In March 1951 ultrahigh capacity field tests were made at Grand Coulee Power Plant on a 230-kv air-blast circuit breaker to further the development of high-voltage circuit breakers incorporating high-speed operation. Development from field tests in recent years has produced 230-kv 10,000,000-kva circuit breakers with the interruption taking place within oil. However, this is the first air-blast circuit breaker to be introduced to the American power industry in this voltage class and interrupting capacity. The tests proved that the air-blast circuit breaker is capable of interrupting the current of overhead lines without any restrikes and, therefore, without raising the voltage above the value on the line before the interruption. The circuit breaker interrupted well over its rated capacity of 6,000,000-kva symmetrical and 9,000,000-kva asymmetrical. The maximum interrupted power being 7,850,000-kva symmetrical and 10,500,000-kva asymmetrical, this was higher than any previous tests. The interrupting time was always well below 3 cycles. The absolute maximum was 2-72 cycles and the average about 2.4 cycles. The circuit breaker also interrupted these high values in high-speed reclosing operations with reclosing times of 13 to 14 cycles. The contacts were not changed or even inspected during the whole test series. After nine interruptions with an average power of substantially more than the rated capacity, contact erosion was extremely slight so that the same contacts could have withstood at least the same tests again. The tests have shown that the air-blast circuit breaker is capable of solving any problems arising in modern high-capacity networks.

Early studies and calculations1,2 concerning the bussing at 230 kv of the generators at the Grand Coulee Power Plant and the lines emanating therefrom indicated that the asymmetrical fault currents to be encountered as the Coulee installation grew would increase very rapidly and the development of ultrahigh interrupting-capacity circuit breakers, as well as means of limiting the fault current, were investigated thoroughly.

Very little has been published in the past regarding the hazard to the integrity of long control circuits due to possible excessive rise in ground potential at one end of the circuits during station faults; and the verification of the safety of such control circuits through actual tests. This paper describes the rather unique arrangement at Grand Coulee power plant wherein the plant is located at the bottom of the Columbia River Channel and one of the associated 230-kv switching stations is located on a rocky plateau approximately 600 feet higher in elevation, thus requiring control cables over 4,000 feet in length. By means of several fault tests staged specifically for this purpose, and as a by-product of subsequent high-interrupting capacity circuit breaker tests, the actual rise in ground potential was determined and the safety of the control circuits positively verified, under 60-cycle fault conditions.