole mode contribution . 100
9.6.6 Combination of harmonic current contributions from converter stations . 100
9.7 Limits for induced voltage criterion . 101
9.8 Experience from existing projects . 101
9.9 Harmonic performance limits for HVDC cables . 102
9.10 Harmonic rating limits for HVDC cables . 104
9.11 Safety aspects . 105
10 Harmonic performance verification and measurements . 105
10.1 General . 105
10.2 Compliance verification . 106
10.3 Measurements on DC conductors . 106
10.4 Measurements of external induction . 108
10.4.1 Overview . 108
10.4.2 Measurements to indirectly assess disturbing current . 108
10.4.3 Verification by magnetic field measurements . 110
10.4.4 Measurements to assess disturbance to an actual telephone line . 111
10.5 Monitoring equipment and analytical software . 112
10.6 Influence of mutual impedance and earth resistivity . 112
10.7 Duration and analysis of measurements . 114
11 Mitigation of adverse effects of DC side harmonics . 114
11.1 General . 114
11.2 Objectives of mitigation measures. 115
11.3 Optimization of cost of mitigation measures . 115
11.4 Mitigation measures in the HVDC system . 115
11.4.1 Overview . 115
11.4.2 Mitigation measures for LCC HVDC . 116
11.4.3 Mitigation measures for VSC HVDC . 117
11.5 Mitigation measures in telecommunication systems . 118
12 DC filter design . 118
12.1 General . 118
12.2 DC filter fundamentals . 119
12.3 Filtering mechanisms for LCCs . 120
12.4 DC filter design process . 123
12.5 Possible DC filter configurations . 125
12.5.1 Single-tuned filters . 125
12.5.2 Multiple-tuned filters . 127
12.5.3 Damping resistors in shunt filters . 133
12.6 Arrangement and switching of filter banks . 135
12.6.1 Overview . 135
12.6.2 Choice of switchable filter units . 136
12.6.3 Capability and speed of switches . 136
12.6.4 Switching of additional filters in monopolar mode. 136
12.6.5 Use of out-of-service pole filters in monopolar mode. 137
12.7 Neutral side smoothing reactors and impact on DC side harmonic flow . 138
12.8 Pole to earth filters . 139
12.9 Series filters . 140
12.10 Active filters . 141
12.11 Paralleling of DC lines and DC filters . 142
12.12 Equipment rating for harmonic stresses . 144
12.12.1 General . 144
12.12.2 Calculation principles . 144
12.12.3 Calculation considerations . 145
12.12.4 Derivation of rating parameters . 145
12.12.5 Outage contingencies . 146
12.12.6 Short-time rating . 147
12.12.7 Re-evaluation of critical cases . 147
12.13 Rating for abnormal operation conditions . 148
12.14 Transient rating . 148
12.15 Protection aspects . 151
12.16 Reliability and availability . 152
12.16.1 Overview . 152
12.16.2 Outage management . 153
12.17 Audible noise . 153
12.18 Losses . 154
12.19 Mechanical and visual design . 155
12.20 Economic aspects . 157
12.20.1 General . 157
12.20.2 Equipment costs . 157
12.20.3 Other costs . 158
12.21 Safety aspects related to harmonic currents. 158
12.21.1 HVDC substations . 158
12.21.2 HVDC transmission lines . 158
12.22 Seismic requirements . 159
13 Elimination or reduction of DC filters. 160
13.1 General . 160
13.2 Refurbishment of existing LCC HVDC schemes . 161
13.3 Overhead line and cable transmissions . 161
13.4 DC filters in a hybrid LCC-VSC system . 162
13.4.1 General . 162
13.4.2 Assessment of vulnerability of the VSC to DC side harmonic currents . 163
13.4.3 With limitations due to telecommunication interference . 163
13.4.4 No limitations due to telecommunication interference . 163
13.5 Back-to-back systems . 164
14 DC side low order resonance . 164
14.1 General . 164
14.2 Parameters affecting low-order resonance . 165
14.2.1 Overhead line transmission . 165
14.2.2 Cable transmissions . 168
14.3 Low order harmonic voltage sources . 170
nd
14.4 Impact of high values of 2 harmonic currents and voltages . 171
14.5 Core saturation instability . 173
14.6 Mitigation measures . 174
14.7 Study and assessment . 176
15 Coupling between DC line and parallel AC lines . 177
15.1 General . 177
15.2 Deleterious effects of fundamental frequency induction . 178
15.3 Factors affecting significance . 179
15.4 Study methods . 181
15.5 Modes and impacts of induced currents . 181
15.6 AC line configuration, loading and phase sequences . 183
15.7 Effect of earth resistivity . 184
15.8 Location along line route . 185
15.9 Multiple AC line exposures . 185
15.10 AC and DC lines sharing a tower . 187
15.11 Transpositions . 188
15.11.1 Overview . 188
15.11.2 DC line transpositions . 188
15.11.3 AC line transpositions . 189
15.12 Cross modulation in LCC and VSC . 190
15.13 Mitigation measures . 190
15.13.1 General . 190
15.13.2 Transposition of DC and/or AC lines . 191
15.13.3 Fundamental frequency filters . 191
15.13.4 Control actions . 192
15.14 Interaction between nearby HVDC links . 193
16 Harmonics in HVDC grids . 193
16.1 General . 193
16.2 Similarities with AC networks . 194
16.3 Harmonic emissions in an HVDC grid . 196
16.4 Network harmonic impedance of an HVDC grid . 196
16.5 Harmonic distortion limits for HVDC grids . 196
16.6 Allocation of headroom . 197
16.7 Use of DC filters . 198
16.8 DC side harmonic stability . 198
17 Impact of geomagnetic storms . 199
17.1 General . 199
17.2 Impact on DC side harmonics . 199
18 Guidelines for requirement specifications . 200
18.1 General . 200
18.2 Environmental licencing considerations . 200
18.3 Vulnerable infrastructure . 201
18.4 Circuit configurations and operating conditions . 201
18.4.1 General . 201
18.4.2 Circuit configurations . 201
18.4.3 Operating modes . 202
18.4.4 Special operating conditions . 202
18.4.5 Range of operating parameters . 202
18.4.6 Environmental parameters . 203
18.5 Basic data to be considered for harmonic calculation . 203
18.5.1 DC transmission line data . 203
18.5.2 AC system data . 203
18.5.3 Deviation of parameters and tolerances to be considered . 204
18.5.4 DC filter outage and redundancy requirements . 204
18.6 Methods for harmonic calculation . 204
18.7 Performance requirements regarding telecommunication interference . 205
18.8 Requirements regarding danger from induced voltages . 205
18.9 Ripple voltage on cables and overhead lines . 206
18.10 Other criteria for filter design . 206
18.10.1 Additional requirements . 206
18.10.2 Standards and test requirements . 206
18.10.3 Filter component rating . 206
18.10.4 Insulation levels . 206
18.10.5 Availability, reliability and switching . 206
18.10.6 Protection and safety . 206
18.11 DC side harmonic field measurements . 207
19 Equipment design and test requirements . 207
19.1 General . 207
19.2 General requirements . 207
19.2.1 Structural . 207
19.2.2 Failure rate . 208
19.2.3 Quality assurance . 208
19.2.4 Equipment test requirements . 209
19.2.5 Technical information submitted by the contractor . 209
19.2.6 Filter component ratings . 210
19.3 Capacitors . 210
19.3.1 Capacitors general . 210
19.3.2 Capacitor design aspects. 211
19.3.3 Capacitor electrical data . 213
19.3.4 Capacitor tests . 214
19.4 Reactors . 215
19.4.1 Reactors general . 215
19.4.2 Reactor design aspects . 216
19.4.3 Reactor electrical data . 217
19.4.4 Reactor tests . 218
19.5 Resistors . 218
19.5.1 Resistors general . 218
19.5.2 Resistor design aspects . 218
19.5.3 Resistor electrical data . 219
19.5.4 Resistor tests . 220
19.6 Arresters . 222
19.6.1 Arresters general . 222
19.6.2 Arrester design aspects . 222
19.6.3 Arrester electrical data . 223
19.6.4 Arrester tests . 223
19.7 Current transformers . 224
19.7.1 Current transformers general . 224
19.7.2 Current transformer design aspects . 224
19.7.3 Current transformer electrical data . 224
19.7.4 Current transformer tests . 225
19.8 Filter switching equipment . 226
19.8.1 Filter switching equipment general . 226
19.8.2 Filter switching equipment design aspects . 226
19.8.3 Filter switching equipment electrical data . 226
19.8.4 Filter switching equipment tests . 227
Annex A (normative) Stray capacitances and the three pulse model . 228
A.1 Overview . 228
A.2 Calculation of three-pulse harmonic driving voltages . 228
A.3 Phase angles of triplen order harmonic generation . 232
A.4 Return path for ground mode currents . 233
A.5 Triplen order harmonic current flow through stray capacitances . 234
A.5.1 General . 234
A.5.2 Odd order triplens . 235
A.5.3 Even order triplens . 238
A.5.4 Odd and even harmonic flow in bipolar mode . 240
A.6 Complete harmonic current paths. 241
A.7 Stray capacitive paths in converter transformer . 244
A.8 Influence of non-idealities . 247
Annex B (informative) Illustrative calculations . 251
B.1 General . 251
B.2 Data . 251
B.3 Results . 252
B.4 Mechanisms of harmonic current flow . 252
B.4.1 Studies for current flow . 252
B.4.2 Case A . 252
B.4.3 Case B . 252
B.4.4 Case C . 252
B.4.5 Case D . 253
B.4.6 Case E . 253
B.5 Filter solutions for different target disturbance limits . 253
B.5.1 Studies for filter solutions . 253
B.5.2 Case F . 253
B.5.3 Case G . 253
B.5.4 Case H . 254
B.5.5 Case I . 254
B.5.6 Case J . 254
B.5.7 Case K . 254
B.5.8 Case L . 254
B.6 General conclusions . 254
B.7 Connection of filters from pole to earth . 255
B.7.1 Case M . 255
B.7.2 Case N . 255
B.8 Smoothing reactor sizing. 255
B.9 Electrode line harmonic disturbing current . 255
B.10 Other operation modes . 256
Bibliography . 258

Figure 1 – DC-voltage for 12-pulse operation . 26
Figure 2 – References and carrier for a two-level converter using PWM with pulse
number
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