I am not experienced in this area, but I will tell you what I think I know.
- Do engineers use 'standard lines' (with a fixed capacity per distance) or do we have a variety of lines to choose between (with different capacity)
There are several standard voltage levels and conductor sizes used for transmission lines. In addition, more than one set of three-phase conductors can occupy one overhead support structure or underground conduit. The voltage levels of overhead lines can be identified by the insulators and spacing used. Larger diameter sizes of conductors have higher current capacity for a given operating temperature and lower voltage drop for a given distance, so the current carried by a given size may be less for longer distance lines.
- How is the need for line capacity in general determined? Do we estimate the maximum load and increase capacity to be slightly above this?
The capacity determination is done in the same general way that the capacity for a manufacturing plant is determined. A study is undertaken to determine the current power demand for the region to be served and estimate the expected growth in demand for some period of time into the future. Consideration is given to the possible use of alternative energy sources. Possibilities for future transmission level expansion are considered. Those and other factors are weighed against the cost of building various levels of transmission capacity.
- If two power grids are coupled by a DC power line, do they influence eachother in terms of stability? Why/why not?
When two power grids are coupled, each appears to the other as a large load or supply. The connection would not be made without the two grids being under the control of one operator or under an agreement between two operators in constant communication with each other. The DC interconnection facility would isolate the two from one another from phase and frequency variations, but load variations in one section would influence the other. The interconnection facility would probably reduce the influence between the two in terms of stability compared to an AC tie, but not eliminate it.