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Modeling and Control of Distributed Energy Systems During MODELING AND CONTROL OF DISTRIBUTED ENERGY SYSTEMS DURING TRANSITION BETWEEN GRID CONNECTED AND STANDALONE MODES A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Md Nayeem Arafat August, 2014 MODELING AND CONTROL OF DISTRIBUTED ENERGY SYSTEMS DURING TRANSITION BETWEEN GRID CONNECTED AND STANDALONE MODES Md Nayeem Arafat Dissertation Approved: Accepted: __________________________ __________________________ Advisor Department Chair Dr. Yilmaz Sozer Dr. Abbas Omar __________________________ __________________________ Committee Member Dean of the College Dr. Tom Hartley Dr. George K. Haritos __________________________ __________________________ Committee Member Dean of Graduate School Dr. Malik Elbuluk Dr. George R. Newkome __________________________ __________________________ Committee Member Date Dr. Ping Yi __________________________ Committee Member Dr. Alper Buldum ii ABSTRACT Distributed generation systems (DGs) have been penetrating into our energy networks with the advancement in the renewable energy sources and energy storage elements. These systems can operate in synchronism with the utility grid referred to as the grid connected (GC) mode of operation, or work independently, referred to as the standalone (SA) mode of operation. There is a need to ensure continuous power flow during transition between GC and SA modes, referred to as the transition mode, in operating DGs. In this dissertation, efficient and effective transition control algorithms are developed for DGs operating either independently or collectively with other units. Three techniques are proposed in this dissertation to manage the proper transition operations. In the first technique, a new control algorithm is proposed for an independent DG which can operate in SA and GC modes. The proposed transition control algorithm ensures low total harmonic distortion (THD) and less voltage fluctuation during mode transitions compared to the other techniques. In the second technique, a transition control is suggested for a collective of DGs operating in a microgrid system architecture to improve the reliability of the system, reduce the cost, and provide better performance. In this technique, one of the DGs in a microgrid system, referred to as a dispatch unit, takes the additional responsibility of mode transitioning to ensure smooth transition and supply/demand balance in the microgrid. iii In the third technique, an alternative transition technique is proposed through hybridizing the current and droop controllers. The proposed hybrid transition control technique has higher reliability compared to the dispatch unit concept. During the GC mode, the proposed hybrid controller uses current control. During the SA mode, the hybrid controller uses droop control. During the transition mode, both of the controllers participate in formulating the inverter output voltage but with different weights or coefficients. Voltage source inverters interfacing the DGs as well as the proposed transition control algorithms have been modeled to analyze the stability of the algorithms in different configurations. The performances of the proposed algorithms are verified through simulation and experimental studies. It has been found that the proposed control techniques can provide smooth power flow to the local loads during the GC, SA and transition modes. iv ACKNOWLEDGEMENTS My sincere gratitude to Dr. Yilmaz Sozer without whose ardent initiatives, constant compassionate advice and astute guidance this research work would not have materialized. Also I would like to thank Dr. Iqbal Husain for his valuable suggestions and guidelines for my research. Many thanks to all of the committee members, Dr. Tom Hartley, Dr. Malik Elbuluk, Dr. Ping Yi and Dr. Alper Buldum, for their excellent suggestions in making this research a success. The financial support of The University of Akron during my research period is also highly appreciated. I would also like to thank my parents, Md. Shafiqul Alam and Khadiza Begum, my wife, Rulia Farzana, my first son, Affan, my lab mates specially Ali Elrayyah and my sister, Jannatul Ferdous, for their invaluable love and encouragement over the years. v TABLE OF CONTENTS Page LIST OF TABLES …………………………………………………………….......... xii LIST OF FIGURES ………………………………………………………………… xiii CHAPTER I. INTRODUCTION ………………………………………………………... 1 1.1 Overview of the Renewable Energy Sources ……………………….. 1 1.2 Overview of the Distributed Generation System …………………… 3 1.3 Overview of the Microgrid System …………………………………. 5 1.4 Literature Review of Mode Transfers between GC and SA Modes .. 7 1.4.1 Literature Review of the Control Strategies for an Independent Utility Interactive Inverter during Transition Operations …………... 8 1.4.2 Control Strategies of for Parallel Inverters Working in a Microgrid during Transition Modes ………………………………… 11 1.5 Modeling of the Microgrid System for Stability Analysis ………….. 16 1.6 Thesis Outline ………………………………………………………. 17 II. MODE TRANSITIONS FOR INDEPENDENT VOLTAGE SOURCE INVERTERS …………………………………………………………….. 22 2.1 Introduction………………………………………………………….. 22 2.2 Bidirectional Inverter Architecture …………………………………. 23 2.3 Components of the VSI for all Modes of Operations...……………... 24 2.3.1 Phase Lock Loop Algorithm ………………………………….. 26 2.3.2 Current Controller Algorithm ………………………………… 28 2.3.3 Design of a Current Controller Gains ………………………… 29 vi 2.3.4 Voltage Control Algorithm …………………………………… 33 2. 3. 4. 1 Modeling and Stability Analysis of a VSI operating in SA Mode ………………………………………………....... 34 2.3.5 Modulation Index ……………………………………………... 35 2.3.6 PWM Generation ……………………………………………... 36 2.4 Transition from SA Mode to GC Mode for VSI ……………………. 36 2.4.1 Trapezoidal Frequency Variation Technique …………………. 37 2.4.2 Closed Loop Frequency Variation Technique ………………... 39 2.4.3 Smooth Frequency Variation Technique ……………………... 40 2.5 Simulation Results ………………………………………………….. 42 2.5.1 Inverter Operation in GC Mode ……………………………… 42 2.5.1.1 Performance of the Current Controller Operating GC Inverter with Full Load ……………………………... 42 2.5.1.2 Performance of the Current Controller with the Effect of Hardware Delay …………………………………. 44 2.5.1.3 Performance of the Current Controller with Abrupt Change in Current Command…...…………………... 45 2.5.1.4 Performance of the Current Controller with Gradual Commanded Change ………………………………... 47 2.5.1.5 Performance of the Inverter in the Face of DC Voltage Fluctuations …………...…………………... 48 2.5.1.6 The Performance of the Inverter during Battery Charging …………………………............................. 49 2.5.2 RMS Voltage Control in SA Mode …………………………… 50 2.5.3 Operation of the VSI in Transition Mode …………………….. 51 2.5.3.1 GC Mode to SA Mode Transition …………………... 51 2.5.3.2 SA Mode to GC Mode Transition …………………... 52 2.6 Conclusion ………………………………………………………….. 54 vii III. SMOOTH TRANSITION TECHNIQUE BETWEEN SA AND GC MODES FOR VSIS OPERATING IN MICROGRID USING DISPATCH UNIT ……………………………………………………………………… 55 3.1 Introduction ………………………………………………………… 55 3.2 Microgrid System Architecture …………………………………….. 56 3.3 Control Strategy for VSIs using Dispatch Unit in Transition Mode.. 58 3.3.1 Droop Control Technique ……………………………………. 60 3.4 Modeling and Stability Analysis of the Microgrid System using Dispatch Unit ………………………………………………………………. 63 3.4.1 Modeling for VSIs Operating in SA Mode …………………… 64 3.4.1.1 The Effect of Droop Coefficients in a Microgrid System ….................................................................................... 76 3.4.1.2 The Effect of the Load Changes in a Microgrid System 78 3.4.1.3 Inductor Sizing for VSIs in a Microgrid ……………… 79 3.4.2 VSIs Operating in Transition Mode using Dispatch Unit …….. 81 3.4.2.1 Impact of the Current Controller Gains on the Stability 87 3.4.2.2 Effect of the Droop Coefficients on the Performance of a Droop Based VSIs …………………………………………... 89 3.4.3 GC Mode of Operation of the VSIs with Dispatch Unit ………. 91 3.5 Simulation Results ………………………………………………….. 98 3.5.1 VSIs Operating during SA Mode …………………………….. 98 3.5.2 VSIs Operating during GC Mode …………………………….. 103 3.5.3 Transition Mode of Operation for the VSIs using Dispatch Unit Technique………………………………………………………. 106 3.5.3.1 GC to SA during Grid Delivering the Power to the System ........................................................................................ 106 3.5.3.2 GC to SA during Grid Taking the Power from a Microgrid System ……………………………………………... 108 viii 3.5.3.3 SA to GC during the Grid Delivering Power to the System ........................................................................................ 110 3.5.3.4 SA to GC during Grid Taking Power from the System .. 114 3.5.3.5 Effect of the Load Change during Mode Transition …. 117 3.6 Conclusion ………………………………………………………….. 118 IV. SMOOTH TRANSITION TECHNIQUE BETWEEN SA AND GC MODE FOR VSIS IN MICROGRID USING HYBRID CONTROL ….. 120 4.1 Introduction ………………………………………………………... 120 4.2 Microgrid System Architecture ……………………………………. 121 4.3 Control Algorithm of the Hybrid Controller ………….…………… 122 4.3.1 Current Control ………….…………………………………… 124 4.3.2 Droop Control ….…………………………………………….. 125 4.4 Modeling and Stability Analysis of the Hybrid Mode Transition Control ……………………………………………………………... 126 4.5 Simulation Results …………………………………………………. 149 4.5.1 VSIs Operate in SA Mode using Hybrid Control Technique ... 149 4.5.2 VSIs Operate in GC Mode …………………………………... 154 4.5.3 Transition Mode of Operation for VSIs ……………………... 159 4.5.3.1 GC
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