The case that HVDC system is connected to the strong AC network has little the considering problems. However, because there are many problems(example for voltage stability, inertia and low frequency resonance) in case of connecting the weak AC system, a detailed attentions are needed. In particular, because HVDC system can control the flow of AC network freely, have to contain the range of stability and safe operation range. This paper deals with HVDC control algorithms, which are operated within 1 second on viewpoint of time domain, that is, transient stability. The target of HVDC introduced in this paper is Jindo-Cheju HVDC system, which will be operated at 2011. And the introduced algorithm can be actually implemented to Jindo-Cheju HVDC system.