The Analysis And Improvement Of Fast Decoupled Load Flow For System With Small Impedance Branches

Fast decoupled load flow is used to improve and simplify the power flow of Newton method under polar coordinates. Since in fast decoupled load flow the dimension of coefficient matrix B’ and B" which belongs to the correcting equation is a constant and relatively low, it makes power flow have faster speed and better convergence.With the growing complexity of power system, the possibility of appearing a morbid system become greater.System containing small impedance branch is a common condition in pathological system. When dealing with this kind of branch system, because of different branch impedance values, fast decoupled load flow is different in convergence for it. For small impedance branch system in which reactance is much greater than resistance, fast decoupled load flow has good convergence, while for small impedance branch system in which reactance and resistance is quite or resistance is far greater than reactance, its convergence is not good.Through the theoretical analysis to the original fast decoupled load flow, the important reasons that will lead to the misconvergence of original fast decoupled load flow can be got. First, node types at both ends of small impedance branch are different. Second, branch parameters are different (the value of the resistance and reactance, whether transformer exists or not). The two kinds of factors will lead to the final result which can’t satisfy the related relation of convergence, and makes the system achieve better convergence.To solve the above problems, on the basis of detailed theoretical analysis, conventional fast decoupled load flow is proposed. The specific improvements are as follows. First, it is to improve the sequence of iteration so that it can get the two conditions that convergence needs, which are that the variation of voltage phase angle on both ends of small impedance branch are equal and the variation should be small, besides, the voltage amplitude on both ends are proportional. Second, the related elements of coefficient matrix B’ of non-standard ratio of side in transformer branch with small impedance divided by the ratio k. Through detailed theoretical derivation, it can be concluded that when x>>r2, the improved fast decoupled load flow can converge in some conditions that the original fast decoupled load flow can’t converge (these conditions include small impedance branch systems in which resistance and reactance were quite or resistance is far greater reactance). Besides, the actual example analysis is also consistent with theoretical analysis. At the same time, example verification results also shows that for small impedance branch system in which reactance is far greater than resistance, its convergence is also improved.