1 CHAPTER 1 INTRODUCTION 1.1 Power Inverter An Inverter is basically a converter that converts DC-AC power. Inverter circuits can be very complex so the objective of this method is to present some of the inner workings of inverters without getting lost in some of the fine details. The word inverter in the context of power electronics denotes a class of power conversion circuits that operates from a dc voltage source or a dc current source and converts it into ac voltage or current. Even though input to an inverter circuit is a dc source, it not uncommon to have this dc derived from an ac source such as utility ac supply. Thus, for example, the primary source of input power may be utility ac voltage supply that is converted, to dc by an ac to dc converter and then inverted back to ac using an inverter. Here, the final output may be of a different frequency and magnitude than the input ac of the utility supply [1]. Typical Applications such as Un-interruptible Power Supply (UPS), Industrial (induction motor) drives, Traction, HVDC. 1.2 Multi Level Inverters (MLI) Numerous industrial applications have begun to require higher power apparatus in recent years. Some medium voltage motor drives and utility applications require medium voltage and megawatt power level [26]. For a medium voltage grid, it is troublesome to connect only one power semiconductor switch directly. As a result, a multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations [65]. A multilevel converter not only achieves high power ratings, but also enables the use of renewable energy sources. Renewable energy sources such as photovoltaic, wind, and fuel cells can be easily interfaced to a multilevel converter system for a high power application [3].
2 The concept of multilevel converters has been introduced since 1975. The term multilevel began with the three-level converter. Subsequently, several multilevel converter topologies have been developed. However, the elementary concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc voltage sources. The commutation of the power switches aggregate these multiple dc sources in order to achieve high voltage at the output; however, the rated voltage of the power semiconductor switches depends only upon the rating of the dc voltage sources to which they are connected [6]. Although multilevel inverters were basically developed to reach higher voltage operation, before being restricted by semiconductor limitations, the extra switches and dc sources (supplied by dc-link capacitors) could be used to generate different voltage levels, enabling the generation of stepped waveform with less harmonic distortion, reducing dv/dt and common-mode voltages. These characteristics have made them popular for high-power medium-voltage applications but the large number of semiconductor switches in these inverters, result in a reduction both of the reliability and efficiency of the drive. Therefore, many power electronic researchers have made great effort in developing multilevel inverters with the same benefits and less number of semiconductor devices [7]. 1.2.1Importance of multilevel inverter The importance of multilevel inverters has been increased since last few decades. These new types of inverters are suitable for high voltage and high power application due to their ability to synthesize waveforms with better harmonic spectrum and with less Total Harmonic Distortion (THD). Numerous topologies have been introduced and widely studied for utility of non-conventional sources and also for drive
3 applications. Amongst these topologies, the multilevel cascaded inverter was introduced in Static VAR compensation and in drive systems [18]. The Multi-Level Inverter [MLI] is a promising inverter topology for high voltage and high power applications. This inverter synthesizes several different levels of DC voltages to produce a stepped AC output that approaches the pure sine waveform. It has the advantages like high power quality waveforms, lower voltage ratings of devices, lower harmonic distortion, lower switching frequency and switching losses, higher efficiency, reduction of dv/dt stresses etc. It gives the possibility of working with low speed semiconductors in comparison with the two-level inverters [19]. 1.2.2 Main feature of Multi-Level Inverter (MLI) 1. Ability to reduce the voltage stress on each power device due to the utilization of multiple levels on the DC bus. 2. Important when a high DC side voltage is imposed by an application (e.g. traction systems). 3. Even at low switching frequencies, smaller distortion in the multilevel inverter AC side waveform can be achieved (with stepped modulation technique). 1.2.3 Advantages of Multi-Level Inverter (MLI) A multilevel converter has several advantages over a conventional two-level converter that uses high switching frequency pulse width modulation (PWM). The attractive features of a multilevel converter can be briefly summarized as follows. 1. Staircase waveform quality: Multilevel converters not only can generate the output voltages with very low distortion, but also can reduce the dv/dt stresses; therefore Electro Magnetic Compatibility (EMC) problems can be reduced. 2. Common-Mode (CM) voltage: Multilevel converters produce smaller CM voltage; therefore, the stress in the bearings of a motor connected to a multilevel motor drive
4 can be reduced. Furthermore, CM voltage can be eliminated by using advanced modulation strategies 3. Input current: Multilevel converters can draw input current with low distortion. 4. Switching frequency: Multilevel converters can operate at both fundamental switching frequency and high switching frequency PWM. It should be noted that lower switching frequency usually means lower switching loss and higher efficiency. Unfortunately, the multilevel converters do have some disadvantages. One particular disadvantage is the greater number of power semiconductor switches needed. Although lower voltage rated switches can be utilized in a multilevel converter, each switch requires a related gate drive circuit [40]. This may cause the overall system to be more expensive and complex. Plentiful multilevel converter topologies have been proposed during the last two decades. Contemporary research has engaged novel converter topologies and unique modulation schemes. 1.3 Types of Multi-Level Inverter Currently, in terms of topology, multilevel inverters can be mainly divided into three major groups [44]: Cascaded multilevel inverters : These inverters include several H-bridge cells (Full-bridge inverters) connected in series. One leg of a cascaded multilevel inverter is shown in Fig1.1. In the same figure, it is possible to observe the structure of one individual cell [48].
5 Fig. 1.1 Cascaded multilevel inverter. Diode-clamped multilevel inverters : These inverters use clamped diodes and dc capacitors in order to generate ac voltage. This inverter is manufactured in 3, 4 and 5- level structures. The 3-level structure is known as neutral-point clamped (NPC) and is widely used in medium voltage, high power drives [23]. One leg of an NPC inverter can be seen in Fig.1.2.
6 Fig. 1.2 NPC inverter. Flying-capacitor multilevel inverter [79][50] : In this topology, semiconductor devices are in series and their connecting points are clamped by extra capacitors, as it can shown in Fig. 1.3.
7 Fig. 1.3 Flying-capacitor inverter. 1.3.1 Other Multi Level Inverter 1. Generalized Multilevel Topology. 2. Mixed-Level Hybrid. 3. Multilevel Converter. 4. Soft-Switched Multilevel Converter. 5. Back-to-Back Diode-Clamped Converter. 1.3.2 Hybrid and Asymmetric Multi Level Inverters The topologies mentioned before are typically called symmetric multilevel inverters, because the dc link capacitors have the same voltages [66]. Asymmetric multilevel inverters have the same topology as symmetric ones; the only difference is in the dc link voltages. However, the asymmetric multilevel inverters
8 can generate higher number of output voltage levels with the same number of semiconductor switchers in symmetric ones. Therefore, in these inverters the efficiency is improved by using less semiconductor devices [32] and more complicated switching algorithms; while, output filters are very small or even removed. One leg of an asymmetric multilevel inverter is shown in Fig.1.4. Since the different cells of asymmetric inverter work with different dc link voltages and different switching frequencies, it is more efficient to use appropriate semiconductor devices in different cells. For example, using IGCT integrated (Gate-Commutated Thyristor) switches which are suitable for high voltages low frequency applications, in higher voltage cells decreases the power losses. These inverters are called hybrid multilevel inverters [31]. A hybrid inverter which uses several types of semiconductors has many advantages Active power is transferred by semiconductors with low losses and high reliability and the output harmonic spectrum is improved by other semiconductors. Fig. 1.4 Asymmetric multilevel inverter.
9 1.3.3 Symmetric Multi Level Inverters symmetric multilevel inverters are characterized by the fact that the voltages across the different dc link capacitors are equal[59], importance to mention that the switches applied in the symmetric inverter have the same off-state voltage. 1.4 Different Switching Methods to Reduce Harmonic Distortion Together with the converter topology, great effort has been addressed from the research community in investigating different switching methods for these inverters. This is mainly due to the fact that the adopted switching strategy impacts the harmonic spectrum of output waveforms as well as the switching and the conduction power losses. In case of multilevel converters, three switching methods are usually used. 1. Selective Harmonic Elimination. In this method, each switch is turned on and turned off once in a switching cycle and switching angles are usually chosen based on specific harmonics elimination or minimization of output voltage Total Harmonic Distortion (THD)[57]. 2. Carrier-Based PWM. In this method, drive signals of switches are derived from comparison of reference signal with carrier signals. 3. Space-Vector PWM. The space vector modulation technique is based on reconstruction of sampled reference voltage with help of switching space vectors of a voltage source inverter in a sampling period [52]. 1.5 Cascade Multi Level Inverter Cascade Multilevel Inverter (CMLI) is more recent and popular type of power electronic converter [62] that synthesizes a desired output voltage from several levels of dc voltages as inputs [24]. If sufficient number of dc sources is used, a nearly sinusoidal voltage waveform can be synthesized. CMLI offers several advantages such
10 as, its capabilities to operate at high voltage with lower dv/dt per switching, high efficiency and low electromagnetic interference [EMI]. CMLI is one of the most important topology in the family of multilevel and multi pulse inverters. It requires least number of Components with compare to diode-clamped and flying capacitors type multilevel inverters and no specially designed transformer is needed as compared to multi pulse inverter. It has modular structure with simple switching strategy and occupies less space. A cascaded multilevel inverter is discussed to eliminate the excessively large number of component. 1. Bulky transformers required by conventional multi pulse inverters, 2. Clamping diodes required by multilevel diode-clamped inverters, and 3. Flying capacitors required by multilevel flying-capacitor inverters. 1.5.1 Features of CMLI 1. It is much more suitable to high-voltage, high-power applications than the conventional inverters. 2. It switches each device only once per line cycle and generates a multistep staircase voltage waveform approaching a pure sinusoidal output voltage by increasing the number of levels. 3. Since the inverter structure itself consists of a cascade connection of many singlephase, Full-Bridge Inverter (FBI) units and each bridge is fed with a separate DC source, it does not require voltage balance (sharing) circuits or voltage matching of the switching devices. 4. Packaging layout is much easier because of the simplicity of structure and lower component count.
11 5. Soft-switching can be used in this structure to avoid bulky and lossy resistor - capacitor-diode snubbers. 6. For real power conversions, (ac to dc and dc to ac), the cascaded-inverter needs separate dc sources. The structure of separate dc sources is well suited for various renewable energy sources such as fuel cell, photovoltaic, and biomass, etc. 7. Connecting separated dc sources between two converters in a back-to-back fashion is not possible because a short circuit will be introduced when two backto-back converters are not switching synchronously. 1.5.2 Advantages of CMLI 1. The regulation of the DC buses is simple. 2. Modularity of control can be achieved. Unlike the diode clamped and capacitor clamped inverter where the individual phase legs must be modulated by a central controller, the full-bridge inverters of a cascaded structure can be modulated separately. 3. Requires the least number of components among all multilevel converters to achieve the same number of voltage levels. 4. Soft-switching can be used in this structure to avoid bulky and lossy resistorcapacitor-diode snubbers. These advantages are our motivation to work on the harmonic analysis of the cascaded Three-level, Five-level, Seven-level, Nine level &Twenty seven level inverters. 1.5.3 Cascaded H-Bridges MLI (Multi Level Inverter) Cascaded H-Bridge (CHB) configuration has recently become very popular in high-power AC supplies and adjustable-speed drive applications. A cascade multilevel inverter consists of a series of H-bridge (single-phase full bridge) inverter units in each of its three phases. Each H-bridge unit has its own dc source, which for an induction motor would be a battery unit, fuel cell or solar cell.
12 Each SDC (separate D.C. source) is associated with a single-phase full-bridge inverter. The ac terminal voltages of different level inverters are connected in series. Through different combinations of the four switches, S1-S4, each converter level can generate three different voltage outputs, +, - and zero. The AC outputs of different full-bridge converters in the same phase are connected in series such that the synthesized voltage waveform is the sum of the individual converter outputs. Note that the number of output -phase voltage levels is defined in a different way from those of the two previous converters (i.e. diode clamped and flying capacitor). In this topology, the number of output-phase voltage levels is defined by m= 2N+1, where N is the number of DC sources. A seven-level cascaded converter, for example, consists of three DC sources and three full bridge converters. Minimum harmonic distortion can be obtained by controlling the conducting angles at different converter levels. Each H- bridge unit generates a quasi-square waveform by phase shifting its positive and negative phase switching timings. Each switching device always conducts for 180 (or half cycle) regardless of the pulse width of the quasi-square wave. This switching method makes all of the switching devices current stress equal. In the motoring mode, power flows from the batteries through the cascade inverters to the motor. In the charging mode, the cascade converters act as rectifiers, and power flows from the charger (ac source) to the batteries. The cascade converters can also act as rectifiers to help recover the kinetic energy of the vehicle if regenerative braking is used. The cascade inverter can also be used in parallel HEV configurations. This new converter can avoid extra clamping diodes or voltage balancing capacitors. The combination of the 180 conducting method and the pattern-swapping scheme make the cascade inverter s voltage and current stresses the same and battery voltage balanced. Identical H-bridge inverter units can be utilized, thus
13 improving modularity and manufacturability and greatly reducing production costs. Battery-fed cascade inverter prototype driving an induction motor at 50% and 80% rated speed both the voltage and current are almost sinusoidal. Electromagnetic interference (EMI) and common mode voltage are also much less than what would result from a PWM inverter because of the inherently low dv/dt and sinusoidal voltage output. The main advantages of multilevel cascaded H-bridge converters are as follows. 1. The number of possible output voltage levels is more than twice the number of dc sources (m = 2s + 1). 2. The series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply. 1.6 Total Harmonics Distortion (THD) Harmonic currents, generated by non-linear electronic loads, increase power system heat losses and power bills of end-users. These harmonic-related losses reduce system efficiency, cause apparatus overheating, and increase power and air conditioning costs. As the number of harmonics-producing loads has increased over the years, it has become increasingly necessary to address their influence when making any additions or changes to an installation. Harmonic currents can have a significant impact on electrical distribution systems and the facilities they feed. It is important to consider their impact when planning additions or changes to a system. In addition, identifying the size and location of non-linear loads should be an important part of any maintenance, troubleshooting and repair program [2]. 1.6.1 Sources of Harmonic Distortion Non-linear equipment or components in the power system cause distortion of the current and to a lesser extent of the voltage. These sources of distortion can be divided in three groups:
14 1. Loads 2. The power system itself (HVDC, SVC, transformers, etc) 3. The generation stage (synchronous generators) Subdivision can also be made regarding the connection at different voltage levels. In general, loads can be considered connected at lower voltage levels, the power system exists at all voltage levels and the generation stage at low and medium voltage levels. The dominating distortion-producing group, globally, are the loads. At some locations HVDC-links, SVC s, arc furnaces and wind turbines contributes more than the other sources. The generation stage can, during some special conditions, contribute to some voltage distortion at high voltage transmission level. The characteristic behavior of non-linear loads is that they draw a distorted current waveform even though the supply voltage is sinusoidal. Most equipment only produces odd harmonics but some devices have a fluctuating power consumption, from half cycle to half cycle or shorter, which then generates odd, even and inter harmonic currents. The current distortion, for each device, changes due to the consumption of active power, background voltage distortion and changes in the source impedance. 1.6.2 Harmonics in Electrical Systems One of the biggest problems in power quality aspects is the harmonic contents in the electrical system. Generally, harmonics may be divided into two types: 1) voltage harmonics, and 2) current harmonics. Current harmonics is usually generated by harmonics contained in voltage supply and depends on the type of load such as resistive load, capacitive load, and inductive load. Harmonic currents can produce a number of problems, namely: Equipment heating, Equipment malfunction, Equipment failure, Communications interference, Fuse and breaker disoperation Process problems, Conductor heating. Both harmonics can be generated by either the source or the load side.
15 Harmonics generated by load are caused by nonlinear operation of devices, including power converters, arc-furnaces, gas discharge lighting devices, etc. Load harmonics can cause the overheating of the magnetic cores of transformer and motors [54]. On the other hand, source harmonics are mainly generated by power supply with non-sinusoidal voltage waveform. Voltage and current source harmonics imply power losses, Electro Magnetic Interference (EMI) and pulsating torque in AC motor drives. Any periodic waveform can be shown to be the superposition of a fundamental and a set of harmonic components. By applying Fourier transformation, these components can be extracted. The frequency of each harmonic component is an integral multiple of its fundamental. 1.6.3 Current distortion On three-phase star systems, current distortion causes higher than expected currents in shared neutrals. A shared neutral is one that provides the return path for two or three-phases. Currents as high as 200% of the phase conductors have been seen in the field. This large level of current can easily burn up the neutral creating an open neutral environment. This open neutral creates voltage swells and overvoltage. These voltage conditions easily destroy equipment, particularly power supplies. Another indirect problem introduced by current distortion is called resonance. Certain current harmonics may excite resonant frequencies in the system. This resonance can cause extremely high harmonic voltages, possibly damaging equipment. There is one additional comment about current distortion. When the current is non-sinusoidal, our conventional ammeters and voltmeters will not respond accurately. To accurately measure currents that are harmonically distorted, use a True- RMS meter. This applies equally to distorted voltages.
16 1.6.4 Voltage distortion Voltage distortion, on the other hand, directly affects loads. Distorted voltage can cause motors to overheat and vibrate excessively. It can also cause damage to the motor shaft. Even non-linear loads are prey to voltage distortion. Equipment ranging from computers to electronically-ballasted fluorescent lights may be damaged by voltage distortion. If damage occurs due to current distortion, except for high neutral current, then one solution is to reduce the distortion. There are three methods for this. First, a passive filter can be used to reduce the current from the one or two specific harmonics. In the second method, an active filter reduces all the harmonic currents. It is more costly and complex to use, but it works better than passive filters. The third method involves the use of transformers. Delta-Star transformers reduce certain harmonics, particularly what are called zero sequence harmonics. Zigzag transformers can also be used to reduce zero sequence harmonics, but without changing the system type between delta and star. In addition, they can help reduce high neutral currents. If there is concern that these special transformers or the regular distribution transformers may overheat, then transformer de-rating, or the use of K-rated transformers, is recommended. If high neutral currents are the culprit, then the first step is to eliminate shared neutrals wherever possible. Where this cannot be done, try over sizing the neutral wire so it won't overheat. If this doesn't work, then the distortion must be reduced as described above. There are two ways to reduce voltage distortion. Remember that internal voltage distortion is the result of the business's non-linear loads interacting with the wiring. The first way to reduce the distortion is to reduce the harmonic current. The second way is to reduce the impedance of the wiring. This is done by increasing the size of the
17 conductors. Where the total voltage distortion is the sum of internal and external distortion, these techniques reduce the internal contribution. 1.6.5 Most common causes While motor drives and commercial power supplies are most often blamed for harmonics, the most likely culprits in the typical commercial power system is "switched-mode-power-supplies [33]" such as those seen in personal computers and other electronically driven devices. The typical office can have as much as 50% of its load being determined by devices of this type. There are several methods to indicate of the quantity of harmonics contents. The most widely used measure in North America is the total harmonics distortion (THD) which is defined in terms of the amplitudes of the harmonics, at frequency n, where is frequency of the fundamental component whose amplitude of H 1 and n is integer. The THD is mathematically given by 1.6.6 The Trouble with Harmonics in Modern Power Systems Harmonics are a distortion of the normal electrical current waveform, generally transmitted by nonlinear loads. Switch-Mode Power Supplies (SMPS), variable speed motors and drives, photocopiers, personal computers, laser printers, fax machines, battery chargers and UPSs [43] are examples of nonlinear loads. Single-phase nonlinear loads are prevalent in modern office buildings, while three-phase, non-linear loads are widespread in factories and industrial plants. A large portion of the non-linear electrical load on most electrical distribution systems comes from SMPS equipment. For example, all computer systems use SMPS that convert utility AC voltage to regulated low-voltage DC for internal electronics. These non-linear power supplies draw current in high-amplitude short pulses that create
18 significant distortion in the electrical current and voltage wave shape harmonic distortion, measured as Total Harmonic Distortion (THD). The distortion travels back into the power source and can affect other equipment connected to the same source. Most power systems can accommodate a certain level of harmonic currents but will experience problems when harmonics become a significant component of the overall load. As these higher frequency harmonic currents flow through the power system, they can cause communication errors, overheating and hardware damage, such as: 1. Overheating of electrical distribution equipment, cables, transformers, standby generators, etc. 2. High voltages and circulating currents caused by harmonic resonance 3. Equipment malfunctions due to excessive voltage distortion 4. Increased internal energy losses in connected equipment, causing component failure and shortened life span 5. False tripping of branch circuit breakers 6. Metering errors 7. Fires in wiring and distribution systems 8. Generator failures 9. Crest factors and related problems 10. Lower system power factor, resulting in penalties on monthly utility bills. 1.6.7 A Technical View of Harmonics Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. If the fundamental power frequency is 60 Hz, then the 2 nd harmonic is 120 Hz, the 3 rd is 180 Hz, etc. When harmonic frequencies are prevalent, electrical power panels and transformers become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When this happens, the power panel or transformer vibrates and emits a buzzing sound for the different
19 harmonic frequencies. Harmonic frequencies from the 3 rd to the 25 th are the most common range of frequencies measured in electrical distribution systems. All periodic waves can be generated with sine waves of various frequencies. The Fourier theorem breaks down a periodic wave into its component frequencies. 1.6.8 Solutions to Compensate and Reduce Harmonics While standards to limit the generation of harmonic currents are under consideration, harmonic control today relies primarily on remedial techniques. There are several approaches that can be taken to compensate for or reduce harmonics in the power system, with varying degrees of effectiveness and efficiency. 1.7 Modulation Topologies for Multilevel Inverter It is generally accepted that the performance of an inverter, with any switching strategies, can be related to the harmonic contents of its output voltage [29]. Power electronics researchers have always studied many novel control techniques to reduce harmonics in such waveforms. Up-to-date, there are many techniques, which are applied to inverter topologies. 1.7.1 Sinusoidal Natural Pulse Width Modulation (SPWM) Sinusoidal pulse width modulation is one of the primitive techniques, which are used to suppress harmonics presented in the quasi-square wave [41]. In the modulation techniques, there are two important defined parameters: 1). The ratio P= known as frequency ratio 2). The ratio M = known as modulation index. Where is the reference frequency, is the carrier frequency, is reference signal amplitude, and is carrier signal amplitude. The amplitude of the fundamental frequency components of the output is directly proportional to the modulation depth.
20 The second term of the equation gives the amplitude of the component of the carrier frequency and the harmonics of the carrier frequency [53]. The magnitude of this term decreases with increased modulation depth. Because of the presence of sin (m /2), even harmonics of the carrier are eliminated. Term3 gives the amplitude of the harmonics in the sidebands around each multiple of the carrier frequency. Fig.1.5 Three-phase two-level natural SPWM with a triangular carrier wave. The presence of sin ((m + n) /2) indicated that, for odd harmonics of the carrier, only even-order sidebands exist, and for even harmonics of the carrier only odd order sidebands exist. In addition, increasing carrier or switching frequency does not decrease the amplitude of the harmonics, but the high amplitude harmonic at the carrier frequency is shifted to higher frequency. Consequently, requirements of the output
21 filter can be improved. However, it is not possible to improve the total harmonic distortion without using output filter circuits. In multilevel case, SPWM techniques with three different disposed triangular carrier wave proposed as follows: 1. All the carriers are alternatively in opposition (APO disposition) 2. All the carriers above the zero value reference are in phase among them, but in opposition with those below (PO disposition) 3. All the carriers are in phase (PH disposition). 1.8 Features of VLSI Considerable research over the past two decades focused on the design of parallel machines and many valuable research contributions were made. The mainstream computer market, however, was largely unaffected by this research. Most computers today are Uniprocessor and even large servers have only modest numbers (a few 10s) of processors. In the first conferences, many of the hard problems of parallel machine design have been solved. The design of fast and efficient networks to connect arrays of the processors together and mechanisms those allow processors to quickly communicate. 1) A grid is an intersection of a horizontal and vertical wire [71]. Hence the number of grids on a chip is the square of the number of wiring tracks that fit along one edge of a chip. As VLSI chips are limited by wiring, not devices, the number of grids is a better measure of complexity than the number of transistors. 2) The key here is to match the design of the network to the properties of the implementation technology rather than to optimize abstract mathematical properties of the network. Odds of programming parallel machines have been demonstrated.
22 Research machines were constructed to demonstrate the technology, provide a platform for parallel software research, and solve the engineering problems associated with its realization. The results of this research resulted in numerous commercial machines that form the core of the high-end computer industry today. 3) MIMD machines are preferable to SIMD machines even for data-parallel applications. Similarly, general-purpose MIMD machines are preferable to systolic arrays, even for regular computations with local communication. Bit-serial processors loose more in efficiency than they gain in density. 4) A good general-purpose network (like a 3-D torus) usually outperforms a network with a topology matched to the problem of interest (like a tree for divide and conquer problems). It is better to provide a general-purpose set of mechanisms than to create a specialized machine for a single model of computation. While successful at the high end, parallel VLSI architectures have had little impact on the main stream computer industry. Most desktop machines are Uniprocessor and even departmental servers contain at most a few 10s of processors. Today s mainstream microprocessor chips are dense enough to hold 1000 of the 8086s or 68000s of 1979, yet all of this area is used to implement a single processor. By many objective measures this would clearly be a more efficient architecture. 1.9 Field Programmable Gate Array (FPGA) The most common FPGA architecture consists of an array of configurable logic blocks (CLBs), Input / Output blocks and reconfigurable matrix of interconnects CLBs are used for realization of main logic in FPGA. It typically consists of 4-input Lookup Tables (LUT), multiplexors and flip-flops. LUT is used for realization of combinational logic. It is a 16-bit configurable memory which is capable to realize all 4-input
23 combination logical functions. The output of the LUT can be registered to realize sequential functions [22]. In modern FPGAs are also CLBs with more than 4 inputs integrated. Multiplexors in CLB can be also used for realization of logic function with more than 4 inputs, they allows combine outputs of LUTs CLBs are used for realization of main logic in FPGA. It typically consists of 4-input Lookup Tables (LUT), multiplexers and flip-flops. LUT is used for realization of combinational logic. It is a 16-bit configurable memory which is capable to realize all 4-input combination logical functions. The output of the LUT can be registered to realize sequential functions. In modern FPGAs there are also CLBs with more than 4 inputs integrated. Multiplexors in CLB can be also used for realization of logic function with more than 4 inputs, they allow combine outputs of LUTs FPGAs is higher. On the other hand SRAM memory does not remember the configuration without the power, thus the configuration must be loaded to the SRAM before the start of the FPGA function. It increases boot-time and power consumption. Nonvolatile FPGAs use antifuses or some type of nonvolatile memory (FLASH, EEPROM). This solution provides the advantages of lower power consumption and higher resistance to radiation. Beside basic blocks described above, there are also some others block integrated in most of the FPGAs. Delay-Locked Loops are used for clock signal generation. They synchronize clock signal in whole FPGA a can be also used as clock divider or multiplier. In some application is necessary to use a lot of memory. In these cases, there are Block RAMs integrated in FPGA. Block RAM is usually a dualport RAM with independent control signals for each port. Because FPGAs is widely used for DSP applications, there are multipliers, adders with Carry chain architecture, MAC units etc. integrated in some FPGAs to increase speed of computation. The main advantage of DSP processor in comparison with FPGA is a simple implementation.
24 DSP processor can be program in assembler or C language. On the other hand the computation performance is still limited by the architecture of DSP processor and program realization. 1.10 Advantages of FPGA Any technique can prove its success if and only if it is been implemented in real-time. In order to have a successful hardware implementation, the various constrains viz. availability of equipment, durability for completion and viability of commercial transactions should be overcome. Field programmable Gate Arrays (FPGA) are found to be the most cost-effective and least time consuming with simple solutions for designers to implement their findings in real-time environment. FPGAs are futureoriented building blocks, which allow perfect customization of the hardware at an attractive price even in low quantities. FPGA components available today have usable sizes at an acceptable price. This makes them effective factors for cost savings and time-to-market when making individual configurations of standard products. Re-design of a board can often be avoided through application-specific integration of our desired circuit in the FPGA - an alternative for the future, especially for very specialized applications with only small or medium volumes. FPGA technology is indispensable wherever long-term availability or harsh industrial environments are involved. Another important aspect is long-term availability. Many component manufacturers do not agree on any long-term availability. This makes it difficult or impossible for the board manufacturer to support his product for more than 10 years. The remarkable advantage of FPGAs and their nearly unlimited availability lies in the fact that, even if the device migrates to the next generation, the code remains unchanged. This is in accordance with norms like the (European standards) EN 50155
25 which prescribes that customized parts like FPGAs must be documented to allow reproduction and that the documentation and the source code must be handed out to the customer. In order to have a customized function, normally a device is programmed and is connected to the logic blocks through the transistors as interconnectors. The major benefit of using FPGA has two fold, one is flexibility in design and the other one is fast time in completion of the task.
26 1.11 Thesis Organization A brief outline of the various chapters of the thesis is as follows. Chapter 1: Provides information about the inverters, multilevel inverters (MLI) and Total Harmonic Distortion. It deals with cascade multi level inverters and real time application of multilevel inverters, features and challenges in VLSI and advantage of FPGA. Chapter 2: Deals with review of literature survey about the generation of PWM, hence it is applied in power inverters. It gives detailed studies of work before done and it reviews the PWM and SPWM to be applied in various multilevel inverters. Chapter 3: Provides information about generation of N-bit counter based PWM using FPGA and it is applied as triggering pulse for multilevel inverters. A comparison of Multi Level inverter is performed and analysis the THD values of seven and nine level inverters using induction motor. Chapter 4: Design and Analysis of Various PWM Techniques for Symmetric and asymmetric multilevel inverters using FPGA. PWM pulses are given as triggering input for multilevel inverters and also provide analyzed results. Chapter 5: Discussion about the Design of proposed MFCWPT based PWM techniques for Twenty Seven Level Inverter is presented. MFCPT and MFCWPT based PWM pulse is given as triggering input for Twenty Seven Level Inverter. Performances of multilevel inverters were analyzed. Chapter 6: It deals with the results and discussions of all the methods. Provides information about application of Twenty Seven Level Asymmetric Inverter. Chapter 7: Concludes the overall work, which has been done. It provides highlights on thesis work and suggestions for future work.