Low-Cost Current-Source 1-ph Photovoltaic Grid-Connected Inverter

Transcription

1 THE UNIVERSITY OF ADELAIDE Low-Cost Current-Source 1-ph Photovoltaic Grid-Connected Inverter by Gürhan Ertaşgın A thesis submitted in partial fulfillment for the degree of Doctor of Philosophy in the Faculty of Engineering, Computer and Mathematical Sciences School of Electrical and Electronic Engineering August 2010

2 Dedicated to my brother, Ahmet Bülent Ertaşgın iii

3 CONTENTS Abstract ix Statement of Originality x Acknowledgements xi List of Figures List of Tables Abbreviations Physical Constants Symbols xiv xix xxi xxiii xxv 1 Introduction Renewable Energy Sources Power Electronics Control Stand-Alone and Grid-Connected Inverters Grid-Connected Inverter Topologies Voltage Source Inverters Current Source Inverters Technical Requirements Total Harmonic Distortion Power Factor Literature Review of CSIs Line-Commutated CSI v

4 Contents Self-Commutated CSI Soft-Switched CSI Three-Phase CSI Research Gap Original Contributions Thesis Layout PV Array Models Background and PV Module Modelling Non-linear Model Piecewise Linear Model Rectangular Model Irradiance and Temperature Curves with Normalisations Experimental Testing Dark I-V method Conclusions Energy Storage Energy Storage Requirement Current/Voltage Ripple Calculation Effect of Energy Storage on Current/Voltage PV Power Loss Calculations PV characteristics Average Power Reduction Application Examples Summary Fundamental Analysis Proposed Concept Ideal Current-Source Grid-Connected Inverter Simulation Model Fundamental Operation of the CSI Idealised Waveforms and Analysis Selection of PV Array to Grid Voltage Ratio Effect of Energy Storage Effect of Modulation Index Effect of Irradiance Summary Low-Pass Filter Design Low-Pass Filter Design Design Criteria Filter Resonance and Damping Filter Normalisations Filter Configurations vi

5 Contents CONTENTS 5.2 Analysis of Low-Pass Filter with Proposed CSI Phase Advance Design Trade-Offs Effect of C n Variation Effect of f c Variation Effect of Q Variation Summary of Effects of Variations Summary W Inverter Simulation and Test Results Proposed Inverter Implementation Inverter Simulation Inverter Control Experimental System First CSI Prototype Verification of the First CSI Prototype Second CSI Prototype Verification of the Second CSI Prototype Performance Analysis of the Second Prototype Modulation Index and Irradiance Adjustments Total Harmonic Distortion Power Factor Efficiency Feedforward Implementation and Results Feedforward Compensation Control Proof of Feedforward Implementation Summary Design and Simulation of a Higher Power Grid-Connected Inverter Photovoltaic Array Selection of Solar Array Voltage PV Array Arrangement DC Link Inductor Required Energy Storage Assumption Switching Component Ratings Low-Pass Filter Design System Simulation Simulation Procedure Voltage and Current Waveforms Total Harmonic Distortion Power Factor Efficiency Summary Conclusion 121 vii

6 Contents 8.1 Background Key Results Future Work A DC Link Inductor Design 125 A.1 Inductor Design for 160 W CSI A.1.1 Inductor Volume Calculations A.1.2 Inductor Airgap and Windings A.1.3 Packing Factor A.1.4 Copper Loss A.2 Experimental Results A.3 Conclusion B Design Schematics and Controller Code 135 B.1 Design Schematics B.2 Microcontroller Code Bibliography 145 viii

7 Abstract This research investigates a complete analysis and investigation of a single-phase currentsource (CSI) grid-connected inverter topology that is based on a photovoltaic array as a supply and a DC link inductor acting as a constant-current source. The proposed low-cost system is implemented using an open-loop control to prove the concept. Then a well-known feedforward compensation control is implemented to achieve acceptable total harmonic distortion of the inverter output current. A single boost switch (based on a switched-mode rectifier) has a duty-cycle that is modulated sinusoidally at the mains (grid) frequency such that it produces an output current that appears a full-wave rectified sinewave that is synchronised to the grid voltage. Additionally, a H-bridge inverter circuit and a capacitive-inductive low-pass grid filter is used to unfold, filter and feed the sinusoidal output current into the grid. A number of detailed PV array models are studied and used in the simulations. The relationship between the PV array output ripple and the DC link energy storage (element) for single-phase grid-connected inverters is analysed. The balanced ripple definition is introduced to estimate the PV array output power reduction due to ripple. The proposed grid-connected CSI topology is idealised which ignores : component losses, voltage drops, PWM switching and low-pass output filter resonance effects. Normalised simulations are carried out to investigate DC link energy storage, modulation index and irradiance variation effects with an emphasis on meeting the power factor (PF) and total harmonic distortion (THD) grid requirements. The low-pass grid filter optimisation for the proposed topology is studied showing a tradeoff between the output current THD, power loss, and quality factor. A 160 W inverter is implemented and a set of comprehensive test results obtained to verify the simulations using open-loop and feedforward compensation control. To conclude, a 1.2 kw gridconnected inverter based on the proposed low-cost topology was designed and simulated. Its simulated efficiency of 95% was higher than that of the 160 W inverter.

8 Statement of Originality This work contains no material which has been accepted for the award of any other degree or diploma in any university or other tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text. I give consent to this copy of the thesis, when deposited in the University Library, being available for loan, photocopying and dissemination through the library digital thesis collection, subject to the provisions of the Copyright Act I also give permission for the digital version of my thesis to be made available on the web, via the University s digital research repository, the Library catalogue, the Australasian Digital Thesis Program (ADTP) and also through web search engines, unless permission has been granted by the University to restrict access for a period of time. Signed: Date: x

9 Acknowledgements I am especially grateful to my supervisor Assoc. Prof. Nesimi Ertu grul for his guidance, insightful conversations and encouragement. I am also sincerely grateful to my supervisor Dr. Wen L. Soong whose constant guidance, support, encouragement, help and advice kept me going in the right direction during the course of my postgraduate studies. It has been very stimulating and enjoyable to work with them. I acknowledge the financial support of the University of Adelaide with the Divisional Scholarship. I thank all the members of the department and especially the members of the Power Electronics and Control Group for their friendliness and help over the years. I would like to thank Dr. David M. Whaley, Dr. Gene S. Liew and Dr. Jingwei Zhu. I would also like to thank Stuart Brand, Ian Linke, Bernard Dumuid and Brandon Pullen for their help in the power electronics laboratory during my research. I thank Pavel Simcik and Rainer Weydert for their help for the microcontroller programming and Rose-Marie Descalzi for her help and support. I thank Dr. L. Bülent Gün for his encouragement and advices. This thesis would not have been possible without the love and support of my lovely wife, Nihan Ertaşgın and my brother A. Bülent Ertaşgın. I am grateful to them and appreciate their continuous encouragement, endless support and understanding during my studies. xi

10 PUBLICATIONS [1] G. Ertasgin, David M. Whaley, N. Ertugrul, Wen L. Soong, Implementation and performance evaluation of a low-cost current-cource grid-connected inverter for PV applications, Proc. IEEE Sustainable Energy Technologies Conf., (ICSET 08), Nov. 2008, pp [2] G. Ertasgin, David M. Whaley, N. Ertugrul, Wen L. Soong, Analysis and design of energy storage for current-source 1-ph grid-connected PV inverters, Proc. IEEE Applied Power Electronics Conf., (APEC 08), Feb. 2008, pp [3] G. Ertasgin, David M. Whaley, N. Ertugrul, Wen L. Soong, A current-cource gridconnected converter topology for photovoltaic systems, Proc. Australasian Universities Power Engineering Conf., (AUPEC 06), Dec [4] David M. Whaley, G. Ertasgin, N. Ertugrul, Wen L. Soong, J. Darbyshire, H. Dehbonei, Chem V. Nayar, Investigation of a low-cost grid-connected inverter for smallscale wind turbines based on a constant-current source PM generator, Proc. IEEE Industry Electronics Conf., (IECON 06), 7-10 Nov. 2006, pp xiii

11 LIST OF FIGURES 1.1 Block diagram of a typical wind turbine power conditioning system PV cell, module and array structures Stand-alone and grid-connected systems Single-phase voltage and current-source inverters Early CSI inverter topology A single-phase H-bridge CSI using series diode-transistor arrangement GTO based H-bridge current-source inverter Early grid-connected CSI topology Thyristor H-bridge soft-switched current-source inverter topology Current-source grid-connected wind inverter topology Current-source three-phase inverter topology Current-source boost inverter topology Proposed grid-connected inverter topology Thesis layout consists of chapters Typical I-V curve of a PV module Voltage variations, reference to (a) irradiance and (b) temperature at MPP Non-linear I-V curves for various irradiance and temperature values Dark I-V block diagram The non-linear model I-V and P-V curves including dark I-V results Block diagram of the power reduction Common single-phase PV inveter topologies Single-phase current-source GCI Demonstration of energy buffering PV array output current variation diagram Normalised power-current curves Definition of centred (a) and balanced assumptions (b) for I Average power reduction versus voltage and current ripples Grid-connected CSI showing previous wind and proposed PV applications. 40 xv

12 List of Figures 4.2 Ideal single-phase current-source grid-connected inverter Grid-connected inverter simulation circuit using PSIM Ideal waveform flow diagram of the GC CSI at MPP PV array power reduction curves due to temperature Normalised output power vs. energy storage for various PV array models PV array voltage vs. current curves showing operating points Effect of modulation index changes on PV array operating point THD and power factor as a function of normalised inverter output power THD and PF curves using different energy storage and control approaches Common grid-connected inverter low-pass filter types Parallel-damped CL type low-pass filter Power loss and THD contours of 5%, for filter configurations The unfolding circuit output current without and with phase advance Analytical phase advance ϕ and its corresponding phase difference α The CSI output power vs. THD, PF and P d by changing C n The CSI output power vs. THD, PF and P d by changing f c The CSI output power vs. THD, PF and P d by changing Q Damping resistance power loss for several THD values Inverter circuit simulation using PSIM MPPT algorithm flow diagram using perturb-and-observe method Simplified behavioural flow diagram of the microcontroller operation Measured PWM signals from controller and SCR pair s drive pulses Photo of the CSI prototypes Grid-connected CSI test arrangement Simulated and measured resistive load current waveforms Comparison of the inverter output current waveforms Simulated and measured CSI output voltage and current Simulated and measured CSI input and output currents Simulated and measured CSI input (P P V ) and output (P G ) powers Simulated and measured CSI THD as a function of output power Simulated and measured CSI power factor as a function of output power Simulated and measured CSI efficiency as a function of output power Calculated rated loss breakdown of the 160 W prototype Measured WS input and output currents at 40% m A CSI circuit diagram showing voltage, current values and the voltage drops PV array simulation model which consists of six series 4-diode models I-V and P-V curves of the 1.2 kw PV array Inductor design trade-off graph Proposed 1.2 kw inverter topology showing component ratings PSIM simulation model of the 1.2 kw CSI using FFD control Simulated CSI output voltage and current waveforms Simulated 4 khz PWM switching effect on the output current waveforms. 112 xvi

13 List of Figures LIST OF FIGURES 7.9 Simulated CSI total harmonic distortion as a function of output power Simulated CSI power factor as a function of output power Simulated CSI efficiency as a function of output power Designed higher power CSI loss breakdown as a function of output power Simulated loss pie chart of the designed grid-connected CSI A.1 Dimensions for the designed inductor A.2 Grid-connected CSI test arrangement A.3 Measured inductance of the two different DC link inductors A.4 Iron loss vs. coil current and voltage A.5 The PV array output and the waveshaper input waveforms B.1 Master file of the PCB design B.2 The CSI circuit schematic B.3 Microcontroller and its connections B.4 The input current and voltage sensors B.5 The output current and voltage sensors B.6 PCB layout of the proposed inverter xvii

14 LIST OF TABLES 1.1 Current Harmonic Limits Specifications of the 80 W BP Solar BP380U module PV simulation models for the BP380U PV module Parameter values of the PV module models under nominal conditions Effect of different normalisations on the I-V and P-V curves Grid-connected PV VSI and CSI examples based on balanced method Ideal and non-ideal components of the proposed inverter Simulation model parameters of the simplified GC CSI Normalised ideal GC CSI waveforms as a function of energy storage Rectangular PV array model as a function of modulation index diode PV array model results as a function of modulation index Normalised ideal GC CSI waveforms as a function of irradiance Resistive damping of second-order CL filter configurations The normalised output currents of the CSI and their FFT spectrums The normalised output currents of the CSI with α First inverter prototype semiconductor properties Parameters of the two CSI prototypes Optimised inverter prototype semiconductor properties Inverter output filter types and parameters Measured output currents of the CSI and their FFT spectrums Summary of the proposed 1.2 kw PV array properties Parameters of the inductor for the 1.2 kw PV CSI The designed higher power inverter component summary Low-pass output filter component values and its parameters Simulation model parameters of the designed higher power CSI Simulated loss calculation summary at the rated output power xix

15 List of Tables A.1 Parameters of the two inductors for the proposed CSI A W inductor measured results xx

16 ABBREVIATIONS AC AS CC CCS CL CSI CWS DC ESR FC FFD GC GCI I-V LC LCL MLT MPP MPPT Alternating Current Australian Standards Constant Current Constant Current Source Capacitive-Inductive Current-Source Inverter Current Waveshaper Direct Current Equivalent Series Resistance Filter Configuration Feedf orward Grid-Connected Grid-Connected Inverter Current vs. Voltage Inductive-Capacitive Inductive-Capacitive-Inductive Mean Length per Turn Maximum Power Point Maximum Power Point Tracking xxi

17 Abbreviations OL PF PM PV P-V PWL PWM T THD UC VSI WS Open-Loop Power Factor Permanent Magnet Photovoltaic Power vs. Voltage Piecewise Linear Pulse Width Modulation Thyristor Total Harmonic Distortion Unfolding Circuit Voltage-Source Inverter Waveshaper xxii

18 PHYSICAL CONSTANTS Boltzman constant k = J K 1 Charge on electron q = C Permeability of free space (air) µ 0 = 4π 10 7 H m 1 xxiii

19 SYMBOLS a temperature coefficient of copper %/ C A W radius value of copper wire m A W T total winding area m 2 B flux density T C capacitance F C DC DC link capacitor F C F filter capacitor F C P V PV array output capacitor F d duty cycle % E energy storage J/W E 0 average energy storage J f 1 grid (inverter fundamental) frequency Hz f c cutoff frequency Hz f R resonant frequency Hz f sw switching frequency Hz g airgap m G irradiance W/m 2 g E equivalent gap m g T total gap m xxv

20 Symbols H 1 the rms value of the first harmonic V or A H n the rms value of the n th harmonic V or A I 0 current at the maximum power A I B base current A i G inverter output or grid current pu I G grid current (fundamental) pu i IN waveshaper input current pu i IN desired PV array output current A i L inductor current A i L CC inductor current (constant-current) A i L Clipped inductor current (clipped) A i L F F D inductor current (feedforward control) A i L MP P inductor current (MPP) A I L0 nominal PV array output current (constant) A i L OL inductor current (open-loop control) A i OUT waveshaper output current pu i P V PV array output current A or pu I rms the rms value of distorted current A I SC short circuit current A i W S F F D waveshaper output current (feedforward control) A i W S OL waveshaper output current (open-loop control) A L inductor H l C mean core magnetic path length m L DC DC link inductor H L F filter inductor H or pu m A modulation index % or pu n diode ideality factor N number of turns N S module cell number P active power W P 0 rated maximum power W xxvi

21 Symbols SYMBOLS P 0 average output power W P CU inductor power loss W P d damping resistance power loss W pf packing factor % p IN waveshaper input power W p LOSS ripple loss W P OUT inverter output power W R CU coil resistance Ω R D damping resistance Ω R S module series resistance Ω S apparent power V A T period s V 0 voltage at the maximum power V V B base voltage V V dcmax ripple voltage (maximum) V V dcmin ripple voltage (minimum) V V g band gap V V G grid voltage pu v IN inverter input voltage V v IN waveshaper input voltage pu v IN CC waveshaper input voltage (constant-current) V v IN Clipped waveshaper input voltage (clipped) V v IN MP P waveshaper input voltage (MPP) V V L F applied voltage to the filter inductor pu V OC open circuit voltage V v OUT waveshaper output voltage pu V P K peak grid voltage V or pu v P V PV array output voltage V or pu V UC unfolding circuit output voltage pu X L inductor impedance pu Z 0 characteristic impedance Ω xxvii

22 Symbols Z B base impedance Ω α phase advance in simulation deg α temperature coefficient of I SC %/ C β temperature coefficient of V OC V m/ C difference η efficiency % λ flux-linkage V s ρ static resistivity V m/a ϕ phase advance or difference deg Φ flux V m ω angular frequency rad/s ω cn normalised cutoff frequency (relative to f 1 ) pu xxviii