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

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8 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.
9 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.
Page 1 and 2: Technology and Development of 800 k
Page 3 and 4: 3 Limits due to transformer With in
Page 5 and 6: 5 on the neutral bus to reduce the
Page 7: 7 Figure 4 : A two winding converte
Page 11 and 12: 11 Fig. 12: UHVDC Double Break Disc

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

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