Power Flow to Grid

​

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.

​

Inputs

---

​

Outputs

---

​

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

​

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. $\eta_{inv}$ is converted from percentage to fraction before use. When discharging ($P_{DC,batt} < 0$), the inverter converts DC to AC with efficiency losses: $$P_{AC,d} = -P_{DC,batt} \times \eta_{inv}$$ When charging ($P_{DC,batt} > 0$), the inverter converts AC to DC, requiring more AC input than DC output: $$P_{AC,c} = \frac{P_{DC,batt}}{\eta_{inv}}$$ The inverter efficiency loss is calculated for reporting during both charge and discharge: $$L_{inv} = (1 - \eta_{inv}) \times |P_{DC,batt}|$$

​

Step 2: ESS MV Transformer

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

​

Discharge

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

​

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: $$P_{MV,c} = P_{AC,c} + L_{MV}(P_{AC,c})$$ 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: $P_{MV,c} = L_{NL,ESS} \times P_{MV,rated,ESS}$.

​

Step 3: Combined PV+ESS Output

When charging or idle, PV output is reduced by the total power diverted to storage, $P_{MV,c}$ (which includes MV transformer loss overhead and collapses to the no-load loss when idle, per Step 2): $$P_{combined} = P_{PV,MV} - P_{MV,c}$$ When discharging, battery power is added to the full PV output: $$P_{combined} = P_{PV,MV} + P_{MV,d}$$ 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: $P_{combined} = P_{MV,d}$.

​

Step 4: Plant-Level Power Flow Re-Run

The combined output ($P_{combined}$) 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 — $P_{combined}$ 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 $P_{HV,out}$ 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:

$$P_{avail} = P_{HV,out} \times \frac{100 - f_{avail}}{100}$$

3. LGIA limit — power exceeding the interconnect capacity ($P_{POI}$, converted from MW to W) is curtailed:

$$P_{grid} = \min(P_{avail},\, P_{POI})$$ 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