Research On The Key Issues Of Grid Seamentation Strategy

Because of the high penetration of infeed dc power and the complicated ac/dc interaction of multi-infeed HVDC systems, the secure and stable operations of the grids are facing challenges. Grid segmentation can improve system stability and prevent cascading failures and large-scale blackouts. Mainly there are three key issues about the strategy of grid segmentation:1) the selection of segmentation methods,2) the development of segmentation schemes,3) the evaluation and selection for optimal segmentation scheme. Based on the questions mentioned above, this paper is to study the strategy of grid segmentation. The main work is organized as follows:(1) The physical models of grid shock absorber and fault current limiter are studied. Meanwhile the models are built in PSS/E respectively. The effectiveness of outer control loop in grid shock absorber and the ac system fault isolation effect of fault current limiter are verified by simulations in test systems.(2) The impacts on system’s performance of different segmentation methods are analyzed. First, based on the four-machine multi-infeed HVDC system, the physical mechanisms of LCC-HVDC (line-commutated converter based high voltage direct current) segmentation and VSC-HVDC (voltage source converter based high voltage direct current) segmentation methods are discussed. Second, LCC-HVDC segmentation method, VSC-HVDC segmentation method and FCL (fault current limiter) segmentation method are applied in real power grids. The impacts of three segmentation methods are compared. This study is useful for grid planners to choose the proper segmentation method.(3) An approach to develop VSC-HVDC segmentation schemes is proposed. First, based on the sensitivity method, the characteristics of multi-infeed HVDC systems are analyzed and two principles of VSC-HVDC segmentation are put forward. Second, a VSC-HVDC segmentation approach based on multi-infeed interaction factor is presented to obtain the target segmentation schemes. Finally, the effectiveness of the segmentation approach is verified through simulations of real power grids.(4) An approach to evaluate VSC-HVDC segmentation schemes is proposed. First, three major stability problems of multi-infeed HVDC systems are pointed out, and three typical evaluation indexes which correspond to the above three stability problems are introduced:multi-infeed interaction factor, multi-infeed effective short circuit ratio and frequency bias factor. Second, in order to solve this multi-objective decision-making problem, linear weighted sum model is built. Based on the stability problems, an evaluation and selecting method is established to choose the optimal VSC-HVDC segmentation scheme. Lastly, a four-machine multi-infeed HVDC system and a practical multi-infeed HVDC system are taken as examples respectively. Target VSC-HVDC segmentation schemes of each system are evaluated by the proposed method and the optimal one is selected. The simulation results verify the validity of the proposed method.