A number of microprocessor-based digital devices with communication capabilities have been manufactured for traction power substations. These devices include protective relays, fault recorders, and metering units. One such device is the SIEMENS SITRAS DPU96 relay that provides over-current and rate-of-rise protection for faults on DC traction power distribution system. To fully utilize this digital technology, Programmable Logic Controller (PLC) was the best mean of collecting and organizing data from these devices locally at the traction power substation and also provide easy interface with the SCADA system. PLC was also used for high rail-to-earth potential monitoring and protection. The trip characteristics of this relay require adjustability over the voltage ranges normally from 40Vdc to 160Vdc and time-delay range from 10ms to 3 seconds which can be easily accomplished with PLC programming. This paper describes the applications of digital relays, human machine interface (HMI), and PLC in a traction power substation.

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