A direct current power supply system includes at least one first input source, a second input source, a phase shifting transformer set, a rectifier set and a monitoring module. The phase shifting transformer set is for converting a medium voltage alternating current of the first input source and/or the second input source to a low voltage alternating current. The rectifier set is for rectifying the low voltage AC to a low voltage DC. The phase shifting transformer set and the rectifier set are disposed either in a power supply housing, or in two adjacent power supply housings. The monitoring module includes a phase shifting transformer monitoring unit for retrieving at least one of temperature data, voltage data and current data of the phase shifting transformer set and a rectifier monitoring unit for retrieving at least one of temperature data, voltage data and current data of the rectifier set.

An apparatus for uninterruptible power supply (UPS) comprises a rotating electric machine having a generator winding and a plurality of motor windings. The motor windings of the plurality of motor windings are electrically insulated from the generator winding and from one another. The UPS further comprises, for each motor winding, an energy storage for electric energy and an inverter which outputs an alternating current and via which the energy storage is connected to the respective motor winding.

A grid interconnection system for a power grid includes: a storage battery; a power conversion device that converts power from the storage battery into AC; and an interconnection control device that supplies power from a commercial power supply to a load and supplies power from the power conversion device to the load when the commercial power supply fails. When the commercial power supply is restored, the interconnection control device starts supply of power from the commercial power supply to the load, and stops supply of power from the power conversion device to the load after the supply of power from the commercial power supply to the load is started.

A ferroresonant transformer system has a core, a shunt, first and second input windings, an inverter winding, a tank winding, a resonant capacitor, an output capacitor, and a plurality of switches. The tank winding defines a plurality of switch tap locations and at least two output tap locations. The resonant capacitor is connected across at least a portion of the tank winding. Each switch is operatively connectable between one of the switch tap locations and the resonant capacitor. Each output terminal is operatively connected to one of the at least two output tap locations.

An uninterruptible power supply for providing an output power signal to a load comprises a ferroresonant transformer, a resonant capacitor, and an inverter. The resonant capacitor is operatively connected to the ferroresonant transformer. The inverter is operatively connected to the ferroresonant transformer. The inverter is configured to generate the output power signal based on at least one inverter control signal such that the output power signal is a quasi square wave having at least one change of phase and an upper limit. The at least one inverter control signal is held in an OFF state during at least a portion of the at least one change of phase, pulse-width modulated during at least a portion of the at least one change of phase, and held in an ON state when the output power signal is at the upper limit.

A ferroresonant transformer system has a core, a shunt, first and second input windings, an inverter winding, a tank winding, a resonant capacitor, an output capacitor, and a plurality of switches. The tank winding defines a plurality of switch tap locations and at least two output tap locations. The resonant capacitor is connected across at least a portion of the tank winding. Each switch is operatively connectable between one of the switch tap locations and the resonant capacitor. Each output terminal is operatively connected to one of the at least two output tap locations.

An uninterruptible power supply adapted to be connected between an AC line and a load, comprising a battery system, an inverter, and a transformer, and a controller. The battery system stores battery power. The inverter is operatively connected to the battery system. The transformer is operatively connected to the AC line, the load, and an inverter winding operatively connected to the inverter. The controller controls the inverter to supply power to the primary winding using battery power stored in the battery system based on a cost value indicative of reduction of life of the battery system.

An uninterruptible power supply for providing an output power signal to a load comprises a ferroresonant transformer, a resonant capacitor, and an inverter. The resonant capacitor is operatively connected to the ferroresonant transformer. The inverter is operatively connected to the ferroresonant transformer. The inverter is configured to generate the output power signal based on at least one inverter control signal such that the output power signal is a quasi square wave having at least one change of phase and an upper limit. The at least one inverter control signal is held in an OFF state during at least a portion of the at least one change of phase, pulse-width modulated during at least a portion of the at least one change of phase, and held in an ON state when the output power signal is at the upper limit.

An uninterruptible power supply adapted to be connected between an AC line and a load a battery system for storing battery power, an inverter, a transformer, and a controller. The inverter is operatively connected to the battery system. The transformer comprises a primary winding adapted to be operatively connected to the AC line, a load winding adapted to be operatively connected to the load, and an inverter winding operatively connected to the inverter. The controller controls the inverter to operate in a first mode in which the inverter supplies power to the battery system, a second mode in which the inverter supplies power to the load winding using battery power stored in the battery system, and, based on a cost value indicative of reduction of life of the battery system, a third mode in which the inverter supplies power to the primary winding using battery power stored in the battery system.

Sharing of load current in desired ratios between multiple electrical voltage sources is achieved without coupling between the sources by controlling a selection switch to select each voltage source for a proportion of the time at a high rate, the switching rate components being prevented from being seen by either the load or the sources through use of low-pass filters.