High voltage transmission systems has a dual purpose, i.e. system interconnection and bulk-energy transfer. With reference to system interconnection, the need to operate the whole system in perfect synchronism often prevents the transfer of power by alternating current.
The reasons are following:
The economic power ratings of such interconnections are often small in relation to the installed capacity of the systems to be interconnected. In such cases an AC tie line may not be able to cope with the power flow and stability control problems.
An alternative DC interconnection provides a fast and flexible power flow control, regardless of the conditions in the AC systems, and can provide stability improvement for the two interconnected systems.