In this document
In photovoltaic (PV) system simulations, it is also required to accurately calculate cabling losses, transformer losses, and grid losses to achieve a reliable energy yield analysis.
Overview
In PV simulation, accurately modeling the power transmission process is essential to assess the efficiency and performance of the entire system. Losses incurred during power transmission—such as cabling losses, transformer losses, and grid interaction—play a crucial role in determining the net energy output and must be carefully considered to provide realistic and actionable insights.
Cabling losses occur due to the resistance in electrical cables, which converts a portion of the transmitted energy into heat. Similarly, transformer losses must be reflected in the energy production estimates. The integration of grid interaction adds complexity to the power transmission model, requiring the consideration of grid limits.
Accounting for this allows the simulation to accurately represent the operational behavior of the PV system under real-world constraints, ensuring compliance and reliability.
Cabling losses
Cabling losses in PV plants refer to the energy dissipated as heat due to the electrical resistance of the cables. These losses occur as current flows through the cables, and the amount of energy lost is governed by Joule’s law, which calculates power loss as a function of the current and the cable's resistance.
To accurately assess cabling losses, it is essential to consider them separately, especially in the context of Maximum Power Point (MPP) tracking. By comparing the output of the MPP algorithm with and without the influence of cable resistances, we can precisely quantify the energy transformed into heat within the cables.
DC losses
DC losses in the direct current path from PV modules to inverters are a crucial factor in energy yield simulations. These losses occur due to the electrical resistance in the DC combiner boxes and DC cables.
Components: The DC electrical network includes DC combiner boxes (also known as junction boxes) and DC cables. These components are modeled using electrical resistance, which includes the resistivity of contacts in strings, connectors, and cables.
Calculation method: DC losses are calculated as losses, where is the electrical current flowing through the components, and is the resistance.
Solargis Evaluate provides a default value of 2% for DC cable losses. Users can adjust this value based on specific project requirements.
Comparison with other software
DC losses are accounted for similarly in other solar simulation software:
Software | Parameter name | Notes |
|---|---|---|
Solargis Prospect | Mismatch and cabling in the DC section | DC current path electrical loss at reference (STC) conditions. |
Solargis Evaluate | DC losses | DC current path electrical loss at reference (STC) conditions. |
PVsyst | Ohmic wiring loss | DC wiring losses. |
SAM (NREL) | DC wiring | DC electrical losses factor consisting of several loss types (page 67). |
SolarFarmer (DNV) | DC collectors | DC collection network resistance. |
AC losses
AC losses in a PV power plant occur in the AC cabling and combiner boxes, affecting the transmission of electricity from the inverters to the grid connection point. These losses are modeled using resistance, similar to DC circuits, and do not take into account the inductive or capacitive effects of the cables or combiner boxes.
AC cables: Losses occur on both the low-voltage side and the medium (high, extra high) voltage side, up to the grid connection point with the electricity meter.
Combiner boxes: All combiner boxes in the path of the AC current also contribute to these losses.
Comparison with other software
AC losses are similarly accounted for in various solar simulation software:
Software | Parameter name | Notes |
|---|---|---|
Solargis Prospect | AC cable losses | AC current path electrical loss at reference (STC) conditions. |
Solargis Evaluate | AC cable losses (low, medium, high voltage) | AC current path electrical loss at reference (STC) conditions, separated for low, medium, and high voltage circuits/connections. |
PVsyst | AC ohmic loss MV line ohmic loss HV line ohmic loss | AC current path electrical loss at reference conditions, related to given reference power (array at STC multiplied by the inverter's efficiency or nominal output of inverters without temperature correction). |
SAM (NREL) | AC wiring Transmission loss
| AC electrical losses factors for AC circuits and transmission line (page 84). |
SolarFarmer (DNV) | AC collectors | AC collection network resistance |
Transformer model
In PV systems, the AC working power is stepped up to a higher voltage by the transformer, a process critical for efficient power transmission and integration into the grid. To accurately model this process, the transformer model simulates both inverter transformers and, if applicable, power transformers used in the system.
In Solargis Evaluate, two different loss models are implemented for transformers:
Variable losses model
Constant losses model
This model accounts for transformer losses driven by their internal properties, which include:
Iron losses (no-load losses): These losses occur at almost a constant value while the transformer is connected to voltage. Iron losses depend non-linearly on the voltage level and appear negative at night.
Copper losses (load losses): These losses vary based on the transformer's load level and are a variable component of total losses.
Calculation method: Transformer losses are applied at all time steps in the temporal resolution of the simulated time series. The calculation for transformer loss at each time step relies on rated values (e.g., rated apparent power, rated voltage on primary and secondary sides, rated no-load and full-load losses) and actual operational conditions (e.g., electrical voltages and currents).
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In this model, transformer losses are represented as a percentage reduction of electrical power at the primary side of the transformer.
Comparison with other software
Transformer performance and loss modeling are comparable across various solar simulation software:
Software | Parameter name | Notes |
|---|---|---|
Solargis Prospect | Transformer losses | Transformer efficiency modeled by simple losses |
Solargis Evaluate | Transformer losses (LV/MV, MV/HV) | Variable or constant losses model for LV/MV (named inverter transformer) or MV/HV transformers (named power transformer), considering copper and iron losses. |
PVsyst | Medium voltage transformer loss High voltage transformer loss | Transformer losses model considering copper and iron losses with possible night disconnection. |
SAM (NREL) | Transformer loss
| Transformer model for distribution or substation transformers with load and no-load loss, considering power factor of 1 (page 84). |
SolarFarmer (DNV) | Transformer | Transformer model with considering of load and no-load loss. |
Grid limits
The grid connection model ensures that the simulation adheres to predefined grid power limits, enabling realistic performance analysis under grid constraints. Two types of power limits are applied in the simulation:
Static Grid Power Limit: A fixed power limit defined for the entire simulation period.
Dynamic Grid Power Limit: Variable power limits specified for individual time slots. The Solargis Evaluate currently uses a more simplified "external unavailability" model.
These limits are distributed across all inverters by setting a maximum power constraint for the inverters' Maximum Power Point (MPP) algorithms. An equal MPP limit is applied to each inverter, which is determined numerically to ensure balanced operation.
To meet the specified grid limits, the simulation iteratively adjusts and recalculates the Inverter-to-Grid segment.