The present invention relates to power supply apparatus and methods and, more particularly, to apparatus and methods for transferring loads among multiple power sources.
A “reverse transfer” or “on-line” uninterruptible power supply (UPS) may be used to provide protected power to a load. As shown in FIG. 1, a typical on-line UPS 110 includes a rectifier 112 that is configured to be coupled to an AC power source 10 and to produce a DC voltage therefrom. The DC voltage is applied to an inverter 114, which generates an AC voltage for a load 20 coupled thereto. An alternative power source, e.g., a battery 116, may be coupled to the intermediate DC link to provide power to the inverter 114 in the event of failure of the AC power source 10. If the UPS 110 fails or is taken off line for maintenance, a high-speed solid-state static switch 120, which, as shown, may include anti-parallel connected silicon controlled rectifiers (SCRs) or other solid-state circuits that provide similar switching capabilities, may be used to provide a bypass path between the power source 10 and power to the load 20. The high-speed nature of the static switch 120 allows the load 20 to be transferred to the main source 10 with little or no interruption. In some applications, the static switch 120 may be integrated with the UPS 110 while, in other applications, the static switch 120 may be a separate device. As shown, a lower-speed switching device 130, such as switch, relay or circuit breaker, may be provided to allow for bypassing of the static switch 120. Other switches/breakers 118 may be provided for isolation and/or circuit protection.
Static switches may also be used to provide for transfer of loads among UPSs. Referring to FIG. 2, a first UPS 110a, here shown as including bypass static switch and other isolation/circuit protection devices, may be coupled to a first power source 10a and switchably coupled to a first load 20a via a static switch 212 of a first static transfer switch (STS) 210a and to a second load 20b via a static switch 212 of a second STS 210b. Similarly, a second UPS 110b may be coupled to a second power source 10b and switchably coupled to the first load 20a via a static switch 212 of the first STS 210a and to the second load 20b via a static switch 212 of the second STS 210b. A switch/breaker 220 may be provided between the outputs of the UPSs 110a, 110b. 
The STSs 210a, 210b allow for transfer of the loads 20a, 20b between the first and second UPSs 110a, 110b by providing for selective coupling therebetween. In particular, at a given time, one static switch 212 in each STS 210a, 210b is activated to provide power to one of the loads 20a, 20b, while the other static switch 212 isolates the same load from the other power source. Each of the STSs 210a, 210b can transfer a load in an uninterrupted manner by substantially simultaneously turning off the active static switch and turning on the inactive static switch. The STSs 210a, 210b are typically built as integrated units that include electronic circuits to monitor voltages applied to the loads 20a, 20b and responsively control the static switches 212. It will be appreciated that, although FIG. 2 illustrates dual STSs 210a, 210b, STSs with more than two static switches 212 may be used in applications in which more than two UPSs (or other power sources) are to be interconnected. A relatively less complex load transfer capability has also been provided in conventional systems by using a single static transfer switch that is coupled between two UPS outputs (or other power sources) and that operates responsive to a voltage at one or both of the outputs.