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Transformer Tests UsingMATLAB/Simulink andTheir Integration IntoUndergraduate ElectricMachinery Courses SAFFET AYASUN, 1 CHIKA O. NWANKPA 2 1 Department of Electrical and Electronics Engineering, Nigde University, Nigde 51100, Turkey 2 Department of Electrical and Computer Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Received 15 March 2005; accepted 29 December 2005 ABSTRACT: This article describes MATLAB/Simulink realization of open-circuit andshort-circuit tests of transformers that are performed to identify equivalent circuit parameters.These simulation models are developed to support and enhance electric machinery educationat the undergraduate level. The proposed tests have been successfully integrated into electricmachinery courses at Drexel University, Philadelphia, PA and Nigde University, Nigde, Turkey. ß 2006 Wiley Periodicals, Inc. Comput Appl Eng Educ 14: 142 À 150, 2006; Published online in WileyInterScience (www.interscience.wiley.com); DOI 10.1002/cae.20077 Keywords: transformers; education; software laboratory; MATLAB/Simulink INTRODUCTION Electrical machinery courses at the undergraduatelevel typically consist of classroom and laboratorysections. The classroom section generally covers thesteady-state operation of transformers in which theper-phase equivalent circuit is used to computevarious quantities such as losses, efficiency, andvoltage regulation. The laboratory section includesopen-circuit, short-circuit tests conducted to deter-mine no-load losses and equivalent circuit parameters,and load test to study transformer performance undervarious loading conditions.The Electrical and Computer Engineering (ECE)Department of Drexel University offers a pre-juniorlevel machine course (ECE-P 352 Electric MotorControl Principles) that concentrates on the funda-mentals of electromechanical energy conversion andrelated control theory. This 5-h course, required for Correspondence to S. Ayasun (
[email protected]).Contract grant sponsor: U.S. Department of Energy; contractgrant number: ER63384. ß 2006 Wiley Periodicals Inc. 142 those who are in the power and control track, has bothlecture (3 h) and laboratory (2 h) sections that must betaken in the same quarter. Similarly, the Departmentof Electrical and Electronics Engineering at NigdeUniversity offers a 5-h (3-h lecture and 2-h laboratorysections) junior-level machinery course, EEM 308Electric Machinery, which mainly focuses on trans-formers and induction motors. The authors’ ex-perience while teaching electric machinery andtransformers at Drexel and Nigde Universities indi-cates that students generally have difficulty when theycome to the laboratory to carry out transformerexperiments even though the corresponding theory isextensively covered in the classroom section with adetailed hand-out describing laboratory facilities andthe procedure of the experiments, given to them atleast a week before the laboratory. Students are notfamiliar with such a laboratory environment thatcontains large machines, transformers; and relativelycomplex measurement methods, and devices ascompared with other laboratories they have been tobefore. The time constraints during the laboratoryexercise are also a difficult adjustment. In a usual 2-hlaboratory section, students are required to set up andperform three transformer experiments, to take thenecessary measurements, and to investigate steady-state performance of the transformer under variousloading conditions. Because of the time limitations,students often rush through the experiments in orderto finish them on time, which unfortunately preventsthem from getting a true feeling of transformeroperation and from appreciating what has beenaccomplished during the laboratory practice.In order to prepare students for physical experi-ments and to give them insight into the experimentalprocedure, a software laboratory is designed anddeveloped. It includes simulation models of transfor-mer, induction motor, and DC motor experiments.This article presents simulation models of openand short-circuit tests of transformers as a part of the software laboratory. The simulation modelsare developed as stand-alone applications usingMATLAB/Simulink [1] and SimPowerSystems tool-box [2]. As will be discussed later in this article, forthe load experiment, students are required to write acomputer program using MATLAB’s M-file program-ming for the equivalent circuit to compute operatingquantities or to modify Simulink model of the no-loadtest by adding a load for simulating load test. Thisassignment improves students’ programming skillsthat would be helpful in other classes as well.Such an approach to enhance the instruction of transformers and induction machines has been sug-gested and employed by various educators [3 À 7].Various software tools such as MATLAB andMathCad have been used to model the steady-stateor transient operation of transformers [5] and induc-tion motors [6,7], which mainly improves the class-room lectures. The authors’ approach [8] differs fromthose in the way that the computational laboratory asa part of laboratory experiments provides studentswith simulation models of physical experiments to beconducted in an actual laboratory environment. Theproposed simulation models developed by usingSimulink/SimPowerSystem toolbox enhances labo-ratory experience by providing students with theopportunity to verify results of laboratory experimentsand compare them with those obtained by computersimulations. Such a comparison opportunity helpsstudents realize the limitations of hardware experi-ments and, as a counterpoint,appreciate that computersimulations cannot substitute for actual hardwareexperiments as they might not exactly represent theoperation of transformers because of some modelingassumptions. Moreover, an undergraduate electricmachinery course that integrates up-to-date computerhardware and software tools in both lecture and labo-ratory sections also meets the expectations of today’sstudents who want to use computers and simulationtools in every aspect of a course, and thus, possiblyattracts more students.The remainder of the article is organized asfollows: Section II describes simulation models of no-load and short-circuit tests together with necessarytheory. Section III compares the equivalent circuitparameters determined using simulation data withthose obtained from experimental data. Section IVexplains how these simulation models are integratedinto undergraduate electric machinery courses atauthors’ institutions, and Section V presents a surveydesigned to evaluate whether simulation models werehelpful in understanding the transformer theory andexperiments while the last section concludes thearticle. TRANSFORMER TESTS: EXPERIMENTAL SETUPS AND SIMULINK MODELS The steady-state operating characteristics of transfor-mers are investigated using an equivalent circuit asshown in Figure 1 [9,10]. In this circuit, R 1 and X l1 represent the primary winding resistance and leakagereactance; R 2 and X l2 denote the secondary windingresistance and leakage reactance; R C resistance standsfor core losses, X M represents magnetizing reactance;and a denotes the transformer turns ratio. Theequivalent circuit is used to facilitate the computation TRANSFORMER TESTS USING MATLAB/SIMULINK 143 of various operating quantities such as losses, voltageregulation, and efficiency. The parameters of theequivalent circuit can be obtained from the open-circuit and short-circuit tests. In the following, theexperimental setup and Simulink/SimPowerSystemsmodel of each test are described.The SimPowerSystems toolbox is a useful soft-ware package to develop simulation models for powersystem applications in the MATLAB/Simulink envir-onment. With its graphical user interface andextensive library, it provides power engineers andresearchers with a modern and interactive designtool to build simulation models rapidly and easily.MATLAB and Simulink/SimPowerSystems havebeen widely used by educators to enhance teachingof transient and steady-state characteristics of electricmachines [5,7,11,12], modeling of power electronicconverters [13], power system transient stability andcontrol [14 À 16]. Other commercial software pack-ages, such as Maple and MathCad, are commonlyused in electrical engineering education with theiradvantages and disadvantages [17,18]. The reason thatMATLAB with its toolboxes was selected is that it isthe main software package used in almost all under-graduate courses in the authors’ institutions as acomputation tool to reinforce electrical engineeringeducation. Therefore, students can easily accessMATLAB, and they already have the basic program-ming skills to use the given Simulink models and towrite computer programs when required beforecoming to the machinery class. Open-Circuit Test The open-circuit test is performed in order todetermine exciting branch parameters (i.e., R C and X M ) of the equivalent circuit, the no-load loss, the no-load exciting current, and the no-load power factor. Asshown in Figure 2, while one of the windings is open-circuited (in our case secondary winding is open-circuited), a rated voltage is applied to the otherwinding (in our case it is the primary winding), andthe input voltage, V OC ; input current, I OC ; and inputpower, P OC , to the transformer are measured.Figure 3 shows the Simulink/SimPowerSystemsrealization of the open-circuit test. A single-phasetransformer model whose equivalent circuit para-meters could be specified using transformer dialogbox is used. A single phase AC voltage source isapplied to the primary side. Since in Simulink environment, all elements must be electrically con-nected, the secondary side of the transformer cannotbe left open and a load has to be connected. In orderto simulate no-load condition, constant impedancemodel to reflect loading is used, and the resistance andinductance values are set to very large numbers whilethevalue of the capacitor is set to avery small number.The resulting secondary current will be approximately Figure 1 Equivalent circuit of a transformer referred to the primary side. Figure 2 Experimental setup of the open-circuit test. 144 AYASUN AND NWANKPA zero. On the primary side, current, and voltagemeasurement blocks are used to measure the instan-taneous current and voltage. The output of each meteris connected to a root mean square (rms) block, signalrms , to determine the rms values of primary currentand voltage. The rms block computes the rms value of its input signal over a running window of the one cycleof the fundamental frequency. The display boxes readthese rms values of the open-circuit current, I OC , andvoltage, V OC . The outputs of the current and voltagemeasurement blocks are connected to a power mea-surement block, power measurement , that measuresthe active power, P OC , and reactive power, Q OC , of theprimary side. The output of this block is connected toa display box to read P OC and Q OC . In order tomeasure the secondary current, which is approxi-mately zero, a current measurement block with an rmsblock and display is used.These measurements either from experiment orfrom simulation enable the approximate computationof the resistance R C and reactance X M of the excitationbranch referred to the primary side. The magnitude of the excitation admittance from the open-circuitvoltage and current is computed as Y E j j¼ G C À jB M ¼ I OC V OC ð 1 Þ where G C is the conductance of the core-loss resistorand B M is the susceptance of the magnetizinginductor. The phase angle of the admittance can befound from the knowledge of the power factor. Theopen-circuit power factor, PF OC , is given by PF OC ¼ cos y ¼ P OC V OC I OC ð 2 Þ Once the power factor angle y is known, R C and X M can easily be computed as follows: G C ¼ Y E j j cos ; R C ¼ 1 G C and B M ¼ Y E j j sin ; X M ¼ 1 B M ð 3 Þ Short-Circuit Test The short-circuit test is conducted by short-circuitingthe secondary terminal of the transformer, andapplying a reduced voltage to the primary side, asshown in Figure 4, such that the rated current flowsthrough the windings. The input voltage, V SC , current, I SC , and real power, P SC , are measured.Figure 5 shows the Simulink/SimPowerSystemsimplementation of the short-circuit test. This model isalmost the same as simulation model of open-circuittest shown in Figure 3. The only difference is thatsecondary side is short-circuited. Several measure-ment blocks are used to obtain short-circuit realpower, voltage, and current. The value of the ACvoltage source is adjusted until the current in theshort-circuited winding is equal to its rated value.Since a reduced voltage is applied to the primarywindings, a negligible current flows through theexcitation branch. Ignoring this current, the magni-tude of the series impedance referred to the primaryside of the transformer can easily be computed as Z eq ¼ Z SC j j¼ V SC I SC ð 4 Þ Neglecting the core loss at the low value of V SC , the Figure 3 Simulink/SimPowerSystems implementation of the open-circuit test. TRANSFORMER TESTS USING MATLAB/SIMULINK 145
