Hi Kaiyan,
Sorry to take so long getting back to you! I missed a batch of emails while I was truing to meet a deadline and didn't see them till now.
Switch has three ways of calculating the cost of transmission lines for new power plants.
You can add a gen_connect_cost_per_MW in generation_projects_info.csv. This usually is used to represent the cost of connecting a new generator to the grid, including substation upgrades, tie lines, etc.
You can specify a cost per MW of additional transfer capability when using the transmission.transport module. This is usually used to represent the cost of increasing transfer capability from one region to another to allow more imports or exports.
You can also specify a cost for local transmission and distribution capacity when using the transmission.local_ts module. This is meant to represent the cost of adding transmission and distribution capacity within the zone to move power from the high-voltage network to customers. This cost is specified per kW of peak demand in the zone, and can be partly avoided by increasing use of distributed generation instead of utility-scale generation.
Switch doesn't calculate any of these coefficients for you - you have to provide them as input data. But if you do provide them, it will take account of these costs when deciding where to build new generation, and it will report the costs in its output.
I hope this is still useful. Let me know if you have any more questions.
Matthias