Environmental Characteristics of HVDC Overhead Transmission
Environmental Characteristics of HVDC Overhead Transmission
Environmental Characteristics of HVDC Overhead Transmission
1. INTRODUCTION
The range of applications of HVDC (High-Voltage Direct Current) technologies for
electric power transmission is defined by the well-known technical and economical advantages
of HVDC technologies. The most important of these advantages are:
Simpler requirements for line tower construction in comparison with HVAC (High Voltage
Alternating Current) transmission lines, and also lower per-unit costs, including costs per km
of line and per MW of transmitted power;
Significantly lower costs for cables of the same transfer capacity (relative to HVAC lines);
The possibility of interconnection of power systems with different nominal frequencies (50
and 60 Hz) and systems using various frequency regulating standards;
There are no limits imposed by stability considerations on the transfer capacity of HVDC
lines;
Additional reactive power compensators are not necessary when using long HVDC
transmission lines;
There is the possibility for independent power flows and frequency regulation in power
systems that are connected via HVDC lines;
Using HVDC power transfer significantly decreases the mutual influence of emergency
processes in interconnected power systems;
There is the possibility that power transfer can continue via one pole of a bipolar line even in
when the second pole trips during an emergency.
In any specific transmission line application, one or several of the advantages listed
above may take precedence in selecting HVDC transmission. At the same time, accounting for
the environmental characteristics of power transmission is also of considerable importance.
The possible influences on the environment caused by High Power Electricity
Transmission Systems can include:
Radio interference;
Audible noise;
The use of land for transmission line and substation facilities that was previously used for
other purposes; and
Visual impacts.
HVDC power transmission systems have particular characteristics related to all of these
environmental influences. These HVDC-specific characteristics have to be taken into account in
the process of choosing transmission line routings and in planning of a transmission line project.
In comparison with HVAC transmission lines, several of these characteristics can be considered
as positive, that is, they are offer some environmental advantages to HVDC transmissions.
Other HVDC characteristics are negatives from an environmental point of view, relative to
corresponding characteristics of HVAC lines.
In the sections of this paper that follow, each of the ecological characteristics of
transmission lines noted above are discussed with specific reference to the technical features of
HVDC transmission systems.
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In addition, the experimental fact that large machines with rubber tires (such as combine
harvesters, automobiles, and some others) are not electrically charged to levels dangerous for a
human when the machines are standing under HVDC overhead lines should be considered a
significant result of the investigation. The electrical resistance in the tires of these machines (at
about 10 MOhm) turns out to be enough to prevent the accumulation of a dangerous charge (via
charge leakage) even when the machine is standing on dry asphalt. In the case of HVAC
overhead lines, inducted capacitive currents on large machines may be lethal in some cases.
These results suggest that electrical fields below HVDC transmission lines are not
sufficiently hazardous as to necessitate significant safety measures, as the environmental
influence.of a HVDC transmission lines electrical field is very limited.
The results of measurements show that in good weather the ion current density under
HVDC overhead lines can vary from 102 to 5102 /m2 (nano-Amperes/square meter), which
lead to an increase in the concentration of positive ions in the air from normal levels of 102 - 103
to 105 - 106 cm-3. During precipitation events, however, this value can rise several times higher.
Positive ion concentrations higher than 104 cm-3 are considered detrimental to health under
conditions in which there is prolonged exposure of humans respiratory tracts.
The level of space charge from HVDC lines is changeable and difficult to predict, as it is
a result of corona activity and depends, among other factors, on weather conditions. The space
charge present in the DC electric field produces an ion current flux. The total electric field and
ion current flux measured near a transmission line are not steady and can be described by
statistical parameters. Due to the statistical character of the impact of electric fields, there are
different approaches used to limit the fields' effects.
The following types of guidelines are usually used to limit the environmental impact of
electrical fields from transmisison lines.
limits imposed on the total electric field of a DC line by a certain level of space charge; and
limits imposed separately on the electrostatic field and on ion current density.
Russian regulations are designed to ensure the safety of people working in DC electric
fields, and prescribe the limits to time of exposure shown in the Table below:
Field conditions
Time of exposure
E=15kV/m, J=20nA/m2
t=8h
E=l5-20kV/m, J = 25 nA/m2
t=5h
E = 20 - 60 kV/m
t=
(E m )2
kVm
t ; = 0,25
2 1
nA
(E + j)
Em = 60 kV/m
t1=lh
E>Em
In the Table above, E = electric field strength, and J = ion current density.
Based on the standards shown in this table, exposure and field strength limits for public
areas can be derived. Providing that people do not stay in exposure areas for more than 8 hours,
the limits are E = 15 kV/m and J = 20 nA/m2.
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Those regulations on transmission line field strength used in other countries that are
known to the author are not easily comparable with Russian regulations, as regulations in other
countries are presented in a very different form.
Codes and regulations limiting the electrical field environment impact exert the largest
influence on the choice of the techniques used for overhead line construction, and on the
resulting technical and economic parameters of transmission lines. At the same time, the
regulations that would be applied to transmission lines used for grid interconnections in
Northeast Asia are still uncertain. The regulations applicable to electric fields from transmission
lines in different countries have differ in a number of essential ways. The Russian regulations
described above are quite stringent, relative to regulations in some other nearby countries.
4. RADIO INTERFERENCE
The radio interference caused by electric power transmission lines is the result of the
corona discharge around conductors, which is generated only at positive voltages. As a result, on
a HVDC line radio interference is generated only by positive pole conductors, whereas with a
HVAC transmission line radio interference is generated by all of the three AC phases.
There is a also difference between HVAC and HVDC lines with regard to the impact that
different weather conditions have on line-induced radio interference. The electric field
intensities recommended for AC lines take into account a 10 dB (decibel) increase in radio
interference under rainy conditions. With DC lines, the radio interference decreases during
rains. In order to assure acceptable radio interference levels, a surface voltage gradient of about
25 kV/cm should not be exceeded for DC lines.
Assuming equal capacity conductors and maximum levels of electrical field intensity on
the conductors surfaces, the radio interference level of HVDC lines is typically lower by 6-8 dB
than of HVAC lines.
5. AUDIBLE NOISE
Audible noise is one of the important design parameters for both overhead lines and
substations. All known measures to decrease audible noise from these sources are quite costly.
In the substations used on HVDC systems, the main source of audible noise the
converter transformercan be surrounded by screens when the noise level at the nearest
premises is not acceptable.
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Audible noise from DC transmission lines is a broadband noise with contributions
extending to high frequencies. The noise is most prevalent in fair weather. Noise levels from a
DC line will usually decrease during foul weather, unlike the noise levels on AC lines. As a rule,
the audible noise from transmission lines should not exceed, in residential areas, 50 dB during
the day, or 40 dB at night.
On the whole, the problem of limiting audible noise during HVDC and HVAC
transmission line operation usually is addressed with the same types of measures on both types
of lines.
4. Quadrapolar transmission
line overloading possibility
Figure 1. Basic variants of return current circuit organization for monopole operation
a)
b)
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The latter path for current return presents a danger to buried metal infrastructure (for
example, footings or pipelines) by way of electrocorrosion. The danger level depends on the one
hand on the quality of electrical insulation and special defenses against corrosion used in the
metal infrastructure present, and on the other hand, on the integral value of the current passing
through the object (in A*h).
Overhead HVDC transmission lines are usually bipolar. That is why the operation of such
lines as monopoles is typically transitory in nature. In all cases except operation with an
additional conductor, however, some operation as a monopole cannot be avoided. Furthermore,
because of the dissymmetry of the bipolar scheme, a prolonged current passing through the
ground exists. Usually the dissymmetry current is estimated as 1-3% of the nominal current
value.
The most complex grounding systems for the HVDC substation exists if there is no
expected possibility of any variant of the metallic return being used. Usually, in these cases,
grounding electrodes are situated in some distance from the substation (see Figure 3) to exclude
the possibility of corrosion of the substations underground components. The grounding
installation must have a design that excludes the possibility of dangerous step voltages appearing
in a grounding electrodes zone. Electrodes are made from the special materials. Special measures
are applied to prevent the ground from drying or becoming fossilized (and thus, in both cases,
losing its properties as a conductor).
If there are pipelines or other underground metal objects near the grounding installation,
it is recommended that additional cathodic protection of such objects be provided to allow
prevent rapid corrosion.
8. VISUAL IMPACT
When transmission lines cross populated areas and especially national parks, resorts and
other territories where conservation of the natural landscape is important, special demands are
placed on the transmission lines dimensions. For example, it is sometimes necessary to limit the
height of the towers with the height of the trees in a woodland, so that the transmission line itself
is largely obscured. Special demands are then placed on the aesthetics of the design of the line.
In cases where there are especially rigid aesthetic requirements, cable inserts have to be
used, which leads to a rise in the cost of a transmission project. In cases where only overhead
lines are utilized, the length of the route that a line must take increases if protected areas need to
be avoided, or if different towers designs must be used. These changes also typically lead to
increases in line expense as well.
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HVDC overhead transmission lines offer several advantages from the point of view of
visual impact relative to HVAC lines of the same capacity. Bipolar HVDC transmission lines
have two conductors and already because of that it are more simple in design in comparison with
the three-phase structure of a HVAC line. HVDC lines require shorter tower heights in
comparison with HVAC lines of equal capacity and comparable voltage levels.
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If quadrapolar HVDC lines are used, then towers can be designed as flat towers or towers
with 2 cross-arms, depending on the specific conditions in the transmission corridor. In Figure 4
a schematic view of these tower types for a 500 kV HVDC line are shown, with approximate
tower dimensions indicated. There is thus a choice of tower design options depending on the
specific requirements for the line. In each case, however, the dimensions of the towers for the
quadrapolar line are smaller than those for comparable double-circuit HVAC lines.
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9. CONCLUSION
During HVDC transmission line project planning, nearly the same environmental impact
characteristics that are considered in planning a HVAC transmission lines project should be
taken into consideration. These characteristics include impacts from electrical and magnetic
fields, radio interference, audio noise, the potential fast corrosion of metal installations due to
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electrical currents in the ground, the land alienation (use changes) involved in siting transmission
line towers and substations and, also, the potential changes in and limits on land use under and
near overhead transmission lines. In some cases, the visual impacts of lines also have to be
considered
The combination of several specific physical characteristics and related technical aspects
related to line construction and operation, HVDC transmission lines have advantages over
HVAC transmission lines for a majority of environmental impact indices. These advantages
allow environmental performance to be improved at lower costs when installing HVDC lines,
relative to HVAC lines. The value of land use changes can be taken as an overall index for the
comparative analysis of the environmental impacts of HVAC and HVDC transmission lines of
the same relative capacity. Based on a rough estimate, this ratio is 1.5 in favor of the (lower cost)
HVDC transmission lines. Thus, from the ecological point of view, a HVDC power transmission
system as a whole is preferable to a system using exclusively HVAC transmission lines.