Model step
MPPT minimum
Array MPP voltage below the inverter's MPPT window
Inputs used: MPPT window minimum, Modules per string
What it is
An inverter tracks the array's maximum-power point only between a minimum and a maximum MPP voltage, its MPPT window. The array's maximum-power voltage falls as the cells warm up, so strings with too few modules in series can fall below the minimum on hot days. The inverter then holds the voltage at its terminals at the minimum MPP voltage, the MPPT minimum. The array operates at that voltage on its own current-voltage curve, above its maximum-power voltage, where the current is lower. The power between the maximum-power point and this operating point is lost (Figure 11, top panel). When even the array's open-circuit voltage is below the floor, the array cannot reach the window. Strings sized for the site's hottest conditions stay inside the window and are not affected. On strings that run below the window most of the year, the options differ by about 0.6 % of the annual energy.
Options
- Hold the MPPT minimum, fitted voltage trim: the array operates at the MPPT minimum on its curve, read 0.33 V lower by a fitted trim
- Hold the MPPT minimum, curve current: the array operates at the MPPT minimum on its curve, with the current the curve gives there
References
- PVsyst 8 help, Inverter model and operating limits
API
inverter.mppt_floor_behavior = displaced | clamp