AC System Losses

General

AC system modeling begins after DC power has been calculated at each combiner. The model applies the remaining DC wiring loss, combines child combiners at each inverter, converts DC power to AC power, applies transformer losses, and combines the result into expected power at the point of interconnection.

The electrical hierarchy stored for the project determines which combiners feed each inverter and which inverters feed each transformer.

Acronyms

• AC: Alternating Current
• DC: Direct Current
• i: Current
• v: Voltage
• p: Power
• mp: Maximum Power
• POI: Point of Interconnection
• STC: Standard Test Conditions

Simulation Pipeline

The following flow diagram shows how AC system losses are calculated in the Proximal expected energy simulation. The flow chart is interactive. Clicking a model step will take you to its external documentation when available.

You may need to zoom in to better see all of the details in the flow chart.

Legend

flowchart LR

  classDef source fill:#6B7A8F, color:#CCCCCC
  classDef previous fill:#4F5B6F,color:#CCCCCC
  classDef model fill:#202020, color:#CCCCCC
  classDef inputs fill:#1A1A1A, color:#CCCCCC
  classDef outputs fill:#B39245, color:#CCCCCC

  database[(Database)]:::source
  previous{{Previous Calculation}}:::previous
  model_step[[
    Modeling Step
    DEFAULT MODEL CHOICE
  ]]:::model
  model_inputs[\
    Input Parameters
    for Modeling Step
  /]:::inputs
  model_outputs([Calculated Parameters]):::outputs

  database --> model_inputs
  previous --> model_inputs
  model_inputs --> model_step --> model_outputs --> model_inputs

Model Chain

flowchart TD

  classDef source fill:#6B7A8F, color:#CCCCCC
  classDef previous fill:#4F5B6F,color:#CCCCCC
  classDef model fill:#202020, color:#CCCCCC
  classDef inputs fill:#1A1A1A, color:#CCCCCC
  classDef outputs fill:#B39245, color:#CCCCCC

  combiner_power{{
    --- COMBINER POWER ---
    i_mp
    v_mp
    i_sc
    v_oc
  }}:::previous
  click combiner_power "dc_system_losses.html"

  system[(
    --- SYSTEM ---
    combiner to inverter assignments
    inverter to transformer assignments
    DC wiring loss at STC
    POI limit
  )]:::source

  inverter[(
    --- INVERTER ---
    nominal AC power
    nominal DC power and voltage
    startup power
    efficiency coefficients
    night tare
  )]:::source

  transformer[(
    --- TRANSFORMER ---
    rating
    no-load loss
    load loss
  )]:::source

  dc_wiring_inputs[\
    combiner IV values
    DC wiring loss at STC
  /]:::inputs
  combiner_power --> dc_wiring_inputs
  system --> dc_wiring_inputs

  dc_wiring[[
    proximal.dc_wiring_to_inverter
    TARGET LOSS AT STC
  ]]:::model
  dc_wiring_inputs --> dc_wiring --> dc_wiring_outputs

  dc_wiring_outputs([
    i_mp
    v_mp after wiring loss
    i_sc
    v_oc
  ]):::outputs

  combine_inverter_inputs[\
    combiner IV values
    combiner to inverter assignments
  /]:::inputs
  dc_wiring_outputs --> combine_inverter_inputs
  system --> combine_inverter_inputs

  combine_inverter[[
    proximal.combine_at_inverter
    ELECTRICAL AGGREGATION
  ]]:::model
  combine_inverter_inputs --> combine_inverter --> combine_inverter_outputs

  combine_inverter_outputs([
    inverter DC power
    inverter DC current
    inverter DC voltage
  ]):::outputs

  inverter_efficiency_inputs[\
    DC power
    DC voltage
    inverter parameters
  /]:::inputs
  combine_inverter_outputs --> inverter_efficiency_inputs
  inverter --> inverter_efficiency_inputs

  inverter_efficiency[[
    pvlib.inverter.sandia
    SANDIA
  ]]:::model
  click inverter_efficiency "https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.inverter.sandia.html"
  inverter_efficiency_inputs --> inverter_efficiency --> inverter_efficiency_outputs

  inverter_efficiency_outputs([
    AC power by inverter
  ]):::outputs

  ac_wiring_inputs[\
    inverter AC power
    nominal inverter AC power
  /]:::inputs
  inverter_efficiency_outputs --> ac_wiring_inputs
  inverter --> ac_wiring_inputs

  ac_wiring[[
    proximal.ac_wiring_to_transformer
    LOAD-DEPENDENT LOSS
  ]]:::model
  ac_wiring_inputs --> ac_wiring --> ac_wiring_outputs

  ac_wiring_outputs([
    AC power after wiring loss
  ]):::outputs

  combine_transformer_inputs[\
    inverter AC power
    inverter to transformer assignments
  /]:::inputs
  ac_wiring_outputs --> combine_transformer_inputs
  system --> combine_transformer_inputs

  combine_transformer[[
    proximal.combine_at_transformer
    SUM
  ]]:::model
  combine_transformer_inputs --> combine_transformer --> combine_transformer_outputs

  combine_transformer_outputs([
    input power by transformer
  ]):::outputs

  transformer_efficiency_inputs[\
    transformer input power
    rating
    no-load loss
    load loss
  /]:::inputs
  combine_transformer_outputs --> transformer_efficiency_inputs
  transformer --> transformer_efficiency_inputs

  transformer_efficiency[[
    pvlib.transformer.simple_efficiency
    SIMPLE EFFICIENCY
  ]]:::model
  click transformer_efficiency "https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.transformer.simple_efficiency.html"
  transformer_efficiency_inputs --> transformer_efficiency --> transformer_efficiency_outputs

  transformer_efficiency_outputs([
    AC power by transformer
  ]):::outputs

  poi_inputs[\
    transformer AC power
    POI limit
  /]:::inputs
  transformer_efficiency_outputs --> poi_inputs
  system --> poi_inputs

  poi[[
    proximal.point_of_interconnection
    SUM AND CLIP
  ]]:::model
  poi_inputs --> poi --> poi_outputs

  poi_outputs([
    expected power at POI
  ]):::outputs

Model Behavior

• DC wiring loss from the combiner to the inverter is scaled from the configured loss at standard test conditions using the operating current.
• Combiner currents are summed at each inverter, while combiner voltages are averaged.
• The default inverter model is the Sandia inverter model. PVWatts is also supported.
• Transformer efficiency accounts for transformer rating, no-load loss, and load loss.
• Transformer power is summed at the project level and limited to the configured point of interconnection capacity. Negative power is limited to zero.

Edits and Additions

If you would like to see support for another algorithm or would like to suggest edits or additions to this documentation page, please open an issue on the Proximal GitHub repository.