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  5. TRANSFORMER SIZING AND IMPEDANCE

Inverter Step-Up Transformer

Inverter Step-Up Transformers are assumed to be pad-mount, oil-filled, two or three-winding (depending on inverter connection), 65C AWR transformers. The winding configuration is currently not considered in the transformer design or cost. The X winding (LV) voltage rating is assumed to be equal to the inverter AC output voltage. The H winding (HV) rating is equal to one of the following:

  • the user defined interconnection voltage (if interconnecting at medium voltage)
  • 34.5kV for projects in Imperial Units (if the system is interconnecting at high voltage)
  • 33kV for projects in Metric Units (if the system is interconnecting at high voltage)

The MVA rating is set equal to the sum of inverter rated maximum power outputs for all inverters supplying the transformer. Additionally, the following transformer characteristics are assumed:

  • Impedance (Z): 6%
  • X/R: 8
  • Core Losses: 0.13%

 

Generator Step-Up Transformer (GSU)

Winding and core losses associated with a GSU transformer are only included when the user defined interconnection voltage is greater than 34.5kV. The losses are estimated based on transformer MVA rating and HV winding voltage rating. The Top MVA rating is assumed equal to the sum of inverter rated maximum power output ratings. This assumption is reasonable for the purposes of estimating losses even though the actual transformer MVA rating may be as much as +/- 10% of this value. The following ranges of transformer voltages and MVA ratings are available within HST’s transformer database:

 

  • Voltage Range: 69kV – 500kV
  • Top MVA Rating Range: 10MVA – 300MVA
  • Base MVA Rating Impedance Range: 8% – 14.5%
  • X/R Range: 15.5 – 50
  • Core Loss Range: 0.1% – 0.8%

 

Note: Inverter Step-Up Transformers within the plant are included in the Bill of Materials and affect total installed system cost. GSU Transformers are only sized as a means to estimate losses during energy simulation and are not included in the system cost. The costs for interconnection facilities including GSU Transformers will vary based on factors currently outside the scope of this software. The user should include costs associated with these interconnection facilities within the appropriate financial inputs.

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