Power distribution systems fall in the category of ill-conditioned power systems due to their various special features which make them different to analyze, when compared to transmission systems. Moreover, distribution systems are inherently unbalanced. Special three-phase load flow methods, known as Unbalanced Distribution System Load Flow (UDSLF) methods, are needed to exploit the special topological properties, as well as to overcome the difficulties arising out of the ill-conditioned nature of the distribution systems. The modeling aspect of the unbalanced distribution system has been discussed. Three established methods suitable for UDSLF have been presented in this paper in some detail, with their respective algorithms developed. The performances of these methods are compared, considering multiphase practical test distribution systems.

Load flow study is an important and basic tool and is used in both the operational and the planning stages of power systems. Certain applications, particularly in distribution automation and optimization of power systems, require repeated load flow solutions. In these applications, it is very important to solve the problem of load flow as efficiently as possible. Since the invention of digital computers in the 1950s and 1960s and their widespread use, many methods for solving the problem of load flow have been developed (Stott, 1974). Most of these methods have ‘grown up’ around transmission systems and, over the years, variations of the Newton method, such as the fast decoupled method (Stott and Alsac, 1974), have become the most widely used. The analysis of a distribution system is an important area of activity as distribution systems provide the final link between the bulk power system and the consumers. A distribution circuit normally uses primary or main feeders and lateral distributors. A main feeder originates from the substation and passes through the major load centers. Lateral distributors connect the individual load points to the main feeder with distribution transformers at their ends. Many distribution systems used in practice have a single circuit main feeder and are defined as radial distribution systems. Radial systems are popular because of their low cost and simple design. Distribution networks, because of the following special features, fall in the category of ill-conditioned power systems, for generic Newton-Raphson and fast decoupled load flow methods:

Due to the inherent unbalanced nature, distribution systems are always analyzed on three-phase basis. Thus, distribution system load flow studies are always performed on three-phase basis, with detailed modeling of the various components of the system, which includes mutual coupling between the feeders, known as Unbalanced Distribution System Load Flow (UDSLF) methods. Because of its application in many distribution system decision algorithms such as network planning, service restoration, feeder reconfiguration, state estimation, etc., distribution system load flow analysis is, nowadays, an integral component of distribution system planning, operation and control functions.

Electrical and Electronics Engineering Journal, Bus Injection to Branch Current, BIBC, Branch Current to Bus Voltage, BCBV, Matrix, Equivalent Current Injection ECI, Forward and Backward Sweep, FBS, Unbalanced Distribution System Load Flow, UDSLF, Unbalanced Distribution Networks.