Algorithm Study And Software Development On Power System Unbalanced Three-phase Load Flow

Generally, power system is analyzed as a balanced system. However, due to manufacturing, construction and other reasons, it is impossible to keep the three phase parameters of equipments completely equal, and the load of one phase also is not exactly the same as those of other two phases. So power system exactly runs in non-symmetric state. In some areas, the asymmetry may even be very prominent. Conventional power flow programs can only solve the symmetrical power grid problem, but cannot accurately simulate and make judgment in the non-symmetric system. So it is necessary to study the algorithm of unbalanced three-phase load flow and develop the relevant program.Based on the previous research materials, algorithm and component models of unbalanced three phase load flow are studied in this paper. The author describes the selected transmission line model of the program and analyzes the converting method of the input parameters. As to the transformer, a new method of computation and implementation, in which the admittance matrix is derived in phase domain, is proposed. In this paper, the generator bus is equivalent described as four buses. Using this model, we can keep the positive sequence voltage or phase A voltage at generator terminals as specified value. Correspondingly, the author derives the new Jacobian matrix elements. Two load models, equivalent admittance matrix and PQ injection, are compared and the latter is selected to be described in detail. The paper proposes a new DC model which can be connected with the unbalanced AC system. According to the commutation voltage and dc parameters, the DC power can be obtained directly. The load flow algorithms in phase domain or in sequence domain are comprehensively studied, and the former is chosen to be implemented finally. The Newton-Raphson solution is adopted in calculation, and the concrete application of the algorithm combined with the actual models is described in detail. In the program, modular programming and sparse storage are adopted. Besides, the author uses semi-dynamic method (T-2) to optimize the order of nodes and uses Gaussian elimination to solve linear equations in the program.Finally, a balanced system and an unbalanced system are taken as examples to test and verify the correctness of the program. Compared with the computation results of PSASP and EMTP, the program in this paper is proven that it can meet the requirement of design, and can be used to simulate both balanced and unbalanced three phase power system.