General Cable Selection and Sizing
The following design assumptions are made with regard to cable selection and sizing:
- All string conductors are Copper.
- All DC Feeder and AC Feeder conductors are Aluminum.
- All insulated conductors have an insulation temperature rating of 90C.
- All conductors are single conductor construction.
- All String conductors are installed in conduit when installed between rows.
- All DC and AC Feeders are direct buried.
- All gen-tie lines are installed overhead.
- The minimum conductor sizing for gen-tie lines at voltages greater than or equal to 115kV is 795kcmil.
Note: DC and AC circuits within the plant are included in the Bill of Materials and affect total installed system cost. Gen-Tie line circuits 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 Gen-Tie lines vary significantly 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.
The following table lists all cables currently available within the software for systems designed in imperial units:
|Circuit Type||Cable Construction||Size Range|
|DC String||1/C, 600V, Copper, Type PV, XLPE, 90C||#12 AWG – #8 AWG|
|1/C, 2kV, Copper, Type PV, XLPE, 90C||#12 AWG – #8 AWG|
|DC Feeder||1/C, 600V, Aluminum, Type PV, XLPE, 90C||#6 AWG – 1000kcmil|
|1/C, 2kV, Aluminum, Type PV, XLPE, 90C||#6 AWG – 1000kcmil|
|AC Feeder||1/C, 5kV, Aluminum, TRXLPE, 90C, 133% Insulation||#2 AWG – 1000kcmil|
|1/C, 15kV, Aluminum, TRXLPE, 90C, 133% Insulation||#2 AWG – 1000kcmil|
|1/C, 25kV, Aluminum, TRXLPE, 90C, 133% Insulation||#1 AWG – 1000kcmil|
|1/C, 35kV, Aluminum, TRXLPE, 90C, 133% Insulation||#1/0 AWG – 1000kcmil|
|Gen-Tie||Bare, Overhead ACSR||397kcmil – 2167kcmil|
The following table lists all cables currently available within the software for systems designed in metric units:
|Circuit Type||Cable Construction||Size Range|
|DC String||1.5kV, Copper, Solar PV Cable, XLPE||1.5mm^2 – 6mm^2|
|DC Feeder||1.5kV, Aluminum, Solar PV Cable, XLPE||10mm^2 – 500mm^2|
|AC Feeder||3.8/6.6 (7.2)kV, Aluminum, XLPE, Single Core, 90C||35mm^2 – 500mm^2|
|8.7/15 (17.5)kV, Aluminum, XLPE, Single Core, 90C||35mm^2 – 500mm^2|
|12.7/20 (24)kV, Aluminum, XLPE, Single Core, 90C||35mm^2 – 500mm^2|
|19/33 (36)kV, Aluminum, XLPE, Single Core, 90C||70mm^2 – 500mm^2|
|Gen-Tie||Bare, Overhead ACSR||200mm^2 – 1100mm^2|
Sizing of DC and AC Circuits Within the Plant
DC and AC circuits within the plant are sized in accordance with the NEC. HST software follows the following basic steps:
Step 1: Calculate the maximum continuous design current that will flow through each circuit.
Step 2: Calculate the the applicable ampacity derate factors as follows:
- Air Ambient Temperature Correction Factor (if installed above ground)
- Soil Ambient Temperature Correction Factor (if installed below ground)
- Soil Thermal Resistivity Adjustment Factor (if installed below ground)
- Number of Conductors Adjustment Factor (if installed in conduit with multiple circuits)
- Grouping Adjustment Factor (if installed in same trench as other circuits)
Step 3: Look up the conductor size from the applicable NEC table and select the smallest conductor that exceeds the maximum continuous design current after derate factors have been applied.
Step 4: Verify the 75C conductor rating without any derates exceeds the maximum design current (if circuit is less than or equal to 2kV). If it is not, increase cable size until this condition is met.
Sizing of Gen-Tie Circuits
AC gen-tie circuits between the PCC and POD (when modeled) are conservatively sized using reasonable industry assumptions. Generally, overhead transmission lines, especially high-voltage overhead transmission lines are oversized for the current they have to carry. This is partially done to minimize losses but also to prevent future upgrades if circuit requirements change as well as for other reasons related to the mechanical properties of the conductor and the span lengths between towers, wind speed, etc. It is beyond the scope of HST software to automatically design overhead transmission lines as this is a complex field. However, for the purposes of estimating losses, HST software does estimate the size of the overhead gen-tie lines using published manufacturer ACSR ampacity tables along with the maximum continuous design current that will be flowing through the gen-tie circuit and the interconnection voltage of the gen-tie circuit. Additionally, a 25% safety factor is applied to the design current to slightly increase the conductor size to a more reasonable size. It is reasonably likely that the final as-built gen-tie line will use an even larger conductor size than estimated by HST, but HST’s approach results in conservative loss assumptions for energy modeling which is considered appropriate for early stage project feasibility and development modeling