Ultra-high voltage DC（UHVDC） system plays an irreplaceable role in long-distance and large-capacity power transmission. However

it also brings new challenges and difficulties to the safe operation of power system. To provide reference and guidance for the future development and application of UHVDC

based on the practice and analyses of UHVDC components in China Southern Power Grid（CSG） and some experience

the impact of UHVDC to existing power networks is analyzed

some typical technical issues are summarized. The main risks of networks brought by UHVDC include the lockdown of heavily-loaded UHVDC system

the possible continuous commutation failure or lockdown induced by the operation failure of protection devices or switches when there is fault at the load center of receiving end. According to these issues

several measures were applied

such as optimizing the location of UHVDC and the scale of single UHVDC

optimizing the AC network structure

and increasing the dynamic reactive power compensation ability of receiving networks. These measures have been proved to be effective by transient electromagnetic simulation which is able to accurately simulate the commutation process of DC systems in CSG. Several typical technical problems occurred in operation and construction of UHVDC in CSG are presented

such as typical faults in ±800 k V system with the valve groups in asymmetric operation

the bipolar blocking of UHVDC system

the extreme faults of multi-infeed HVDC system

and the difficulties in the location of the DC grounding system. Analysis and advice of countermeasures is given. In the end

it is suggested that the voltage source converter based HVDC（VSC-HVDC） technology is effective for instability at receiving networks caused by multi-infeed HVDC

enhancing the controllability of AC system

and will play an essential part in the development of future power system.