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PPT ON Recent Trends in Power Electronics and Electric Drivers

Recent Trends in Power Electronics and Electric Drivers Abstract On the basis of the analysis of global trends, a comprehension review of the recent advances of Power electronics that include Power Semiconductor devices, converters, drives control and energy storage systems. Application areas are outlined both for the systems and for the devices incorporated. The perspectives of structural and electronic circuit developments of power electronic systems are presented, taking account of the properties of devices involved, and specific features of potential consumers of the electric power. Keywords MOSFET , IGBT , SCR , GaN , IGCT, Electric Vehicle , Wind Turbine system, DC Drives flywheel, hydrogen, super capacitors, superconducting magnetic energy storage (SMES), Pumped-Hydroelectric Storage (PHS), Global warming. Introduction The technology of Power electronics gone through rapid technological development during the last four decades and recently its applications are fast expanding in industrial, Commercial, military , and utility environments especially industrial drives and electric traction drives, A major hallmark of this unfolding drive development history has been an accelerating trend away from dc commutator machines toward various types of ac brushless machines as a direct result of the continually improving cost-effectiveness of “electronic commutation” made possible by modern power electronics. This progress in power electronics technology has been largely driven by the appearance of successive generations of gate-controlled power switches beginning with bipolar junction transistors (BJTs), followed by MOSFETs and insulated gate bipolar transistors (IGBTs). These power switches have gradually taken over more and more of the applications and power ratings previously dominated by silicon controlled rectifiers (SCRs) and gate turnoff thyristors (GTOs). The availability of these new switches has made it possible to shrink the size of industrial ac adjustable-speed drives (excluding the machine) by an order of magnitude during the past 20 years while halving their cost per kilowatt and also open doors many new technologies like HVDC, Electric Vehicles , Solar Energy . So there is necessity of updating the knowledge with state-of-the-art developments in this field. 1) Recent Power Semiconductor Materials GaN (Gallium Nitride) The use of a GaN device provides many advantages for the user, including reduced switching losses in both the diode and the MOSFET, elimination of active snubber components due to there being no voltage overshoot at turn-off, increased efficiency, and improved temperature performance. The reduction in switching losses in GaN devices can be applied in a number of ways to optimize the user’s circuit design; by increasing efficiency, reducing heatsink requirements, or reducing the current rating of the transistor. The operating frequency can be increased to allow the use of smaller passive components, or to achieve acoustic requirements. The absence of high frequency oscillation at turn-off reduces RFI filter requirements. In October 2007 Velox announced that it will develop 1200V, 100A, GaNon-Silicon transistors for automotive and power supply applications. Rohm of Japan announced the successful development of GaN devices in November 2007, but will use this technology for voltages up to 200V and silicon carbide (SiC) for breakdown voltages of 1000V and above. The below graph will shows roll of different Power semiconductor materials for SMPS design w.r.to time. Power technology and device driven evolution in power electronics with IR’s HEXFET as the starting point 2) Trends in Power Semiconductor Devices A) MOSFET Progress in modern power electronics is provided by the appearance of high-power, practically ideal switches based on transistors (MOSFET, IGBT) and fully controllable thyristors (GTO, IGCT, SGCT). The classification of the types of modern high-power semiconductor devices is shown in Fig. 1. Figure Within the low-voltage region (200–1000 V), the market is winningly occupied by the MOSFET transistors, modules and intellectual power integrated circuits based on them, by replacing bi polar transistors. This is mainly connected with the decrease of the cost of these devices due to technological improvements (trench-gate, Cool MOSTM) and high performance characteristics of the MOSFET transistors: high commuting rate, low static and dynamic losses, low control power, high stability to overload. The limiting parameters of power MOSFET are doubled every two years, while the annual increase of their production is 45–50 %. The outburst made by the Siemens Company in the area of the creation of high-voltage MOSFET with the specific resistance about 3 mm2 (MOSFET 600 V with Rdc(on)= 70 MW in TO-218 housing) will broaden the application area of the devices of this class within commuted voltage range 600–1000 V and power up to 10 kW. The application area of the MOSFET transistors includes uninterrupted power supplies, electronic ballast of modern light sources, automobile electronics (electric rudder amplifiers, starter-generator devices, etc.). frequency and voltage transformers with the power up to 10 kW and transformation frequencies up to several mega hertzes. Fig-1 B) IGBT Bipolar transistors with insulated gate (IGBT) first appeared on market in 1985 and began conquering it quickly. A successful combination of the MOSFET properties (small control power, high commute rate, rectangular region of safe operation, ability to operate parallel without aligning elements, small voltage drop in the open state, high limiting voltage) made IGBT practically ideal power switch. IGBT parameters are continuously improved by manufacturers (direct voltage drop decreased from 4 V in the devices of the 1st generation to 1.2 V at present in the devices of the 4th generation, limiting frequency of switching similarly increased from 5 to 150 kHz and above); limiting characteristics of IGBT modules exhibit a three-fold increase every two years. At present, the company Fuji Electric Co. Achieved the parameters 2000 A, 4.5 kV in the press pack IGBT-module. Infineon Technologies Co. The sales volume of IGBT modules increases continuously (by up to 25 % per year) and has already exceeded US $ 10 billion, which is several times higher than the sales volume of thyristors. The area of IGBT module application is most vast and using in voltage inverters , matrix transformers (These transformers are used in electrical drives fig-2 for different purposes) uninterrupted power supplies, powerful converters of combined automobiles, in electro technologies (various versions of welding equipment) and ion plasma technologies. Fig-2 C) IGCT Some shortcomings of GTO were the reason why they have found only limited application. These shortcomings include: high (up to 4 V) direct drop in voltage, complicated control circuits, necessity to use large snubbers, high dynamic losses. However, modernization of GTO due to the application of novel technologies and due to combining in one device with the control circuit allowed one to increase fast operation of the new type of confinable thyristors (IGCT), decrease static and dynamic losses, providing operation without snubber within the voltage range up to 4.5 V and current 1–10 kA. IGCT overcome IGBT modules in many important parameters: static and dynamic losses, reliability, stability to thermocycling, limiting characteristics, commute power-to-price ratio, possibility of sequential switching without levelling circuits. One more important advantage of IGCT over IGBT is the possibility to block reverse voltage, which is especially important for their use in high-power high-voltage current inverters. Because of this, the leading position in high voltage applications (above 3.5 kV) is likely to be occupied by IGCT. D) SCR Classical thyristors (SCR) still hold the highest “commuting power-to-price” index, in spite of the fact that their application area is constantly decreasing. Because of this, preferable areas of their application are those in which the price is the determining factor, as well as super high-power and super high-current transformers with natural commuting (power engineering, electro technologies) 3) Recent Developments in Power Electronics Systems A).Wind Turbine Systems The wind turbine technology is one of the most emerging renewable technologies. It started in the 1980’es with a few tens of kW production power to today with Multi-MW range wind turbines that are being installed. This also means that wind power production in the beginning did not have any impact on the power system control but now due to their size they have to play an active part in the grid. The technology used in wind turbines was in the beginning based on a squirrel-cage induction generator connected directly to the grid. By that power pulsations in the wind are almost directly transferred to the electrical grid. Furthermore there is no control of the active and reactive power, which typically are important control parameters to regulate the frequency and the voltage. As the power range of the turbines increases those control parameters become more important and it is necessary to introduce power electronics as an interface between the wind turbine and the grid. The power electronics is changing the basic characteristic of the wind turbine from being an energy source to be an active power source. The electrical technology used in wind turbine is not new. It has been discussed for several years but now the price pr. produced kWh is so low, that solutions with power electronics are very attractive. A typical wind systems is shown in below Fig-3 Wind turbines capture the power from the wind by means of aerodynamically designed blades and convert it to rotating mechanical power. The number of blades is normally three. As the blade tip-speed typically should be lower than half the speed of sound the rotational speed will decrease as the radius of the blade increases. For multi-MW wind turbines the rotational speed will be 10-15 rpm. The most weight efficient way to convert the low-speed, high-torque power to electrical power is using a gear-box and a standard fixed speed generator as Illustrated in Fig. 3.The gear-box is optional as multi-pole generator systems are possible solutions. Between the grid and the generator a power converter can be inserted. The possible technical solutions are many and Fig. 5 shows a technological roadmap starting with wind energy/power and converting the mechanical power into Electrical power. It involves solutions with and without gearbox as well as solutions with or without power Electronic conversion. The electrical output can either be ac or dc. FIXED SPEED WIND TURBINES The development in wind turbine systems has been steady for the last 25 years and four to five generations of wind turbines exist. It is now proven technology. The conversion of wind power to mechanical power is as mentioned before done aerodynamically. It is important to be able to control and limit the converted mechanical power at higher wind speed, as the power in the wind is a cube of the wind speed. The power limitation may be done either by stall control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed), active stall (the blade angle is adjusted in order to create stall along the blades) or pitch control (the blades are turned out of the wind at higher wind speed).The wind turbines technology can basically be divided into three categories: the first category is systems without power electronics, the second category is with Power electronics. But in this paper will focus on second category. The wind turbines with a full-scale power converter between the generator and grid, which are the ultimate solutions technically. It gives extra losses in the power conversion but it may be gained by the added technical performance. Fig. 4 shows four possible, but not exhaustive, solutions with full-scale power converters. (a) (b) (c) (d) Fig-4 The solutions shown in Fig. 4a and Fig. 4b are characterized by having a gear. A synchronous generator solution shown in Fig. 4b needs a small power converter for field excitation. Multi-pole systems with the synchronous generator without a gear are shown in Fig.4c and Fig. 4d. The last solution is using permanent magnets, which are still becoming cheaper and thereby more attractive. All four solutions have the same controllable characteristics since the generator is decoupled from the grid by a dc-link. The power converter to the grid enables the system to control active and reactive power very fast. However, the negative side is a more complex system with more sensitive electronic parts. By introducing power electronics many of the wind turbine systems get a performance like a power plant. In respect to control performance they are faster but of course the produced real power depends on the available wind. The reactive power can in some solutions be delivered without having any wind producing active power. Fig. 4 is also indicating other important issues for wind turbines in order to act as a real power source for the grid. They are able to be active when a fault appears at the grid and where it is necessary to build the grid voltage up again; having the possibility to lower the power production even though more power is available in the wind and thereby act as a rolling capacity for the power system. Finally, some systems are able to work in island operation in the case of a grid collapse. A).Electrical Vehicles (EV) EV technologies are multidisciplinary - involving electrical engineering, mechanical engineering, and chemical engineering. Among them, the technology of power electronic drives plays an important role in electrical engineer. The above fig shows the general electrical configuration of EVs, including the BEV, HEV and FCEV. It consists of three major subsystems — electric propulsion, energy source, and auxiliary. The electric propulsion subsystem comprises the electronic controller, power converter, electric motor, mechanical transmission, and driving wheels. The energy source subsystem involves the energy source, energy management unit, and energy refuelling unit. The auxiliary subsystem consists of the power steering unit, temperature control unit, and auxiliary power supply. In the figure, a mechanical link is represented by a double line, an electrical link by a thick line and a control link by a thin line. The arrow on each line denotes the direction of electrical power flow or control information communication. Based on the control inputs from the brake and accelerator pedals, the electronic controller provides proper control signals to switch on or off the power devices of the power converter which functions to regulate power flow between the electric motor and energy source. The backward power flow is due to regenerative braking of the EV and this regenerative energy can be stored provided that the energy source is receptive. Notice that most available EV batteries as well as capacitors and flywheels readily accept regenerative energy. The energy management unit cooperates with the electronic controller to control regenerative braking and its energy recovery. It also works with the energy refuelling unit to control refuelling and to monitor usability of the energy source. The auxiliary power supply provides the necessary power with different voltage levels for all EV auxiliaries, especially the temperature control and power steering units. Besides the brake and accelerator, the steering wheel is another key control input of the EV. Based on its angular position, the power steering unit can determine how sharply the vehicle should turn. So many types of motors and their drive systems will be present in an typical EV system, but this paper will focus on different development of DC. Drives in a EV system. These are explained below. Power converters for DC Motor Drives DC motor drives take the advantages of mature technology and simple control. However, their commutates and brushes make them less reliable and unsuitable for maintenance-free operation. Thus, the DC motor drives are mainly applied to low-cost EVs, such as motorcycles and mini EVs. The corresponding DC-link voltage and power ratings are 24-48 V and 1-3 kW, respectively. The power converters for DC motor drives are traditionally based on hard-switching which results in pulsating currents and voltages, thus imposing high voltage and current stresses on power devices and contributing to electromagnetic interference (EMI). Although there have been many soft-switching DC-DC converters developed for switched-mode power supplies, these converters cannot be directly applied to DC motor drives for EV propulsion. Apart from suffering excessive voltage and current stresses, they cannot handle backward power flow during regenerative braking. It should be noted that the capability of regenerative braking is very essential for EVs as it can extend the vehicle driving range by up to 25%. Fig2 Q-ZVS-MR converter for DC motor drives. In recent years, some soft-switching DC-DC converters have been specially developed for EV propulsion, which offers the capability of bidirectional power flow for motoring and regenerative braking. As shown in above fig a two-quadrant zero-voltage-switching multi-resonant (2Q-ZVS-MR) converter has been applied to DC motor drives [21]. The major advantages of this converter are ZVS operation of both power switches, full ranges of both Voltage conversion-ratio and load, constant-frequency operation, capability of short-circuit operation, and absorption of all major parasitics. However, the high circulating energy and hence the conduction losses are significantly increased, resulting the power devices and other circuit components to be rated for higher VA ratings, as compared with their PWM counterpart. Fig2 Q-ZVTconverter for DC motor drives. Consequently, above shown in fig, a two-quadrant zero-voltage-transition (2Q-ZVT) converter has been specially developed for DC motor drives [22].It possesses the advantages that both the main switches and diodes can switch with ZVS and unity device stresses during both the motoring and regenerating modes of operation. It also offers a simple circuit topology and low cost, leading to achieve high switching frequency, high power density, and high efficiency. Other key features are the use of the same resonant tank for both forward and backward power flows and the full utilization of all built-in diodes of the power switches, thus minimizing the overall hardware count and cost. This 2Q-ZVT converter is particularly useful for those power MOSFET-based DC motor drive applications, which generally suffer from severe capacitive turn-on switching losses. On the other hand, a 2Q-ZVS isolated converter has been proposed for DC motor drives. 2Q-ZVS isolated converter for DC motor drives. As shown in above Fig. converter adopts a dual half-bridge topology which can offer ZVS with neither a voltage-clamping circuit nor additional switching devices and resonant components, leading to a reduced number of devices and hence compact packaging. As an extension from the 2Q-ZVT converter, a two-quadrant zero-current-transition (2Q-ZCT) converter has also been proposed for DC motor drives [24]. As shown in Fig. 6, this 2Q-ZCT 2 Q-ZCT converter for DC motor drives. converter possesses the advantages that both the main and auxiliary switches can operate with zero-current switching (ZCS) during both the motoring and regenerating modes. It takes the role to be particularly useful for those IGBT-based DC motor drive applications, which generally suffer from severe inductive turn-off switching losses. At present, most commercially available electric motorcycles and mini EVs utilize DC motor drives for propulsion, and all of them adopt hard switching DC-DC converters. Since motorcycles and mini vehicles are widely accepted in densely populated cities such as Tokyo, Beijing and Taipei, the development of soft-switching DC-DC converters for electric motorcycles and mini EVs has a definite market value. The major challenges are to elevate the power level of those soft-switching converters up to 3 kW, and to cater at least 2Q but preferably four-quadrant (4Q) operation. 3) Trends in Energy Storage Systems Flywheels In order to improve the quality of the generated power, as well as to support critical loads during mains’ power interruption, several energy-storage technologies have been investigated, developed, proved, and implemented in renewable energy systems. However, flywheels are very commonly used due to the simplicity of storing kinetic energy in a spinning mass. For approximately 20 years, it has been a primary technology used to limit power interruptions in motor/generator sets where steel wheels increase the rotating inertia providing short power interruptions protection and smoothing of delivered power. One of the first commercial uses of flywheels in conjunction with active filtering to improve frequency distortion on a high-voltage power-system line. There are two broad classes of flywheel-energy-storage technologies. One is a technology based on low-speed flywheels (up to 6000 r/min) with steel rotors and conventional bearings. The other one involves modern high-speed flywheel systems (up to 60 000 r/min) that are just becoming commercial and make use of advanced composite wheels that have much higher energy and power density than steel wheels. This technology requires ultralow friction bearing assemblies, such as magnetic bearings, and stimulates a research trend . Most applications of flywheels in the area of renewable energy delivery are based on a typical configuration where an electrical machine (i.e., high-speed synchronous machine or induction machine) drives a flywheel, and its electrical part is connected to the grid via a back-to-back converter, as shown in Fig Such configuration requires an adequate control strategy to improve power smoothing. The basic operation could be summarized as follows. When there is excess in the generated power with respect to the demanded power, the difference is stored in the flywheel that is driven by the electrical machine operating as a motor. On the other hand, when a perturbation or a fluctuation in delivered power is detected in the loads, the electrical machine is driven by the flywheel and operates as a generator supplying needed extra energy. A typical control algorithm is a direct vector control with rotor-flux orientation and sensor less control using a model-reference-adaptive-system (MRAS) observer. Experimental alternatives for wind farms include flywheel compensation systems connected to the dc link, which are the same as the systems used for power smoothing for a single or a group of wind turbines . Usually, a control strategy is applied to regulate the dc voltage against the input power surges/sags or sudden changes in the load demand. A similar configuration can be applied to solar cells. Another renewable energy resource where power oscillations need to be smoothed is wave energy. Recent proposals on using flywheels to regulate the system frequency include the disposal of a matrix of several flywheels to compensate the difference between the network’s load and the power generated. Recently, there has been research where integrated flywheel systems can be encountered. Those systems use the same steel rotor of the electrical machine as energy-storage element .Two of the main advantages of a system like that are its high power density and its similarity with a standard electrical machine. It seems that a new trend for energy storage in renewable energy systems is to combine several storing technologies (as what occurs in uninterruptible power system (UPS) application), where a storage system integrates compressed-air system, thermal storage unit, and flywheel energy storage. Hydrogen This section aims to analyze new trends in hydrogen-storage systems for high-quality back-up power. The hydrogen-fuel economy has been rapidly increasing in industrial application due to the advantages of the hydrogen of being storable, transportable, highly versatile, efficient, and clean energy carrier to supplement or replace many of the current fuel options. It can be used in fuel cells to produce electricity in a versatile way, for example, in portable applications, stationary use of energy, transportation, or high-power generation. The use of fuel cells in such applications is justified since they are a very important alternative power source due to their well-known specific characteristics such as very low toxic emissions, low noise and vibrations, modular design, high efficiency (especially with partial load), easy installation, compatibility with a lot of types of fuels, and low maintenance cost. The increase of the penetration of renewable energies worldwide makes the storage issue critical both in stand-alone and grid-connected application. Hydrogen could be stored as compressed or liquefied gas or by using metal hydrides or carbon nano tubes. For a particular application, the choice of a storage technology implies a trade off between the characteristics of available technologies in terms of technical, economical, or environmental performance. Applications must also include a discussion of the lifecycle efficiency and cost of the proposed storage system. This analysis should consider the total life of the proposed hydrogen storage system including raw-material requirements, manufacturing and fabrication processes, integration of the system into the vehicle or off-board configuration, useful service life, and removal and disposal processes including recycling. Recently, research and development are focused on new materials or technologies for hydrogen storage: metal hydrides (reduce the volumetric and pressure requirements for storage, but they are more complex than other solutions), chemical hydrides, carbon-based hydrogen-storage materials, compressed- and liquid-hydrogen tank technologies, off-board hydrogen-storage systems (a typical refuelling station will be delivering 200–1500 kg/day of hydrogen), and new materials and approaches for storing hydrogen on board a vehicle. Applications to identify and investigate advanced concepts for material storage that have the potential to achieve 2010 targets of 2 kWh/kg and 1.5 kWh/L. Compressed-Air Energy Storage (CAES) Energy storage in compressed air is made using a compressor that stores it in an air reservoir (i.e., an aquifer like the ones used for natural-gas storage, natural caverns, or mechanically formed caverns, etc.). When a grid is operating off peak, the compressor stores air in the air reservoir. During discharge at peak loads, the compressed air is released to a combustor where it is mixed with oil or gas driving a gas turbine. Such systems are available for 100–300 MW and burn about one-third of the premium fuel of a conventional simple cycle combustion turbine. An alternative to CAES is the use of compressed air in vessels (called CAS), which operates exactly in the same way as CAES except that the air is stored in pressure vessels rather than underground reservoirs. Such difference makes possible variations consisting of the use of pneumatic motor acting as compressors or driving a dc motor/generator according to the operation required by the system, i.e., storing energy when there is no extra demand of energy or delivering extra power at peak loads. Recent research is devoted to the maximum-efficiency point tracking control or integrated technologies for power supply applications. Super capacitors Super capacitors, which are also known as ultra capacitors or electric double layer capacitors (EDLC), are built up with modules of single cells connected in series and packed with adjacent modules connected in parallel. Single cells are available with capacitance values from 350 to 2700 F and operate in the range of 2 V. The module voltage is usually in the range from 200 to 400 V. They have a long life cycle and are suitable for short discharge applications and are less than 100 kW. New trends focused on using ultra capacitors to cover temporary high peak power demands, integration with other energy-storage technologies, and development of high-voltage applications. Superconducting Magnetic Energy Storage (SMES) In an SMES, a coil of superconducting wire stores electrical energy in a magnetic field without resistive losses. Also, there is no need for conversion between chemical or mechanical forms of energy. Recent systems are based on both general configurations of the coil: solenoid or toroidal. The second topology has a minimal external magnetic field but the cost of superconductor and coil components is higher than the first topology. Such devices require cryogenic refrigerators (to operate in liquid helium at −269 ◦C) besides the solid-state power electronics. The system operates by injecting a dc current into the superconducting coil, which stores the energy in magnetic field. When a load must be fed, the current is generated using the energy stored in the magnetic field. One of the major advantages of SMES is the ability to release large quantities of power during a fraction of a cycle. Typical applications of SMES are corrections of voltage sags and dips at industrial facilities (1-MW units) and stabilization of ring networks (2-MW units). New trends in SMES are related to the use of low temperature superconductors (liquid-nitrogen temperature), the use of secondary batteries, and the integration of STATCOM and several topologies of ac–dc–ac converters with SMES. Battery Storage The use of batteries as a system to interchange energy with the grid is well known. There are several types of batteries used in renewable energy systems: lead acid, lithium, and nickel. Batteries provide a rapid response for either charge or discharge, although the discharge rate is limited by the chemical reactions and the type of battery. They act as a constant voltage source in the power systems. New trends in the use of batteries for renewable energy systems focused on the integration with several energy sources (wind energy, PV systems, etc.) and also on the integration with other energy-storage systems complementing them. Also, there are attempts to optimize battery cells in order to reduce maintenance and to increment its lifetime Pumped-Hydroelectric Storage (PHS) As batteries, PHS is a mature technology where a swamp of water stored at a certain high elevation is used to generate electric energy by hydro turbines, whenever there is an additional power demand in the grid. When no extra generation is needed, the water is pumped back up to recharge the upper reservoir. One limitation of PHS is that they require significant land areas with suitable topography. There are units with sizes from 30 to 350 MW, with efficiencies around 75%. New trends in PHS are focused on the integration with variable-speed drives (cycloconverters driven doubly fed induction machine) and the use of underground PHS (UPHS), where the lower reservoir is excavated from subterranean rock. Such a system is more flexible and more efficient but requires a higher capital cost. Conclusion Innovation of new power semiconductor materials and huge development in fabrication techniques leads to introduce new power devices with higher configuration and optimum performance this again leads to emerge so many advanced developments in the field of Power Electronics. This paper discussed only two recent developments. But Power Electronics is playing very good role in different sectors like Medical, Military, and Aerospace, Transport, Industrial heating, Automobile, HVDC and so many. By Innovating of electric vehicle shows great contribution of Power Electronics to Global Warming References 1) Development of devices and systems in Power Electronics GEORGY V. GRABOVETSKY, SERGEY A. KHARITONOV 2) Power Electronics Europe – a Magazine, issue 7 ,2008 3)Innovative Power electronics technology by H umida 4) Overview of Power Electronics drives for EV –K T Chau

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