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

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2 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
3 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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Technology and Development of 800 kV HVDC ... - Siemens

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