24-07-2012, 01:15 PM
GRID CONNECTED INVERTER
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INTRODUCTION
GENERAL INTRODUCTION
Proper use of freely available renewable energy sources is a global need today. There are two major global challenges ahead; one is to reduce the carbon emission to mitigate the reduction in global warming and another is to allow easy access to the basic need for power to all human beings. The proper use of effective power electronics technology will increase the use of widely distributed power sources which addresses both above mentioned challenges. Advanced power electronics also play a vital role to interconnect different types of power sources efficiently, increase power transfers and reduce losses, allow flexible transmission of energy, variable speed operation of electric generators and low engineering cost Grid Connected inverter etc . The thesis goal is the development of a simple grid connected current source inverter which will boost the use of renewable energy systems in remote and urban locations.
The purpose of this thesis is to develop a cheap and simple inverter system to interconnect small scale power generator to the low voltage grid. Grid connected current source inverter systems often require a Digital Signal Processor (DSP) and complex engineering. Depending upon the nature of sources, the inter-connection systems require AC to DC and DC to AC converters. Herein after the AC to DC converter is called generator side converter and DC to AC converter is called AC side converter.
The generator side converters are relatively cheap and available in different standards. They directly connect generator output to the AC side converter through a DC link capacitor. The work is focused on the development of the AC side converter. The purpose of AC side converter is not only to invert DC into AC voltage, but also to interconnect output power to a utility grid. The GC CSI systems should be featured with interconnection standards acceptable by the utility systems.
Generally renewable power sources are connected to a distribution level grid. Penetration of both active and reactive power is significantly useful for stability of the grid. Reactive power control might not be very effective to control voltage at the low voltage grid in comparison to
a high voltage systems. Nevertheless, reactive power injection is important to support the inductive load and reactive part of grid itself. Use of reactive power control helps to stabilize the grid voltage to some extent. Optimum active power injection is effective to maintain the voltage level in such smaller systems. Generally low voltage systems have a high impedance. The proposed inverter system is intended to support the grid at unity power factor during normal grid voltage. Upon decreasing or increasing the grid voltage level, the inverter system either generates or absorbs reactive power. The consequence of reactive power compensation from individual system may be negligible. However, large numbers of such small systems at suitable locations can provide substantial support for the system stability. This thesis doesn't outline the rating and number required to compensate the reactive power at different voltage level. It does, however, discuss how much the proposed system can compensate reactive power at different voltage levels. The thesis proposes a GC CSI system to connect at distribution voltage level from small scale power generators for a range of 1KW in single phase systems.
The minimum requirements of source voltage and operating characteristics are outlined. Single Cycle Controller is explained and power control methodologies are presented. The logic implementation for inverter switching strategies are presented and current zero-crossing problems are addressed for each strategy. A simulation is performed on PSCAD=EMTDC program. A laboratory hardware prototype is built. Both simulation and hardware results are presented in the thesis.
RESEARCH OBJECTIVES
The objective of this research is to develop a cheap and simple grid connected inverter system which allows the connection of dispersed small scale renewable sources or DC energy storage to the grid. Available inverters are relatively expensive and use complex circuitry for the purpose. The thesis investigates different single cycle logic switching methods for the H-bridge converter to find efficient and less current distorting methods for easier implementation in hardware. The performance for different switching methods are investigated through simulation. The inverter is proposed to support the grid at unity power factor, during healthy network conditions. However, if the system voltage level changes, it will inject the reactive current to compensate the reactive loss in the filter impedance and the grid itself. The converter will be equipped with power control and allows bidirectional flow of both active and reactive current.
THESIS OUTLINE
provides background on renewable energy integration into low voltage grids. Grid
connected inverter topologies are reviewed and interactions of the inverter with low voltage grid are discussed.
Chapter 3 describes Single Cycle Control theory.
Chapter 4 presents switching logic for Single Cycle Control. Three different switching strategies are presented and ranges of switching methods are given for switching of full bridge inverter.
Chapter 5 illustrates current performance in Single Cycle Control. Zero crossing performance of the current is shown for the ranges of switching methods.
Chapter 6 presents power controller using Single Cycle Control. A waveform generation method is proposed. A phase synchronization method implementable in microcontroller is proposed in the chapter.
Chapter 7 provides results. Circuit design is presented. A laboratory prototype development is given and experimental results are presented.
INTRODUCTION
A number of grid connected inverter topologies and switching strategies have been developed depending upon the nature of distributed power sources. The aim of the inverter is to transfer maximum available energy to the power network despite having strong interactions during connection. An economic and reliable interconnected system is a challenging issue for a weak grid network.
This chapter presents a brief summary of recent work in the field of interconnection of small
distributed power sources through a inverter into the low voltage network. It contains a brief introduction to distributed energy sources, a review on inverter topology and controller strategies, integration of inverter and source into a distribution network and probable interactions between inverter and grid for single phase systems.
DISTRIBUTED POWER SOURCES
Distributed Power Sources (DPS) are defined as "demand and supply side resources that can be deployed throughout an electric distribution system (as distinguished from the transmission system) to meet the energy and reliability needs of the customers served by the system. Distributed resources can be installed on either the customer side or the utility side of the meter". Various kinds of renewable and non renewable distributed power sources exist. Due to the increasing global crisis in non-renewable energy resources, renewable and green sources have been attracting more attention recently. Established distributed energy sources are; micro-hydro, wind, solar power fuel cell, combined heat and power, bio-gas, bio-mass, geothermal and diesel power.
In order to improve the reliability of the generation and injection of power into the grid during overload, several energy storage systems have been investigated and are implemented primarily in renewable energy integrated systems. The well developed energy storage systems are; flywheels, hydrogen storage, battery storage, super-capacitors, compressed air energy storage, super-conducting magnetic energy storage and pump hydro-electric storage. This thesis is concerned with grid connection of a renewable or storage energy sources but does not outline its characteristics and nature. Generally the renewable energy sources have a stochastic nature and require advanced power electronics control to maintain the power output as required by the utility standards. In order to be incorporated into the grid certain technical guidelines need to be met before interconnection so that the distributed power sources do not strongly affect the normal operation of network. The proposed system can be implemented for any type of energy source which satisfies the general input requirements of the proposed inverter.
A REVIEW OF GCI TOPOLOGY AND CONTROLLER STRATEGY
The proposed GC CSI system primarily controls three major parameters; current magnitude, phase and frequency. Several types of inverter topology and controller strategy have been used for three phase and single phase systems in the past. Synchronization of the systems with the utility supply is a challenging issue. It is often achieved by using DSP or field programmable gate array (FPGA). It adds complexity to the circuit and increases the engineering cost. However, the complex digital control ensures a delivery of a good power quality into the grid and intelligent quick controller systems. Various single stage and multi stage GC CSIs are used. The single stage inverters offer simple structure and low cost but suffer from limited input voltage range, while the multi stage inverters are complex, expensive and less efficient. The proposed system uses a single stage inverter with a full bridge topology and a Single Cycle Controller (SCC) strategy for single phase system.