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Summary

Power Flow to Grid computes the combined PV+ESS net power delivered to the grid. Battery DC power is converted to AC through the storage and traced through the . During charging, PV output is reduced by the power diverted to storage; during discharging, battery power is added to PV output. The combined output at MV then triggers a re-run of the plant-level power flow—the transformers losses, transmission lines losses, , and limit are re-calculated and applied; the rest of the PV pipeline is not re-run.

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Inputs

NameSymbolUnitsDescription
Battery DC PowerPDC,battP_{DC,batt}WDC power at battery terminals, positive = charge, negative = discharge (from Battery Model)
ESS Inverter Efficiencyηinv\eta_{inv}%ESS inverter conversion efficiency
ESS MV Transformer RatingPMV,rated,ESSP_{MV,rated,ESS}MVAESS MV transformer nameplate capacity
ESS MV No-Load LossLNL,ESSL_{NL,ESS}%ESS MV transformer no-load loss as a percentage of PMV,rated,ESSP_{MV,rated,ESS}
ESS MV Full-Load LossLFL,ESSL_{FL,ESS}%ESS MV transformer full-load loss as a percentage of PMV,rated,ESSP_{MV,rated,ESS}
PV MV Transformer OutputPPV,MVP_{PV,MV}WPV power output from MV transformer, before HV losses
Grid Limit (LGIA)PPOIP_{POI}MWMaximum allowed power at point of interconnection
Availability Factorfavailf_{avail}%Flat percentage deduction for estimated downtime

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Outputs

NameSymbolUnitsDescription
Combined PV+ESS PowerPcombinedP_{combined}WMerged PV and ESS power at MV level before HV losses
Grid PowerPgridP_{grid}WFinal plant output at the POI after HV losses, availability, and LGIA limit

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Detailed Description

The power flow traces battery power from the DC terminals to the grid in four steps:
  1. ESS Inverter — converts DC to AC (or AC to DC) with efficiency losses
  2. ESS MV Transformer — applies transformer losses to the inverter AC power
  3. Combined PV+ESS Output — merges PV and ESS power at the MV level (subtracting charge or adding discharge)
  4. Plant-Level Power Flow Re-Run — applies HV losses, availability, and LGIA limit to the combined output

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Step 1: ESS Inverter

The inverter converts between DC power at the battery terminals and AC power on the low-voltage side of the MV transformer. ηinv\eta_{inv} is converted from percentage to fraction before use. When discharging (PDC,batt<0P_{DC,batt} < 0), the inverter converts DC to AC with efficiency losses: PAC,d=−PDC,batt×ηinvP_{AC,d} = -P_{DC,batt} \times \eta_{inv} When charging (PDC,batt>0P_{DC,batt} > 0), the inverter converts AC to DC, requiring more AC input than DC output: PAC,c=PDC,battηinvP_{AC,c} = \frac{P_{DC,batt}}{\eta_{inv}} The inverter efficiency loss is calculated for reporting during both charge and discharge: Linv=(1−ηinv)×∣PDC,batt∣L_{inv} = (1 - \eta_{inv}) \times |P_{DC,batt}|

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Step 2: ESS MV Transformer

The storage MV transformer uses the same quadratic loss model as PV transformers (see Transformer Loss Model), parameterized by PMV,rated,ESSP_{MV,rated,ESS}—converted from MVA to VA—LNL,ESSL_{NL,ESS}, and LFL,ESSL_{FL,ESS}.

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Discharge

During discharge (power flows battery → grid), the transformer reduces the power delivered to the MV bus: PMV,d=PAC,d−LMV(PAC,d)P_{MV,d} = P_{AC,d} - L_{MV}(P_{AC,d})

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Charge (or Idle)

During charge or when the battery is idle (power flows PV → battery via the MV bus), the transformer increases the power drawn from the MV bus—the PV must supply the charge power plus the transformer loss: PMV,c=PAC,c+LMV(PAC,c)P_{MV,c} = P_{AC,c} + L_{MV}(P_{AC,c}) When the battery is idle, the transformer still draws power from the grid to maintain core magnetization. The charge power reduces to the no-load loss alone: PMV,c=LNL,ESS×PMV,rated,ESSP_{MV,c} = L_{NL,ESS} \times P_{MV,rated,ESS}.

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Step 3: Combined PV+ESS Output

When charging or idle, PV output is reduced by the total power diverted to storage, PMV,cP_{MV,c} (which includes MV transformer loss overhead and collapses to the no-load loss when idle, per Step 2): Pcombined=PPV,MV−PMV,cP_{combined} = P_{PV,MV} - P_{MV,c} When discharging, battery power is added to the full PV output: Pcombined=PPV,MV+PMV,dP_{combined} = P_{PV,MV} + P_{MV,d} When PV is negative and the battery is discharging—possible at night due to auxiliary loads and transformer energization losses—only the battery discharge is delivered: Pcombined=PMV,dP_{combined} = P_{MV,d}.

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Step 4: Plant-Level Power Flow Re-Run

The combined output (PcombinedP_{combined}) replaces the sum of PV block outputs, and the plant-level power flow is re-run with the new PV + ESS combined power value to determine revised HV losses. The upstream portion of the PV pipeline (irradiance, DC performance, inverter, array-level losses) is not run again; those results are retained. The re-run applies the same three stages as the PV-only pass:
  1. HV equipment — PcombinedP_{combined} enters the HV chain and passes through each transformer and transmission line in series; each element reduces the power by its losses and feeds the result to the next element. The final output PHV,outP_{HV,out} is the power upstream of the after all HV losses.
  2. Availability — a flat percentage deduction for estimated downtime due to maintenance and unplanned outages:
Pavail=PHV,out×100−favail100P_{avail} = P_{HV,out} \times \frac{100 - f_{avail}}{100}
  1. LGIA limit — power exceeding the interconnect capacity (PPOIP_{POI}, converted from MW to W) is curtailed:
Pgrid=min⁡(Pavail, PPOI)P_{grid} = \min(P_{avail},\, P_{POI}) can occur when:
  • PV alone exceeds the LGIA limit and the battery does not charge—either because the dispatch intent is not to charge (e.g., discharge target period or non-charge custom instruction), or because hardware limits prevent it (inverter at capacity or battery full)
  • The battery discharges while PV is already near the limit, pushing the combined output above the cap (only possible with Custom Dispatch algorithm—other algorithms enforce LGIA headroom on discharge)
  • Slight inaccuracies in the simplified loss estimates used by the charge and discharge limits cause the combined output to marginally overshoot