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

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.