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

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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