Stability and Bandwidth Implications of Digitally Controlled Grid-Connected Parallel Inverters

The increasing use of grid-connected inverter systems is resulting in a desire for parallel-connected inverters that offer greater power capacity while maintaining the high control bandwidth achieved by individual inverters. This paper demonstrates that, in addition to the traditional stability and bandwidth limitations of digitally controlled inverters, further stability and bandwidth limitations occur when LCL inverters with a common set point are connected in parallel to a grid. This paper provides detailed discrete-time derivations for parallel grid-connected inverters and uncovers stability and bandwidth limitations that only occur in grid-connected applications and are not apparent if the system is studied in continuous time. This paper demonstrates that, in a typical application, the voltage bandwidth of an LCL parallel inverter array is 25% lower than a single module or LC parallel configuration. Both simulations and hardware demonstrations on a 105-kVA parallel three-module grid-connected system confirm the findings.