ISO/DIS 24695

ISO/TC 67/SC 2

Secretariat: UNI

Date: 2026-02-02

Oil and gas industries including lower carbon energy — The effects of High Voltage DC interference to buried pipelines — Measures to be implemented

This draft is submitted to a parallel vote in ISO, CEN.

DIS stage

© ISO 2026

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Contents

Foreword v

Introduction vi

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Abbreviations and symbols 5

5 Definition of an HVDC transmission system 6

6 Sources of Interference 7

6.1 General 7

6.2 DC Interference 7

6.3 Electromagnetic Interference (EMI) 8

6.4 Converters 8

6.5 HVDC Overhead Power lines (normal and fault conditions) 9

6.6 HVDC buried power cables (normal and fault conditions) 9

6.7 Earth Electrodes 11

6.8 Earth electrode line 11

6.9 Corona 11

7 Measurements and calculations of Interference to buried pipelines 12

7.1 General 12

7.2 Calculations during the planning and design phases 12

7.3 Measurements during operation stage 14

7.4 Zone of influence and separation distances (DC interference) 15

7.5 Zone of Influence and separation distances (EMI) 16

8 Effects of HVDC interference and Classification of Risk 16

9 Interference to buried pipelines by monopolar HVDC systems 16

9.1 DC Interference risk level – Monopolar systems 16

9.2 EMI Risks –Monopolar systems 17

10 Interference to pipelines by Bipolar HVDC systems 19

10.1 DC Interference risk level – Bipolar systems (normal operation) 19

10.2 EMI Risks –Bipolar systems 19

11 Protection Criteria 21

11.1 Protection criteria (DC Interference) 21

12 Thermal influence of the HVDC system 22

13 HVDC projects and existing adjacent infrastructure 22

14 Operational Information Exchange and Coordination for Pipeline and HVDC Systems 23

14.1 Operational information exchange 23

14.2 Normal operation 24

14.3 Emergency operation 25

14.4 Changes to crossings and proximity points 26

Annex A (informative) HVDC CONFIGURATIONS 27

Annex B (informative) CORONA 33

Annex C (informative) Competence requirements for modellers assessing HVDC interference on pipeline systems 35

Annex D (informative) Earth Potential Rise 38

Annex E (informative) BARNES LAYER 41

Annex F (informative) Method to determine the soil potential to remote earth through voltage gradient measurements 43

Annex G (informative) Principles of the influence of direct currents from external sources on buried metal pipelines 46

Annex H (informative) Technical measures for design phase of the HVDC system 49

Annex I (informative) Separation distances under DC interference 51

Annex J (informative) Separation distance under DC side fault conditions (inductive coupling) 55

Bibliography 59

Foreword

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Introduction

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Oil and gas industries including lower carbon energy — The effects of High Voltage DC interference to buried pipelines — Measures to be implemented

This document describes technical measures to be carried out at crossings and parallelisms of buried metal pipelines influenced by HVDC systems. It provides guidance on how the design, construction, operation, maintenance, and decommissioning phases of HVDC systems affect buried metal pipelines. Electromagnetic, DC interference and thermal influences on pipeline coatings are described.

Acceptable levels of interference are discussed.

Guidance is provided for calculation methods to establish an acceptable separation distance between the pipeline and the source of interference.

The following aspects are not covered in this document:

• Interference from other AC sources
• Contractual responsibilities
• Personnel safety.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 15589-1:2015, Petroleum, petrochemical and natural gas industries — Cathodic protection of pipeline systems — Part 1: On-land pipelines

ISO 18086:2019, Corrosion of metals and alloys — Determination of AC corrosion — Protection criteria

ISO 21857:2021, Petroleum, petrochemical and natural gas industries — Prevention of corrosion on pipeline systems influenced by stray currents

IEC 60479-1:2018 ED1:2018, Effects of current on human beings and livestock - Part 1: General aspects

IEC 61936-2:2023 ED1:2023, Power installations exceeding 1 kV AC and 1,5 kV DC - Part 2: DC

EN 50443:2011, Effects of electromagnetic interference on pipelines caused by high voltage a.c. electric traction systems and/or high voltage a.c. power supply systems

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp

High-voltage direct-current system (HVDC system)

power transmission system that transfers energy using direct current at high voltage between two or more grid connection points. The system consists of at least two converter substations with corresponding earthing systems and direct current transmission lines (overhead lines or buried or immersed cables)

conductive coupling

transfer of energy occurring when a part of the current belonging to the interfering system returns to the system earth via the interfered system

Note 1 to entry: Also, when the voltage to the reference earth of the ground in the vicinity of the influenced object rises because of a default in the interfering system, and the results of which are conductive voltages and currents.

Electromagnetic Interference (EMI)

interference phenomenon resulting from conductive, capacitive and inductive coupling between systems and which can cause disturbance, malfunction, damage and danger

[SOURCE: EN 50443:2011]

earth electrode

structure with a conductor or a group of conductors embedded in the soil or immersed in sea water, directly or surrounded with a specific conductive medium

[SOURCE: IEC TS 62344 ^[1]]

pond electrode

electrode usually placed outside but within 100 m of the waterline, having electrodes directly in contact with the seawater, within a small area which is usually protected against waves and possible ice damage by a breakwater

[SOURCE: IEC TS 62344 ^[1]]

sea electrode

electrode located away from the shoreline at a distance deeper than 100 m into the sea

[SOURCE: IEC TS 62344 ^[1]]

electrode station

whole facility which transfers current to/from the electrode line (3.8) to/from the earth or sea water, usually including the feeding cable, towers, switchgear, fencing an any necessary auxiliary equipment in addition to the electrode itself

[SOURCE: IEC TS 62344 ^[1]]

electrode line

overhead line or underground cable used to connect the neutral bus in a converter station to the earth electrode (3.4) station

[SOURCE: IEC TS 62344 ^[1]]

earth return operation mode

operation mode in the HVDC power transmission system, using DC lines and earth (or seawater) as the current loop

[SOURCE: IEC TS 62344 ^[1]]

earth return system

set of devices designed and built specifically for earth return operation mode (3.9)

Note 1 to entry: It mainly consists of the electrode line (3.8), earth electrode (3.4), current guiding system, and other auxiliary facilities.

[SOURCE: IEC TS 62344 ^[1]]

unbalanced current

difference of current between two poles during operation of a bipolar DC system (3.16)

Note 1 to entry: For balanced bipolar operation mode, the unbalanced current flowing can be controlled automatically by the control system with about 1% of the related current.

Note 2 to entry: For balanced bipolar operation mode, the current flowing through the earth electrode (3.4) is the difference in currents between the two poles.

[SOURCE: IEC TS 62344 ^[1]]

cathode

electrode capable of emitting negative charge carriers to and/or receiving positive charge carriers from the medium of lower conductivity

Note 1 to entry: The direction of electric current is from the medium of lower conductivity, through the cathode to the external circuit.

Note 2 to entry: In some cases (e.g. electrochemical cells), the term “cathode” is applied to one or another electrode, depending on the electric operating conditions of the device. In other cases, (e.g. electronic tubes and semiconductor devices). The term “cathode” is assigned to a specific electrode.

[SOURCE: IEC 60050-151 ^[2]]

anode

electrode capable of emitting positive charge carriers to and/or receiving negative charge carriers from the medium of lower conductivity

Note 1 to entry: The direction of electric current is from the external circuit, through the anode, to the medium of lower conductivity.

Note 2 to entry: In some cases (e.g. electrochemical cells), the term “anode” is applied to one or another electrode, depending on the electric operating condition of the device. In other cases (e.g. electronic tubes and semiconductor devices), the term “anode” is assigned to a specific electrode.

[SOURCE: IEC 60050-151 ^[2]]

DC neutral point

common point of two monopoles forming a bipole converter or the earthed point of a monopole converter

[SOURCE: IEC 61936-2:2023 ED1:2023]

DC electrode line

electrical connection between a DC earth electrode (3.20) and the DC installation

[SOURCE: IEC 61936-2:2023 ED1:2023]

DC system

all interconnected parts of a power system installation that is installed between and including the DC side windings of the interface/converter transformers at each terminal except for the valve hall or converter hall

Note 1 to entry: Components connected to the AC side windings of the converter/interface transformers including the AC windings themselves are not considered to be part of the DC system as defined for this standard.

[SOURCE: IEC 61936-2:2023 ED1:2023]

high voltage

DC voltage exceeding 1500 V DC

[SOURCE: IEC 61936-2:2023 ED1:2023]

converter unit

indivisible operative unit comprising all equipment between the point of connection on the AC side (or DC side for DC/DC converters) and the point of connection on the DC side, essentially one or more converters, together with converter transformers, control equipment, essential protective and switching devices and auxiliaries, if any, used for conversion

[SOURCE: IEC 61936-2:2023 ED1:2023]

converter station

part of a DC system (3.16) which consists of one or more converter units (3.18) including DC switchgear, DC fault current controlling devices, if any, installed in a single location together with buildings, reactors, filters, reactive power supply, control, monitoring, protective, measuring and auxiliary equipment

[SOURCE: IEC 61936-2:2023 ED1:2023]

DC earth electrode

array of conductive elements placed in the earth, or the sea, which provides a low resistance path between a point in the DC system (3.16) and the earth and is capable of carrying continuous current for some extended period

Note 1 to entry: An earth electrode (3.4) may be located at a point some distance from the HVDC substation.

Note 2 to entry: Where the electrode is placed in the sea, it may be termed as a sea electrode (3.6).

[SOURCE: IEC 61936-2:2023 ED1:2023]

Voltage source converter (VSC)

electronic device that converts DC voltage into AC voltage, or vice versa, by controlling the voltage and frequency of the output AC waveform

Note 1 to entry: Usually using an Insulated Gate Bipolar Transistor (IGBT) and an AC filter.

thermal conductivity

measure of the ability of a material to conduct heat

normal operation

operational state in which the system functions within its prescribed design parameters, without activation of protective actions or alarms, while all subsystems operate as intended

maintenance operation

operational state in which the system functions within its prescribed design parameters for routine maintenance

Note 1 to entry: This can include a bipolar system operating in monopolar configuration.

abnormal operation

operational state in which the system functions outside its prescribed specified design parameters

Note 1 to entry: This includes commissioning, and testing.

fault conditions

condition initiated by the detection of an electrical fault within an HVDC system, where automated protection systems activate to isolate, clear, or mitigate the fault while preserving equipment integrity and grid stability

A typical HVDC (High Voltage Direct Current) transmission system consists of two main terminals (Figure 1), the sending terminal and the receiving terminal.

At the sending terminal, there is a converter that functions as a rectifier, converting alternating current (AC) power from the grid into direct current (DC).

Conversely, at the receiving terminal, there is another converter acting as an inverter, which converts the DC power back into AC for distribution into a local grid.

The connection between the sending and receiving terminals can be established using overhead lines, underground/submarine cables, or a combination of both, depending on factors such as distance, environmental considerations, and project requirements.

Inside the converters, power electronic valves, which are essentially high-powered electronic switches, facilitate the control of power flow. These valves enable precise regulation of the electricity being transmitted, allowing for efficient and reliable operation.

Modern HVDC systems use two main converter technologies,  conventional line-commutated current source converters (CSCs) and self-commutated voltage source converters (VSCs), which can be part of different systems like back-to-back, monopolar, bipolar, homopolar, and multi-terminal setups.  Additional schematic diagrams and explanatory notes can be found in Annex A.

Depending on the technology, HVDC systems can operate differently under normal, emergency, and fault condition.

One key feature of HVDC systems is their ability to reverse the direction of power flow as needed. This is achieved by configuring the converters at the terminals to operate as either rectifiers or inverters interchangeably. As a result, power can be transmitted bi-directionally, providing flexibility and enhancing the system's overall reliability. More details on HVDC configurations can be found in Annex A.

Key

1 converter transformer

2 converter (I)

3 converter (II)

4 converter station

5 DC Line

6 AC grid (I)

7 AC grid (II)

8 DC power transmission system

Figure 1 — HVDC Power Transmission Structure (Simplified Layout)

The technology of HVDC systems entails topologies, components and operating characteristics that affect the interference situations (DC interference and Electromagnetic Interference (EMI)) and impacts on pipeline systems.

DC interference, caused by HVDC systems, is a disturbance that primarily affects metallic pipelines through conduction via the earth or other electrolytes. For buried or submerged pipelines, this interference can accelerate corrosion on their external surfaces. The corrosive effect is mainly due to the combination of the earth/seawater return current's magnitude and the duration of exposure ^[3].

DC interference can arise from:

• The normal operation of unbalanced monopole configurations, using electrodes to facilitate current return through the earth or sea.
• The normal operation of bipolar configurations, where electrodes are used to conduct any unbalanced current that arises between the two poles of the system. This unbalanced current, typically limited to 0.5% to 1% of the rated current of each pole, results from minor differences between the converter units at each station.
• Temporary operation (e.g. maintenance or emergencies) of bipolar configurations that are run as monopolar systems, using electrodes to facilitate current return through the earth or sea.

• Close proximity of HVDC and pipeline systems can result in circumstances that give rise to increased external corrosion risks to the pipeline, hazardous electrical conditions (e.g. touch voltages) on the interfered pipelines and associated systems.
• Consideration should be given to:
• the impact of harmonic currents and voltages on the DC side, under the normal operating conditions of the HVDC systems
• the impact of DC-side fault conditions of the HVDC systems

NOTE The DC-side fault conditions are described in §10.3.2 of IEC 61936-2:2023 ED1:2023

• monopolar operations.

DC side faults within the HVDC installation site shall be assessed to identify the worst-case scenario for Electromagnetic Interference (EMI) affecting nearby pipeline systems. The types of faults that can be relevant, include:

• Pole-to-earth fault
• Two-pole-to-earth fault
• Metallic-return-to-earth fault
• Electrode-line-to-earth fault.

The HVDC operator can provide the pipeline operator or owner with waveform data for short-circuit currents for each type of DC fault type, including the fault time duration for each case.

The normal operation of HVDC systems fundamentally entails a constant or static DC magnetic field emission. However, consideration should be given to the effect of harmonic currents and harmonic voltages on the HVDC system. The generation and propagation of harmonics is dependent on the type of DC converters of the HVDC system. The harmonic profile of the DC converters is influenced by whether the converters are line-commutated converters (LCC) or voltage-sourced converters (VSC) [§ 4.2.9 IEC 61936-2:2023 ED1:2023].  

Harmonics can cause interference in nearby pipeline systems through inductive coupling. While most inductive coupling calculations focus on the fundamental frequency (50/60 Hz), the presence of harmonics introduces the risk of unacceptable levels of higher-frequency inductive coupling, on pipelines. Thus, the effect of harmonics generated by the HVDC system on nearby pipelines and other infrastructure located close to DC overhead lines or cables should be examined.

To avoid unacceptable risks to the integrity of the pipeline the operator will need to know the induced voltages. The HVDC system operator and the pipeline operator shall collaborate to provide all the information necessary for the induced voltage calculations.

Interference situations on pipeline systems from HVDC overhead power lines can occur under normal, maintenance, abnormal operation and fault conditions.

Where applicable, inductive and conductive effects shall be combined, to comprehensively address any adverse effects on the pipelines.

The inductive coupling is possible because DC faults, even of short duration, are generally fast transient phenomena. Thus, the fault current comprises a wide frequency spectrum, ranging from relatively low frequencies to several thousand hertz (kHz). Therefore, significant inductive interference can occur under DC fault conditions, on nearby pipelines.

When a DC ground fault occurs at an overhead line structure, a nearby pipeline can be also influenced through conductive coupling. This can occur when a large fault current flowing into the earth from the structure (e.g. lattice tower) increases the earth potential local to the pipeline. This causes the earth around the pipeline to be at a relatively high potential with respect to the pipeline metal potential. This can result in increased coating stress voltages on the pipeline.

Some degree of inductive coupling with buried pipelines can be expected. Inductive coupling is typically more of a concern with AC systems in the vicinity of pipeline systems. It can still occur under the normal operation of the HVDC systems, particularly when the harmonics generated by the converters are propagated in the DC overhead power lines. The presence of harmonics introduces the risk of unacceptable levels of higher-frequency inductive coupling.

A higher level of interference is to be expected since the operating parameters are different from normal operation. Under monopolar operation some interference mechanisms will be exacerbated.

Abnormal conditions cover many scenarios. Control systems will quickly adjust the operating conditions to safe limits, which can result in exacerbated interference. Under monopolar operation some interference mechanisms will be exacerbated.

During DC-side fault conditions on HVDC overhead power lines, buried pipelines can experience both inductive and conductive coupling effects.

Interference situations on pipeline systems from HVDC buried cables can occur under both normal and fault conditions. These interference situations originate from the rich frequency spectrum of the DC fault current as well as from possible injections of the fault current into the earth in areas that are common to buried pipeline routings.

For assessing EMI from HVDC cable networks, it is necessary to understand and account for the electrical-circuit behaviour and topology/technology of the systems involved. To comprehend the behaviour of a HVDC system under DC-side faults, it is essential to know:

• the functional and protective earthing configurations of the system,
• winding configurations of the interface/converter transformers,
• the type of cables and their bonding configuration,
• the converters’ operational configuration.

These elements will principally determine the stages of a DC-side fault response as well as its peak value. Typically, the DC fault analysis of such systems can be confined into two stages ^[4]:

1) the capacitive discharge stage

2) the AC grid side current feeding stage.

During Stage 1 the stored DC side capacitive energy of the system (including cables) will be discharged through the ground fault. This will result in a transient current flowing for some milliseconds.

During Stage 2, it is possible to have AC grid current feeding the DC side fault (through the IGBT anti-parallel diodes or protective thyristors), until the converter’s AC breaker is tripped. If the converter is of the fault-blocking type, the contribution to the DC fault current is blocked or considerably reduced by the converter.

Under normal operating conditions, some inductive coupling with buried pipelines can occur when harmonics propagate through the HVDC cable. However, the cable's metallic sheath can act as a shield if it is grounded at both ends. This limits the inductive coupling, thereby reducing the severity of the interference situation.

During DC-side fault conditions on HVDC DC power cables, buried pipelines can experience both inductive and conductive coupling effects.

Inductive coupling is possible because DC faults, even of short duration, are generally fast transient phenomena. Thus, the fault current comprises a wide frequency spectrum, ranging from relatively low frequencies to several thousand hertz (kHz). Therefore, significant inductive interference can occur under DC fault conditions, on nearby pipelines.

NOTE Current is also induced on the metallic sheath of the cable (since it is in close proximity to the core), the level of which depends on the insulation size and properties of the cable and type of sheath bonding. The metallic sheath may therefore offer some cancellation effect on the induced voltage on a nearby pipeline.

Conductive coupling arises from the fault current injections into the earth, at the locations where the metallic sheath of the HVDC cable is either directly bonded to earthing electrodes or has damaged insulation.  These injections raise the local earth potential, this creates a voltage gradient in the earth that allows for the conduction of current. The earth potential rise with respect to remote earth decreases with distance from the current injection points. See Annex D for further information.

Electrical interference will be different under emergency and maintenance conditions.

Where applicable, inductive and conductive effects shall be combined, to comprehensively address any adverse effects on the pipelines.

The conductive coupling arises from the current injections into the earth, during unbalanced conditions or monopolar operation with earth return. These injections raise the local earth potential. This forms a voltage gradient in the earth that allows for the conduction of current. The earth potential rise, with respect to remote earth, decreases with distance from the current injection points. See Annex I for further information.

Electrical interference will be different under emergency and maintenance conditions.

In HVDC asymmetric monopole configurations, and in HVDC bipolar configurations with an earth/sea return operation mode, earth electrode lines are used. An electrode line is an overhead line or underground cable used to connect the neutral bus in a converter station to the earth electrode station (IEC TS 62344 ^[1]). Annex C IEC TS 62344 ^[1] describes the electrode’s line design requirements and purpose.

Under the normal operation of HVDC systems, the earth electrode line is considered as a source of electromagnetic interference to nearby pipeline systems, when/if the current flowing through this line contains harmonics resulting from the DC conversion process.

Under DC-side fault conditions, when the fault current uses the electrode line as part of the return current circuit (e.g. pole to earth fault or DC electrode line fault) the earth electrode line is considered as a source of electromagnetic interference to nearby pipeline systems, due to the wide frequency spectrum of the DC fault waveform. Both inductive and conductive effects should be considered.

Corona discharge is a phenomenon associated with aerial conductors. The intense electric field around a conductor ionizes the surrounding air, causing a visible glow, audible noise, and energy loss. This effect is more pronounced at high voltage (e.g. ≥ ± 500 kV), due to the constant polarity of DC electric fields and the environmental conditions it creates.

Co-located steel pipelines can be affected by the electromagnetic interference generated by the corona.

The risks of corona discharge can be mitigated by the design of the conductors. 

Key factors are:

• Conductor size,
• Bundle configuration,
• Conductor spacing,
• Height above ground,
• Mast grounding.

Additional information is provided in Annex B.

The assessment and management of corrosion risks requires collaboration between designers, constructors and operators of the HVDC system and adjacent pipelines.  It is recommended that a formal collaboration group is formed as early as possible during the design phase.  This collaboration should endure for the entire operating life of the systems.

During the planning and design phases, calculations shall be conducted to evaluate the risks of unacceptable electrical interference to the buried pipeline.

In certain cases, a comprehensive analysis using specialised software can be necessary. However, it is recognised that commercially available software often rely on inherent modelling assumptions. Therefore, it is crucial to understand the limitations and underlying assumptions of any software employed to analyse these complex interference scenarios. This requires well-informed modelling practices, including iterative processes and “what-if” analyses. Annex C provides recommended competence levels for persons who carry out the modelling.

Such exercises can involve incorporating conservative allowances to account for modelling limitations and examining the sensitivity of results to variations in calculation parameters. Moreover, the integration of professional experience and sound engineering judgment is required. These elements ensure that conclusions err on the side of caution, thereby enhancing the reliability of the analysis.

If the HVDC system is already operational, measurements shall be performed on the pipeline to evaluate the risks of interference.

Special attention should be given to HVDC harmonic effects, as these can induce interference in nearby pipeline systems through inductive coupling [see Clause 6].

Accurate field measurements of pipeline interference requires specialised instrumentation. It is important to note that many instruments designed for measuring voltage, current, and power in AC systems assume that waveforms are approximately sinusoidal. When this assumption does not hold—due to the presence of significant harmonic content or load characteristics—unexpected measurement errors can occur. To mitigate these risks, measurement techniques and equipment that can account for non-sinusoidal waveforms should be employed, ensuring reliable assessment of harmonic-induced interference.

Baseline pipe-to-soil potential and coupon measurements (Clause 7.2) shall be undertaken over a period of time that is representative of the anticipated interference. This will provide a baseline of data that can be used for comparison when the HVDC system is commissioned and placed into operation.

During commissioning the baseline measurements shall be repeated and the data used to evaluate the levels of interference.  For bipolar systems with earth return operations the HVDC system operator should test under monopolar operation conditions (ISO 21857:2021), with each electrode operating both as an anode and as a cathode. Pipeline pipe-to-soil potential measurements should be carried out during both bipolar and monopolar operations.

DC interference calculations should consider the following conditions:

• The normal operation of asymmetric monopole configurations, using electrodes to facilitate current return through the earth or sea.
• The normal operation of bipolar configurations, where electrodes are used to conduct any unbalanced current that arises between the two poles of the system. This unbalanced current, typically limited to 0.5% to 1% of the rated current of each pole, results from minor differences between the converter units at each station.
• Temporary operation (e.g. maintenance or emergencies) of bipolar configurations that are run as monopolar systems, using electrodes to facilitate current return through the earth or sea.

Voltage gradients, and current density across pipeline coating defects (also known as holidays) are important factors when assessing pipeline integrity risks under DC interference. Typically, a pipeline can either collect, conduct, or discharge stray currents depending on factors such as the pipeline's orientation, routing, the polarity (cathode or anode) of the HVDC electrode, and the soil/seawater characteristics.

Table 1 — Typical information required to carry out preliminary assessment of DC interference

Further details are given in Annex I.

EMI calculations should consider the following conditions:

• the impact of harmonic currents and voltages on the DC side, under the normal operating conditions of the HVDC systems;
• the impact of DC-side fault conditions of the HVDC systems.

During normal operation, harmonics can cause interference to nearby pipeline systems through inductive coupling. A method to calculate the voltage induced in the pipeline system by using the mutual impedance method is described in Annex J.

During fault conditions, the integrity of the pipeline faces potential threats, including risks to electrical equipment connected to the pipeline (e.g., cathodic protection units, telemetry devices). Insulation joints, insulating flanges  and  the pipeline coating are also at risk.

Assessing the impact on pipeline coatings involves evaluating the RMS coating stress voltage, defined as the voltage between the pipeline metal and the earth immediately outside the coating. This stress voltage encompasses the voltage induced on the pipeline steel by induction and the Earth Potential Rise (EPR) resulting from fault current injections (conductive coupling).

Table 2 — Typical information required to carry out preliminary assessment of EMI

For existing systems the interference values under operating conditions can be directly measured.

Key parameters that can be measured and observed on the pipeline are:

• DC pipe-to-soil potentials
• AC (RMS) TO BE CHECKED pipe-to-soil potentials
• Coupon current and potentials (AC and DC)
• Soil potential gradients
• Soil resistivity
• Voltage waveform.

Pipe-to-soil potentials shall be measured over a period of time to establish the overall effect on the pipeline.  The duration of the measurements should be representative of the normal operating conditions (steady state conditions) and include periods of peak demand.  Guidance for potential measurements are given in ISO 21857:2021 and ISO 15589-1:2015 series.

Solid state data logging instruments should be used for the collection of data.  The instrument should have sufficient memory to store all of the data for the required measurement period.  The instrument should be capable of measuring at programmed sampling intervals at least between 0.1 s and 60 s.  The data should be in a format to allow mathematical analysis e.g. comma separated value (csv).

For coupon measurements the datalogger should incorporate a switcher to provide coupon on and off voltage and current measurements.

Soil potential gradients can be measured between portable reference electrodes.  The type of reference electrode is determined by the electrolyte e.g. copper/copper sulphate for soil applications.  The electrodes shall be calibrated on a daily basis. 

Soil resistivity measurements can be made using the Wenner 4 pin technique.  The depth of measurement (i.e. pin spacing) is determined by the depth of the pipeline and the interference source.  The resistivity at different depths can be calculated using the Barnes Layer analysis.  Soil resistivity measurements and the Barnes Layer analysis are described in Annex E, with an example of the calculation.

Voltage waveform analysis can be necessary if the harmonic interference is to be assessed.  A digital oscilloscope with the facility to store waveforms is required.  The instrument should preferably have a Fast Fourier Transform facility.  If the instrument does not have a Fast Fourier Transform facility (FFT) then the analysis of the waveform frequencies and magnitudes can be calculated. 

When current is injected into the earth as part of an earth return process—common in systems like asymmetric monopolar or bipolar with earth return—it creates a voltage gradient in the soil. Metallic pipelines located near this area can be affected, experiencing issues such as accelerated corrosion or interference with their cathodic protection (CP) systems.

The DC interference "zone of influence" is the area around an HVDC earth or shoreline electrode system where electrical or electrochemical effects can significantly impact nearby pipelines.

The extent of this zone depends on factors such as:

• soil resistivity and composition,
• magnitude of earth current injections,
• orientation of the pipeline relative to the electrode,
• type of pipeline metal,
• presence of coating defects on the pipeline.

The zone of influence establishes the area affected by DCinterference, providing a basis for determining the appropriate separation distance.

Separation distances refer to the minimum required space between pipelines and HVDC electrodes. This ensures that the corrosion rate on the pipeline remains within acceptable limits, or is manageable by the pipeline's cathodic protection system.

A preliminary calculation and methodology for separation distances, is outlined in Annex I.

Determining appropriate separation distances is a complex task that often requires multiple iterations to refine the model and achieve reliable conclusions. Each project is inherently unique due to the numerous variables and the scale of the problem, necessitating a tailored approach.

When evaluating the effects of electromagnetic interference (EMI) on pipeline systems, two separate zones of influence should be considered.

The first zone relates to EMI effects caused by DC-side harmonics of the HVDC power system, which increase the risk of corrosion due to inductive coupling. The boundaries of this zone can be determined based on criteria such as an acceptable corrosion rate or the current leakage density through coating defects, or pipe-to-soil potential shifts.

Separation distance in this instance refers to the minimum required space between pipelines and HVDC power lines to ensure that the impact on the pipeline remains within acceptable limits/criteria or is manageable by the pipeline's cathodic protection system.

The second zone is associated with transient fault conditions in the HVDC system, which can induce both inductive and conductive couplings. For this case, separate sub zones of influence can be defined for each type of coupling. However, a more comprehensive approach would involve combining inductive and conductive couplings effects into a single, unified zone of influence. The boundaries of this zone can be defined based on an acceptable coating stress voltages (coating dielectric strength breakdown). Safety is not considered in this standard.

Separation distance in this instance refers to the minimum required space between pipelines and HVDC power lines to ensure that impact on the pipeline’s coating remains within acceptable limits/criteria. A preliminary calculation and methodology for defining this distance are provided in Annex J.

NOTE Important: Determining appropriate separation distances is a complex task that often requires multiple iterations to refine the model and achieve reliable conclusions. Each project is inherently unique due to the numerous variables and the scale of the problem, necessitating a tailored approach.

Clause 6 outlines the sources of interference, which include DC interference and electromagnetic interference (EMI). DC interference can accelerate electrochemical corrosion in pipeline systems, while EMI may contribute to corrosion under normal HVDC system operation due to the presence of harmonics in HVDC power lines. During the HVDC transient fault conditions, there is also a risk of damage to pipeline coating. Protection criteria for each source are specified in 7.1 and 7.2 respectively.

If HVDC interference affects electrical safety, protective measures against electric shock in accordance with EN 50443:2011 and IEC 60479-1:2018 ED1:2018 shall be also considered to ensure that safety of humans and livestock remains the primary consideration.

The level of corrosion risk to buried pipelines near monopolar HVDC systems is influenced by several factors:

• Distance from electrode stations
• Earth electrode configuration
• Orientation of pipeline relative to electrode stations
• Soil resistivity and structure
• Pipeline material and coating (including the location of coating defects/ holidays)
• Magnitude of earth return current
• Polarity changes of earth electrodes
• Presence of other metallic structures in the area
• Effectiveness of Cathodic Protection.

The corrosion risk associated with each type of monopolar system is outlined in Table 3. During the planning stages, a preliminary assessment of risk can be performed using approximate separation distances or zones of influence calculations, as detailed in Annex I and Annex J. However, for more precise evaluation, numerical calculations and modelling using appropriate software tools may be necessary during the design, commissioning, and operational phases of the HVDC system.

Table 3 — Classification of DC interference risks – Monopolar HVDC systems (normal operation)

The electromagnetic interference (EMI) risk during the normal operation of a monopolar HVDC system is difficult to define in advance. This difficulty arises due to the presence of harmonic currents and voltages on the DC power lines, which can vary depending on converter technology. As outlined in CIGRE 811 ^[5], switching actions in HVDC converters generate a broad spectrum of harmonics, spanning from the fundamental frequency to the radio frequency range.

The harmonic effects can contribute to the corrosion risk of pipelines during the normal operation of monopolar HVDC systems. The risk applies to all types of monopolar systems, as illustrated in Table 4.

During the planning stages, a simplified approach for evaluating the effects of DC-side harmonics on pipeline systems is provided in Annex J. However, for a more accurate assessment, numerical calculations and modelling using advanced software tools are recommended during the design, commissioning, and operational phases of the HVDC system.

Table 4 — EMI risks normal operation – Monopolar HVDC systems

The risk of electromagnetic interference (EMI) during HVDC transient fault conditions is influenced by the type of DC fault, which is, in turn, determined by the specific HVDC configuration, as outlined in Table 5. During DC-side fault conditions on HVDC power lines, buried pipelines can experience both inductive and conductive coupling effects. (see 6.2 and 6.3 for overhead and cable power lines respectively). Moreover, the following factors related to pipelines can influence the level of risk:

• Separation distance and orientation of pipeline relative to power lines
• Soil resistivity and structure
• Pipeline material and coating dielectric strength
• Fault duration.

Table 5 — EMI risks transient fault conditions operation - Monopole HVDC systems

Assessing the combined effects of conductive and inductive coupling on pipelines presents significant challenges when using simplified calculations. However, simplified methods can be employed during the HVDC planning stages for a preliminary risk assessment, as illustrated in Annex J.

Commercially available software can support this analysis, but such tools often rely on proprietary computational methods. Thus, iterative modelling with "what-if" scenarios is essential, including comparisons between software outputs and simplified calculations to assess reliability. Ultimately, sound conclusions depend on engineering judgment and experience. These are critical to ensure cautious and reliable conclusions.

The corrosion risk to buried pipelines near bipolar HVDC systems is influenced by factors described in this section. The corrosion risk for different types of bipolar systems are outlined in Table 6. During the planning stages, the risk can be estimated using approximate separation distances or zones of influence, following principles similar to those used for monopolar systems, as detailed in Annex I and Annex J. However, for more accurate assessments, numerical calculations and modelling with appropriate software tools may be required during the design, commissioning, and operational phases of the HVDC system.

Table 6 — DC Interference risk level – Bipolar systems

Normal operation includes fault-free operation and temporarily permissible operation of faulty system parts. Temporary faults without effects on third-party installations also fall under this category. If faults and measures occur in crossing or proximity areas and affect third-party installations, an early information obligation exists.

Notwithstanding the above, the instructions, requirements and agreements described in the context of crossing agreements shall be observed.

Assessing electromagnetic interference (EMI) risks caused by harmonics in bipolar HVDC systems is a complex task. All harmonics should be taken into account.

In bipolar HVDC systems, DC harmonics are classified into balanced mode and residual mode components. Bipolar HVDC lines have two poles or conductors. Balanced mode currents (also called pole mode currents) are equal in magnitude but flow in opposite directions in the two lines, resulting in field cancellation. In contrast, residual mode currents are in phase and do not cancel out, making them the primary source of interference. This interference behaves similarly to zero-sequence interference in AC transmission lines. Further technical details can be found in the CIGRE 811 Technical Brochure ^[5].

The overall risk is difficult to define in advance, as it requires substantial information. Simplified methods, such as those outlined in Annex J, can be utilised during the HVDC planning stages to provide a preliminary risk assessment. However, these methods are not a substitute for the detailed calculations necessary during the design, commissioning, and operational phases of the HVDC system.

Table 7 — EMI risks normal operation – Bipolar HVDC systems

The approach to assessing electromagnetic interference (EMI) risk during HVDC transient fault conditions is similar to the method outlined for monopolar systems in 10.2.2, with adjustments to account for the fault behaviour specific to each bipolar system.

The risk of EMI is influenced by the type of DC fault, which depends on the HVDC configuration, as shown in Table 7 and Table 8.

During DC-side fault conditions on HVDC power lines, buried pipelines can be subjected to both inductive and conductive coupling effects (see 6.2 and 6.3 for overhead and cable power lines, respectively). Additionally, the following pipeline-related factors contribute to the level of risk:

• Orientation of the pipeline relative to power lines
• Soil resistivity and structure
• Pipeline material and coating dielectric strength.

The overall risk is difficult to define in advance, as it requires substantial information. Simplified methods, such as those outlined in Annex J, can be utilised during the HVDC planning stages to provide a preliminary risk assessment. However, these methods are not a substitute for the detailed calculations necessary during the design, commissioning, and operational phases of the HVDC system.

Table 8 — EMI risks transient fault conditions operation–Bipolar HVDC systems

Cathodic protection criteria under electrical interference conditions from a DC source are given in ISO 21857:2021.

An interference that a metallic buried structure experiences in a cathodic voltage gradient as shown in Figure G.2 is called anodic interference. ISO 21857:2021 specifies limit values for this case. These can be taken from Table 9.

Table 9 — Permissible positive potential shift ΔU in the case of anodic interference for buried metal structures that are not cathodically protected

Table 9 contains the maximum permissible threshold values of the positive potential shift of non-cathodically protected buried metallic structures due to the anodic interference of direct currents from foreign sources. However, a corresponding value for copper is not listed there. To a first approximation, it can be assumed that the values for steel can also be applied to copper, as they offer enough safety reserves to prevent an unacceptably high anodic influence on the copper in the vast majority of cases.

An interference that a metallic buried structure experiences in an anodic (i.e.positive) voltage gradient as shown in Figure G.1 is called cathodic interference. In order to ensure that the anodic voltage gradient does not create an unacceptably high interference on a buried metallic structure (e.g. pipeline) , the voltage gradient at the location of the buried structure should not exceed a value of 500 mV, measured against remote earth.

Protection criteria under AC electrical interference are given in ISO 18086:2019 and ISO 21857:2021.

Under fault conditions the risk of damage to the pipeline coating depends on the dielectric strength of the coating. 

Electrical strength threshold values vary according to the type of coating and its condition and shall be specified by the pipeline operator.

HVDC electrodes are designed not to raise the ground or water temperatures to levels that will have an adverse effect on flora and fauna. Nevertheless, under certain circumstances it is feasible that the pipeline can be affected.

Buried cables and electrodes are the principal heat sources in an HVDC system.

Acceptable temperature increases to the pipeline are determined by the operating temperature (e.g. the product) and the coating type.

The magnitude of the thermal effect is directly related to the buried depth, length of parallelism, the separation distance and the thermal conductivity and thermal capacity of the soil.

Temperature threshold values for acceptance shall be provided by the pipeline operator, since they are product, site and seasonally dependent.

The HVDC designer shall provide temperature profiles where the threshold values prescribed by the pipeline operator are not met.

If the temperature rise caused by the HVDC is beyond acceptable threshold limits provided by the pipeline operator, then suitable mitigation shall be applied.  This can be either at the source (e.g. thermally conductive backfill) or on the pipeline (e.g. additional thermal isolation).

HVDC systems collocated with cathodic protection anodes or earthing system electrodes shall be considered. Drying out of the soil can affect the functionality of the anodes and earthing systems. The temperature rise can also change the cathodic protection criteria (ISO 15589-1). The effect of thermal influence on the earth resistance and the pipe surface shall be evaluated.

Further information is provided in IEC TS 62344 ^[1], IEC 60287-1-1 ^[6] and ^[7].

Corrosion protection and AC mitigation systems associated with buried metal infrastructure can be adversely affected by electrical interference from HVDC systems. Devices can include systems such as:

• AC mitigation electrodes
• AC mitigation discharge devices
• Cathodic protection electrodes
• Cathodic protection test facilities
• Cathodic protection power supplies
• Stray current drainage equipment
• Remote monitoring and SCADA systems
• Pipeline telecommunication system cables (e.g. copper, fibre-optic).

The presence of such systems shall be considered in advance, and the interference risks assessed by the HVDC system designers. Collaboration with the operators of the systems that can be affected is a prerequisite to resolving any issues.

NOTE This can include oil or gas pressured cables.

The following shall be included in the obligatory exchange of information between the companies operating the facilities:

• central management bodies,
• regional bodies accompanying the operation.

The exchange of information shall include the continuous updating of technical and other documents on the crossing and proximity points and of the technical contact persons.

The operators involved are obliged to provide information on the following conditions:

• Initial commissioning(s) of systems, installations, piping systems.

As a result of this information, the commissioning date, the respective contact persons and the associated documents are to be jointly agreed.

• Trial operation(s).

For this purpose, the time periods and the work planned for this purpose, the respective contact persons and the associated documents shall be jointly agreed by the parties involved in advance. Any additional measures that may be necessary are to be determined within the scope of this coordination.

• Commissioning(s).

The time, the respective contact persons and the associated documents of a planned permanent commissioning shall be notified to all parties involved.

• Decommissioning(s).

The time, the respective contact persons and the associated documents of a planned permanent decommissioning / decommissioning / shutdown shall be notified accordingly to all parties involved.

• Transfer of legal entity(ies).

In the event of changes in legal entities, the companies involved shall be notified accordingly. The contact persons and associated documents shall be updated immediately.

• Change of operating mode(s).

In the event of a change in the operating mode(s), the parties involved must be informed. These are, for example:

• Transition to other performance-enhancing operating modes,
• significant change in the composition of the product in the pipeline.

The applicable industry-specific regulations on personal protection, health protection and work safety as well as the general regulations must be complied with.

Existing risk assessments for crossing and proximity points must be observed. If any changes become known that have significant effects on other operators, they must be communicated. If necessary, adjustments to the technical design of the crossing or proximity points and an adjustment of the crossing agreements are to be agreed.

Further possible requirements and provisions of third parties are to be implemented accordingly.

Normal operation includes fault-free operation and temporarily permissible operation of faulty system parts. Temporary faults without effects on third-party installations also fall under this category. If faults and measures occur in crossing or proximity areas and affect third-party installations, an early information obligation exists.

Notwithstanding the above, the instructions, requirements and agreements described in the context of crossing agreements must be observed.

Inspection of pipelines

A classification of the crossing points and the zone of influence of HVDC systems to pipelines as an "operationally important point" is recommended. This can result in more frequent inspection intervals and additional measures. If the pipeline is susceptible to temperature changes, this should be taken into account. (CHECK CLAUSE 12)

For the inspection of HVDC systems and pipeline systems, the initial requirements for normal operation are already defined in the design and construction phase. In the process of normal operation, however, new findings usually arise. These findings, which may also result from third party information sources, are to be shared with the pipeline operators involved in case of suspected mutual interference and appropriate measures are to be derived accordingly. Furthermore, these findings may contain the following additional or differing aspects:

• Establishment of additional measuring points to record temperatures and temperature-dependent findings near crossing points.
• Necessity of carrying out special measurement examinations, e.g., with the help of CP measurement methods or other special electrical measurements.
• Adaptation of maintenance cycles resulting e.g., from the recommended categorisation as operationally important points.
• Mining subsidence areas require additional measures to be jointly coordinated.
• Additional measures due to soil compaction work, particularly following construction works.
• Replacement of bedding materials with different technical properties.

This is necessary in individual cases; effects on certain measurements and properties of the overall system must be considered, e.g., with regard to the change in specific soil resistance or local mechanical load increases.

Special requirements of CP

During the inspection measurements according to ISO 15589-1:2015 the following additional measures shall be considered in coordination with the HVDC system operator:

• Measurements on existing earthing systems of the HVDC systems for recording and assessing possible interference parameters on both sides, if applicable, taking into account safety-related aspects.
• Interference measurements near HVDC overhead line systems and their associated earthing systems.

If HVDC operators introduce technologies to diagnose their transmission lines or systems during the HVDC service life, which may have a recognizable impact on the crossing or nearby pipelines and their earthing systems, joint consultations are required as a matter of principle.

This refers to the operating state that does not correspond to normal operation according to 10.2. This is initiated, for example, by scheduled or unscheduled maintenance measures. Emergency operations are limited in time.

In the following, the most probable events are considered which can lead to corresponding changes in operation and information is provided on which aspects must be considered.

Malfunctions

The pipeline operator shall inform the HVDC system operator of any changes in the status or operation of the pipeline that can affect the operation of the HVDC system.

Change in the operation of HVDC systems

If a deviation from the agreed operation of the HVDC system occurs after initial commissioning of HVDC systems (e.g., introduction of technologies to increase availability), a new process in accordance with Clause 8 and Clause 9 needs to be initiated. The same applies to the introduction of technologies to increase transmission capacity, such as an operating mode for HVDC overhead lines based on thermal mmonitoring or short circuit level. and protection settings.

Maintenance works on HVDC systems

Both planned and unplanned maintenance measures that can impact on the electrical interference to the pipeline shall be immediately communicated.to the pipeline operaator. In addition, if necessary, an agreement shall be reached between the operators on the measures to be carried out, the supervision of these works and the period during which they are to be carried out.

Planned structural changes at intersections and proximity points shall be communicated to the pipeline operator in good time. They automatically initiate a new process according to Clause 8 and Clause 9.

1. 
   (informative)
   
   HVDC CONFIGURATIONS
   1. General

Based on the number of nodes connected to AC transmission systems, HVDC power transmission systems are categorised into two types: point-to-point (or two-terminal) systems and multi-terminal systems.

Point-to-point HVDC systems are further divided into three configurations: monopolar, bipolar, and back-to-back. Typical monopolar and bipolar configurations are summarised in Figure A.1.

Figure A.1 — Typical monopolar and bipolar configurations

• 1. DC Neutral Point Earthing

In HVDC systems, DC neutral point earthing refers to the practice of connecting the neutral point of the DC circuit to the ground (earth).

The method of DC neutral point earthing influences the voltage stress experienced by equipment and insulation systems under normal operation and fault conditions, guiding the selection of appropriate insulation levels.

Additionally, it determines the magnitude and path of fault currents, which is essential for ensuring effective system protection and maintaining reliability.

When a DC neutral point is present, it can be earthed using one of three methods:

1. isolated neutral,
2. resistive earthing, or
3. solid earthing.

The selection of the method is guided by the system’s safety, reliability, and cost requirements.

NOTE According to IEC 61936-2:2023 ED1:2023 AC and DC systems are typically galvanically isolated. Where this is not the case, the compatibility between the AC side and the DC side earthing methods needs to be considered.

• 1. HVDC system configuration

The selection of the HVDC system configuration and the DC neutral point earthing method is a critical decision influenced by several key factors, including, according to IEC 61936-2:2023 ED1:2023, the following:

• Functional requirements for managing overvoltages and fault currents during DC-side faults.
• Choice between earth return and metallic return for current flow.
• Compliance with local or environmental regulations regarding the use of the earth as a return path.
• The level of service continuity required by the system.
• The ability to selectively isolate faulty sections of the HVDC system.
• Considerations for operation and maintenance practices.
  • 1. Monopolar systems
       1. Symmetric monopole

Figure A.2 illustrates the configuration of a symmetric monopole system.

In this configuration, the two DC terminals are connected to the positive and negative poles, which operate at equal but opposite DC voltages. The system can be earthed through a high impedance, such as by connecting the midpoint of shunt capacitors. Two high-voltage power lines are required, each fully rated for the current load and insulated to the necessary standards. It is important to note that in the event of a pole-to-ground fault, the voltage on the healthy pole can theoretically rise to twice the normal operating voltage. A fault that causes the failure of any major component results in the complete loss of power transfer. Since the DC voltage is symmetrical, the transformer does not experience any steady-state DC voltage stress. Additionally, there is no earth current return during normal operation nor fault current contribution from the AC grid in the event of a DC-side fault.

Figure A.2 — Symmetrical monopole

• • • 1. Asymmetric monopole earth (seawater) return mode

Figure A.3 shows the arrangement of an asymmetrical monopolar point-to-point HVDC system with earth or seawater return mode. This is a d.c. system with one overhead line or cable and uses the earth or seawater as the return circuit. The failure of any major component in the converter or power transmission line, whether caused by a fault or during maintenance, results in a total interruption of power transfer. Such a system can reduce investment costs in transmission lines. However, it may cause DC interference, leading to electrochemical corrosion in nearby third-party metal infrastructure within the zone of influence.

Figure A.3 — Schematic illustration of an asymmetric monopole HVDC system with earth or seawater current return mode

• • • 1. Asymmetric monopole metallic return mode

Figure A.4 shows a simplified schematic arrangement of an asymmetrical monopolar point-to-point HVDC system with a metallic return mode. This topology requires only a single high-voltage conductor (overhead line or cable) and a neutral conductor (metallic return), which is earthed at a single point. It is important to note that the earth reference depicted in Figure A.4 does not carry DC current and, therefore, does not need to be designed as an earth or shoreline electrode. The neutral conductor, or metallic return, is fully rated to handle the load current but requires less insulation, making it less expensive and less prone to dielectric failure compared to the pole conductor. However, the failure of any major component in the converter or power transmission line, whether due to a fault or maintenance, results in a complete disruption of power transfer. Using a monopolar system with a metallic return conductor may increase transmission line costs but eliminates the risk of DC interference, as no current flows through the earth.

Figure A.4 — Schematic illustration of an asymmetric monopole HVDC system with metallic return mode

• • 1. Bipolar systems

The bipolar line configuration operates with two conductors of opposite polarities: a positive pole and a negative pole. On the AC side, the two converters are connected in parallel, with each pole having its own transformer interface to the AC grid. One converter is linked to the positive pole, while the other is connected to the negative pole.

• • • 1. Bipolar with earth or seawater return operation

Figure A.5 illustrates a simplified schematic of a bipolar system designed to allow earth or seawater return operation. This capability arises from the neutral being earthed at both ends—specifically, at the neutral points of the rectifier and inverter stations. The system can be viewed as two asymmetrical monopoles interconnected through the shared earth-connected pole.

It is important to note that the earth references depicted in Figure A.5 do carry DC current and, therefore, they need to be designed as earth or shoreline electrodes.

Electrodes are used to carry unbalanced currents that may arise between the two poles of the system. These currents, typically ranging from 0.5% to 1% of the rated current of each pole, result from slight differences in the performance of the converter units at each station.

In the event of a failure or maintenance of one pole, the operational pole can utilise earth or seawater as a return path, maintaining up to 50% of the total power transmission. However, during temporary monopolar operation—such as during maintenance or emergencies—bipolar systems rely on electrodes to return the full (rated) current through the earth or seawater. This can lead to DC interference, potentially causing electrochemical corrosion in nearby third-party metal infrastructure within the zone of influence. As noted previously, under normal operation, the unbalanced current remains within the 0.5% to 1% range, thus significantly reducing the risk of DC interference. Nevertheless, this risk should not be entirely disregarded.

Figure A.5 — Schematic illustration of bipolar HVDC system earthed at both ends (earth, seawater return mode)

• • • 1. Bipolar with metallic return operation

Figure A.6 presents a simplified schematic of a bipolar system with a metallic return operation. In this configuration, a metallic conductor serves as the neutral conductor, which is in addition to the positive and negative poles.

The neutral conductor interconnects the neutral points of the two converter stations and is earthed at a single location. However, this earthing does not require the design of an earth or shoreline electrode, as it does not carry any DC current. This mode eliminates the risk of DC interference, which can cause electrochemical corrosion, by avoiding the use of earth or seawater as the current return path. It also enables continuous monopolar operation, maintaining up to 50% of the total power transmission capacity.

Figure A.6 — Schematic illustration of bipolar HVDC system earthed at one end (metallic return mode)

• • • 1. Bipolar earthed at one end (rigid bipolar)

Figure A.7 illustrates a simplified schematic of a rigid bipolar system, earthed at one end only. In this configuration, the earth reference depicted in Figure A.7 does not conduct any DC current, eliminating the need for it to be designed or function as an earth or shoreline electrode. Consequently, there is no DC current flow through the earth or seawater, which effectively eliminates the risk of DC interference, which can cause electrochemical corrosion to third-party infrastructure.

However, this arrangement has significant disadvantages. It cannot operate if a failure occurs on one of the transmission lines, and it requires perfectly balanced currents in the poles, as there is no return path for any unbalanced current. While this configuration is theoretically and technically feasible, there are no reported projects to date that have implemented it.

Figure A.7 — Schematic illustration of bipolar HVDC system earthed at one end (rigid bipolar)

• • 1. Back-to-back converter stations

Figure A.8 illustrates a simplified arrangement of a back-to-back converter station. In this setup, the two converters are located in close proximity. They are typically used to separate two independent power systems. This configuration eliminates the need for DC power transmission lines and earth electrodes. The DC voltage in the intermediate circuit can be set to a lower voltage rating, thus reducing the overall investment costs. The primary purpose of a back-to-back converter station is to limit short-circuit currents during the interconnection of electric grids. Additionally, it serves as a frequency conversion station when connecting grids operating at different frequencies.

Figure A.8 — Schematic illustration of a back-to-back converter station

• 1. Multi-terminal HVDC systems

Multiterminal HVDC systems (Figure A.9) connect three or more converter stations via a shared DC network, enabling power exchange between multiple locations. Common configurations include point-to-point, ring, and meshed topologies, each offering varying levels of redundancy and operational flexibility.

Multiterminal HVDC is typically used for connecting multiple power sources or grids, such as offshore wind farms, interconnecting regional power networks, or integrating renewable energy from diverse locations.

Therefore, evaluating DC ^[8] or electromagnetic interference (EMI) requires a thorough understanding of the entire system’s operational characteristics and response under normal conditions, emergency situations, and during DC-side transient fault events.

Figure A.9 — Schematic illustration of multiterminal HVDC systems

1. 
   (informative)
   
   CORONA
   1. General

The electric field produced by the line voltage attains the highest magnitude at the conductor surface and falls off rapidly away from the conductors, the lowest magnitude occurring near the ground surface. When the electric field at the conductor surface exceeds a critical value, known as the corona onset gradient, ionization takes place in the layer of surrounding air, leading to the formation of corona discharges ^[9].

Ionization occurs when the critical field strength on an ideally smooth conductor surface is greater than 29.8 ... 30 kV/cm. Since in practice the conductor surface is not exactly smooth, e.g. due to irregularities, ionization processes can also occur at an average critical field strength between 15 and 29.8 kV/cm. The critical field strength is locally more than 29.8 kV/cm ^[10]. The corona is characterized that with exceeding the corona onset gradient corona discharge occurs, but the electrical conduction is not that intense to cause breakdown or electrical arcing to the other nearby area.

The intensity and characteristics of corona discharges on conductors are greatly influenced by atmospheric conditions such as air density, wind, relative humidity and precipitation, as well as by environmental factors such as organic or inorganic pollution deposits on the conductors ^[9].

In bipolar HVDC systems, a partial charge balance occurs between positively and negatively charged ions which have emanated from the respective conductor, so that only a part of the total corona discharge leads to a ground current. In monopolar systems no charge equalization can occur, so that compared to bipolar systems significantly higher current densities occur.

As per ^[11] 100 to 200 nA/m²can be considered as reference for the ion current density at ground. The maximum current density can be assumed to be vertical below the conductor and decreasing rapidly towards the sides.

The corona discharge causes a visible glow on the conductors and is accompanied by audible noise and energy loss.

• • 1. Summary of HVDC corona effects

Ionization and Air Breakdown: Corona occurs when the electric field near the conductor exceeds a critical value, causing the ionization of surrounding air molecules. This breakdown is influenced by the conductor shape, surface condition, voltage level, humidity, temperature, and altitude.

Noise and Audible Hum: Corona discharges from HVDC lines can produce a noticeable "hissing" or "crackling" noise.

Ozone and Nitrogen Oxides Formation: Corona discharge on HVDC lines generates ozone (O₃) and nitrogen oxides (NOₓ), which can affect air quality near transmission lines. Ozone forms when the ionized air molecules recombine, especially in humid or polluted environments.

Energy Losses: Corona results in energy losses, which can reduce the transmission efficiency. These losses, while often small relative to the total power transmitted, become more significant at higher voltages or when lines are affected by rain or fog.

Ground Level Currents: ionic currents emitted by the HVDC transmission lines will cause a current flow in the ground following the return path to HV groundings. 

Impact on Nearby Infrastructure: HVDC corona can lead to electromagnetic interference (EMI), affecting nearby electronic equipment, especially in sensitive installations like communication lines. Proper grounding and shielding in line design and around the transmission corridor can help mitigate these issues.

Corona discharge in HVDC is a significant factor in both the design and environmental impact assessment of high-voltage lines. Effective mitigation strategies, like larger conductors, increased spacing, and controlled insulator designs, are key in managing corona effects for HVDC transmission.

• • 1. Summary of HVDC corona mitigation measures

Transmission Line Design: HVDC transmission line design should take corona effects into account. Conductor size, bundle configuration, spacing, and height above ground are optimized to reduce corona onset.

Corona Rings and Grading: HVDC lines use corona rings and grading devices to manage and distribute the electric field, especially near transmission line terminations or insulators. These devices help reduce corona effects, extending the equipment’s longevity and improving system performance.

1. 
   (informative)
   
   Competence requirements for modellers assessing HVDC interference on pipeline systems
   1. General

The assessment of high-voltage direct current (HVDC) interference on pipeline systems requires specific competence. This annex provides guidance on the knowledge, skills, and experience recommended for personnel carrying out such modelling activities. The information given is intended to support organizations in ensuring that suitably competent individuals are engaged.

• Auditing numerical analyses remains consistently challenging, as software source code is often inaccessible and users are frequently reluctant to share comprehensive details of their HVDC numerical models.
• Prequalifying modeling engineers is considered as an effective method for auditing model results. This standard offers simplified calculations and assumptions to facilitate a preliminary review of model outcomes.
• At present, standardized competence levels for this type of modelling are not yet available, despite some companies offering training programs that include certification and re-validation. This annex presents a method for assessing the capability of engineers engaged in modelling, as detailed below.
  1. Knowledge requirements
• principles of pipeline engineering, including:
• construction, operation and maintenance of buried pipelines;
• pipeline materials, coatings and cathodic protection systems;
• fundamentals of HVDC transmission systems, including:
• operating characteristics of typical configurations under normal and fault conditions;
• electromagnetic theory relevant to interference, including:
• influence of soil resistivity and conductive media;
• electrical and magnetic field coupling with pipelines;
• corrosion science, with emphasis on electrochemical processes related to interference and cathodic protection;
• modelling methodologies, including:
• numerical approaches (e.g. finite element, boundary element, circuit models);
• underlying assumptions, applicability and limitations.
  1. Skills requirements
• use and apply modelling software appropriate for HVDC interference assessment;
• develop models that represent pipeline, soil, and HVDC system interactions with sufficient accuracy;
• ability to validate model results by comparing them with field data and measurements, identify and explain any discrepancies, and implement refinements to improve model accuracy;
• interpret outputs to assess potential risks and compliance with applicable criteria;
• apply relevant standards, specifications, and industry guidance;
• prepare clear documentation of modelling inputs, assumptions, results and conclusions.
  1. Experience requirements
• practical application of cathodic protection design and monitoring techniques;
• participation in interference studies involving AC, DC or mixed sources;
• collection and use of site-specific data such as soil resistivity, pipeline coating condition and cathodic protection measurements;
• successful application of models in comparable projects.
  1. Professional attributes
• analytical and problem-solving capability;
• awareness of the limitations of models and the importance of validating results against fundamental theory;
• commitment to professional ethics and safety;
• ability to work effectively with multidisciplinary teams, including pipeline operators, cathodic protection specialists, and power transmission engineers.
  1. Confirmation of competence
• formal qualifications: evidence of relevant academic or professional education in engineering, corrosion science, or a related discipline;
• professional certification: recognition by an appropriate professional or technical body, where available;
• training records: documented completion of structured training related to cathodic protection, AC/HVDC interference, or modelling techniques;
• project references: evidence of participation in projects where HVDC or electrical interference studies were undertaken, including documented outcomes.

NOTE Competence is normally confirmed through an independent technical peer review of modelling work conducted by a qualified specialist.

• 1. Consolidated Technical Abilities

This sub clause consolidates the technical abilities expected of personnel engaged in the assessment and modelling of HVDC interference on pipeline systems. The abilities can be grouped into baseline abilities, which represent the fundamental competence required for effective performance of modelling tasks, and advanced/expert abilities, which build on the baseline and reflect higher levels of expertise needed for complex analysis and design of mitigation measures. The intent of this consolidation is to provide clear guidance on the progression of competence, to assist organizations in defining role requirements, training needs, and qualification pathways.

• • 1. Baseline abilities
• Specify all soil resistivity measurements required along joint-use corridors for EMI assessment.
• Specify appropriate equipment and test procedures.
• Accurately interpret and refine soil/sea water resistivity measurements.
• Determine appropriate soil structure models, accounting for seasonal and geographical variations.
• Build accurate models of transmission HVDC lines entering substations in rural, semi-urban, and urban areas.
• Model HVDC and AC power lines.
• Select appropriate models for gas and oil pipelines.
• Evaluate electrical safety and electrochemical concerns for pipelines.
• Understand and apply mitigation techniques for pipelines, recommending economical solutions for safe operation.
• Carry out comprehensive analysis of joint-use corridor performance under steady state and fault conditions.
  • 1. Advanced/expert abilities
• Calculate self and mutual impedances of arbitrary 3D conductor and complex HVDC transmission systems (including multi-terminal HVDC systems.
• Determine interference (EMI) effects on pipelines under HVDC systems’ fault and transient conditions.
• Design mitigation measures to mitigate or reduce EMI levels.

1. 
   (informative)
   
   Earth Potential Rise
   1. General

Modelling the effects of stored energy in an HVDC system—particularly the contribution from capacitance—is a significant factor when considering transient phenomena such as faults. In an HVDC system, the capacitance of the cables, along  other stored energy components, plays a crucial role during faults, affecting both the magnitude and duration of phenomena like Ground Potential Rise GPR (EPR).

To account for stored energy due to capacitance within an HVDC system under fault conditions a more detailed approach is required.

• 1. Understanding Capacitance in HVDC Systems
• Capacitance Sources: In HVDC systems, capacitance is inherent in components like transmission cables, transformers, and filter banks. The cables themselves act as capacitors, storing electric energy due to the potential difference between the conductor and ground.

Effect During Fault

During a fault, the stored energy in the system discharges into the fault node, which increases the fault current. This additional current affects the GPR (EPR) and can lead to a higher potential rise compared to what is estimated by just considering resistive properties.

• 1. Modelling Capacitance Contribution

To model this more accurately, we need to consider the total stored energy in the system and how it discharges during a fault. Here are the key parameters to consider:

• Cable Capacitance (C): Typically measured in farads per unit length, this capacitance is a key factor in how much energy is stored in the cable.
• Initial Voltage (V₀): The pre-fault voltage, which represents the potential across the capacitance before the fault occurs.
• Discharge Current: The discharge from the capacitance contributes to the overall fault current.
  1. Mathematical Representation

The energy stored in the capacitance of a transmission line can be given by:

(D.1)

where

E energy stored in the capacitor (in joules);

C capacitance (in farads);

V₀ voltage across the capacitor before the fault (in volts).

When a fault occurs, this stored energy will discharge into the fault location. The transient fault current contribution from the capacitance can be estimated by considering the discharge time constant of the capacitor-resistor (CR) circuit formed by the line capacitance and the soil resistance.

The transient current I_t due to capacitance discharge can be approximated as:

(D.2)

where

R equivalent resistance of the ground (including soil resistivity effects);

C capacitance of the cable;

t time after the initiation of the fault.

• 1. Incorporating Capacitance in GPR (EPR) Calculations

To account for capacitance in the GPR (EPR) calculations:

Total Fault Current (I_f(t)): Consider both the steady-state fault current (from resistive components) and the transient current (from capacitive discharge). The total fault current is:

(D.3)

where I_resistive is the fault current due to resistive components, and I(t) is the transient component due to capacitance.

• 1. Ground Potential Rise GPR (EPR)

The GPR (EPR) can now be modeled as:

(D.4)

where R_eq is the equivalent resistance looking into the soil layers.

• 1. Calculation Methodology
     1. Define the Fault Current as a Function of Time

Use the formula for I(t) to account for the transient effect of capacitance.

Calculate the total fault current I_f(t) by adding the steady-state current and the transient current.

• • 1. Calculate GPR (EPR) Over Time

Use the formula for V_GPR(t) to find the GPR (EPR) over time as the capacitance discharges.

• 1. Key Considerations
• Accuracy of Capacitance Values: Make sure the capacitance values are accurately measured or calculated based on cable specifications.
• Transient Duration: The transient current due to capacitive discharge usually decays quickly, but it can significantly impact the peak GPR (EPR) during the initial phase of the fault.
• Numerical Solutions: In more complex scenarios involving multiple capacitances and resistances, numerical simulation software tools can be required for more detailed modelling.

This method incorporates the energy stored in the system due to capacitance, which significantly impacts the peak GPR (EPR) during faults in HVDC systems.

1. 
   (informative)
   
   BARNES LAYER
   1. General

Soil resistivity measurements made in accordance with IEEE Std 81, provide an average value of the soil resistivity at a depth equivalent to the spacing of the measuring and injection pins.

The Barnes method is a mathematical procedure that approximates the resistivity of the soil in incremental layers.  This is achieved by determining the conductivity of each layer and then converting it to resistivity.

• 1. Limitations

The Barnes method cannot be universally applied, because the method assumes that the layers are of uniform thickness and parallel to the surface. Nevertheless, it can provide useful information about the layer resistivities because inconsistent results indicate that the layers are not uniform and further measurements are required.

• 1. Example Calculation:

There are six steps to follow to collect and evaluate the resistivity

1. Perform standard 4-electrode Wenner tests, measuring the resistance at increasing electrode spacings
2. Calculate the apparent resistivity
3. Calculate the factor (Factor = 628 x Spacing (m))
4. Calculate R1
5. Convert R1 to conductance (1/R1) by taking the reciprocal of the apparent resistivity
6. Find the incremental conductance by subtracting consecutive values
7. Calculate the layer resistance, R2
8. Calculate layer resistivity.

Table E.1 — Example Calculation

1. 
   (informative)
   
   Method to determine the soil potential to remote earth through voltage gradient measurements
   1. General

In practice, the absolute potential to remote earth at a given point cannot be measured directly. A reference electrode placed in the ground provides only local potential, which still contains residual interference and is influenced by soil resistivity conditions. Voltage-gradient measurements offer a practical method to derive the soil potential relative to remote earth and to support reliable interference assessment, particularly where the exact HVDC electrode location or the magnitude of injected current is not known. The method described herein is valid under the assumptions of:

• uniform soil resistivity and
• steady earth-injected current.
  1. Measurement Principle

To determine the potential field, voltage differences are measured at three successive locations lying along the radial axis of the current flow.

The potential at distance r from the current source is described by:

(F.1)

where

I is the injected current (A);

ρ is the soil resistiviti (Ωm);

is an unknown parameter.

At the ground surface, the measurable quantity is the voltage difference between two locations separated by a known spacing x

(F.2)

• 1. Measurement Configuration

Three measurement points are arranged along a straight line in the direction of expected current flow, each separated by the known spacing x:

(F.3)

The measured potentials at these points are:

(F.4)

The corresponding voltage differences are:

(F.5)

(F.6)

• 1. Determination of distance to the current source

Eliminating the unknown parameter K yields:

(F.7)

Substituting and :

(F.8)

(F.9)

This nonlinear equation is solved iteratively to obtain the distance r_a from the first measurement point to the effective electrode position. No prior knowledge of electrode location or injected current is required.

• 1. Determination of the Proportionality constant

Once r_a is known, the parameter K is obtained from the measured voltage difference:

(F.10)

• 1. Reconstruction of the potential field

With K known, the soil potential at any radial distance r is calculated as:

(F.11)

1. 
   (informative)
   
   Principles of the influence of direct currents from external sources on buried metal pipelines
   1. General

If a buried metal pipeline is located in a voltage gradient from an external source, then this part of the pipeline will experience a voltage difference. The magnitude and polarity, with respect to remote earth, of the voltage difference depends on the orientation of the pipeline perpendicular to the equipotential lines and on the distribution of the voltage gradient in this area. CHANGE ALL STRUCTURES TO PIPELINE

If the buried system has a high-resistance coating (e.g., PE coating), the above effect can only take place if the coating has more than one coating defect.

Figure G.1 and Figure G.2 show the case of a buried pipeline section with 2 coating defects, which is located in an anodic (Figure G.1) and cathodic (Figure G.2) voltage gradient generated by a point source.

Key

1 buried object e.g. pipeline

2 electric field lines

3 equipotential lines

4 coating defect 2

5 coating defect 1

6 pipeline

A detail view buried pipeline

Figure G.1 — Presentation of a pipeline section with 2 coating defects in an anodic voltage gradient caused by a point source

Key

1 buried object e.g. pipeline

2 electric field lines

3 equipotential lines

4 coating defect 2

5 coating defect 1

6 pipeline

A detail view buried pipeline

Figure G.2 — Presentation of a pipeline section with 2 coating defects in a cathodic voltage gradient caused by a point source

While anodic voltage gradients are generated by corrosion protection devices, poorly coated cathodically protected buried pipelines cause cathodic voltage gradients.

• 1. Principles of the influence of direct currents from external sources on buried metallic structures

Figure G.1 illustrates that in the case of an anodic voltage gradient. There is current injection at the defect that is closer to the point source (defect 1), while there is current discharge at the defect that is further away from the point source (defect 2). This means that in this case the corrosion process takes place at the defects that are far away from the centre of the voltage gradient. Figure G.2 on the other hand, clearly shows that with the same arrangement in a cathodic voltage gradient, the corrosion takes place at coating defects in the vicinity of the voltage gradient maximum.

While anodic voltage gradients are generated by corrosion protection devices, poorly coated cathodically protected buried pipelines cause cathodic voltage gradients.

1. 
   (informative)
   
   Technical measures for design phase of the HVDC system
   1. General

During the design phase an Electrical Interference Assessment (EIA) will be required to assess the risk of unacceptable electrical interference from the HVDC system.

• 1. Technical Measures

At this stage it is likely that key criteria for the EIA may not be available. In such case the EIA shall be undertaken using agreed estimates of the key values with the designer of the HVDC system.  Such criteria can include:

• Separation distances
• Earthing impedances
• AC and DC load currents and voltages
• Soil resistivities
• Mean time between failures
• Mean time to repair
• Location of substations
• Location of joint bays
• Location of converter stations
• Harmonics
• Pipeline coating condition
• Status of cathodic protection.

The design phase is the time to establish a formal liaison between the pipeline and HVDC system operators.

To consider the possible influences during the route planning, at least the following preliminary works/basics must be carried out (the following list does not claim to be complete):

• Query of third-party pipelines:

The inventory of third-party pipelines and installations (e.g. public records, GIS portals, land registers, ) should be queried. The query should include information on the following topics: Pipeline location, depth, material, diameter, maintenance work, easement restrictions, earthing systems, and other critical information.

• Data validation:

The data of the operators of the third-party installations should be validated. It may be necessary to verify the location of the third-party installations.

• Evaluation of restrictions:

 The easement access (horizontal/vertical) imposed on the pipeline operators should be documented and graphically displayed. The need for action at crossings and parallel routes is to be determined. Depending on the result, the following steps can be required:

• Additional expert reports/calculations:
• Especially in the case of deviations from the specified easement/minimum distances, calculations must be carried out to exclude unacceptable influences or to prove the deviations are acceptable.
• Further coordination between the parties concerned (e.g., crossing agreements).

1. 
   (informative)
   
   Separation distances under DC interference
   1. General

This Annex outlines an approximate, simplified calculation method for determining the appropriate separation distance during the planning stage of an HVDC project, where information is limited, and uncertainty is high. When current is injected into the earth as part of an earth return process — common in asymmetric monopolar or bipolar systems with earth return — it creates a voltage gradient in the soil, which in turn influences the pipeline potential. This method approximates the resulting pipeline potential, leading to leakage current density over a coating defect and subsequent metal loss due to corrosion, as governed by Faraday’s electrolytic law. Ultimately, the separation distance can be defined based on the acceptable annual metal loss threshold.

• 1. Description of Methodology

We consider an HVDC electrode and a pipeline routed at any orientation, positioned at a distance y from the electrode, as illustrated in Figure I.1.

Key

1 HVDC Ground electrode

2 pipeline

I₀: current injected into the earth

y: shortest distance of HVDE electrode to pipeline

x: distance along the length of pipeline

point P: located at any distance ∓ x from the origin

Figure I.1 — Pipeline routed at any orientation, positioned at a distance y from the electrode

• 1. Calculation of pipe to soil potential

With reference to Figure I.1 If the soil is either inherently uniform or approximated as an equivalent uniform medium from a multi-layer structure, and the earth electrode is modelled as a point current source, the potential V at a given point P on the metal pipe is given by Formula (I.1).

(I.1)

where

V is the potential of Point P on the metal pipe (V);

I_o is the HVDC electrode equivalent earthing current (A);

ρ is the global soil resistivity either inherently uniform or approximated as an equivalent uniform medium (Ω.m);

y represents the shortest distance between the pipeline and the HVDC electrode (m);

x is the distance from point P along the pipeline, measured from the location where the pipeline is closest to the HVDC electrode (m).

NOTE Formula (I.1) is derived from Laplace’s formula, which governs the distribution of electric potential in a uniform soil medium.

In Formula (I.1) y needs to be significantly larger than the diameter of the HVDC earth electrode array.

The resistivity valued used in Formula (I.1) is the deep layer value, not the surface value.  See annexe E for information about the Barnes Layer calculation.

• 1. Calculation of coating defect resistance

The coating holiday resistance is defined in ^[12] as given in Formula (I.2).

(I.2)

where

A is the coating holiday disc area in contact with the local soil;

ρ_s is the soil resistivity of the area near the coating holiday location (Ωm);

ρ_h is the resistivity of the electrolyte filling the small cylindrical coating defect (Ωm);

d is the pipeline coating thickness (m) filled with local soil.

NOTE 1 The coating holiday is assumed to be a small cylindrical vacancy in the coating, having a cross section A and the same height as the pipeline coating thickness d.

NOTE 2 To simplify Formula (I.2), it may be assumed that d is very small so that ρ_hd/A can be neglected.

• 1. Calculation of current through the coating defect

The coating defect current (I_d) is estimated as the ratio of the pipe-to-soil potential (Formula (I.1)) to the holiday resistance (Formula (I.2)), as shown in (Formula (I.3)).

(I.3)

• 1. Corrosion Estimation Using Faraday's Law

The output of equation (Formula (I.3)) can be used to calculate the metal loss as described by Faraday’s law of electrolysis, which relates the electric current passing through a metal to the amount of material that corrodes over time. The mass of corroded metal and the thickness of metal loss are determined by equations (Formula (I.4)) through (Formula (I.6)):            

(I.4)

where

I_d is the current through the coating defect, given by Formula (I.3);

Q is the total corrosive charge;

t is the time duration of the earthing current flow for as specified period in seconds, (e.g. 1 year = 31 536 000 s).

The metal loss (m) in g/time (e.g. g/year) is given as in (Formula (I.5)) where the thickness of metal loss (t_m) in m/time (e.g. m/year) is given in (Formula (I.6)).

(I.5)

(I.6)

where

t_m is metal loss per year (m/year);

M is the atomic mass;

dm is mass density volume of metal which depends on the material;

n is the number of electrons per molecule reacted;

F is the Faraday electrolytic constant, (96485 C/mol);

A is the coating holiday area in contact with the local soil.

• 1. Estimation of required separation distance

The procedure for estimating the required separation distance is based on the acceptable annual metal loss threshold for the pipeline. The steps are as follows:

1. Determine the acceptable annual metal loss threshold for the pipeline.
2. Perform calculations defined in Formula (I.1)-Formula (I.3) at multiple distances y from the electrode (as shown in Figure I.1.
3. Estimate the metal loss at each distance using the calculations described in equations (4)-(6).
4. Identify the value of y as the separation distance that results in an annual metal loss within the acceptable threshold.
5. 
   (informative)
   
   Separation distance under DC side fault conditions (inductive coupling)
   1. General

DC side fault conditions in an HVDC system can generate both inductive and conductive couplings. As a result, distinct zones of influence can be established for each type of coupling. In this context, separation distance refers to the minimum required space between pipelines and HVDC power lines to ensure that the impact on pipeline coating remains within acceptable limits. This annex provides a preliminary calculation for determining separation distances, specifically considering only the inductive coupling caused by DC-side faults in an HVDC system with overhead lines ^[13].

• 1. Notes
• The calculation is based on an estimation of the induced voltages on the pipeline, expressed as V = I × Z_m, where I represents the fault current and Z_m is the mutual impedance between the two circuits (e.g., the overhead line conductor that carries the fault current and the pipeline).
• In this annex, the mutual impedance Z_m is determined using the complex image method.
• A DC side fault current may consist of multiple frequencies, meaning it contains a rich spectrum of harmonics. These harmonics influence the value of the mutual impedance. Depending on the converter technology, there may also be a fault current contribution from the AC side, (e.g. 50/60 Hz).
• The effect of harmonics on the calculation of Z_m can be accounted for using the complex image method. The total induced voltage is the vector sum of the voltages calculated for each harmonic.
• In the following worked example, harmonic effects of the fault current waveform are not considered in the analysis.
• Superposition and vectorial addion represents a worst case scenario and can be used to establish an estimated separation distgance.
  1. Worked example
     1. Layout

We consider the following layout:

Key

1 faulted power line conductor that carries current i

2 pipeline

Figure J.1 — Layout

• • 1. Limit length concept

The limit length (L) concept is a visual graphical method used to determine whether a specific combination of two factors— the length of parallelism between a pipeline and a faulted overhead line conductor and the separation distance between them—exceeds the acceptable induced coating stress voltage for the pipeline.

• • • 1. Calculation of mutal impedance Zm

With reference to Figure J.1 the mutual impedance is calculated as expressed in (Formula (J.1)):

(J.1)

(J.2)

where

p is the complex skin depth for uniform soil resistivity of fictitious mirror plane which depends on frequency and earth resistivity (m);

d is the lateral separation distance between the conductor and pipeline (m);

h is the vertical distance of the conductor (m);

f is the frequency in (Hz);

ω is the angular frequency equals to ;

ρ is the earth resistivity ;

σ is the earth conductivity, equals to ;

μ₀ is the magnetic permeability of free space .

• • • 1. Calculation of limit length

The procedure for calculating the limit length (L) is based on the acceptable induced voltage and other parameters defined in Formula (J.3). The calculation and processing steps are as follows:

1. Set the coating stress voltage threshold – Define the acceptable induced voltage for a given pipeline coating.
2. Perform mutual impedance calculations (– Compute the necessary values from Formula (J.1) and Formula (J.2) for various separation distances (d).
3. Determine the limit length(L) – Using the relationship in Formula (J.3), calculate the limit length for each horizontal distance (d) based on the defined voltage threshold in step 1.

NOTE The mutual impedance can be calculated as given in Formula (J.1) and Formula (J.2).

(J.3)

where

V_i is the coating stress voltage threshold limit (V);

r is the reduction factor for the fault current magnitude;

I is the fault current (A);

Z_m(d) is the mutual impedance (Ω/m) at each separation distance;

d is the separation distance (m) (see Figure J.1).

• • • • 1. Create a table containing the calculated values of d Z_m(d) and L

Table J.1 — Table containing the calculated values of d Z_m(d) and L

• • • • 1. Generate logarithmic plots of L (limit length) vs d (separation distance)

In the plots:

• The y-axis (limit length) inherently represents the parallel length between the faulted power line and the pipeline.
• The x-axis represents the horizontal separation distance between them.

Figure J.2 provides an example of this plot using the data in Table J.2.

Table J.2 — Input Data

Key

X separation distance (m)

Y limit length L (km)

1 area where the coating stress voltage of the pipeline is > 5 kV

2 area where the coating stress voltage of the pipeline is < 5 kV

Figure J.2 — Sample Plot with the data of Table J.2

• • • • 1. How to interpret the graph

The solid line represents the boundary for the coating stress voltage limit (e.g., 5 kV).

• Points A1 and A2 on the graph indicate combinations of parallel length and separation distance that result in exactly 5 kV induced voltage.
• Point B represents a combination where the induced voltage is less than 5 kV.
• Point C represents a combination where the induced voltage exceeds 5 kV.

The exact induced voltage for points B and C can be calculated using Formula (J.4).

(J.4)

Further details are provided in ^[8].

Bibliography

[1] IEC TS 62344:2022 ED2:2022, Design of earth electrode stations for high-voltage direct current (HVDC) links - General guidelines

[2] IEC 60050-151:2001/AMD3:2019 ED2:2019, Amendment 3 - International Electrotechnical Vocabulary (IEV) - Part 151: Electrical and magnetic devices

[3] C. A. Charalambous and A. I. Nikolaidis, "Complete Method to Assess the DC Corrosion Impact on Pipeline Systems During the Planning and Approval Stages of HVDC Systems With Earth Current Return," in IEEE Access, vol. 10, pp. 127550-127562, 2022, doi: 10.1109/ACCESS.2022.3226940

[4] C. A. Charalambous, "Interference Activity on Pipeline Systems From VSC-Based HVDC Cable Networks With Earth/Sea Return: An Insightful Review," in IEEE Transactions on Power Delivery, vol. 36, no. 3, pp. 1531-1541, June 2021, doi: 10.1109/TPWRD.2020.3011128

[5] CIGRE Working Group Number – B4-68 DC side harmonics and filtering in HVDC transmission systems – TB 811

[6] IEC 60287-1-1:2023 ED3:2023, Electric cables - Calculation of the current rating - Part 1-1: Current rating equations (100 % load factor) and calculation of losses - General

[7] CIGRE 675 GENERAL GUIDELINES FOR HVDC ELECTRODE DESIGN January 2017

[8] A. M. Demetriou, A. Nikolaidis and C. A. Charalambous, "DC Corrosive Challenges in Radial Multi-Terminal HVDC Systems: Understanding the Role of HVDC Electrodes Under Emergency Operations," 2025 IEEE International Conference on Environment and Electrical Engineering and 2025 IEEE Industrial and Commercial Power Systems Europe (EEEIC / I&CPS Europe), Chania, Crete, Greece, 2025, pp. 1-7

[9] Electric Field and Ion Current Environment of HVDC Overhead Transmission Lines, CIGRE 473, August 2011

[10] Hochspannungsgleichstromübertragung - Eigenschaften des Übertragungsmediums Freileitung, Fuchs, Karsten; Novickij, Aleksandr; Berger, Frank; Westermann, Dirk, Ilmenauer Beiträge zur elektrischen Energiesystem-, Geräte- und Anlagentechnik - Band 7, 2014

[11] Corona Characteristics and Audible Noise of Hybrid AC/DC Transmission Lines, Hedtke, Sören, ETH Zurich (DISS. ETH NO. 26534), 2020

[12] CIGRE 290 AC corrosion on metallic pipelines due to interference from AC power lines - Phenomenon, modelling and countermeasures - 2006

[13] C. A. Charalambous, A.-M. Demetriou, K. Lax, and N. Kioupis, “Calculation of separation distances between pipelines and HVDC systems,” in Proc. CEOCOR Conf., Ancona, Italy, May 23–26, 2025.