Power Electronics, converters, research and Applications: A Brief overview

Power electronics is the application of solid-state electronics for the control and conversion of electric power. It also refers to a subject of research in electronic and electrical engineering which deals with design, control, computation and integration of nonlinear, time varying energy processing electronic systems with fast dynamics. The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices. Power electronic devices may be used as switches, or as amplifiers. An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load. Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the mercury valve and thyratron) allow control of the start of conduction, but rely on periodic reversal of current flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source. As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz. Devices such as MOSFETS and BJTs can switch at tens of kilohertz up to a few megahertz in power applications, but with decreasing power levels. Vacuum tube devices dominate high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of megahertz) applications. Faster switching devices minimize energy lost in the transitions from on to off and back, but may create problems with radiated electromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating. Power electronic circuit’s process and control electrical energy, and are critical elements in many kinds of systems. The rapid evolution of technology is generating a demand for power electronics whose capabilities greatly exceed what is presently achievable. Challenges of particular importance include miniaturization and integration of power electronics, and improving their cost and dynamic performance. Miniaturization is difficult in part because the magnetic components used in most power circuits scale down poorly in size. Likewise, achieving integration and low cost is difficult because of the diverse materials and assembly methods that are required for contemporary designs. My research interest includes working to address these challenges through a combination of new technologies. One research focus is on the development of improved power passive components. Passive components such as inductors and capacitors often dominate the size and cost of power circuits, and limit their efficiency, noise attenuation, and transient performance. In one effort, we are developing means to improve the performance of passive filter components by compensating for their parasitic. These efforts have led to new integrated filter components with much better performance than conventional passives. Likewise, we are developing new types of power passive components that better scale to small sizes and high frequencies. Construction of these components using micro fabrication techniques is also being explored, with the goal of enabling integrated fabrication of power converters. A second research focus is the development of techniques to achieve greatly increased switching frequencies in power converters. Higher frequencies are desirable because they enable faster transient response and reduce passive component requirements. Moreover, at sufficiently high frequencies, batch fabrication of many circuit components may become possible, enabling higher levels of integration to be achieved. We are exploring new system architectures, circuit designs, and control methods that together enable substantial increases in operating frequency over the present state of the art. It is anticipated that the technologies under development will lead to miniaturized, highly integrated power electronics. In addition to developing fundamental power conversion technologies, we are applying them in a variety of applications. Automotive power generation and control is one such area. For example, we have investigated the application of power electronics to enhance the efficiency, power, and transient performance of automotive alternators. We have also developed dc/dc converters and other power electronics for automotive applications, with the goal of enabling improved performance, safety, and comfort in vehicles. Other areas of interest include power components and circuits for industrial, commercial, consumer, and medical applications where improved size, efficiency, and performance are of importance. Applications of power electronics range in size from a switched mode power supply in an AC adapter, battery chargers, fluorescent lamp ballasts, through variable frequency drives and DC motor drives used to operate pumps, fans, and manufacturing machinery, up to gigawatt-scale high voltage direct current power transmission systems used to interconnect electrical grids. Power electronic systems are found in virtually every electronic device. For example: • DC/DC converters are used in most mobile devices (mobile phones, PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of the battery is. These converters are also used for electronic isolation and power factor correction. A power optimizer is a type of DC/DC converter developed to maximize the energy harvest from solar photovoltaic or wind turbine systems. • AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television etc.). These may simply change AC to DC or can also change the voltage level as part of their operation. • AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids. • DC/AC converters (inverters) are used primarily in UPS or renewable energy systems or emergency lighting systems. Mains power charges the

DC battery. If the mains fail, an inverter produces AC electricity at mains voltage from the DC battery. Solar inverter, both smaller string and larger central inverters, as well as solar micro-inverter are used in photovoltaic as a component of a PV system. Motor drives are found in pumps, blowers, and mill drives for textile, paper, cement and other such facilities. Drives may be used for power conversion and for motion control. For AC motors, applications include variable-frequency drives, motor soft starters and excitation systems. In hybrid electric vehicles (HEVs), power electronics are used in two formats: series hybrid and parallel hybrid. The difference between a series hybrid and a parallel hybrid is the relationship of the electric motor to the internal combustion engine (ICE). Devices used in electric vehicles consist mostly of dc/dc converters for battery charging and dc/ac converters to power the propulsion motor. Electric trains use power electronic devices to obtain power, as well as for vector control using pulse width modulation (PWM) rectifiers. The trains obtain their power from power lines. Another new usage for power electronics is in elevator systems. These systems may use thyristors, inverters, permanent magnet motors, or various hybrid systems that incorporate PWM systems and standard motors.

How to Write a Great Research Paper

MATLAB Electric Power System

Solar photo voltaic power generation system

Photovoltaic (PV) technology converts one form of energy (sunlight) into another form of energy (electricity) using no moving parts, consuming no conventional fossil fuels, creating no pollution, and lasting for decades with very little maintenance. The use of a widely available and reasonably reliable fuel source the sun with no associated storage or transportation difficulties and no emissions makes this technology eminently practicable for powering remote scientific research platforms [2]. Indeed, numerous examples of successfully deployed systems are already available around the globe [7, 8]. The completely scalable nature of the technology also lends itself well to varying power requirements i.e. from the smallest autonomous research platforms to infrastructure-based systems. This technology can be limited, however, by annual fluctuations in solar insolation, especially at extreme latitudes. Based on semiconductor technology, solar cells operate on the principle that electricity will flow between two semiconductors when they are put into contact with each other and exposed to light (photons). This phenomenon, known as the Photovoltaic effect was first discovered by

Fig. 2.1 Solar cell Edmund Becquerel in 1839. Actual development of PV technology began in the 1950s and gained greater impetus through the NASA space program during the 1960s. Research continues today at national laboratories and within private industry, focusing on increasing conversion efficiencies and mass production strategies to further lower the cost of producing PV modules. Solar photovoltaic systems convert solar energy directly into electrical energy. Basic conversion device used is known as a solar photovoltaic cell or a solar cell as shown in fig. 2.1. Although other light sources may also produce photovoltaic electricity, only sunlight based PV cells are considered in this chapter. A solar cell is basically an electrical current source, driven by flux of radiation. Solar cells were first produced in 1954 and were rapidly developed to provide power for space satellites based on semiconductor electronics technology. Its terrestrial applications were considered seriously only after the oil crisis of 1973, when a real need of alternative energy source was felt globally. Efficient power utilization depends not only on efficient generation in the cell but also on the dynamic load matching in the in the external circuit. Major advantages of solar PV systems over conventional power systems are as under: 1. It converts solar energy directly into electrical energy without going through thermal mechanical link. 2. Solar PV system is reliable, modular, and durable and generally maintenance free. 3. These systems are quiet, compatible with almost all environmental, respond instantaneously to solar radiation and have an expected life span of 20 years or more. 4. It can located at the place of use i.e. onsite and hence no distribution network is required. It also suffers from some drawbacks which can be overcome by the technology in future, such as: 1. At present the cost of solar cells are high, making them economically uncompetitive with other conventional power sources. 2. The efficiency of solar cell is low. 3. As solar energy is intermittent, some kind of electrical energy storage e.g. battery is required which makes the whole system more expensive. 2.2 Components of Photovoltaic system 2.2.1 PV Panels PV panels tend to work much better in cold weather than in hot climates (except for amorphous silicon panels). Add a reflective snow surface and the output can sometimes exceed the rating for the panel. Array currents up to 20% greater than the specified output have been reported. In general, PV materials are categorized as either crystalline or thin film as shown in fig. 2.2 and they are judged on two basic factors: efficiency and economics. For remote installations where the actual space available for PV panels is often quite limited, the greater conversion efficiency of crystalline technology seems to have the advantage. It is also worth noting that the conversion efficiency of thin-film panels tends to drop off rather rapidly in the first few years of operation. Decreases of more than 25% have been reported. This performance deterioration must be taken into account when sizing the array for a multi-year project. However, there are still applications where the lighter weight and greater flexibility of the thin-film panels may be more suitable. Which PV technology is more appropriate for a given application will need to be determined on a case-by-case basis [14].

Fig. 2.2 Types of solar cells

Fig. 2.3 OFF grid Residential Solar PV system Mono-crystalline silicon panels should be utilized when a higher voltage is desirable. This would be in an instance where the DC power has to travel some distance before being utilized or stored in a battery bank as shown in fig. 2.3. These panels are also the most efficient PV technology, averaging 14% to 17%. New technology charge controllers, which allow for a higher array voltage than the battery bank voltage, somewhat obviate the advantages of the mono-crystalline panels. Polycrystalline silicon panels have efficiencies of 12% to 14% and can often be purchased at a lower cost per watt than mono-crystalline silicon panels. This type of panel sees the widest use in polar applications. Thin-film technologies include amorphous silicon, cadmium telluride, copper-indium dieseline, and others (refer fig. 2.2). Although the cost of these panels appears attractive at first, it is important to note that the efficiencies are comparatively low. The 8% to 10% efficiencies seen in new panels quickly degrade to about 3% to 6% after several months of exposure to sunlight. Furthermore, amorphous silicon and cadmium telluride modules are sensitive to a much narrower band of colors, and the winter shift to redder sunlight results in slightly poorer performance. Newer, triple-junction thin film technologies appear to have higher efficiencies and less degradation over time, but they are still subject to the same problems mentioned above, if to a lesser degree. The somewhat flexible nature of thin-film technology may make it appropriate for some applications, but in general, the higher efficiencies and more robust nature of the crystalline silicon modules make them a better choice for polar applications. Regardless of the technology employed, the researcher would be well advised to look for modules with heavy-duty aluminum frames, UL ratings, easy-to-use junction boxes, and a long warranty (20+ years). All of these are indicative of a quality unit that will withstand the rigors of the polar environment.

Fuel cell for automobiles and power generation

A fuel cell is a device that generates electricity by a chemical reaction. Every fuel cell has two electrodes, one positive and one negative, called, respectively, the anode and cathode. The reactions that produce electricity take place at the electrodes. Every fuel cell also has an electrolyte, which carries electrically charged particles from one electrode to the other, and a catalyst, which speeds the reactions at the electrodes. Hydrogen is the basic fuel, but fuel cells also require oxygen. One great appeal of fuel cells is that they generate electricity with very little pollution–much of the hydrogen and oxygen used in generating electricity ultimately combine to form a harmless byproduct, namely water. One detail of terminology: a single fuel cell generates a tiny amount of direct current (DC) electricity. In practice, many fuel cells are usually assembled into a stack. Cell or stack, the principles are the same. The purpose of a fuel cell is to produce an electrical current that can be directed outside the cell to do work, such as powering an electric motor or illuminating a light bulb or a city. Because of the way electricity behaves, this current returns to the fuel cell, completing an electrical circuit. (To learn more about electricity and electric power, visit "Throw The Switch" on the Smithsonian website Powering a Generation of Change.) The chemical reactions that produce this current are the key to how a fuel cell works. There are several kinds of fuel cells, and each operates a bit differently. But in general terms, hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons. The hydrogen atoms are now "ionized," and carry a positive electrical charge. The negatively charged electrons provide the current through wires to do work. If alternating current (AC) is needed, the DC output of the fuel cell must be routed through a conversion device called an inverter. Oxygen enters the fuel cell at the cathode and, in some cell type, it there combines with electrons returning from the electrical circuit and hydrogen ions that have traveled through the electrolyte from the anode. In other cell types the oxygen picks up electrons and then travels through the electrolyte to the anode, where it combines with hydrogen ions. The electrolyte plays a key role. It must permit only the appropriate ions to pass between the anode and cathode. If free electrons or other substances could travel through the electrolyte, they would disrupt the chemical reaction. Whether they combine at anode or cathode, together hydrogen and oxygen form water, which drains from the cell. As long as a fuel cell is supplied with hydrogen and oxygen, it will generate electricity. Even better, since fuel cells create electricity chemically, rather than by combustion, they are not subject to the thermodynamic laws that limit a conventional power plant (see "Carnot Limit" in the glossary). Therefore, fuel cells are more efficient in extracting energy from a fuel. Waste heat from some cells can also be harnessed, boosting system efficiency still further. Alkali fuel cells operate on compressed hydrogen and oxygen. They generally use a solution of potassium hydroxide (chemically, KOH) in water as their electrolyte. Efficiency is about 70 percent, and operating temperature is 150 to 200 degrees C, (about 300 to 400 degrees F). Cell output ranges from 300 watts (W) to 5 kilowatts (kW). Alkali cells were used in Apollo spacecraft to provide both electricity and drinking water. They require pure hydrogen fuel, however, and their platinum electrode catalysts are expensive. And like any container filled with liquid, they can leak. Molten Carbonate fuel cells (MCFC) use high-temperature compounds of salt (like sodium or magnesium) carbonates (chemically, CO3) as the electrolyte. Efficiency ranges from 60 to 80 percent, and operating temperature is about 650 degrees C (1,200 degrees F). Units with output up to 2 megawatts (MW) have been constructed, and designs exist for units up to 100 MW. The high temperature limits damage from carbon monoxide "poisoning" of the cell and waste heat can be recycled to make additional electricity. Their nickel electrode-catalysts are inexpensive compared to the platinum used in other cells. But the high temperature also limits the materials and safe uses of MCFCs–they would probably be too hot for home use. Also, carbonate ions from the electrolyte are used up in the reactions, making it necessary to inject carbon dioxide to compensate. Phosphoric Acid fuel cells (PAFC) use phosphoric acid as the electrolyte. Efficiency ranges from 40 to 80 percent, and operating temperature is between 150 to 200 degrees C (about 300 to 400 degrees F). Existing phosphoric acid cells have outputs up to 200 kW, and 11 MW units have been tested. PAFCs tolerate a carbon monoxide concentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must be removed. Platinum electrode-catalysts are needed, and internal parts must be able to withstand the corrosive acid. Proton Exchange Membrane (PEM) fuel cells work with a polymer electrolyte in the form of a thin, permeable sheet. Efficiency is about 40 to 50 percent, and operating temperature is about 80 degrees C (about 175 degrees F). Cell outputs generally range from 50 to 250 kW. The solid, flexible electrolyte will not leak or crack, and these cells operate at a low enough temperature to make them suitable for homes and cars. But their fuels must be purified, and a platinum catalyst is used on both sides of the membrane, raising costs. Solid Oxide fuel cells (SOFC) use a hard, ceramic compound of metal (like calcium or zirconium) oxides (chemically, O2) as electrolyte. Efficiency is about 60 percent, and operating temperatures are about 1,000 degrees C (about 1,800 degrees F). Cells output is up to 100 kW. At such high temperatures a reformer is not required to extract hydrogen from the fuel, and waste heat can be recycled to make additional electricity. However, the high temperature limits applications of SOFC units and they tend to be rather large. While solid electrolytes cannot leak, they can crack.

Multi level inverters topologies and their simulation in Matlab/Simulink

In this study, a review of cascaded H-bridge multilevel inverter topology and control schemes was conducted. Multilevel inverter topology (MLI) H-bridge cascade is implemented to reduce harmonic for high power applications. Applications of multilevel converters are able to reduce the number of harmonics contained in the system of low-voltage electrical distribution. Each topology has their own advantages and disadvantages. The cascaded H-bridge multilevel inverter topology requires only a single DC power source with both input and output, high availability, and the control of power flow in the regenerative version. The selected switching technique to control the inverter will also have an effective role on harmonic elimination while generating the ideal output voltage. Intensive studies have been performed on carrier-based, sinusoidal, space vector and sigma delta PWM methods in open loop control of inverters. The results from this study represent a beneficial basis for matching of inverter topology and the best control scheme according to different application areas. In general, increasing the switching frequency in voltage source inverters (VSIs) leads to better output voltage and current waveforms. Harmonic reduction in controlling a VSI with variable amplitude and frequency of the output voltage is important, and thus, conventional inverters which are referred as two-level inverters require increased switching frequency along with various PWM switching strategies. The multilevel fundamental switching scheme is used to control the needed power electronics switches. Also, a method is presented where switching angles are computed such that a desired fundamental sinusoidal voltage is produced and at the same time certain higher order harmonics are eliminated. The generalized multilevel inverter topology can balance each voltage level by itself regardless of the inverter control and load characteristics. The concept of multilevel converters has been introduced since 1975. The usage of these applications has become more diverse and affects a wide field of electrical engineering from a few watts to several hundred megawatts. Converting static structures that comprise mainly applications of power electronics is becoming increasingly powerful, and the technology has had to adapt to the growth of the power to convert. Multilevel inverter topologies are the Neutral-Point Clamped (NPC) inverters (or DiodeClamped inverters), the cascaded H- bridge inverters (CHB), and the Flying Capacitor (FC) inverters (or Capacitor Clamped inverters), as shown in Figure 1. In this paper, a review of multilevel inverter based on cascaded h-bridge topology and control schemes was conducted. The advantages of this multilevel approach include good power quality, good electromagnetic compatibility (EMC), low switching losses, and high voltage capability.

Matrix Converter An Introduction

Variable-speed motor drive that uses an AC Drive has enjoyed widespread use because of its great energy-saving effect. What is yet unsolved are the suppression of a power harmonic current and the effective use of regenerative energy during deceleration. In order to fully solve these technical issues, we employ the Matrix converter technology, which directly converts from AC power source to AC output. Matrix Converter Basic Principle The matrix converter directly converts AC to AC rather than AC to DC to AC as in conventional voltage source PWM AC Drives. Matrix converters have capability to regenerate power and suppress input current harmonics and noticed as optimum drives for applications ranging from cranes, elevators and centrifuges where regeneration occurs, to air-conditioning fans and feed-water pumps where high harmonic countermeasures are required. Figure 1 shows main circuit configurations of the matrix converter and the conventional voltage source PWM AC Drive. The main circuit of the matrix converter consists of small input filters, which consist of reactors and capacitors, and 9 bi-directional switches. The bi-directional switches consist of the combination of IGBTs shown in Fig.1. On the other hand, the voltage source PWM AC Drive consists of a power AC Drive circuit with the combination of a rectifying circuit on the input side, a smoothing circuit with capacitors on the intermediate part, and IGBTs on the output side. Matrix Converter Voltage source PWM AC Drive

Fig.1 Main circuit configurations of matrix converter and voltage source PWM AC Drive PWM method The matrix converter directly PWM switches 3-phase AC power supply voltage and outputs any given voltage and frequency. Figure 2 shows an example of switching waveform of the matrix converter. Zone 1 of Fig.1 shows that the switching is made from phase-T voltage which is the standard, through phase S which is the intermediate voltage phase, to phase R which is the highest voltage phase. This allows smaller voltage fluctuation than as in conventional AC Drives and therefore suppresses surge voltage and leakage current. PWM waveform example

Fig.2 PWM waveform example Input current control The matrix converter uses electric potential at all the three input phases to PWM switch. Therefore, output voltage and input current can be controlled at the same time. Matrix converters hold two advantages; its energy-saving regeneration and high harmonics suppression effect by controlling a current. Figure 3 shows I/O waveforms of a conventional AC Drive and the matrix converter.

Fig.3 I/O waveforms of conventional AC Drive and matrix converter Figure 4 shows the comparison of input current harmonics. The input current of the matrix converter is almost sine wave and problems associated with harmonics can be easily solved by the matrix converter alone.

Fig.4 Comparison of input current harmonics Effects of matrix converters Matrix converter circuits offer the following effects comparing to conventional AC Drive circuits. Suppression of power harmonics Realizes less than 7% THD of input current and more than 98% input power factor without any specific measures taken. Longer operating life The main circuit does not have endurable parts such as an electrolytic capacitor. This makes the operating life of the main circuit longer and the maintenance interval longer. Elimination of derating With the elimination of current constriction on any specific device, the reduced operation during low-frequency operation is unnecessary. Power regeneration Unique bi-directional switches for directly connecting the power supply and loads enable continuous regeneration. High-efficiency Only the bi-directional switches are used to connect the power supply and loads, allowing higher-efficient operation than as in conventional AC Drives.