Evolution of the Automatic Network Relay

Low-vottage 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.