Abstract
Recently the number of distributed generators (DG)being integrated in to the distribution system has increased. Thedistribution system is no longer a single source system. Thusthere has to be a change in strategy for distribution analysis. Thiswould require a fast power flow solution with DG. This paperpresents the developed three phase unbalanced power flowalgorithm with the choice of modeling DG as PQ or PV node.This gives flexibility in modeling various types of DGs. Thedistribution system model includes modeling of lines, cables,transformers, switches, capacitors, loads and DGs. IEEE testcases are designed for testing the three phase unbalancedalgorithm for single source systems. A DG was introduced in theIEEE 13-node test case and the results demonstrate the effect theDG has on voltage profile and currents. Some of the results wereverified using Radial Distribution Analysis Package (RDAP).Index Terms— Distributed Generation, RDAP, DistributionSystem Analysis, IEEE 13-nod
I. INTRODUCTION
Power Flow analysis is very important and basic tool for theanalysis of any power system as it is used in the planning anddesign stages as well as during the operations. Someapplications in distribution automation and distributionmanagement like VAR planning, switching, state estimationand especially optimization need repeated fast power flowsolutions. In these applications it is imperative that powerflow be calculated as efficiently as possible. Typically, adistribution system originates at a substation and continues toa lower voltage for delivery to the customers. Unlike atransmission system, a distribution system typically has aradial topological structure. The radial structure, along withthe higher resistance/reactance (R/X) ratio of the lines, meansdistribution systems are ill conditioned and hence the Fast-Decoupled Newton method is unsuitable for most distributionpower flow problems. Most of the conventional power flowmethods consider power demands as specified constantvalues. This should not be assumed because in distribution system bus voltages are not controlled. Loads are specified byconstant power, current or impedance requirements. Results ofload flow analysis include steady state voltages at all buses inthe system, real and reactive power flows in cables,transformers and loads, power losses and reactive powergenerated or absorbed on voltage controlled buses. The loadflow equations are non linear equations and therefore iterativemethods must be utilized in solving the equations.The techniques for three phase power flow analysis cannotbe developed by simply extending the balanced methods tothree phases. A three phase method has to address issues likemodeling different types of component connections,determining starting point for three phase power flow solutionas there are shifts and transformation ratios for each phase anddifferent nodes. For untransposed lines and cables thebalanced models are no longer useful. The symmetriccomponent transform cannot decouple the three phases. Thusa fully coupled impedance matrix in three phases is needed.M.S. Srinivas [1] gives a brief review of distribution loadflows. There are several power flow methods based onbackward/forward technique. They may be classified as1. Current summation methods2. Power Summation methods3. Admittance summation methodsThe current summation method is more convenient and fasterthan the power summation method because it uses only V andI instead of P and Q. A lot of work has been done for radialpower flow solution [2-11].The propagation of DG in distribution systems will havemajor effects on distribution studies. The impact of DG onthe existing system must be studied because of its significantcontribution in the power flow and short circuit studies. Thereare currently less contributions to study the impact DG willhave on distribution systems, especially unbalanceddistribution systems. Handling of PV nodes for three-phaseunbalanced system has been proposed by Cheng andShirmohammadi [12]. While reference [12] provided someinitial discussions on a PV node concept in unbalanced powerflow, our work provides a detailed description of the PV nodemodel integration into the unbalanced distribution power flow.Our work extends the concept comparing the results from thePQ versus PV node models.The results are shown for an IEEE distribution system testcase. This test case was developed for radial distributionsystem analysis with one source node and hence there are nodistributed generators. Thus, DG was added into the systemand its effect on the test case system is illustrated.



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