Automatic Transformer Substations Of the Edison Electric Illuminating Company of Boston

The Edison Electric Illuminating Company of Boston has adopted a policy of numerous relatively small automatic a-c. substations located at the various individual load centers, this policy being enhanced by the development of a new method of automatic control and by the standardization of the station design. The rating of a substation is based on the sum of three specially designed transformers each having an inherent 50 per cent overload capacity. Thus, the maximum load is normally carried by the three units operating at their nominal 100 per cent rating, but in emergencies the load can be handled by two banks, the transformer out of service, representing only 33 per cent of the station capacity instead of 60 per cent as in the usual case. The automatic control consists essentially of load-current relays which switch in or out the follow-up transformers as needed. This operating point for maximum station efficiency equals √2 × core losses per bank/copper losses. Serious trouble on any bank will bring in the succeeding transformer. Each transformer has its own sequence switch so that it may be made leading, follow-up, second following, or manual, irrespective of the set-up for the remaining units. The simplicity of this control equipment is particularly interesting; in fact only the current control and one auxiliary relay are directly chargeable to the automatic control. The circuits and apparatus have several special features promoting safety and reliability.