DC Field Aggregation

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Summary

When multiple DC fields with non-uniform I-V curve characteristics are connected in parallel to a common inverter input, PlantPredict calculates the combined DC output using a weighted voltage averaging approach. Each DC field may have different I-V characteristics due to variations in module type, irradiance, temperature, or shading. Since parallel-connected DC fields share a common operating voltage, the algorithm determines this shared voltage, recalculates the current contribution from each DC field at that voltage, and sums the currents to obtain total inverter input current and power.

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Inputs

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Outputs

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Detailed Description

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Step 1: Weighted Average Voltage Calculation

Each DC field calculates its maximum power point voltage ($V_{mpp,i}$) independently based on its specific operating conditions. When multiple DC fields are connected in parallel to a common inverter input, a weighted average determines the shared operating voltage: $$V_{op} = \frac{\sum_{i} N_{p,i} \cdot n_{rep,i} \cdot V_{mpp,i}}{\sum_{i} N_{p,i} \cdot n_{rep,i}}$$ The weighting factor ($N_{p,i} \cdot n_{rep,i}$) reflects the relative current contribution capacity of each DC field. Open circuit voltage and initial voltage are calculated using the same weighting approach.

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Step 2: Voltage Reassignment

The common operating voltage is assigned to all DC fields connected to the inverter: $$V_{i} = V_{op} \quad \forall \, i$$ This ensures all parallel-connected DC fields operate at the same voltage, as required by Kirchhoff’s voltage law for parallel circuits.

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Step 3: Current Recalculation

At the common operating voltage, the current from each DC field is recalculated using its I-V curve. The module-level voltage is determined by dividing the common voltage by the number of modules in series: $$V_{mod,i} = \frac{V_{op}}{N_{s,i}}$$ The module-level current ($I_{mod,i}$) is solved from the single diode model at $V_{mod,i}$ using Newton-Raphson iteration. See Single Diode Model for the diode equation formulation. The DC field current is then: $$I_{op,i} = I_{mod,i} \cdot N_{p,i}$$

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Step 4: Current Summation

The total inverter input current is the sum of currents from all parallel DC fields, accounting for repeaters: $$I_{inv} = \sum_{i} I_{op,i} \cdot n_{rep,i}$$

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Step 5: Power Calculation

The DC power delivered to the inverter is: $$P_{inv} = V_{op} \cdot I_{inv}$$

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Mismatch Effect

When DC fields have non-uniform I-V characteristics, the common operating voltage forces each DC field to operate at a point that may differ from its individual maximum power point. The resulting power is less than the sum of individual maximum powers: $$P_{inv} < \sum_{i} V_{mpp,i} \cdot I_{mpp,i} \cdot n_{rep,i}$$ This difference represents the mismatch loss arising from heterogeneous DC field combination.

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Loss Tree Reporting

The power loss from heterogeneous DC field combination is reported in the loss tree as “Inverter Limitation Loss” (also referred to as “Off-MPP Power Loss”). This loss is calculated at the inverter level: $$L_{off-MPP} = P_{initial} - P_{inv} - L_{deg}$$ where:

• $P_{initial}$ is the sum of individual DC field maximum power point powers
• $P_{inv}$ is the DC power after aggregation and degradation
• $L_{deg}$ is any degradation loss applied at the DC level

The degradation loss term is subtracted because $P_{inv}$ already reflects degradation. This isolates the off-MPP loss so it captures only the mismatch effect from heterogeneous DC field combination, separate from the Degradation Loss line item in the loss tree. This loss is also distinct from the user-specified “Module Mismatch” loss coefficient, which is a separate input parameter applied as a percentage reduction at the DC field level.

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Nodal Data Outputs

PlantPredict reports DC electrical parameters at both the DC field and inverter levels. DC Field nodal data represents pre-aggregation values—each field’s individual maximum power point voltage, current, and power before parallel combination. Inverter nodal data represents post-aggregation values—the combined voltage, current, and power after all connected DC fields are aggregated to a common operating point. The difference between “DC Power at MPP” and “DC Power” at each level reflects the power lost due to operating off the individual maximum power points.

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References

• De Soto, W., Klein, S. A., & Beckman, W. A. (2006). Improvement and validation of a model for photovoltaic array performance. Solar Energy, 80(1), 78-88.