Convergence Analysis On The Power Flow With Small Impedance

Along with national economy's development,the electric power is getting more and more important.Power system's security and reliability service and its analysis have been the electric power scholars' research hot spots.The power flow which is applied to this research is most widespread,is most basic and most important electrical operation. Because there are some ill-conditions in the power system,which often cause the power flow unsolved,or make the power flow divergent with the conventional methods,for example,there is frequently divergent of power flow in the systems with small impedance.Based on the analysis of the effect of small impedance branches on convergence of Newton load flow,three effective methods named zero load flow of choosing initial voltage,varied-jacobi Newton and fast decoupled power flow dealing with the divergence were briefly introduced.Proceeding from three methods' power mismatch equations,by contrastive analyzing the correction value of voltage magnitude and phase angle at both ends of small impedance branches,three methods' similarities and differences on the convergence mechanism has been studied.It is found that the active power mismatch equations of three methods are equivalent,for the reactive power mismatch equations,the jacobi-matrices are equivalent,but the right vector is different. Thus it can be seen,the key factor that affects the convergence of power flow with small impedance branches is the jacobi-matrices,But the right vectors or the calculated powers have less influence on convergence of power flow with the small impedance branches.The practical examples confirm the validity of conclusion.Based on the thorough research of the backward/forward sweep(BFS) method for distribution networks,three situations,as the small impedance line,the small impedance transformer with standard equivalent model and the small impedance transformer with Pi-equivalent model that influence the BFS method for distribution networks have been analyzed.The analysis indicated that the small impedance line cannot cause adverse effect on convergence of the BFS method;the small impedance transformer adopts the standard equivalent model,also will not cause adverse effect on convergence of the BFS method,but adopts Pi-equivalent model,will affect the convergence of the BFS method,the degree of influence depends on the calculation method of the power loss.Therefore,it is very important to choose the transformer model when analyzing the power flow of distribution network with BFS method.To adopt the standard equivalent model may guarantee the convergence of the BFS method for distribution network.