Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1992-01-22
1993-09-28
Stephan, Steven L.
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 68, 363125, 363144, 257177, H02J 336
Patent
active
052491145
DESCRIPTION:
BRIEF SUMMARY
It is known that in a valve hall for generation of high voltage direct current, the elongated stacks of rectifier valves may be positioned vertically in an upright or suspended manner. So far, this has been considered to lead to the lowest cost for valves and valve hall. However, for economic reasons, the necessary bushings must be placed in the walls of the hall in such an arrangement. The bushings will then have an almost horizontal extension and will be positioned, at least partially, in the rain shadow of the roof of the valve hall. In case of direct voltage levels in excess of about 400 kV, this may lead to flashover caused by dirt, which is due, in part, to that part of the insulator of the bushing, which lies in the rain shadow, not being cleaned by precipitation, and, in part, to the lower side of the horizontal insulator also being protected against rain and consequently risking a continuous "stripe of dirt" on the surface. To prevent this, it has hitherto been necessary to grease the bushings--an action which has been looked upon with great disapproval by the plant owner since it has constituted a heavy part of the maintenance of the valve hall and has made a high voltage direct current plant appear particularly maintenance-demanding in comparison with an alternating current plant.
It is known from experience that insulators which are placed vertically outside the rain shadow run a very small risk of flashover up to a direct voltage of at least 600 kV. Thus, it should be a suitable solution to place all the bushings on the roof, but in known arrangements of valve stacks this infallibly leads to a considerably larger valve hall with an ensuing higher total cost.
According to the invention, there is now proposed a new method of placing the valve stacks in the valve hall. This method permits the bushings to extend through the roof without the volume of the valve hall having to be increased. The invention comprises arranging each valve stack, which is composed of a number of electrically series-connected valve modules arranged one after the other and which in operation have a voltage to ground which increases along the stack, in a substantially lying position. The valve stack is to lie at such a distance from the floor and roof of the valve hall, respectively, that the smallest electrical flashover distance between live parts on the stack and the floor and roof, respectively, is contained and that electrical connection to the stack is arranged at bushings through the roof of the valve hall. In principle, this means that the entire valve hall is overturned, so to speak, and, contrary to prior art arrangements, its volume need not be increased in spite of the fact that all the bushings--both for alternating and direct voltage--have now been arranged through the roof.
According to a special alternative, the volume of the valve hall may even be reduced if the roof of the valve hall is made sloping towards the horizontal plane and this is combined with the fact that the flashover voltage to ground increases along the valve stack from one end thereof to the other.
The volume of the hall may be further reduced if the valve stack is suspended in such a way that the distance between live points on the valve stack and the roof and floor, respectively, of the valve hall is only kept somewhat greater than the current flashover distance between the corresponding points and the roof and floor, respectively, of the valve hall.
A further simplification of the erection may be made if three valve stacks in a valve hall are arranged in a star- or Y-configuration with the high voltage ends of the valve stacks brought together and electrically interconnected at the center of the star and connected to a common bushing for the high voltage direct current. In addition to this embodiment providing a saving of bushings, the volume of the hall may be further reduced by the roof now being given a dome-like shape which makes it possible to make all the walls in the hall lower in comparison with an arrangement in which the stacks
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Asplund Gunnar
Ekwall Olle
Asea Brown Boveri AB
Berhane Adolf
Stephan Steven L.
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