The effect of short-term inverter saturation on modeled hourly PV output using minute DC power measurements

Abstract

PV system modeling is primarily done on hourly timescales and so cannot capture subhourly effects, including inverter saturation. Inverter saturation occurs when the potential dc power, Pdc, produced by the collectors is greater than the inverter capacity, and some of the PV power is lost or “clipped.” The inverter clips power rapidly, and calculations based on hour-averaged Pdc will overestimate ac power output for hours in which clipping occurs intermittently. Clipping is greater in systems with high ratios of collectors to inverter capacity, “dc:ac ratio.” We studied this modeling error using minute-scale Pdc measurements from a test site in Birmingham AL that had four different mounting configurations equipped with oversized inverters; i.e., 0.8 dc:ac ratio. PV output was calculated using minute- and hour-averaged Pdc and modeled inverters with dc:ac ratios up to 2.0. The modeling errors due to short term inverter saturation were approximately 2% of the annual output for 1.4 dc:ac ratios. We present the effect of mounting type, dc:ac ratio, season, and hour-of-day on these errors for the study site.