Technology and Development of 800 kV HVDC ... - Siemens

Technology and Development of
800 kV HVDC Applications
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M. HAEUSLER H. HUANG V. RAMASWAMI D. KUMAR
KEYWORDS
Siemens AG, Power Transmission and Distribution
Transgridsolutions
Inc.
91056 Erlangen, Germany Winnipeg, Canada
UHVDC, Equipment Design and Testing, Converter Station
ABSTRACT
Bulk Power HVDC transmission schemes over distances of up to 2000 km are currently under planning for
various large hydropower stations in India and China. Ultra high dc voltage (UHVDC) up to 800 kV is the
preferred dc voltage level for these applications. Currently world-wide existing HVDC schemes are limited to
maximum voltage levels of 500 kV to 600 kV. Therefore, the impact of increased steady state and transient
voltage stresses on the design of the main equipment for UHVDC stations has to be carefully investigated. This
paper focuses on specific design aspects for key UHVDC equipment and system to be taken into consideration.
The state of art of HVDC equipment technology and its application in UHVDC system are described and
discussed in detail along with some examples from research and development works.
UHVDC APPLICATIONS
In the last decades many HVDC projects have been realized for power rating of 2000 – 3000 MW at 500 kV
voltage levels. The first HVDC project with voltage above 500 kV, the Cahora-Bassa between Mosambique and
South Africa (533 kV, 1920 MW), was built almost 30 years ago. The second HVDC project with voltage
above 500 kV, constructed about 20 year ago, is the Itaipu HVDC project (600 kV, 3150 MW).
Several UHVDC projects, with preferred operating voltage in the range 750-800 kV have been identified in
China and India with power transmission requirements up to about 6400 MW over a single bipolar dc line. The
length of dc line may be up to 2000 km and even more.
The motivations for choosing UHVDC are very similar to the ones for HVDC projects and are briefly outlined
here:
• Reduction of overall project cost compared to alternatives such as UHVAC
• Transmitting power from remote hydro / thermal power stations over long lines where maintaining stable
transmission may be difficult over long ac lines.
• Conservation of right of way for transmission lines. Transmission of bulk power from narrow restricted right
of way.
• Controlled transmission of power with perhaps preplanned matching with expected load cycles.
• Supporting the grid through frequency control and power modulation.
• Transmission of power across asynchronous power systems.
• Capability to provide flexible VAR support.
The feasibility of UHVDC up to 800 kV has also been conducted by the international organizations such as
IEEE and Cigre [1, 2]. All these working groups reported that the UHVDC system is technically feasible with
some research and development efforts in few key areas.

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2.0 UHVDC PROJECT CONFIGURATIONS
Limits of equipment and comparison of series parallel configurations
The design for UHVDC projects would follow several practices established for the existing HVDC projects of
rating 3000 MW. However, as the power rating of a project is increased, some limit on equipment / devices may
reach, thereby requiring change in the one 12 pulse bridge configuration per pole of a common bipolar HVDC
system. By the term configuration here is meant number of twelve pulse bridges per pole, series / parallel
connections of twelve pulse bridges, type of converter transformer, number of poles etc. Simple single line
diagrams for basic configurations are shown at Figures-1 to 3. It may be noted that in these figures only single
phase two winding converter transformers are shown as these are the most likely type to be employed for
UHVDC projects.
In addition to the reasons on account of limits of equipment and devices within HVDC project, the project
configuration may also be influenced by a need for stage wise development of a project, connected ac system
limitations or environmental, location based conditions.
12 pulse group
DC Filter
To the other pole
Figure-1: Single twelve pulse group per pole
12 pulse group
DC Filter
To the other pole
Figure 2: Two twelve pulse groups in series
DC Filter
12 pulse group
To the
other pole
Figure-3: Two twelve pulse groups in parallel

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Limits due to transformer
With increase in the rating of the project, the most likely reason for adopting two twelve pulse groups per pole
would be the weight or size of converter transformer which may reach to values that it is no longer feasible to
transport the converter transformer to the destined site. For a project of 4000 MW transmission capacity under
bipolar mode, the size of even a single phase two winding transformer is about 394 MVA. This transformer may
be about 12x4.5x5 meter, weighing around 400 tons and might already be approaching the limits of
transportation, which would depend upon the specific location of the converter terminals and influenced by the
strengths of bridges en route, limits of roads and rails etc.
Limits due to thyristor
A UHVDC project is generally required to meet steady state as well as overload conditions as determined by
planners. The employed thyristors must be able to meet these conditions. With the present day capacity of about
4 kA for thyristors the overload capacity is not a problem even with series connected valves of about 6000 MW
capacity bipolar project which has a rated current of 3.75 amps at 800 kV. With higher rating projects or lower
rated voltage a parallel arrangement of bridges or a use of bigger diameter thyristor may be considered.
Comparison of Series / Parallel arrangement
When the need for two twelve pulse groups arise, generally the series group may tend to be adopted mainly
because the series groups maintain balance in the two pole currents even during an outage of a series group and
hence there are potentially no ground currents when one of the two series groups is out. In case of parallel
groups the current in a pole is the addition of currents in each group, and hence, on outage of a group there
would be an unbalance in the two pole currents giving rise to ground currents.
With parallel groups, the currents in each group is only half of the pole current and there would be sufficient
overload capacity left in the thyristor for the maximum size of projects under consideration. Therefore very high
capacity links may find application of parallel groups on account of current rating of thyristors.
On outage of group, the line losses would be considerably lower in case of a parallel group as compared to
series group under similar outage, on account of fact that the pole current reduces to half in case of a parallel
group outage.
The parallel groups are highly adaptable to stage wise development. The parallel groups can easily be added to
an existing pole with minimum of disturbance during commissioning. With parallel groups each group can
easily be located at different places far from each other as per requirement or to reduce the cost of ac
transmission system that connects at either end of the transmission for collecting and delivering power at
different locations in the grid. This may have several system related advantages with regard to maintaining
voltage profile, reliability, stability etc.
From the discussion above it is seen that the operating voltage, current and configuration of a project will be
strongly influenced by the amount of power that has to be transmitted, location and the connected ac systems.
FOCAL POINTS
The major focal points for a UHVDC project briefly discussed in the following sections are:
• System Integration
• Insulation Coordination
• Converter Transformer and Bushings
• Wall Bushings
• DC Yard Equipment

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System Integration
Due to the large amount of real and reactive powers associated with UHVDC projects each of the following
aspects needs to be examined in detail and ensured:
1. the sending and receiving ac systems have adequate (transmission) capacity, taking into considerations the
outage criteria of the system planner.
2. there is an adequate effective short circuit ratio for operation up to overload levels and under the outage
conditions in the ac network. Strategies to cope with abnormal situations where effective short circuit ratio
may reduce below at which project is designed.
3. suitable reactive power support under steady state and dynamic conditions.
4. ensure voltage / power stability under all operating conditions. Steady state, temporary and transient
overvoltages beyond the specified values be avoided under unexpected conditions and following faults.
5. avoid low order resonance under all conditions such as maximum allowed reactive power support and low
short circuit level conditions
6. for multi infeed HVDC configurations possibilities of any adverse interactions among HVDC or actively
controlled projects should be examined and avoided.
7. system stability must be maintained for a group / pole outage as per specifications. System stability is also to
be maintained following specified disturbances, faults and outages in the ac system.
Insulation Coordination
With respect to the UHVDC projects it would be important to reduce the size of equipment to the extent
possible so that mechanical duties and costs are reduced and reliability is increased. This can be achieved by
controlling the steady state, temporary, transient and lightning overvoltages with the help of system design,
control systems, operating logic and arresters.
The utilities have to play a major role in this by formulating a specification that is not overly pessimistic in
assumptions about variations of system parameters such as short circuit levels and operating voltage range for
determining the rating of equipment or for determining the overvoltages in the system. Similarly, it would be
prudent to judiciously select the air clearances and creepage distances.
Since the insulation withstand levels are directly dependent upon the protective levels provided by the arresters,
the use of ZnO arresters would be very important to limit these overvoltages at precise levels for equipment
such as converter transformers, smoothing reactors and valves. This may require placement of arresters near the
valve side windings of converter transformer in the upper bridge or across smoothing reactors. An indicative
placement of arresters for a series connected UHVDC scheme is shown below at Figure-7.
Figure – 7: Placement of arresters in a series connected UHVDC scheme
This scheme is basically similar to the existing HVDC scheme with an addition of arrester for the converter
transformer secondary side connected to the high voltage dc side. The smoothing reactors have also been placed

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on the neutral bus to reduce the ripples on dc voltage at the dc high voltage bus. The arresters shown across
smoothing reactors are also not generally employed in normal HVDC schemes but are indicated here.
Following insulation levels for dc equipment are expected.
Table 2 : Expected Insulation Levels
Equipment / Location Units SIWL LIWL
Transformer valve side, connected in the upper bridge kV 1600 1800
Transformer Valve side, other in the upper bridge kV 1300 1550
DC yard equipment, line side of smoothing reactor kV 1600 1900
DC yard equipment, valve hall side of smoothing reactor kV 1600 1800
External Insulation of the Equipment
Designing equipment for correct external insulation means to take care of proper
flash distances and
creepage distances
of the equipment housings.
Flash distances
Required flash distances determine the axial length of the equipment. Flash distances can be calculated fairly
well based on the specified insulation levels for the equipment. For UHVDC equipment the switching impulse
level will become the dimensioning factor. DC voltages are not decisive with respect to the flash distance.
Corrections will be included for equipment to be installed at higher altitudes above sea level. Flash distances
increase more than linearly with increasing switching impulse voltages. Finally the correct design will be
verified by corresponding type tests of the equipment.
Creepage distances
As far as equipment will be installed outdoors the external insulation with respect to creepage distances is more
complex compared to the flash distance as it severely depends on environmental and weather conditions, i.e. on
the degree of pollution collected on the equipment and wetting conditions (rain, fog etc.) of the polluted housing
surfaces. It must be accepted that pollution combined with wetting as it occurs under service conditions can
hardly be determined for the purpose of test conditions. Artificial pollution tests as per the standards can only
show the capability of a housing to cope electrically with a certain degree of completely wetted pollution.
Another important role concerning the pollution flashover performance of housings is played by the shed profile
of the insulators. From experience two types of shed forms should be taken into consideration for DC
application which are the deep under-rib and the alternate shed profile. A decisive property of a shed profile is
the ability to prevent wetting of the pollution, i.e. to maintain the effectiveness of the creepage distance on the
surface as much as possible. Publications show that there is no straightforward advantage for one of the shed
types. A well designed alternating shed profile can perform as good as a deep under-rib profile. The decisive
understanding seems to be that an improvement in pollution performance cannot be achieved by only increasing
the creepage distance without increasing the axial length of the housing. In other words: increasing the creepage
distance by increasing only the overhang of the sheds and/or reducing the shed spacing is not effective. Hence,
increasing the creepage distance must go coincidently with increasing the axial length of the housing.
This shows that general rules to select the appropriate equipment housing in order to prevent external flashovers
resulting from pollution and wetting conditions cannot be established. Site conditions, if available, with respect
to amount of pollution collected on the housing and the ability of natural cleaning should be taken into
consideration, as well.
Such reflections are mainly applicable for porcelain type equipment housings. Alternative solutions are known
and have to be looked at.
From long-term experience with 500 kV DC outdoor equipment one alternative solution is housings with
hydrophobic type of surfaces. The hydrophobic properties need not to be explained in detail here.
Hydrophobicity is an intrinsic property of silicone rubber and is not achieved on any long term by additives or

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surface treatment. Elements of the silicone rubber which cause hydrophobicity diffuse into contamination layers
on the surface of the housing and thus are transferring this property to the contamination. A further advantage is
that hydrophobic material prevents the formation of larger coherent wet zones on the surface under rain and
humidity. This is an important precondition in order to maintain the effectiveness of the creepage distance
against pollution flashovers. Silicone rubber housings having such advantages are available and successfully in
service in DC stations since a very long time. From experience with existing projects the specific creepage
distance of composite insulators can even be reduced by up to 25% compared to that of porcelain insulators.
However, for some equipment porcelain type insulators are advantageous because of mechanical reasons. This
refers to the support insulators in the DC yard and for air-insulated smoothing reactors and to the insulators of
disconnect switches. Without taking remedy measures for such UHVDC equipment pollution flashovers cannot
be excluded. Alternatives for improvement are needed. One possibility could be to coat the porcelain housings
with hydrophobic material. This technology has been improved quite a lot in recent years. Yet, details need still
to be verified especially the long-term behavior of the coating material.
Another alternative for post insulators under investigation is in a very early stage. The idea is to use porcelain as
core material providing sufficient mechanical strength and to equip this core with silicone rubber sheds, as it is
done with composite housings (silicone rubber sheds on epoxy resin tube, mechanically reinforced with fiber
glass).
The method of booster sheds is also a well known possibility to improve the performance of porcelain insulators
under DC stresses.
As far as converter valves and associated equipment are concerned the design of creepage distance is not a
problem as such equipment will be installed indoors in the converter valve hall like in the existing projects. The
valve hall provides a controlled environment. For UHVDC equipment inside the valve hall the same specific
creepage distance can be selected as for the existing 500 kV DC equipment.
Consistent to this it seems reasonable to consider the installation of UHVDC yard equipment also indoors in a
so-called DC hall. If this alternative is followed the DC hall has to be designed very thoroughly with the
objective of achieving defined low pollution and humidity conditions inside the hall, conditions which do not
depend on the outside environment. For indoor installation the specific creepage distance can be reduced to
some extent compared to outdoor installation.
Relative to existing equipment it can be stated that specific creepage distances neither for outdoor nor for indoor
installation need not to be increased for UHVDC equipment in order to ensure safe performance against
pollution flashovers. Furthermore, higher altitudes above sea level are no issue for the creepage distance. If the
flash distance of the equipment is properly corrected with respect to the altitude above sea level influences of
the altitude on the creepage distance, should any exist, will be covered.
Converter Transformer and Transformer Bushings
The operating conditions of HVDC converter transformer differ from conventional ac transformers in several
ways such as presence of harmonic currents, possible small amount of dc currents, complex voltage waveform
that has ac and dc components on the valve side windings, cleat leads and bushings having dc component
stresses, stringent mechanical duties due to commutation process etc. Due to the combined ac and dc stresses
the internal insulation system design comprise of considerable amount of paper insulation in addition to oil. The
converter transformers may also have considerable duty on account of reactive power consumption by HVDC
projects. As a result the size and weight of converter transformer is large compared to normal ac transformer of
equivalent rating.
The valve windings of the converter transformers connected with the upper six pulse group would see
maximum rated dc voltage of transmission which may be +/- 800 kV plus the allowed tolerances. These
windings may also see overvoltages originating from dc as well ac side. Thus the bushings, leads and winding
insulation shall have to be rated for high voltage resulting in large dimensions for these equipment. It will be
very important to uniformly distribute the steady state and transient voltage stresses across the insulation within
the transformer and also in bushings.

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Figure 4 : A two winding converter transformer for a +/- 500 kV dc project
In line with the generally acceptable practice for modern high capacity projects dry type bushings with
composite insulation are expected.
Increased insulation levels in combination with larger rated power may lead to new solutions for the transformer
design. A possible approach may
be splitting up of the secondary windings and a new design of the connections between windings and bushings
on the valve side as indicated in Figure-5. This part of the transformer will be crucial for its transportation data.
As an example, transformer rating and preliminary shipping data for a system rating of 5000 MW with 800 kV
DC are as follows:
Table-1: Typical parameters for converter UHVDC
transformer of 5000 MW rating
one 12p-group two 12p-groups
per pole per pole
rating (MVA) 494 247
length / width / 13.3 / 4.3 / 5.0 9.4 / 4.0 / 4.9
height(m)
weight (t) 480 300
These data refer to the unit of the highest DC voltage level at the sending end; all other units of the system will
not exceed such dimensions and weights. With respect to typical limits which might exist in case of railway
transportation it is quite evident that improvement is required especially as far as the transformer width is
Figure-5: Preliminary outline Drawing UHVDC Converter
Transformer

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concerned. If railway transportation is the only possibility, then in case of one 12-pulse group per pole
arrangement additional investigations have to be done to achieve adequate transport conditions.
Some important points that require attention in converter transformer are summarized below:
• to ensure uniform distribution of voltage for the leads taking into considerations the effects of harmonic
currents and harmonic impedance
• to ensure that radial stress on components e.g. bushings is within the design limits under all operating
conditions including the effects due to external pollution if these are exposed to atmosphere.
• achieve a balance between mechanical duties and electrical requirements. For example, the creepage distance
should be chosen such that it meets the electrical requirements but does not result in unduly large mechanical
dimensions.
• to manage external electrical stresses by proper design of electrode, shape of corona rings etc.
• the use of considerable paper insulation should not affect the thermal design of the transformer and proper
cooling of all parts must be ensured.
DC Wall Bushing
Doubtless, the existing technology of wall bushings provides the best solution for UHVDC applications.
Existing technology means composite housing with silicone rubber sheds for external insulation and internally
with condenser core of oil-free resin impregnated paper and SF6 for insulation between core and inner surface
of the housing. Both parts of the wall bushing, indoors and outdoors, are of the same design and are connected
by means of a SF6 filled duct. This type of wall bushing is successfully in operation since more than 15 years
under various pollution conditions.
Figure-6 shows an outline drawing of a wall bushing as suitable for UHVDC applications.
Figure – 6: DC Wall Bushing
Most important for this bushing technology is the appropriate coordination between internal and external
insulation. The manufacturing capabilities in terms of length of housings and length of condenser cores play an
important part in this context. Another area of concern with wall bushings is the mechanical stresses which
needs thorough investigations.
Further, it has been observed that the wall bushings require particular attention in their mounting since these are
particularly prone to uneven wetting during rain which increases the risk of flashovers. For UHVDC projects, it
would be necessary to check this aspect carefully with regard to the above mentioned radial stresses and
probability of external flashover. The hydrophobic silicon material used for these bushings has proved to be is
extremely effective in mitigating the problem of flashovers.
Proper design of external clamps, connectors, corona rings etc would be essential.
DC Switchyard Equipment
The main DC yard equipment that would require special attention is the following:
Smoothing Reactor
Both, air- and oil-type smoothing reactors, have to be considered. Know-how and experience for both
technologies are available.
Oil-type Smoothing Reactor
Two different arrangements are sown in Figure-8 and 9 for indoor and outdoor arrangements.
For the outdoor part of this smoothing reactor bushing similar requirements as for the wall bushings regarding
pollution conditions exist. Preliminary shipping data for a 300 mH, 800 kV smoothing reactor can be given as:
length / width / height (m) 5.5 / 4.0 / 4.8
weight (t) 150.

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Fig.-8: Oil-type Smoothing Reactor (Indoor)
Fig.-9: Oil-type Smoothing Reactor (Outdoor)
Air-type Smoothing Reactor
The operating dc current determines the largest smoothing reactor coil size which can be manufactured. As an
example for a dc current of 3.3 kA a maximum coil size of approx. 100 mH is feasible. Fig.-10 shows the
preliminary drawing of such an air-type smoothing reactor suitable for UHVDC application. The available
technology and know-how for the coils of existing HVDC schemes can be fully used for UHVDC application.
Assuming typical total smoothing reactor sizes of 250 to 350 mH a series connection of several coils might be
needed as illustrated in Fig.-11. For reasons stated above parts of the coils might also be installed at the neutral
bus.
Porcelain support insulators are the main issues to be solved for air-type smoothing reactors.

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Fig.10: UHVDC Dry-type Smoothing Reactor Coil
Fig. 11: Example for Series Connection of Dry-Type Smoothing Reactor Coils
DC Switchgear
Disconnect Switch
Disconnect switches are typically designed based on porcelain insulators specifically because of mechanical
stresses resulting from operation. Such stresses are even increased if grounding switches are directly attached to
the disconnect switch. However, designs based on composite insulators are currently under investigation. Fig.-
12 shows the preliminary drawing of an UHVDC double break disconnect switch.

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Fig. 12: UHVDC Double Break Disconnect Switch
Bypass Switch
If a transmission system consists of two 12-pulse groups per pole breakers and disconnect switches are needed
for each 12-pulse group. In case of failures related to a 12-pulse group and its associated equipment this group
can be by-passed with the other 12-pulse group of the pole still in operation. Regarding the disconnect switches
needed for the by-pass operation the section above is relevant.
The breaker to be used as bypass switch will be a proven standard AC breaker adapted for the DC application. It
should be kept in mind that for the voltage-wise higher 12-pulse group the voltage between terminals is the
group voltage whereas the voltage to ground is the UHVDC bus voltage.
The steady state DC voltage capability between the terminals of the bypass switch, i. e. the DC voltage
capability of the interrupting units is another very important subject which needs further investigation. This is
part of the modification for DC application mentioned before.
DC Yard Bus Work, Conductors
In addition to current ratings of the diameters of dc bus work, conductors and their bundling would be
influenced by operating voltage, corona, RI, audible noise, electric field considerations etc.
CONCLUSIONS
From above discussions general conclusions read as follows:
The configuration of a UHVDC project would be decided from the rating and system considerations which
influence converter transformer size and overload capability of the project
From the main equipment point of view UHDC systems of up to 800 kV are technically feasible. In general
UHVDC equipment can be designed and manufactured based on existing technology and know-how. For most
of the station equipment only some or even no R&D is anticipated. However, converter transformers and
bushings need more thorough R&D.
External insulation of UHVDC equipment, especially for outdoor installation has been identified as one of the
key issues.
REFERENCES
1. HVDC Converter Stations for Voltages Above ±600 kV, WG 14.32, CIGRE Publication, December 2002.

2. HVDC Converter Stations for Voltages Above ±600 kV, - EPRI EL-3892-Project 2115-4 Final Report –
Feb 1985.
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BIOGRAPHICAL DETAILS OF THE AUTHORS
Marcus Haeusler, born in 1966, received his Dipl.-Ing. Degree in Electrical Engineering from University of
Karlsruhe, Germany, in 1992. Since he joined Siemens AG in 1992, he has been working on HVDC system
engineering for many HVDC projects world wide.
Ramaswami Velpanur, born in 1946 received his Bachelors Degree in Electrical Engineering from the
University of Bangalore in 1969. He has been working in the HVDC field since 1980, first with Brown Boveri
& Cie and then with Siemens
Hartmut Huang, born in 1963, received his Dipl.-Ing. degree and PhD in Electrical Engineering from
University of Braunschweig, Germany, in 1986 and 1992, respectively. Since he joined Siemens AG in 1992,
he has been working on HVDC system engineering for many HVDC projects world wide. Currently he is
manager of HVDC system engineering department at Siemens Power Transmission and Distribution Group.
Devinder Kumar, born 1958, received his B.Tech degree in Electrical Engineering from IIT Delhi, in 1980.
He joined NTPC, India in 1980 and later POWERGRID, India in 1990. Since 2005 he has been working with
TransGrid Solutions Inc., Canada. He has worked on HVDC projects in the areas of planning, specification
preparation, studies, and operation & maintenance.