Inverter Operating Regions

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Summary

Inverter Operating Regions define the operational state of the inverter based on DC voltage and power relative to inverter specifications. PlantPredict divides the voltage-power plane into 13 regions using four voltage thresholds ($V_{min}$, $V_{MPP,max}$, $V_{max}$, $\infty$) and three power thresholds ($P_{min}$, $P_{max}$, $\infty$). Region 6 represents normal MPPT operation. Other regions trigger specific control actions: voltage adjustment (regions 5, 7, 9, 11), clipping (region 10), or shutdown (regions 1-4, 8, 12, 13). This page documents the operating region logic for Version 7 and later. Earlier logic versions (V3-6) used a different point-in-polygon approach and are not described here.

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Inputs

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Outputs

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Detailed Description

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Voltage and Power Thresholds

When user-defined time-series adjustments are provided, the voltage and power thresholds are adjusted at each timestep. Adjusted voltage limits: $$V_{min,adj} = \frac{V_{min}}{1 + \Delta V_{MPP}}$$ $$V_{MPP,max,adj} = \frac{V_{MPP,max}}{1 + \Delta V_{MPP}}$$ $$V_{max,adj} = \frac{V_{max}}{1 + \Delta V_{MPP}}$$ Adjusted power limits: $$P_{min,adj} = \frac{P_{min}}{(1 + \Delta I_{MPP})(1 + \Delta V_{MPP})}$$ $$P_{max,adj} = \frac{P_{AC,rated}}{\eta(V_{DC}, P_{AC,rated}) \times (1 + \Delta I_{MPP})(1 + \Delta V_{MPP})}$$ If no time-series adjustments are defined, $\Delta V_{MPP} = 0$ and $\Delta I_{MPP} = 0$, and the thresholds equal the inverter specification values.

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Region Definitions

Region 1: $P \leq P_{min}$ and $V \leq V_{min}$ (Under-voltage, under-power) Region 2: $P \leq P_{min}$ and $V_{min} < V \leq V_{MPP,max}$ (Under-power, normal voltage) Region 3: $P \leq P_{min}$ and $V_{MPP,max} < V \leq V_{max}$ (Under-power, over-voltage) Region 4: $P \leq P_{min}$ and $V > V_{max}$ (Under-power, severe over-voltage) Region 5: $P_{min} < P \leq P_{max}$ and $V \leq V_{min}$ (Under-voltage) Region 6: $P_{min} < P \leq P_{max}$ and $V_{min} < V \leq V_{MPP,max}$ (Normal MPPT operation) Region 7: $P_{min} < P \leq P_{max}$ and $V_{MPP,max} < V \leq V_{max}$ (Over-voltage) Region 8: $P_{min} < P \leq P_{max}$ and $V > V_{max}$ (Severe over-voltage) Region 9: $P > P_{max}$ and $V \leq V_{min}$ (Clipping, under-voltage) Region 10: $P > P_{max}$ and $V_{min} < V \leq V_{MPP,max}$ (Clipping, normal voltage) Region 11: $P > P_{max}$ and $V_{MPP,max} < V \leq V_{max}$ (Clipping, over-voltage) Region 12: $P > P_{max}$ and $V > V_{max}$ (Clipping, severe over-voltage) Region 13: Undefined (no regions matched)

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Control Actions by Region

Regions 1, 2, 3, 13: Set voltage to open-circuit voltage (shutdown) Regions 4, 8, 12: Set voltage to 0 (shutdown - safety) Region 5, 9: Increase voltage to $V_{min,adj}$, recalculate region Region 6: Normal operation (MPPT tracking) Region 7, 11: Decrease voltage to $V_{MPP,max,adj}$, recalculate region Region 10: Clipping algorithm - find voltage where $P_{DC} = P_{max,adj}$

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Clipping Algorithm (Region 10)

When the operating point falls in Region 10 (over-power), the algorithm finds a voltage that reduces power to the maximum limit. With time-series adjustments: Generate a power-voltage curve from $V_{oc}$ to 0 in 0.1 V steps. Find voltage $V_{clip}$ where:

1. $V_{min,adj} \leq V_{clip} \leq V_{MPP,max,adj}$ (preferred range)
2. $P(V_{clip}) \leq P_{max,adj}$ and $P(V_{clip}) \geq P_{min,adj}$
3. $V_{clip}$ minimizes $|P_{max,adj} - P(V_{clip})|$

If no solution is found in the MPPT voltage range, the search expands to $V < V_{MPP}$. Without time-series adjustments: Iterative search from $V_{oc}$ downward:

1. Calculate total DC power from all fields at each voltage step
2. Find voltage where $|P_{DC} - P_{max,adj}|$ is minimized
3. Refine with progressively smaller voltage steps (five iterations)

If the clipped power is sufficiently close to the maximum limit, the region is reassigned to Region 6 (normal operation).

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References

• King, D. L., Boyson, W. E., & Kratochvil, J. A. (2004). Photovoltaic array performance model. SAND2004-3535, Sandia National Laboratories.