Power Flow Model And Method Of Three-Phase Four-Wire Distribution Networks

Three-phase four-wire distribution network is at the end of the power system network,and is also the largest one among all voltage level networks. The power flow analysis is the foundation of the distribution network planning, operation and quality control. It is necessary to consider not only asymmetry of load and structure in the distribution network, but also the effects of the iterative earth, the neutral and the ground wires on the power flow. Thus, the computation of the power flow for the three-phase four-wire distribution network is very complicated. Currently, research in this area is not perfect enough.In this paper,the power flow model and algorithm are studied in detail, fully considering the characteristics of the structure and the load in three-phase four-wire distribution network. The main contents are as follows:(1)In this paper, an exact power flow model of three-phase four-wire distribution network are established. In the model, not only the three-phase wires, the neutral and the ground wires are considered; but also the mutual inductance among the wires, the shunt capacitance and the multi-grounded are taken into account. Some new concepts, such as geographical node, self-admittance matrix and mutual inductance matrix of geographical node, are presented. Based on the geographical terminal, this paper forms the node admittance matrix and establishes the current injection equation. Since the approach to the formation of symmetric system node admittance matrix can be used to form the node admittance matrix, it is convenient to set up power flow equation of three-phase four-wire distribution network.(2)Considering three-phase four-wire, three-phase three- wire, two-phase and three- wire, and single-phase two-wire, as well as single-phase load, two-phase load, wye and delta connected three-phase load, this paper corrects the node admittance matrix and the node current injection equation. Based on them, this paper implements backward/forward sweep algorithm and Newton-Raphson method for power flow of three-phase four-wire distribution network. Bcause of the accurate power flow model, the results of the power flow have a better reference value. Moreover, the two methods can be flexible to deal with various connection types and various load types in distribution network, so that they are very useful in practice.(3)In this paper two standard examples and an actual example, which are IEEE 13-bus distribution network, IEEE 123-bus distribution network and a real 111 nodes distribution network, are simulated and their power flow distribution is computed by the above two methods. Through the detailed simulation and analysis of the effects of the ground wires, grounding resistance and the number of the iterative earth on the power flow of the distribution network, a lot of important reference valuable results are obtained. Meanwhile, this paper also simulates and analyses the effects of the above factors on the convergence of the two methods. This paper points out that the meshes, among the grounding branches, the neutral wires and the ground wires, have a major effect on the convergence of backward/forward sweep algorithm. With grounding nodes being increased or grounding impedance being reduced, the convergence of backward/forward sweep algorithm becomes worse. Comparatively, the Newton-Raphson method has better convergence, and could deal with loops more easily. The more iterative earth or the smaller grounding impedance, the better convergence Newton-Raphson method has.