Operation strategy of tree bilateral interconnected power supply system and the negative-sequence effect on external power supply

The tree bilateral interconnected power supply system can realize long-distance interconnected power supply without electrical phase separation, which improves the utilization rate of regenerative braking energy of the system. When the tree bilateral interconnected power supply scheme is only composed of single-phase connection transformers, the three-phase voltage unbalance generated by the train at the common connection point will be aggravated. Once the negative sequence exceeds the standard, it will face adverse effects caused by the negative sequence. In order to ensure the good operation of the system and avoid the adverse effects of negative sequence, it is necessary to evaluate the negative sequence of the system. The tree bilateral interconnected power supply system and the operation strategies were studied which including the system using single-phase transformer and the system using combined co-phase power supply device. A local control strategy was proposed for the method of using co-phase power supply device. The three-phase voltage unbalance at the point of common coupling was used as the evaluation index to evaluate the negative sequence influence of the tree bilateral interconnected power supply system on the external power supply, and negative sequence evaluation model of traditional unilateral power supply system based on Vv connection transformer, the tree bilateral interconnected power supply system composed of single-phase connection transformers and the tree bilateral interconnected power supply system composed of combined co-phase power supply devices were constructed respectively. Taking a line as an example, based on the negative sequence evaluation model constructed, the threephase voltage unbalance degree of the system after adopting the tree bilateral interconnected power supply scheme was evaluated, and compared with the results of the existing power supply scheme. After the tree bilateral interconnected power supply transformation scheme of the co-phase power supply was adopted, the maximum value of the three-phase voltage unbalance was 1.65%, and the maximum value of the 95% probability was 1.20%, which were all in line with the national standards. The results show that the negative sequence problem has been greatly improved under the tree bilateral interconnected power supply scheme, which meets the requirements of national standards, and also shows the effectiveness of the operation strategy of the co-phase power supply device in this paper. © 2023, Central South University Press. All rights reserved.