Active Splitting Strategies Based on Severe Faults of Hybrid AC/DC Power Systems

Abstract

Recently, the development of the ultra-high-voltage hybrid AC/DC power systems brings not only positive effect such as large-scale resources’ optimal allocation to release the intense resource situation in China, but also negative one such as great challenges to the safe and stable operation of power systems, which brings great strength to the whole power systems. Especially with the larger scale and capacity of DC feeding to the same receiving end, one AC fault in the power system may cause cascading AC/DC failure, which may be worse cause the simultaneous commutation failure of multiple DC lines. The simultaneous commutation failure of multiple DC lines will increase the risk of large capacity loss in the power systems. To maximally guarantee the power supply of local important load, active splitting is deemed to be an effective method. In this paper, the receiving-end grid with multiple DC lines feeding to is considered. Firstly, the research results of active splitting strategies got by the researchers are listed, and the different technologies of active splitting strategies are also mentioned. Secondly, the scenarios in which transient instability of the whole power system happen in the severe AC and DC cascading failure are analyzed. One is the instability of the whole system caused by DC blocking of multiple DC lines, along with the invalid out-of-step splitting, the other is the failure of stability control device which is deal with the problem of transient instability of local power grid. To avoid the local important loads going instability with main power systems when the extreme severe faults happen, the general framework of active splitting strategies is designed. The framework is according to the specific instability characteristic of the scenarios mentioned before, and in the meantime, the trigger criteria of active splitting strategies are also proposed, as well as the chosen principles of the active splitting surfaces. When the transient instability of whole power system occurs, the active splitting strategies take responsibility for the safe and stable operation of the local important loads by splitting the decided active splitting surfaces triggered by the active splitting signals (including the capacity of DC blocking provided by the active splitting primary control stations, the signals of line voltage and power angle provided by wide-area measurement system (WAMS)), and along with the additional emergency control (usually including the load shedding) which is taking advantages of the WAMS. Finally, the active strategies are confirmed to be effective by verifying in an actual power system, and the simulation results turn out that the active splitting strategies with specific selection of active splitting surfaces and emergency control can be utilized to deal with the severe faults in ultra-high-voltage hybrid AC/DC power systems.