Abstract This paper describes a new approach for modelling of automatic voltage regulator in the forward/backward sweep-based algorithms for unbalanced radial distribution systems. The automatic voltage regulators (AVRs) are included into the sweep based methods and tested by using two distribution test systems. From the studies, it can be concluded that all results are in agreement, the proposed approach is valid and reliable, and the performance of the algorithms is less affected from the inclusion of regulator modelling. I. Introduction Accurate and fast load flow analysis is an important task in power system planning and operational studies. Accordingly there are number of methods in the literature to solve this task. These methods may be listed as Newton-Raphson, Gauss- Seidel and Fast Decoupled load flow algorithms. It has repeatedly been shown that these classical methods may become inefficient in the analysis of distribution systems with high R/X ratios or special network structures [1]. Therefore, there are a number of reported studies in the literature [2-18] specially designed for the solution of power flow problem in radial distribution networks. These methods can be categorized as the Z bus based methods [2-3, 13-14], the Newton Raphson-based methods [5, 7, 9, 10, 15, 16, 17], and the Forward/ Backward sweep-based methods [4, 6, 8, 11, 12, 18]. Since it is the utilities’ responsibility to keep the customer voltage within specified tolerances, voltage regulation is an important subject in electrical distribution engineering. One of the performance criteria for a distribution system and the quality of the provided service are the maintenance of satisfactory voltage levels at the customers’ premises. However, most equipment and appliances operate satisfactorily over some ‘reasonable’ range of voltages; hence, certain tolerances are allowed at the customers’ end. Thus, it is common practice among utilities to stay within preferred voltage levels and ranges of variations for satisfactory operation of apparatus as set by various standards [19]. One of the most important devices to be utilized for the voltage regulation is the AVRs which can be operated in manual or automatic mode. In the manual mode, the output voltage can be manually raised or lowered on the regulator’s control board and it could be modelled as a constant ratio transformer in power flow algorithms. In the automatic mode, the regulator control mechanism adjusts the taps to assure that the voltage being monitored is within certain range. In distribution systems, voltages along the primary feeders are often controlled by voltage regulators. These regulators are generally auto-transformers with individual taps on their windings and must be incorporated into the load flow algorithms. Some distribution system power flow algorithms have been made to incorporate voltage regulator in manual or in automatic mode [6, 13, 16, 17]. Although the Forward/ Backward sweep-based methods are mostly used for the load flow analysis of distribution systems, only a sweep-algorithm, given in [6], incorporated AVRs to the load flow analysis. In the study, AVRs are included into the forward voltage calculation of a particular forward/backward substitution method. However the authors did not model the automatic voltage regulators for the backward voltage calculation as it is not required for their particular algorithm. In distribution load flow analysis, there are number of power flow algorithms which has backward voltage calculation such as; Ratio-Flow method [12], Ladder Network theory [18]. In this paper, the modelling of voltage regulators in the backward/forward sweep-based power flow methods is demonstrated. In section 2, a brief description of these algorithms is provided. The implementation of the automatic voltage regulators into the sweep-based algorithms is given in Section 3. Comparisons are undertaken to verify the approach, and the results are presented in Section 4. II. Sweep-Based Algorithms Sweep-based algorithms are based on forward-backward sweep processes using Kirchooff’s Laws or making use of the well-known bi-quadratic equation which, for every branch, relates the voltage magnitude at the receiving end to the voltage at the sending end and the branch power flow for solution of ladder networks. Due to its low memory requirements, computational efficiency and robust convergence characteristic, sweep-based methods have gained the most popularity for distribution load flow analysis in recent years. Three-Phase Voltage Regulator Modeling for Forward/ Backward Sweep- Based Distribution Systems Power Flow Algorithms U. Eminoglu and M. H. Hocaoglu e-mail: u.eminoglu@gyte.edu.tr, e-mail: hocaoglu@gyte.edu.tr Gebze Institute of Technology, Department of Electronics Engineering, Kocaeli 41400, TURKEY Key words: Distribution system, power flow, sweep-based algorithm, voltage regulators