Evolution of the automatic network relay

Low voltage a-c. networks are coming into use as a means of providing a source of power having the dependability of the Edison d-c. network, and at the same time having the efficiency which can be obtained by supplying power from high voltage feeders with transformers located near the point of utilization. As in the case of d-c. networks, it is assumed that all trouble on the network itself will be burned clear without seriously interfering with any of the service. In case of trouble on a primary feeder or in one of its transformers, however, it is necessary to disconnect the feeder from the system. The a-c. network unit has been developed to connect these transformers to the network, and the “brains” of this unit, which is the automatic network relay, is described in this paper. This relay not only opens the network breaker whenever there is trouble in any of the high tension equipment or when the power feeds back into the high tension feeder but also recloses the breaker when conditions are restored to normal and the feeder is in condition to supply power to the network. The difficulties which were encountered and the relay characteristics necessary to overcome them are explained briefly. The first installation of network units was made in April 1922 on single-phase, three-wire and two-phase, three-wire networks fed by three-phase, 2750-volt primary feeders. Since that time numerous improvements have been made and new designs created to extend the field of application, but the same general scheme of operation has been retained. The automatic network relay is now past the experimental stage, approximately 1000 of them having been in service for some time. The probable future development of the network relay based upon operating experience to date is pointed out. Attention is called to the close relation existing between the operating characteristics of the automatic network relay and the characteristics of the system on which it is to be installed.