| The development of Chinese electric power industry has accessed age of wide area electric network and realized the resource allocation in greater range. Compare with AC transmission system, the HVDC transmission system possesses greater dominance in long-distance transmission system, large-scale electric network interconnection, inhibiting low frequency oscillation and prevention accident chain reaction. Research on operation, controls and power flow algorithm in HVDC system possess vital theory meaning and actual value.In the power flow solution of AC/DC systems, the problem of how to describe the transformation of HVDC system control method effectively must be considered for operation and analysis of actual power system. In this paper, the program for power flow are presented, which based on the method of equivalent power and principle of alternant decouple solving in AC/DC system. The control effect and mutual conversion of the main control modes in HVDC system are considered in this program, and proving it through the function of user program interface (UPI) of power system analysis software package (PSASP). The result shows that this algorithm can deal with transformation of HVDC system control mode caused by the change of operating conditions and are used to power flow calculation and simulation of large scale power systems.Based on research of two-terminal HVDC system, further studies on mode of connection, control method, operation mechanics and power flow algorithm of Multi Terminal HVDC (MTDC) system. The HVDC die block in PSASP only can define two-terminal HVDC system and can not define MTDC system. In this paper the function of PSASP/UPI are used and the power flow programs are written. This program realizes secondary development and application of MTDC system power flow of PSASP. Through calculation and analysis the modificative EPRI-36 system, the result shows that the UPI program possesses better accuracy and computational efficiency.
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