Selective relay system of the 66,000-volt ring of the Duquesne Light company

The selective relaying of a high-voltage 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 circuit 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. These studies showed the advisability of using the scheme of balanced directional relays for short-circuit protection, and selective differential current relays for ground protection, on the 66,000-volt ring of the Duquesne Light Company. Before putting the relays into service on this system it was decided to make a series of actual service tests on the lines. Accordingly a series of dead grounds were put on the ring and the relays allowed to function to clear the grounded section of line. The dead ground was replaced by a simulated fallen line wire lying on the surface of th- ground. This was in turn replaced by a series of arcing grounds accomplished by fusing over a suspension insulator to ground. A total of 28 grounds were thrown on the 66,000-volt ring and successfully cleared by the relays. A description of the tests and a summary of the results are given.