Application of phase sequence principles to relaying of low-voltage secondary networks

The authors present an improved network relaying system utilizing a conventional single-phase relaying element and contactor type overcurrent elements excited by symmetrical phase sequence components to represent completely power flow conditions and control protector operation on a threephase four-wire secondary network system. The low-voltage alternating-current network was conceived and developed to meet the need for an alternating-current system of distribution that would provide high quality service and compete with the direct- current system in reliability and economy. In its present form, it consists of a distribution network, served by a plurality of feeders, with intermediate voltage transformation and network protectors. The use of the present arrangement dates from the development of the network protector which fulfilled several functions and may be described as a means to connect and disconnect each of the feeders to the network under proper conditions. The protector consists of a circuit breaker and necessary operating relays located in each of the feeds to the network. Each main feeder provides a multiplicity of feeds to the network through low-capacity transformers, (50 to 1,000 kva) and it is the function of the protector to connect to or disconnect from the feed to the network under proper conditions. For discussion of this paper see page 626.