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代写英语论文
AN IMPROVED PID AND REPETITIVE CONTROL FOR SINGLE PHASE INVERTERS OF PHOTOVOLTAIC POWER SYSTEMS
作者:Muhammad Arslan 日期:2018/11/5 10:35:13 点击:

Abstract

Inverters, which are installed in photovoltaic (PV) power systems, are key devices to turn output direct current (DC) of PV arrays to alternative current (AC) with a specific waveform required by power load. With their widespread application and increasing large-scale of PV power systems in utility power network, the disturbances from load and line faults or external interferences often cause serious problems in inverters, operating in a safe and steady condition and inverters control problems on suppressing the disturbances have always been focused by the industry and academic circles. Many efforts and academic research have been carried out to improve the flexible performance of PV-connected inverters. This thesis presents a controlled mechanism of single-phase inverter for PV system. Proportional Integral Derivative (PID) controller performs better in the existence of noise and high error while difficult in fine-tuning. The proposed method involves the advantages of PID with Repetitive Control (RC) controllers, helping PID controller to decrease error from last period while still using PID controller for handling with noise and error.

First of all this dissertation establishes the conversion method of single-phase inverter of the photovoltaic power system by using the LC filter and find a novel method to control the system with improved performance and stability. Secondly, this study will dive deeper in controlling mechanism of PID and RC control to make the system efficient. Therefore, Repetitive Control is embedded because it possesses an effective and efficient way of tracking/rejecting periodic signals. Performance of the RC scheme has been measured in terms of steady-state tracking error and Total Harmonic Distortion (THD). Meanwhile, the main controller along with an integrator is used in feedback closed-loop system for achieving steady state error condition and reduced harmonics in output wave. Finally, the experimental results indicate that RC controller with proportional integral derivative is capable of achieving a near zero error steady-state tracking of a reference signal.

The performance of the RC controller and power quality of the converter is measured and presented in terms of the THD. In this proposed method, THD % is reduced to a very low level in comparison to preceding traditional proposed techniques. The simulation results testify to the validity and significance of the proposed method.

 

Keywords: Renewable Energy Resources; DC Component; Total Harmonic Distortion (THD); Photovoltaic Power Systems; Repetitive Control.

Table of Contents

Abstract I

Table of Contents II

List of Figures IV

List of Tables VI

Nomenclature VII

Chapter 1 Introduction 1

1.1 Overview 1

1.2 Problem Statement 1

1.3 Objectives 1

1.4 Methodology 2

1.5 A Comprehensive View of Solar System Configuration 3

1.5.1 Single-Stage Centralized Inverter 3

1.5.2 String Inverter 4

1.5.3 Two-Stage String Inverter 4

1.5.4 Two-Stage Centralized Inverter 5

1.6 DC/AC Inverter Topologies 5

1.7 Thesis Outline 6

Chapter 2 Preliminaries and Basic Technology 7

2.1 Classification of DC to AC Conversion 8

2.1.1 Square Wave Inverters 8

2.1.2 Modified Square Wave Inverters 8

2.1.3 Sine Wave Inverters 8

2.1.4 Pulse Width Modulation & Sinusoidal Pulse Width Modulation 8

2.1.5 Three-Level PWM 9

2.2 Filter Topologies of Inverter Systems 9

2.2.1 Principle of Operation 10

2.2.2 Photovoltaic Systems 10

2.2.3 Cataloging of PV Power System 11

2.2.4 OFF-Grid PV System 11

2.2.5 Grid-Connected PV System 12

2.3 PV Module 13

2.4 Photovoltaic Array 13

2.4.1 The Pattern of PV Array and MPPT Algorithm 14

2.4.2 Maximum Power Point Tracking 14

2.5 Control Techniques for Single-phase Inverter Systems 15

2.6 Comparative Analysis of Proposed Scheme with other Techniques 17

2.6.1 Variable Structure Control Scheme 17

2.6.2 Fuzzy PID Control Scheme 18

2.6.3 Deadbeat Control Scheme 19

2.6.4 Repetitive Control Scheme 19

2.7 Designing of PWM Inverter Circuit 20

2.8 A Brief Summary 22

Chapter 3 Proposed Controller for a Single Phase Inverter Power Systems 23

3.1 Proposed System Model 23

3.1.1 Modeling of a Single-Phase Inverter 23

3.1.2 Full Schematic Diagram of Proposed Single Phase PV Inverter System 26

3.2 PID Controller 27

3.3 Repetitive Control Model 28

3.4 Proposed PID-RC Controller 30

3.5 Stability Analysis of Proposed Controller 33

3.6 A Brief Summary 34

Chapter 4 Numerical Simulations of a Controlled Single-phase Inverter 35

4.1 Simulation Parameters 35

4.2 Proposed Simulink Diagram 35

4.3 Tuning of PID and RC Controller 37

4.4 Performance Evaluation 38

4.5 A Brief Summary 44

Chapter 5 Conclusion and Future Work 45

5.1 Conclusion 45

5.2 Future Work 45

Appendix 46

Bibliography 49

攻 读 硕 士 学 位 期 间 取 得 的 研 究 成 果 54

Acknowledgment 55

List of Figures

Figure 1-1 Research Scheme Diagram 2

Figure 1-2 Single-Stage Centralized Inverter 4

Figure 1-3 Photovoltaic String Inverter 4

Figure 1-4 Two-Stage Sting Inverter 5

Figure 1-5 Two-Stage Centralized Inverter 5

Figure 2-1 Global Electricity Production Report[11] 7

Figure 2-2 Formation of SPWM with Triangulation Technique 9

Figure 2-3 Block Diagram of a Photovoltaic System 11

Figure 2-4 Cataloging of a PV System 11

Figure 2-5 Stand-alone PV System without Batteries 12

Figure 2-6 Stand-alone PV system with Batteries 12

Figure 2-7 A Comparison of Cell, Module and an Array 14

Figure 2-8 Basic Feed Back Control System 15

Figure 2-9 Classification of Controlling methods for PWM Inverter System 17

Figure 2-10 Sliding Mode Control Scheme 18

Figure 2-11 Fuzzy PD-Repetitive Control Block Diagram 18

Figure 2-12 Single Phase Full-bridge Inverter Topology and its Waveforms [50] 21

Figure 3-1 Single Phase Inverter Model with LC Filter 24

Figure 3-2 Proposed Schematic Diagram 27

Figure 3-3 Classical PID Controller 28

Figure 3-4 Repetitive Controller 28

Figure 3-5 Standard Memory Loop in the Discrete Domain of RC 29

Figure 3-6 Block Diagram of Single Phase Inverter System 30

Figure 3-7 Controller Design for given System 31

Figure 3-8 Controller Design for given System with External Disturbance 32

Figure 4-1 Simulink Model of the Proposed Scheme 36

Figure 4-2 PID Controller 37

Figure 4-3 Inverter Output Waveform with a Conventional PID Controller 38

Figure 4-4 FFT of Inverter Output Current using a Conventional PID Controller 39

Figure 4-5 Inverter Output Current with the Conventional PI Controller 40

Figure 4-6 FFT of Inverter Output Current using PI Controller 40

Figure 4-7 Reference Sinusoidal Current Waveform 41

Figure 4-8 Output Current Waveform with PID-RC Scheme 41

Figure 4-9 FFT Analysis of Output Waveform using Proposed PID-RC Scheme 42

Figure 4-10 Output Current Waveform of the Proposed Scheme 42

Figure 4-11 PWM Inverter Output Current Waveform of the Proposed Scheme 43

Figure 4-12 FFT Analysis of the Proposed Scheme PI-RC 43


List of Tables

Table 2-1 Comparative Analysis of Control Techniques 20

Table 2-2 Combination of Switches for Inverter System 22

Table 4-1 Simulation Parameters 35

Table 4-2 Comparison of Techniques using Conventional and Proposed Schemes 43


Igrid Iref IPV

 

Grid Current Reference Current

Photo-Generated Current

Nomenclature

VPV

DP

Output Voltage of the PV Array

Increment in the Output Power of Solar Cell/Array

GA Genetic Algorithm

T Solar Cell Operating Temperature

RC Repetitive Control

Vd

Vgrid

Constant Voltage Source Grid Voltage

1.1 Overview

Chapter 1 Introduction

Photovoltaic (PV) power systems, an easy and available way to utilize the solar power, are most efficient and convenient way to generate power as compared with other renewable energy resources for example wind, tide, biomass and hydro-power, which have formed an environment-friendly power industry and paid more attention. Recently, the demand of electric supply is increasing tremendously; therefore the energy supplied by photovoltaic systems to the electric grid is gaining high visibility [1]. Inverters are used to create single or three-phase AC voltages from a DC supply. High-performance control strategies need to be accurate, fast, robust and implementable for designing of the inverter system. A single-phase inverter using Pulse Width Modulation (PWM) rectifier has been simulated using a conventional proportional integral derivative with plugin repetitive control scheme. Periodic control techniques, also known as interactive learning control techniques, work repeatedly and the repetition enables the system to improve tracking accuracy from one repetition to another.

1.2 Problem Statement

It has been commonly observed in developing countries that power shut down for several hours. It has the detrimental effect on the operation of critical loads. A power inverter is an optimized solution with reduced DC offset and good system stability during this type of power cut. The ongoing issues of distorted output signals with harmonics, higher frequency response, and the steady state errors are crucial and needs to overcome in an inverter system. A photovoltaic power inverter with pure sinusoidal output wave is required because the majority of appliances need pure sine wave for their working.

1.3 Objectives

The objective is to propose an improved topology of inverter systems used with PID and repetitive control for the suitable required results at the output. Different types of strategies have been developed for controlling the inverter systems. The proposed scheme provides the optimal solution for the controlling of the inverter systems.

· Mitigation of DC offset and good tracking of the output waveform.

· To produce a pure sine wave with the proposed design.

· To overcome the problems of depleting traditional energy resources, a photovoltaic system is requisite that is efficient, low cost and environment-friendly.

· Designing of an algorithm that has minimum complexity.

· The proposed algorithms can effectively tradeoff between complexity and performance.

· Experimental measurements, observation, analysis and comparison of results.

 

1.4 Methodology

 

代写英语论文

 

Figure 1-1 Research Scheme Diagram

The photovoltaic power systems are gaining popularity worldwide. Solar energy farms can generate convincing amount of electricity to feed the electrical systems [2-4]. The basic equation that is describing the behavior of a photovoltaic array as shown in (1-1) [4]. Source current is dependent on the solar irradiance. The thermal voltage, reverse saturation current and the output of the PV current are dependent on temperature.

I0 : Diode Reverse Saturation Current

q : Electron Charge

M : Numbers of Cells in parallel

N : Numbers of Cells in series

RS : Series resistance

RP : Parallel resistance

The PV output current is actually a function of irradiance and environmental temperature. The practical solar cell model, which is based on the PV output current-voltage and power-voltage whose curves are plotted with temperatures and different irradiances. The P-V curve is changed accordingly as the irradiance changes. Maximum Power Point Tracking (MPPT) is a tracking on the input DC stage of the inverter, this is a function that attempts to hold the voltage and current of a solar panel at the optimal point of its performance curve which is the highest efficiency. T. Esram et al [5] have proposed many MPPT algorithms which fulfilled the basic requirements of a photovoltaic system. The most common and basic MPPT techniques are incremental conductance algorithm, Perturb and Observe (P&O) algorithm and fractional open-circuit voltage algorithm [6].

1.5 A Comprehensive View of Solar System Configuration

Photovoltaic modules feed DC current and voltage as their inputs, to the inverters into the power electronics system. There are so many converters to amplify the low voltage produced by the Photovoltaic modules; the DC to DC inverters are often used. The inverters are utilized for conversion of the DC voltage to the AC voltage to run the normal loads. There are a lot of configurations for the residential environment and cost budget that affects the solar panel design. Four basic solar system configurations are listed and discussed in the following session.

1.5.1 Single-Stage Centralized Inverter

PV panels are connected in series to form a PV string, for the sake of achieving higher voltage. The PV strings are then connected in parallel with power diodes to get higher power generation. This configuration is shown as in figure 1-3. In this configuration, it can be seen that all the PV strings are in parallel, and thus all the PV strings share the same voltage. Because of the irradiation shading or panel mismatch problems, the operating voltage may not be the maximum power point for all the PV strings [7]. 

 

1.5.2 String Inverter

Figure 1-2 Single-Stage Centralized Inverter

In this configuration, for each PV string, each inverter is supposed to handle its own maximum power point tracking and power conversion control, if there is any panel mismatch or shading. String inverter configuration is superior compared to the single-stage centralized inverter for the power harvesting performance, However, an inverter is applied to each PV string inverter configuration that increases the total installation cost [8].

 

Figure 1-3 Photovoltaic String Inverter

 

1.5.3 Two-Stage String Inverter

This configuration is in the mainstream because of its improved energy harvesting capability, modularity, and design flexibility [7]. The system robustness is increased as each PV string contains less solar panel. The first step is to amplify the low DC voltage which is generated by solar panels to a higher level DC bus. DC to DC converter should also handle

the maximum power point tracking. The second stage controls the power conversion from DC to AC.


1.5.4 Two-Stage Centralized Inverter

In the first stage of this configuration, it is a modulated DC voltage amplification stage. For the connected PV string, the DC to DC converter handles the maximum-power-point- tracking. In the second stage, there is a need for centralized the DC to AC. Using this configuration the cost is reduced; however, the centralized inverter can become large.

 

1.6 DC/AC Inverter Topologies

The single-phase inverter topologies are differentiated in so many ways. Based on the switch leg numbers, inverters are divided as a half-bridge inverter or a full-bridge H-bridge inverter. Inverters can be branched into a voltage source inverter or current source inverter based on the input sources [1] [9]. Inverters can be classified on the basis of peak voltageamplitude and input voltage amplitude and also from comparing the inverter output wave [10]as a buck inverter, buck-boost inverter or boost inverter.

 

1.7 Thesis Outline

The arrangement of the thesis is as follows:

Chapter 1 This chapter establishes the overview of energy resources and importance of renewable energy, research objectives and goals, research methodology, analysis of depleting energy resources, a comprehensive view of solar system configuration and thesis outline.

Chapter 2 This chapter contains some preliminaries along with some technical background and literature review of the research. It explains some basic technologies of the inverter-based system and about renewable energy system i.e. photovoltaic system. Related work about control techniques of a photovoltaic inverter system.

Chapter 3 This chapter presents on the controlling schemes for the inverter system the principle of working, plant modeling, principle of PID-RC controlling technique and how the repetitive technique work with the proportional integral derivative with its mathematical modeling, discusses the PWM technique how PWM triggers the switches and analysis of a proposed scheme, how the system works for achieving efficient topology.

Chapter 4 This chapter focuses on the simulation of the proposed method and analyzes the results and checking the different output waveforms and calculates the FFT and analyzed it with other methods. Representing THD, one is without the proposed method and other is with proposed technique and analyzes both of them, discuss the importance and beneficial aspect of using the repetitive control with the main controller and how it is efficient with other controlling techniques.

Chapter 5 This chapter presents the conclusion of the proposed work scheme summarizes the results and limitation of the research work and ends the chapter with the future works that explore the scope and proposes plans for future research of this thesis.

Chapter 2 Preliminaries and Basic Technology

According to the report on estimated renewable energy, it observed that the production of electricity from renewable sources are depleted with the increase of energy consumption, so it’s the requirement to give more attention on the renewable energy resources for the energy production. The controller block is responsible for taking in the actual and reference signals and producing the correct PWM signal which can reduce the tracking error in upcoming periods. The criterion for the selection of the appropriate control scheme usually involves a trade-off between cost, complexity, and quality. This section summarizes some of the previously reported control approaches for converters and their impact.

The design of the specific inverter systems varies according to various parameters like the frequency, output voltage, input voltage, and power adjustment. Power is not produced by the inverter; it’s required a DC source which is either batteries or PV array which will be discussed in detail in this thesis. Not a specific maintenance and fuel energy are required. Inverters output waveforms are classified into many forms. Out of whack, inverters are either square wave or modified sine wave. Pure sine waves are much complex in design and more expensive but they are more accurate and have less unused harmonics energy delivered to load because of accurate power devices with more accuracy, less power loss, and less heat generation. Boosting is done by either DC-DC boost inverter or by a transformer after the AC stage. The output is usually 120 or 240 voltage. Inverters are not fully electronic some have static parts and some have mechanical effects. There are some issues like size, switching losses and gate capacitance losses to overcome these issues, a technique has been made by Wilson et al [12] for mitigation of transient times switching losses, heat losses that usually comes in Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) drive. The type of inverter which are static inverters they have no moving parts in the conversion process. Based on the input sources, inverters can be divided into a current source inverter or a voltage source

inverter which are classified as boost inverter, buck inverter, or buck-boost inverter [10]; by comparing the inverter output peak voltage amplitude and input voltage amplitude. Some of the examples where voltage source inverters are used are Adjustable-Speed-Drives (ASD) for AC motors, Uninterrupted Power Supply (UPS) units, electronic frequency changer circuits etc. The UPS or photovoltaic inverter output voltage needs to be sinusoidal with minimum Total Harmonic Distortion (THD) [13]. Different range of input voltages, some of them are listed below:

· 12V, smaller commercial inverters.

· 24V, 36V, and 48V, common for home energy systems.

· 200V to 400V, photovoltaic solar panels.

· 300V to 450V, grid systems.


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