Hold the MPPT minimum, fitted voltage trim

Method

The inverter holds the voltage at its terminals at the MPPT minimum, the inverter's minimum MPP voltage. The voltage at the modules is the floor plus the cable drop at the maximum-power current; with the drop at the operating current, $R_w I_{mpp}$ becomes $R_w I$. The array delivers the current its curve gives at that voltage lowered by a fitted trim of 0.33 V. Above the maximum-power voltage a lower voltage means a higher current, so the trim raises the current slightly. The array power at the floor is the floor voltage times that current, and the rest of the maximum power is lost.

The array would have clipped when its maximum power, converted at the floor voltage, exceeds the AC limit. Three cases change the outcome.

1. The open-circuit voltage less the cable drop is below the floor, and the array would not have clipped. The inverter stops: no current flows, the whole maximum power is counted in the voltage-threshold row and no night consumption is drawn.
2. The open-circuit voltage less the cable drop is below the floor, and the array would have clipped. The operating point is taken at the floor on the extension of the curve past the open-circuit voltage, without the trim. The current and the power there are negative, and that negative power reaches the inverter output unchanged. Note: PVsyst's own results show the same negative values in these hours; the option keeps them to reproduce them. The rest of the maximum power is counted in the power-limit row.
3. The floor point delivers less than the inverter's power threshold. The output is zero, the floor power is counted in the efficiency row and the rest in the voltage-threshold row.

When the floor point itself exceeds the AC limit, the AC-limit rule of the operating point on the I–V curve raises the voltage from the floor.

Settings

Setting	Value
Trim on the voltage at which the curve is read $U_{trim}$	0.33 V

The math

1. Current at the floor
2. Clipping test
3. Power at the floor
4. Lost power

$$I = \begin{cases} I_{curve}\big(V_{min} + R_w\,I_{mpp} - U_{trim}\big) & U_{oc} - R_w\,I_{mpp} \ge V_{min} \\ 0 & U_{oc} - R_w\,I_{mpp} < V_{min},\ \text{no clipping} \\ I_{curve}\big(V_{min} + R_w\,I_{mpp}\big) & U_{oc} - R_w\,I_{mpp} < V_{min},\ \text{clipping} \end{cases} \tag{1}$$ $$P_{out}(P_{mpp},\ V_{min}) > P_{lim} \tag{2}$$ $$P_{op} = V_{min}\,I \tag{3}$$ $$\Delta P = P_{mpp} - P_{op} \tag{4}$$

where

Symbol	Meaning	Unit
$I$	the array current at the floor	A
$I_{curve}(V)$	the current the array delivers at voltage $V$ at the modules, from its current-voltage curve	A
$V_{min}$	the minimum MPP voltage, the bottom of the inverter's MPPT window	V
$R_w$	the resistance of the DC cables between the modules and the inverter	Ω
$I_{mpp}$	the array's maximum-power current of the timestep	A
$U_{trim}$	the fitted trim on the voltage at which the curve is read	V
$U_{oc}$	the array's open-circuit voltage at the modules	V
$P_{out}(P, U)$	the AC output of the efficiency curves at DC power $P$ and voltage $U$	W
$P_{mpp}$	the array power at its maximum-power point after the DC losses	W
$P_{lim}$	the AC limit of one inverter at the timestep: its nameplate, derated with the ambient temperature and lowered by a grid power limitation	W
$P_{op}$	the array power at the floor	W
$\Delta P$	the power lost at the MPPT minimum	W

Note

The 0.33 V trim is a fitted constant with no known physical mechanism. It applies to the array voltage, whatever the number of modules per string.

References

1. PVsyst 8 help, Inverter model and operating limits
2. Analysis of PVsyst 8.1.5 outputs shows this convention in its results.

API

inverter.mppt_floor_behavior = displaced

Symbols follow the symbol registry.