DC Field Aggregation

Summary

When multiple DC fields with non-uniform I-V 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, shading, among other factors. Since parallel-connected DC fields share a common operating voltage, the algorithm assesses 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.

Inputs

Name	Symbol	Units	Description
DC Field $i$ Max Power Voltage	$V_{mp,i}$	V	Voltage at maximum power point for DC field $i$ (from single-diode model)
DC Field $i$ Open-Circuit Voltage	$V_{oc,i}$	V	Open circuit voltage for DC field $i$ (from single-diode model)
DC Field $i$ Number of Parallel Strings	$N_{p,i}$	—	Number of series strings wired in parallel in DC field $i$
DC Field $i$ Modules in Series	$N_{s,i}$	—	Number of modules wired in series in DC field $i$
DC Field $i$ Repeater Count	$n_{rep,i}$	—	Number of identical DC field instances

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Outputs

Name	Symbol	Units	Description
DC Operating Voltage	$V_{DC}$	V	Shared operating voltage at inverter input
DC Open-Circuit Voltage	$V_{DC,oc}$	V	Weighted-average open-circuit voltage at inverter input
DC Operating Power	$P_{DC}$	W	DC power at inverter input

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Detailed Description

Step 1: Weighted Average Voltage Calculation

The maximum power point voltage ($V_{mp,i}$) for each DC field is calculated 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:

$$V_{DC} = \frac{\sum_{i} N_{p,i} \cdot n_{rep,i} \cdot V_{mp,i}}{\sum_{i} N_{p,i} \cdot n_{rep,i}}$$

The weighting factor ($N_{p,i} \cdot n_{rep,i}$) reflects the relative current contribution capacity of each DC field. The aggregated open circuit voltage is calculated using the same weighting approach:

$$V_{DC,oc} = \frac{\sum_{i} N_{p,i} \cdot n_{rep,i} \cdot V_{oc,i}}{\sum_{i} N_{p,i} \cdot n_{rep,i}}$$

Step 2: Voltage Reassignment

The common operating voltage is assigned to each DC field $i$ connected to the inverter:

$$V_i = V_{DC}$$

This ensures all parallel-connected DC fields operate at the same voltage, as required by Kirchhoff’s voltage law for parallel circuits.

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:

$$V_{mod,i} = \frac{V_{DC}}{N_{s,i}}$$

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:

$$I_i = I_{mod,i} \cdot N_{p,i}$$

Step 4: Current Summation and Power Calculation

The total inverter input current is the sum of currents from all parallel DC fields, accounting for repeaters, and the DC power delivered to the inverter follows directly:

$$I_{DC} = \sum_{i} I_i \cdot n_{rep,i}$$ $$P_{DC} = V_{DC} \cdot I_{DC}$$