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Summary

Plant-Level AC Losses account for power dissipation between the block-aggregated AC output and the point of interconnection (POI). The loss chain proceeds in sequence: HV equipment (transformers and transmission lines in user-defined order), availability loss, and grid limit (LGIA). The result is the final power delivered to the grid.

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Inputs

NameSymbolUnitsDescription
Block PowerPblockP_{block}WAggregated block-level AC power (from Array-Level AC Losses)
HV Transformer RatingSHVS_{HV}kVAHV transformer nameplate capacity
HV No-Load LossLNLL_{NL}—HV transformer no-load loss fraction
HV Full-Load LossLFLL_{FL}—HV transformer full-load loss fraction
Transformer High-Side VoltageVHSV_{HS}VLine-to-line voltage on transformer high side
Transmission Line Lengthℓ\ellmConductor length
Transmission Line ResistanceRlineR_{line}mΩ/ftConductor resistance per unit length
Conductors per PhaseNcondN_{cond}—Number of parallel conductors per phase
Power Factorcos⁡ϕ\cos\phi—System power factor
Availability Lossfavailf_{avail}%Percentage reduction for system availability
LGIA LimitPPOIP_{POI}WMaximum allowed power at point of interconnection

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Outputs

NameSymbolUnitsDescription
Grid PowerPgridP_{grid}WPower delivered to the grid

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Detailed Description

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HV Equipment

HV equipment consists of one or more transformers and transmission lines connected in series. The user defines the ordinal (sequence) of each element. The prediction iterates through the elements in ordinal order, passing the output of each element as the input to the next.

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HV Transformer

Each HV transformer applies the shared Transformer Loss Model. The calculation is identical to the MV transformer, using the HV transformer’s specific rating and loss fractions. During nighttime disconnect (when all arrays in the plant trigger disconnect), the HV transformer no-load loss is set to zero, eliminating standby core losses.

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Transmission Line Model

Transmission line losses are calculated from the I²R dissipation in the three-phase conductors. The model uses the line-to-line voltage on the high side of the preceding transformer (or the maximum MV transformer voltage if no HV transformer precedes the line). The total line resistance in ohms is computed from the line length and per-unit resistance: Rtotal=ℓ0.3048×Rline1000R_{total} = \frac{\ell}{0.3048} \times \frac{R_{line}}{1000} where ℓ/0.3048\ell / 0.3048 converts the line length from meters to feet to match the resistance units (mΩ/ft), and dividing by 1000 converts milliohms to ohms. The three-phase line current is: I=Pin3 VHS cos⁡ϕI = \frac{P_{in}}{\sqrt{3}\, V_{HS}\, \cos\phi} The total three-phase dissipation, accounting for parallel conductors, is: Lline=3 I2 RtotalNcondL_{line} = \frac{3\, I^2\, R_{total}}{N_{cond}} The output power is: Pout=Pin−LlineP_{out} = P_{in} - L_{line} If the result is negative, power is being sourced from the grid to energize the transformers (nighttime operation).

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Availability Loss

After all HV equipment losses, a flat percentage availability deduction is applied: Pavail=PHV,out×100−favail100P_{avail} = P_{HV,out} \times \frac{100 - f_{avail}}{100} This deduction represents expected downtime due to maintenance, grid curtailment, and unplanned outages. It reduces power uniformly across all timesteps.

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Grid Limit (LGIA)

The final step enforces the capacity constraint at the point of interconnection. If an LGIA limit is configured, the output power is capped: Pgrid=min⁡(Pavail, PPOI)P_{grid} = \min(P_{avail},\, P_{POI}) The LGIA limit can be specified as:
  • Constant value: a fixed MW cap applied to all timesteps.
  • Time series: a timestep-varying MW limit loaded from a time series input.
  • Off: no cap is applied (Pgrid=PavailP_{grid} = P_{avail}).
The loss due to grid curtailment is reported as: LLGIA=max⁡(0, Pavail−PPOI)L_{LGIA} = \max(0,\, P_{avail} - P_{POI})

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References

  • IEEE Std C57.12.00. IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.