The design of high-voltage high-power mercury-arc convertors

The realization of high-voltage d.c. transmission has been prevented by the lack of reliable rectifiers and inverters, although the possibilities of mercury-arc-convertor operation up to 100 kV and 100 MW have been demonstrated by some test installations surveyed in the paper. The same type of valve may be used for both rectification and inversion; the stresses present during both operations are analysed in terms of the random failures which have so far prevented a large-scale practical transmission. High-voltage operation by means of series-connected low-voltage valves is prohibitive, and no more than three valves should be used in series. However, experience has shown that a series of gaps within a single envelope can be used for very-high-voltage conversion. The basic concept of forced voltage subdivision of the anode space is examined, and the conditions necessary to ensure sufficiently high breakdown thresholds are outlined. None of the present high-voltage valves fulfils all the requirements for trouble-free operation, owing to the various phenomena which lower the threshold and cause random failures. Therefore, a valve is described which is expected to meet these requirements, and which, when used in a 3-phase 6-pulse bridge, is designed to convert 250 MW of power at 250 kV. No radically new concepts are involved; the proposed techniques extend modern rectifier experience but will require careful development for the large dimensions. The paper concludes with a brief survey of the anticipated performance of the valve under backfire conditions, of the auxiliary and control devices necessary, and of the future trends of development.