A Decomposition Method To Calculate Available Transfer Capability Of The Large Interconnected Power Grid

The energy sources in China are unbalanced distributed among the regional grids, the energy centers and load centers are far apart from each other. In order to meet the supply-demand balance of power in the entire country, it is necessary to implement large-capacity long-distance electric power transmission form large energy bases. Therefore, to calculate the available transfer capability (ATC) of large interconnected power grid is of great significance for high-capacity and long-distance transmission of the electricity. The methods of ATC calculation of a single system are relatively mature, however, the regional power grids of a large interconnected grid are independent of each other, the needed integrated and comprehensive data for ATC calculation is difficult to collect, and there is flow calculation convergence difficulty in ATC calculation. Therefore, there is great difficulty of calculating ATC of the large interconnected power grid directly.To solve this problem, this paper presented a decomposed calculation method to calculate ATC of the large interconnected grid. The idea of this method is calculating ATC of the interconnected grid indirectly according to ATC of its sub-regions using the studied relevant models. Here are the basic principles and calculation processes:Firstly, according to the partition of China’s natural power system, a large interconnected power system can be divided into a number of interconnected subsystems, of which ATC can be calculated through the flow calculation directly and easily; Secondly, combine every two subsystems to a sub-region according to their series or parallel connection; And then, calculate ATC of each sub-region using the ATCs of subsystems, according to the deterministic and probabilistic ATC combined calculation models; Finally, regard the sub-regional grids which ATC have been calculated as subsystems, and calculate ATC of every sub-region repeatedly according to the proposed combined models until the subsystems are merged into the studied interconnected power grid. In this way, ATC of the large interconnected power grid can be calculated indirectly by the combination of ATCs of its subsystems.In addition, in the calculation of subsystem available transmission capacity, Ward equivalent is used to deal with the external system equivalence, which can effectively solve the problem of low calculation accuracy due to the nonlinear variation of the contact line power flow when the subsystems are in complex connection.At last, this paper has used numerical simulation to study the ATC decomposed calculation method mentioned above. The simulation results have verified the accuracy of the ATC calculation method based on Ward equivalent, the correctness of the calculation principles and the feasibility of the decomposed calculation methods.