Selective Relay System of the 66,000-Volt Ring of the Duquesne Light Company

The selective relaying of a highvoltage transmission system gives rise to many interesting problems. This is particularly the case when the system is grounded through a high resistance, as the amount of energy available on ground faults is limited and the ground protection introduces a problem entirely separate from that of the short-circuit protection of the system. It is also interesting to note that it is only in recent years that the design of high-voltage large capacity control equipment has been perfected to such a state as to place the protection of high-voltage systems on a par with that of comparatively lower voltage systems. The relay problems involved in the protection of low or high-voltage systems are not materially different except as affected by the equipment available, or by the ever present problem of economics. The system in question presented no unusul engineering features except as presented by the problem of securing circtit breakers of the required voltage and with sufficient capacity to rupture the large blocks of power presented by short-circuit conditions. This problem was successfully solved by the manufacturer. A ring system of duplicate feeders lends itself to various relay schemes to secure protection. A number of these schemes was thoroughly investigated and a summary of the conclusions is given, showing the advantages and disadvantages of the schemes considered.