Abstract: An uninterruptible power system having a main power path between input and output terminals monitors the phase of the voltages at the input and output terminals while power is applied to the input terminals from an AC power source. The phase difference between the voltages at the input and output terminals is determined and, upon occurrence of a fault in the power supplied to the input terminals, an inverter is turned on to provide power to the load at a frequency which matches the frequency of the input voltage before the fault and at a phase which is the phase of the voltage at the input terminals before the fault corrected in accordance with the phase difference determined before the fault. Prior to transfer of the supply of power back to the main AC power source when the fault ends, the phase and frequency of the voltage at the input terminals is again measured.

Abstract: Connection of batteries to the DC bus lines across which capacitors are connected in power systems, such as uninterruptible power systems, is carried out using an auxiliary switch which is controlled to connect the batteries to a DC bus line through a precharge resistor momentarily before connection is made through a main connector. The preconnection of the batteries through the precharge resistor provides a controlled rate of charge of the capacitors before closing of the main connector to thereby avoid excessive surge currents. When the main connector is opened, as when the power system is prepared for transportation or long term storage, the auxiliary switch interrupts the current path from the battery through the precharge resistor and provides a current path from the DC bus line through a discharge resistor to ground to bleed down the charge on the capacitors to a desired level within a selected period of time.

Abstract: The run-time available from an uninterruptible power system is accurately determined both during supply of power from the battery to the load and during the time where power is supplied directly from an AC power system to the load. During power outages, where power is supplied from the battery through the inverter to the load, the remaining run-time is first estimated by calculation and then corrected by modification of the calculated value according to the reciprocal of the run-time slope. Alternatively, a scaling factor is used as part of the first run-time calculations and a correction factor which applies more corrective weight as the time on inverter progresses may be used. During power outages and when power is supplied from the AC power system to the load the maximum available run-time is extrapolated from a table of values representing maximum available run-times at various levels of power being supplied to the load from a fully charged battery.