Computational Techniques for Autonomous Microgrid Load Flow Analysis

This paper attempts at developing simple, efficient, and fast converging load flow analysis techniques tailored to autonomous microgrids. Two modified backward forward sweep techniques have been developed in this work where the largest generator is chosen as slack generator, in the first method and all generator buses are modeled as slack buses in the second method. The second method incorporates the concept of distributed slack bus to update the real and reactive power generations in the microgrid. This paper has details on the development of these two methodologies and the efficacy of these methods is compared with the conventional Newton Raphson load flow method. The standard 33-bus distribution system has been transformed into an autonomous microgrid and used for evaluation of the proposed load flow methodologies. Matlab coding has been developed for validating the results. 1. Introduction Aggregation of generating units and loads, at medium and low voltage levels, forms small power islands called microgrids. Most researchers concentrate on the design and control aspects of these microgrids with respect to the resource availability and dispatchability of power to the loads. Design issues, generation planning, and economic dispatch in an autonomous microgrid need dedicated and robust power flow computations. Load flow analysis of an autonomous microgrid is necessary for ascertaining the adequacy of supply from DGs without compromising the voltage profile and to determine the state of the system. Different load flow techniques adopted in the literature are classified into three categories, namely, direct methods, Newton Raphson (NR) based methods, and backward forward sweep based methods. Direct methods involve impedance matrix where the numbering of nodes and lines decides the efficacy and convergence criteria. Chen et al. proposed a rigid power flow method based on series impedance model [1] and Carpaneto et al. suggested a loss allocation technique, based on decomposition of the branch currents [2]. Tedious computation is the basic drawback in these methods. NR method was used in certain methodologies to determine the bus voltages and power flows in distribution networks. A modified Newton method had been discussed by Zhang and Cheng [3] and as an extension Teng and Chang [4] suggested a novel fast three phase load flow analysis for unbalanced radial distribution systems. Bijwe and Kelapure [5] proposed a nondivergent load flow analysis based on NR method. NR-based method was extended to unbalanced systems by Zimmerman and Chiang [6] and further