A fault-tolerant DC UPS system based on a battery charger with an automatic load transfer function

A direct current uninterruptible power supply (DC UPS) system, designed for operation in a hydro power plant, is presented in this paper. Standard DC power supply configurations for large power plants consist of two AC/DC converters with accompanying storage batteries. To improve the system’s reliability, an active fault tolerant control system (FCTS) was utilised. Aside from the two primary AC/DC converters, a third device was implemented with the same nominal characteristics, albeit with some additional advanced functions. This secondary AC/DC converter has two high-power contactors on its output, enabling it to operate (either manually or automatically) in parallel with one of the primary rectifiers. In order to achieve the high reliability of this facility, a serial communication was not established between battery chargers. Instead, a procedure for the turnout detection of a remote battery charger was utilised, based only on data from the auxiliary contacts of its main switch and the input contactor. Based on these logic conditions, an algorithm for the automatic load transfer was devised for implementation in cases when some of the inoperative primary battery chargers would automatically be replaced by a secondary AC/DC converter operating in a “hot reserve”.