An electronic curve tracing system for evaluating the impact of cloud shadow movement on PV panels and microinverters

It is well known that cloud shadows cause rapid variations in PV output. Ramp time between full power and shadowed power (i.e., approximately 30% of full power) is typically about five seconds. This paper describes a PV curve tracing system, designed and built at a university, that sweeps the current-voltage curve of a 130W test panel pair twice every five seconds to find the true maximum power output. Simultaneously, the testing system takes DC-side measurements on a nearby 190W panel connected to 240Vac through a commercially-available microinverter. These data are then interpolated off-line using cubic splines to produce accurate estimates of maximum power output at GPS time-stamped one second intervals. It is shown that the microinverter follows the maximum power point very well during rapid cloud shadow movements.