Why are DC high-voltage transmission lines replacing AC high-voltage transmission lines at some places ???

         With AC systems the peak voltage is 2^0.5 (1.4142) times the RMS (nominal) voltage.  At transmission voltages that additional .4142 x voltage makes the insulation systems much more critical, lengthening the insulator strings and so increasing the cost of the insulators and poles required to separate the lines.

With DC systems the peak voltage is the nominal voltage, so the insulator strings need only deal with the nominal voltage reducing capital and maintenance costs (washing from helicopters etc.).

With very high voltage transmission where maximum voltage is limited by available technology, this fact is used to increase the nominal voltage on a given insulation type, resulting in a system which would be operated at 750,000 volts nominal (RMS) AC being used for 1,000,000 volts actual DC with 25% additional capacity for the same amperage (cable) design.

The other factors are

1. In HVDC no corona loss is there as incase of HVAC.
2. For longer distance,HVDC system is very economical as compared to the HVAC system.
3. Allowing power transmission between unsynchronized AC distribution systems.
4. HVDC increases the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
5. For  DC  frequency is 0.Therefor there is no inductive reactance drop, result of it is improve voltage regulation.
6. There is no problem of stability as in AC.
7. There is no skin effect in DC.
8. AC require 3 wire for transmission but DC require only 2 .
9. There is no ferranti effect in DC{it doesn't  mean that capacitance is absent in DC,capacitance act as open CKT for DC}