Parallel operation of single phase inverter modules with no control interconnections

To provide reliable power under scheduled and unscheduled outages requires an uninterruptible power supply (UPS) which can be easily expanded to meet the needs of a growing demand. A system suck as this should also be fault tolerant and include the capability for redundancy. These goals can be met by paralleling together smaller inverters if a control scheme can be designed to allow them to operate independently yet still share the load. We have developed a control technique for operating two or more single phase inverter modules in parallel with no auxiliary interconnections. This technique uses frequency, fundamental voltage, and harmonic voltage droop to allow independent inverters to share the load in proportion to their capacities. Simulation results are provided to prove the concept.