Parallel Restoration Method for AC-DC Hybrid Power Systems Based on Graph Theory

High voltage direct current (HVDC) transmission systems have been used for interconnection among multiple power grids. As a result, the conventional AC power systems are transforming into AC-DC hybrid power systems. In this paper, a parallel restoration method for AC-DC hybrid power systems is proposed. First, the effects of line-commutated-converter-based (LCC) and voltage-source-converter-based (VSC) HVDC systems on service restoration are explored. The startup characteristics of HVDC systems are analyzed. Second, an optimization model for service restoration of AC-DC hybrid power systems is developed, aiming at maximizing the generating power and minimizing the restoration time. The startup constraints of HVDC systems are incorporated into the proposed model. Third, a graph-theoretic algorithm is proposed to obtain a near-optimal parallel restoration scheme, verified by power flow calculations. An 11-bus test system is used to illustrate the effects of using HVDC systems for system restoration and a modified IEEE 39-bus test system is used to verify the effectiveness of the proposed parallel restoration method. The simulation results indicate that the proposed method can make full use of the benefits of HVDC systems, allocate cranking power flexibly during the parallel restoration, and reduce the restoration time.