Sealed Ignitron Rectifiers for Urban Transit Power Supply

Today, the mercury arc rectifier has virtually superseded the older forms of conversion equipment for supplying power to trolley coach and street railway transit systems. The present trend when selecting a rectifier rated 1,000 kw or less is to choose the sealed ignitron type. The sealed ignitron rectifier is metal-enclosed, which makes possible a unit substation type of equipment when it is combined with metal-enclosed switchgear. This gives increased safety to personnel and a reduction in installation cost. A number of substation arrangements are possible, including installation of part or even all of the equipment outdoors in weatherproof enclosures, thus reducing building costs. Operating experience with close to 4,000 sealed ignitron tubes has demonstrated that they can be expected to have an average life of alt least six years. Their 1-minute rating of 200 per cent load is more than adequate to handle an urban transit load composed of a number of small increments, where diversity serves to reduce the magnitude of peak swings. In keeping with the smaller equipment ratings possible with sealed ignitrons, light-duty switchgear has been introduced having the necessary current and interrupting ratings at lower initial cost. The high reliability of rectifiers makes it possible in many cases to install a single-unit substation where before two smaller conversion units would have been used. Operators are beginning to recognize this and to take advantage of the savings this accomplishes.