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Various innovations and changes have occurred to underground coal mine power systems during the past ten years. Some of these are the use of higher utilization and distribution voltages and associated switchgear, the use of programmable logic controllers (PLC's) for control, monitoring, and diagnostic applications, improved protective relaying with built-in test circuitry, power-factor correction near loads for improved voltage regulation, and modifications to power-system component arrangement. Many of the innovations were necessitated by significant increases in power requirements of face equipment, particularly longwall equipment, while others are the result of improved technology. In either case, improved power-system operating characteristics, as well as enhanced levels of safety, have resulted. This paper describes many of the major innovations that have evolved during recent years. Index Terms - Coal mine power systems, mine power systems.

Radioisotope thermoelectric generators (RTGs) continued to supply power efficiently to a number of spacecraft loads during 1992. Pioneer 10, with its four RTGs, remainsfunctional even after 20 years since it was launched. TRW Inc finished the advanced photovoltaic solar array prgram for the Jet Propulsion Laboratory after a series of ground vibroacoustic tests on the stowed array wing. During the year, the development of an electric power system for the space station also continued and NASA-Lewis Research Center signed a deal with a group of companies for the design, manufacture and testing of a 2-kWe solar dynamic power system.