IEC PAS 62344:2007

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PAS 62344
SPECIFICATION
First edition
Pre-Standard
2007-05
General guidelines for the design
of ground electrodes for high-voltage
direct current (HVDC) links
Reference number
IEC/PAS 62344:2007(E)
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PUBLICLY
IEC
AVAILABLE
PAS 62344
SPECIFICATION
First edition
Pre-Standard
2007-05
General guidelines for the design
of ground electrodes for high-voltage
direct current (HVDC) links
PRICE CODE
Commission Electrotechnique Internationale X

International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
For price, see current catalogue

– 2 – PAS 62344 © IEC:2007(E)
CONTENTS
FOREWORD .4
INTRODUCTION .5
1 Scope. 6
2 Basic concepts .6
2.1 Monopolar system .6
2.2 Bipolar system .7
2.3 Mixed or combined systems .8
3 Electric field as the decisive factor for selection of site.9
3.1 Why is the electric field the decisive factor? .9
3.2 Data necessary to determine the field.9
3.2.1 Reference currents (or electrode rating).9
3.2.2 Resistivity data .9
3.3 Considerations on site selection .9
3.4 Calculation of field.10
3.5 Apparent resistivity.11
4 Impact of the field on buried metallic objects .11
4.1 Impact on non-insulated buried metallic objects .11
4.2 Impact on insulated metallic objects .12
4.3 Impact on an a.c. grid .12
4.4 Reduction of impact due to polarization .14
4.4.1 Polarization on non-insulated metallic objects .14
4.4.2 Polarization on insulated metallic objects .16
4.4.3 Reduction of star-point currents due to polarization .16
5 Compass errors.17
6 Types of electrode stations .17
7 Design aspects for land electrodes .18
7.1 General .18
7.2 Heating of soil .19
7.3 Moisture content of soil/electric osmosis .20
7.4 Material for land electrodes.20
7.4.1 Inner conductor .20
7.4.2 Coke or graphite powder filling .21
7.5 Geometric layout of the electrode .22
7.5.1 Horizontal arrangements.22
7.5.2 Vertical arrangements.24
7.6 Step voltage.24
7.7 Touch voltage .25
8 Design aspects for sea electrodes .25
8.1 General .25
8.2 Sea electrodes using titanium as active part (anodic operation).26
8.2.1 Material.26
8.2.2 Current density and gradients.26
8.2.3 Geometric layout .26
8.3 Sea electrodes using coke or SiFeCr-rods as active parts (reversible operation) .27
8.3.1 Overheating risk .27
8.3.2 Material for sea electrodes .27

PAS 62344 © IEC:2007(E) – 3 –
8.3.3 Geometric layout of reversible sea electrodes .28
8.4 Sea electrodes using bare copper conductors as active parts (cathodic
operation only) .28
9 Design aspects for shore electrodes .30
9.1 General .30
9.2 Beach electrode stations.30
9.3 Pond electrode stations .32
10 Chemical aspects .34
11 Connection converter station – Electrode station .35
11.1 Separation of a.c. grid from electrode station .35
11.2 Constructional principles of electrode connection .36
11.3 Detection of faults on electrode connections .36
11.4 Electrode connection terminations.37
12 Operating experience – Reliability, availability, maintainability.37
13 Commissioning.38
14 List of references.39
Appendix 1 .41
Appendix 2 .42
Appendix 3 .43
Appendix 4 .44
Appendix 5 .47

– 4 – PAS 62344 © IEC:2007(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GENERAL GUIDELINES FOR THE DESIGN OF GROUND ELECTRODES
FOR HIGH-VOLTAGE DIRECT CURRENT (HVDC) LINKS

FOREWORD
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all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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A PAS is a technical specification not fulfilling the requirements for a standard but made available
to the public.
IEC-PAS 62344 was submitted by the CIGRÉ (International Council on Large Electric
Systems) and has been processed by subcommittee 22F: Power electronics for electrical
transmission and distribution systems, of IEC technical committee 22: Power electronic
systems and equipment.
The text of this PAS is based on the This PAS was approved for publication by
following document: the P-members of the committee
concerned as indicated in the following
document:
Draft PAS Report on voting
22F/116/NP 22F/128/RVN
Following publication of this PAS, which is a pre-standard publication, the technical committee
or subcommittee concerned will investigate the possibility of transforming it into an
International Standard.
This PAS shall remain valid for an initial maximum period of three years starting from
2007-05. The validity may be extended for a single three-year period, following which it shall
be revised to become another type of normative document or shall be withdrawn.

PAS 62344 © IEC:2007(E) – 5 –
INTRODUCTION
Most of the world's HVDC links have been (or still are) in a first monopolar stage, because this
solution gives the lowest costs. If the connection between a monopolar pair of converter terminals
consists of an overhead line construction, the extra costs of a return conductor on the pylons are
moderate. This is certainly not the matter if the connection mainly consists of a long submarine
cable, because the return cable, which must have about the same cross-section as the main
cable, but much lower design voltage, may easily cost 30-50 % of the main cable.

The evaluation of the additional losses in the return path must be included when costs of different
possible solutions are compared. A return path via ground electrodes will normally have a
considerably smaller resistance than any reasonable metallic conductor return.

When a monopolar link becomes bipolar, the use of the return path and the number of hours of
operation with nominal current decrease. At this stage the evaluation of losses in the return path
loses importance; but the return path will be important for raising the overall reliability/availability
of the link.
The sites chosen for converter stations belonging to a specific HVDC scheme under
design/construction are generally finalized at an early stage of the time schedule of the project,
while a choice of electrode station sites, or even a general analysis, whether ground return is
feasible (or possible), is often postponed to a later stage in the time schedule.

The summary of existing electrode stations [0] shows distances from converter stations to
electrode stations ranging from 8 km to 85 km. The need for a minimum distance will be explained
in 4.3. The need for a maximum distance is a matter of economy. The selection of a site for an
electrode station should generally involve the following considerations.

a) The possibility of obtaining permission to establish and operate the station at the intended site,
and to obtain the ownership of the area, if appropriate.
b) The distance to metallic objects such as pipelines, cables, grounding networks at a.c. stations
(including the converter station itself), and other infrastructure.
c) The geology of the site must fulfil certain limits for resistivity, moisture content, thermal
conductivity, water exchange, water depth, etc.

The technical circumstances which could be problematic when establishing a ground return may
roughly be divided into two groups.

d) Problems at some distance or far from the station: The field, produced by the current in the
earth, might have an unacceptable influence on other infrastructure.

e) “Local” constructional difficulties, such as high resistivity and too dry soil. Furthermore,
chemical aspects such as chlorine production may cause local difficulties. This is further
described in Clause 10.
There is good reason to mention the “distance” field problem as the most important, because the
remote field produced by an electrode is independent of the construction of the station, and only
depends on the geology of the subsoil. This will be explained further in Clause 3.

As a general rule, local constructional difficulties may be handled to a great extent by making the
size of the station greater, the number of subelectrodes larger, etc.

Following the definition in [3], the electrode stations are divided into three groups:

– land electrodes, located far away from the sea;
– shore electrodes, located on a shore against (salt) seawater. Shore electrodes can be located
either on the beach at a short distance (<50 m) from the waterline or in the water, but
protected by a breakwater;
– sea electrodes, located in the water at some distance (>100 m) from the coastline.
—————————
Figures in square brackets refer to Clause 14.

– 6 – PAS 62344 © IEC:2007(E)
GENERAL GUIDELINES FOR THE DESIGN OF GROUND ELECTRODES
FOR HIGH-VOLTAGE DIRECT CURRENT (HVDC) LINKS
1 Scope
The purpose of this PAS is to provide a guide for the design of electrode stations for HVDC links
intended for ground return. This design guide was prepared by the CIGRÉ Working Group 14.21:
HVDC Ground Electrode Design during the period 1995-1998.

It is not the purpose of this report to provide detailed instructions on how to work out an HVDC
link from the initial idea to final decisions on sites, ratings, constructional principles for converter
stations and for connecting lines/cables. In the often hectic planning phase of a new link, the main
emphasis will be concentrated on converter stations and the line/cable, while less attention is paid
to a simultaneous evaluation of possible current return principles.

2 Basic concepts
2.1 Monopolar system
New HVDC schemes often first have a monopolar stage. The use of ground return necessitates
the presence of an anodic ground electrode adjacent to one of the converter stations and a
cathodic ground electrode adjacent to the other converter station.

Figure 2.1.1 – A 250 MW, 250 kV, 1 000 A monopolar HVDC scheme
transmitting power from converter 1 to converter 2

Normally, converter equipment is constructed for a uniform direction of current. However, the
direction of power transmission can be changed by changing the polarity of voltage; see Figure
2.1.2.
Figure 2.1.2 – A 250 MW, 250 kV, 1 000 A monopolar HVDC scheme
transmitting power from converter 2 to converter 1

PAS 62344 © IEC:2007(E) – 7 –
Thus, the basic concept of a monopolar scheme is characterized by the following.

a) Each electrode station remains in a constant mode, anodic or cathodic.
b) Each electrode station must be able to carry the rated system current continuously.

2.2 Bipolar system
In principle, the bipolar scheme consists of two monopolar systems which generally have the
same rating and where the converter equipment for both monopoles is located in a common
converter station.
Figure 2.2.1 – A 500 MW, ±250 kV, 1 000 A bipolar HVDC scheme
transmitting power from left to right

The basic concept of a bipolar scheme is characterized by the following.

a) Normally, the current in the electrode stations can be kept at a low balanced value (<3 % of
system current).
b) If one of the poles is out of service due to maintenance or fault, the pole is switched off, and
operation may be continued in a monopolar mode with the still operational pole.
c) The electrodes in that case must be able to carry the system current for the period foreseen or
necessary for monopolar operation. Both electrode stations must be able to operate in either
anodic or cathodic mode, depending on which pole is operating.
A bipolar system having one of the poles out of service can be arranged for metallic return,
provided that the high-voltage conductor belonging to the pole is undamaged. To do this, a
number of switches are necessary. Because the resistances of the normal conductors are
usually much higher than the resistance of the electrode circuit, the use of metallic return
raises the conductor losses to the same level as the total bipolar scheme, but with only one
pole in operation. This means a double loss percentage for the line losses.

– 8 – PAS 62344 © IEC:2007(E)
Figure 2.2.2 – A bipolar scheme using metallic return – Pole 1 is operating
and pole 2 is out of service
There will be another emergency operational mode for a bipolar system if one conductor is out of
service. In this case the bipolar system changes to a monopolar system, needing the electrode
circuit to be used for the total system current.

Figure 2.2.3 – Operational mode for a bipolar system where main conductor 1
is out of service – Pole 1 is out of service, even if not damaged

2.3 Mixed or combined systems
A balanced bipolar system consisting of two identical monopoles is normally specified in a single
contract and is constructed and put into operation within a short period of time. If pole 2 of a
bipolar system is not constructed together with the first pole but at a later stage, the technical
evolution may result in differences of ratings, voltages, etc. between the old and the new pole.

This might result in unbalanced technical solutions of different kinds. For instance, the Konti-Skan
scheme consists of two poles of unequal ratings, but using the same pair of reversible electrode
stations. The Skagerrak scheme, originally consisting of a balanced bipole, now consists of three
poles, of which the youngest, pole 3, is opposite to a parallel connection of the two old poles. For
the electrode stations this has resulted in less favourable (unbalanced) operation.

PAS 62344 © IEC:2007(E) – 9 –
3 Electric field as the decisive factor for selection of site
3.1 Why is the electric field the decisive factor?
The field (voltage and gradient) at a point with a certain distance from an electrode station is
dependent only on two parameters once the possible site area has been selected:

a) the current transmitted from the station;
b) the resistivity conditions in the underground/sea as seen from the site of the electrode.

The way the electrode station is constructed has no influence on the magnitude/direction of the
distant field, whether it is superficial or deep, small or large in size, linear, star- or ring-
configurated, etc.
It is crucial, therefore, to reach agreement, or at least have a positive discussion with
environmental authorities, with other utilities having metallic infrastructure in the ground, or
whoever might have an influence on the possibility of using ground return. If an agreement on
acceptance of the field is not likely at any suggested site, the intended HVDC scheme must be
based on return principles other than ground return. The authorities, other utilities and others
being against ground return should bear in mind the following consdierations:

c) ground return, maybe with limitations, is operating successfully in about 25 HVDC schemes
throughout the world;
d) the saving in investment and capitalized loss costs when using ground return is normally much
greater than expenses for changes or modifications to existing infrastructure.

3.2 Data necessary to determine the field
3.2.1 Reference currents (or electrode rating)
The most important piece of data needed for designing electrode stations is the system current or
the reference current (for the schemes in the summary this ranges from 880 A to 4 000 A). There
is no absolutely clear understanding whether the reference current is the maximum current to be
handled under any circumstances, or if we speak of a general rating which under certain
circumstances may be surpassed in a limited time period. In the following, the reference current
will be understood as a general rating and it is up to the designer of the electrode station to
include for elevated levels of current for specified periods of time.

3.2.2 Resistivity data
The interference (magnitude of electric field around the station) must be calculated in the design
phase, based on the best obtainable information on resistivities of the different strata in the
underground and, if the earth is not uniform, in all directions. If the electrode is a shore or sea
electrode, design values must be set up for resistivity of seawater and for the bathymetry (depth
conditions) in a sufficient zone around the intended site.

3.3 Considerations on site selection
Basically, the current rating and the data for resistivities in different directions and depths are the
only parameters necessary to determine the field. It makes no sense to include as a criterion that
the voltage or gradient in a certain point at a considerable distance must not exceed a prescribed
limit. This fact is obvious if we look at a very simple case, that of uniform earth having uniform
ρ ⋅ I
resistivity in all directions and all depths. The voltage against remote earth is V = and the
2π ⋅ x
dV ρ ⋅ I
gradient =
dx
2π ⋅ x
where x is the distance from the midpoint of the electrode.
It is not that obvious, but still true, that the constitution of the underground/sea and the current are
the only field-determining factors for distances which are at least five times the diameter, length or
burial depth of the electrode.

– 10 – PAS 62344 © IEC:2007(E)
This means that if an electrode in the design phase or during commissioning tests turns out to
have an unexpectedly high field influence on a metallic structure, then this problem cannot be
cured by demanding modification of the electrode station.

Let us assume, as an example, a shore electrode located on a long straight coast. The slope
angle of the seabed is 0.05, the resistivity of the seawater 0,2 Ωm and the resistivity of land and
seabed 100 Ωm. Reference current 1 000 A.

At a point off the coast line, 10 km from the station, the potential is calculated at 0,178 V and the
gradient at 0,0178 mV/m. The potential in the sea 1 000 m from the coast as well as the potential
1 000 m deep below the shore stations are 1,78 V.

Now, if the voltage 0,178 V or the gradient at the distance 10 km are deemed to be too high, then
two suggestions might be brought up as a remedy:

a) to move the electrode from the shore to a position 1 000 m outside the shore in a water depth
of 1 000 × 0,05 = 50 m;
b) to transfer the shore electrode to a deep hole electrode 1 000 m below the shore line.

It is readily calculated that none of these suggestions will have any notable effect at a distance of
10 km, because the voltage is reduced by only 0,5 % from 0,178 V to 0,177 V. In both cases, the
resulting voltage at a point on the coast line at a distance of 10 km is calculated by interchanging
the cause and the effect. It is actually the resulting voltages at a position 1 000 m outside the
shore and 1 000 m below the shore line that is calculated with the electrode positioned on the
coast line 10 km away.
Of course, the local voltages and gradients will be significantly changed by moving a shore
electrode to a sea position, or to drill the electrode deep down. In both of the above suggestions
the voltage on the original beach position will be 1,78 V, while the electrode on the beach will
produce voltages of a higher level, depending on the size and physical layout of the station.

A general piece of advice, seen in literature about electrode stations, is that at least three different
sites should be investigated. If any problems are located 10 km from a site, as in the previous
example, the possibilities of moving the electrode 1 km aside or 1 km vertically down do not
represent genuine alternatives to the basic site. It is the horizontal distance from sites to points or
zones with problematic infrastructure that should distinguish the suggested site from the choice of
several sites.
3.4 Calculation of field
It is not the intention of this PAS to include a comprehensive set of formulas or methods for
calculation of the field from design data. As already said, the size and the physical layout are not
important when distances from the electrode are 3-5 times the diameter, length or depth of the
electrode. The electrode should be treated as a point, mathematically speaking, from which the
current emanates. Dr Kimbark’s book “Direct Current Transmission” [2] contains formulas and
viewpoints of calculation, including more complicated conditions such as 2- and 3-layer earth. If
the subsoil resistivity conditions are rather irregular, the modern FEM (finite element method)
provides the possibility of extensive computer-aided calculations.

Most of the existing formulas and methods take into account only the conditions around the
electrode dealt with, although no field exists without a counter-electrode. It is fairly easy to include
the influence of the counter-electrode by means of superposition of the fields from each of two
conjugated electrodes. The formula for the voltage in a two-dimensional room of height h

ρ ⋅ I d
V = ⋅ ln
2π ⋅ h d
contains in this very simple expression the distance to the electrode (d ) and to the counter-
electrode (d ). The expression has no mathematical solution if the counter-electrode is ignored.
When judging whether a pair of conjugated electrodes have an acceptable field, there may be
cases where interference from two pairs of conjugated electrodes mixes and forms a super-

PAS 62344 © IEC:2007(E) – 11 –
positioned field. In [10], the interaction of fields from the Baltic cable scheme and the Kontek
scheme is shown. The anodes of these two schemes are located about 50 km apart. It has been
calculated that the potentials at certain points raise as much as 80 % when both schemes are
running at rated currents, compared with the values when only one scheme is operating.

3.5 Apparent resistivity
If the calculated field (surface potential and gradient) is plotted in a double logarithmic diagram, it
can be compared to the field from existing electrode stations. See Appendix 1 for plotting of
surface potential, and Appendix 2 for plotting of gradients, both against distance from electrode
stations. In these diagrams inclined lines for the apparent resistivity are shown. The apparent
resistivity is the resistivity that fits the formula for a uniform semi-sphere field with current
emanating equally in all directions (the counter-electrode not defined).

I
V = ρ ⋅
2π ⋅ x
dv −I
= ρ ⋅
dx
2π ⋅ x
The use of such diagrams is restricted to distances within about 15 % of the distance to the
counter-station because of the impact of the counter-station.

It can be seen clearly on these diagrams whether the current for a given distance has a tendency
to plunge to deeper good conducting strata (inclination of the gradient curve >2) or has a
tendency to flatten out horizontally because of high resistivity of deeper strata (inclination of the
gradient curve = 1).
4 Impact of the field on buried metallic objects
Metallic objects in the ground can be divided into three categories:

– non-insulated objects, i.e. the metal is directly and continuously in contact with the
surrounding soil;
– objects coated with insulating material such as polyethylene and normally cathodic protected;
– the earthing grids of substations which are interconnected by the power lines.

4.1 Impact on non-insulated buried metallic objects
Examples of non-insulated objects are cables with a conducting layer, lead or steel armouring,
against the soil or, in the case of submarine cables, against the water. Naked metallic conducts
for water supply, buried tanks and sheet piling in harbours are also examples.

Depending on the orientation of the metallic object, its size (length) and the strength of the field,
the object picks up current in the part closest to the anodic electrode and discharges the current
from the part closest to the cathodic electrode.

To judge the impact, it is normal to calculate the distribution of current density, often expressed
2 2 2
in μA/cm (1μA/cm = 0,01 A/m ). The Swedish professor S. Rusck [15] treated these calculations
as early as 1962. Dr Kimbark [2] also gives comprehensive consideration to the corrosion due to
picked up/discharged d.c. ground current.

For formulas and methods for calculation of current density, reference is made to the reference
list last in Clause 14, mainly [1], [2] and [3]. Rusck concludes that a current density of 1μA/cm
can be permitted. It corresponds to a rate of corrosion of 0,174 mm per year removed from the
surface of an iron object.
Apart from d.c. ground currents, metallic unprotected objects in the ground corrode for “natural”
reasons, which is due to local differences in soil composition along the metallic object and/or to
the fact that naturally generated currents (called telluric currents) also take a path via the metallic
objects. STRI in its report [3] concludes that the impact of natural telluric currents is greater than
that from an electrode station for distances greater than 66 km to 110 km from the electrode

– 12 – PAS 62344 © IEC:2007(E)
station.
If an HVDC connection contains land cables or submarine cables it is important to investigate the
corrosion danger of the cable armouring, which normally deliberately is not insulated electrically
against the surrounding soil or seawater.

As for other metallic infrastructure, the necessary precaution consists of having sufficient distance
between the main cable(s) and the electrode. A general suggestion has been 8-10 km. As an
example of a closer location, the distance between the main cable of the Kontek scheme and the
cathode station Graal-Müritz outside Warnemünde is 5,5 km. Curiously, the main cable for
another scheme, the Baltic cable, passes the Kontek cathode at a distance of 7 km.

As shown in Kimbark’s text (p. 429), the presence of a cathode at a certain distance from a
submarine cable or buried pipe results in a worse condition, because the surface of the closest
part of the cable/pipe will be anodic which means corrosion. If the electrode station is an anode,
the current density of dangerous directions is reduced by a factor of 4,95, compared with the
impact of a cathode.
Generally, it is assumed for bare metallic objects that there is continuous contact with the
surrounding soil along the object, that is, all of the surface of the object participates in the
formation of current paths. There are polyethylene-coated submarine pipelines for oil or gas which
may have bracelets of zink or magnesium at about 100 m intervals. With this semi-continuous
contact to surrounding water, the picked-up/discharged current is concentrated on a small part of
the total surface. The expected corrosion of the bracelets in an HVDC field must be judged taking
the greater current density into consideration.

4.2 Impact on insulated metallic objects
The insulated metallic objects are mainly coated pipelines for oil or gas, located on land. It is
normal to have insulating joints, at 10-100 km spacing, which divide the metallic tube into non-
interconnected sections. Each section is equipped with a device for cathodic protection,
generating a voltage which measured against the surrounding soil through a Cu-CuSO half-cell is
about -1,0 V. The preferred voltage level may vary according to the soil composition in a range of,
say, -0,85 V to -1,1 V; but under anaerobic conditions (lack of oxygen), the margin is limited to
about ± 0,05 V. If the voltage is “too positive”, there is a danger of discharge of current, which
means corrosion. If the voltage is “too negative”, hydrogen embrittlement of the steel may occur
on a faulty spot. Faulty spots are often unavoidable pinholes in the coating, due to imperfect
production, or damage during installation or later.

When a section of an insulated pipeline has to cross an HVDC ground current field, the largest
difference from the varying voltage in the soil to the constant voltage impressed on the tube must
be limited to the above-mentioned margin. If the field which the section of the pipeline covers has
greater differences than the margin, further insulating points must be inserted. Insertion of a
further insulating point necessitates the pipeline to be emptied, then refilled with an inactive gas
such as CO or He, then cut and the new point inserted. This procedure may take many days,
costing loss of gas blown out, and cost of interruption of supply. To limit expenses, especially
those connected with outage of the pipeline, the required voltage difference along the pipeline can
be generated by introducing a current (2-20 A) in the metallic tube over about 1-2 km. This current
causes a voltage drop in the longitudinal resistance (~ 0,01 Ω/km) of the tube. The current can be
controlled, even in the reverse direction, by means of an inverter. The inverter is regulated from a
signal picked up as a voltage difference from two points along the pipeline, or the inverter gets a
direct signal from the converter station or the electrode station, proportional to the HVDC ground
return current.
Current compensating devices of this kind are running successfully on a Swedish main gas
pipeline which passes the electrode station Risø (the Konti-Skan scheme) at a distance of about
10 km. In Denmark, the uprating of the electrode current for the Skagerrak scheme, from 1 000 A
to 2 300 A, demanded two insulating joints and two current compensating devices to be installed
on a 508 mm (20 inches) main gas pipeline, which gave costs of a total of about USD 600,000.
The gas pipeline passes the electrode station Lovns at a distance of 6 km.

4.3 Impact on an a.c. grid
The impact of the ground current is shown on the drawing in Appendix 3 in a simplified way. The

PAS 62344 © IEC:2007(E) – 13 –
current emanating from the anode partly enters the earthing grid of substation A, flows further in
the phases and in the shielding wire(s) to substation B, where the current is discharged via the
earthing grid, and flows further to the cathode.

On condition that the shielding wire(s) is/are continuous from A to B, part of the picked-up current
follows these wires. Intermediate pylons closest to the anode pick up further fractions of current,
while pylons closest to the cathode discharge corresponding fractions. The situation is similar to
the continuous metallic pipe or cable, which over the total length is divided into a cathodic zone
(left) and an anodic zone (right on the drawing). The principal risk is corrosion of the anodic part,
and may be judged, for instance, by the viewpoint of S. Rusck (max. 1 μA/cm current density).

There is another principal path for the current. It enters the grounded starpoint of transformer A,
follows the high-voltage phases to transformer B and leaves via the starpoint connection and
earth grid for substation B. The d.c. component in the overhead line, which might be 0.1-1 A per
phase, runs independently of the a.c. phase current, which might be of a magnitude of 1 000 A
per phase.
The d.c. component through the transformer windings provokes a constant magnetizing of the
core, which, superpositioned on the symmetrical a.c. magnetizing, lets the flux vary in an
unbalanced way, which in one flux direction may lead to saturation of the core. The wave form of
the current is destroyed mainly due to a rise in the content of 2nd harmonics.

This vulnerability to d.c. magnetizing is different for different core types. Monophase transformers
with magnetic return equal in area to the wound leg are strongly affected. Three-phase, five-
legged transformers also react to some degree, because the d.c. flux, which is unidirectional in
the three-phase legs, finds a low-reluctance path in the outer two legs. Three-phase, three-legged
transformers will withstand a high level of d.c. current excitation, because the d.c. flux is
developed only to a small degree due to the high magnetic reluctance from the top yoke to the
bottom yoke.
Returning to the five-legged, three-phase transformers, the return legs with an area of, say, 58 %
of each of the main legs, each have to carry the d.c. flux from 1½ main legs. This indicates that
0,58
the return legs saturate by a flux which is about ≈ 0,39 times the flux that would saturate a
1,5
phase leg. Once the outer legs are saturated, the transformer for further excitation acts almost
like a three-legged transformer.

It is only transformers having a direct grounded starpoint of the winding system that pass d.c.
current through the windings. Delta-connected windings, for instance, the converter side windings
of converter transformers, cannot be hit by through-passing d.c. current. As was the case for bare
metallic objects, the necessary and effective protection for ground d.c. excitation of transformers
is to locate the electrode station at a certain distance from any vulnerable substation, including
the converter station.
Unfortunately, converter transformers are often of the monophase type, depending on price,
transportation problems, and the desire to have a spare unit. For instance, the transportation
infrastructure in Western Europe will normally permit transportation of units up to 250-300 tonnes
to be handled. For a rating per pole of 600 MW or greater, monophase transformers are the
solution, while two three-phase transformers per pole are manageable for ratings up to about 500
MW. Three-phase, five-legged transformers have lower transportation height than three-legged
ones.
Three-phase converter transformers need by no means, except transportation, be five-legged,
because three-legged, three-phase transformers, less vulnerable to d.c. excitation, are quite
possible. For instance, pole 2 of the Konti-Skan scheme, in operation since 1989
...