CHAPTER1INTRODUCTIONIn recent years, application of Distributed Generation (DG) sources has increased significantly. Microturbine- Generator (MTG) is well suitable for different distributed generation applications, because it can be connected in parallel to serve larger loads, can provide reliable power and has low-emission.MTGs have the rated power from 30 to 250 kW, generating electricity in ac, and they can be installed in isolated conditions or synchronized with the electrical utility. The main characteristics of MTG can be summarized in low maintenance, capacity of operation with liquid and gas fuels (including natural gas) and small area required for installation [1]. MTGs are available as single-shaft or split-shaft units. Single-shaft unit is a high-speed synchronous machine with the compressor and turbine mounted on the same shaft. While, the split-shaft design uses a power turbine rotating at 3000 rpm and a conventional generator connected via a gearbox for speed multiplication [2]. In this paper, the single-shaft structure is considered. Single-shaft MTGs are usually composed of gas turbines, electric power generators (usually a permanent magnet synchronous generator-PMSG, frequency converters interface converters), and protection and control systems (Fig. 2.1) [1]. The interface converter is used to convert PMSG output voltage frequency (high frequency) to power system (50/60 Hz) frequency.

CHAPTER-2MICRO TURBINEMicro turbines are small combustion turbines approximately one third the size of its equivalent diesel engine with outputs of 25 kW to 500 kW. They evolved from automotive and truck turbo-chargers, auxiliary power units for air planes, and small jet engines and comprise a compressor, combustor, turbine, and recuperatorMicro turbines can run on a number of fuels which include; hydrogen, CNG / LPG, alcohol, Kerosene recycled oil, possibly vegetable oil all which reduces dependence on diesel or petrol. Indeed using CNG / LPG the micro turbine is a micro gas turbine. It is the replacement of a conventional thermal engine by a micro turbine which is the significant technical innovation described in this documentMicro turbines offer a number of potential advantages compared to other technologies f or mobile power generation. These advantages include:a) a small number of moving parts,b) compact size with the potential to be located with strict space limitationsc) light-weightd) lower energy costse) lower emissions with multi-fuel capabilityf) improved overall vehicle design due to weight and size savingsg) the opportunities to utilize otherwise waste fuelsAs energy demands increase and the associated costs increasing with the demand, newer energy alternatives are becoming more important to society and also consumers want an economical and uninterrupted electric power. Recently, distributed generation (DG) has become an attractive method of providing electricity to consumers and retailers. In addition, from the viewpoint of economic feasibility, the costs of installing the generators and producing the electricity can be comparatively inexpensive using the DG method. Furthermore, electrical or thermal efficiency can also be improved if the utilities use co-generation or a combined heat cycle [1].One of DG sources is micro turbine generation systems. Micro turbines are small and simple-cycle gas turbines with outputs ranging from around 25 to 300 kW. They are one part of a general evolution in gas turbine technology. The micro turbine is a high-speed single-shaft unit with the compressor and turbine mounted on the same shaft as the electrical alternator. Turbine speeds mainly range from 50000 to 120000 rpm.As microturbines will likely become major DGs in the near future, it is necessary to deal with dynamic models of micro turbine. This paper describes the development of a dynamic model of a micro turbine system. The micro turbine unit consists of a compressor and a turbine connected on a single shaft to a high-speed generator. Moreover there is a combustion chamber, a recuperator and a gas/water heat exchanger. A control system regulates the speed, the temperature and the electric power. To control the frequency, voltage and current of the outgoing power, the micro turbine uses power electronics. Since the potential applications are so different, the emphasis throughout the paper has been on a general model that can be used in as many different operating ranges as possible.The emphasis has been on the functionality and accuracy of the complete model over more detailed modeling of each component. In this paper also, the functional theory of each component is described and how it is modeled in Matlab/Simulink environment. Microturbine System Components

Fig 2.1: micro turbine generation systemA block diagram of a single shaft microturbine system is shown in Figure. 2.2. In a microturbine, a radial flow (centrifugal) compressor compresses the inlet air that is then preheated in the recuperator using heat from the turbine exhaust. Next, the heated air from the recuperator mixes with fuel in the combustor and hot combustion gas expands through the expansion and power turbines. The expansion turbine turns the compressor and, in single shaft models, turns the generator as well. Two-shaft models use the compressor drive turbines exhaust to power a second turbine that drives the generator. Finally, the recuperator uses the exhaust of the power turbine to preheat the air from the compressor. Single-shaft models generally operate at speeds over 60,000 revolutions per minute (rpm) and generate electrical power of high frequency, and of variable frequency.This power is rectified to direct current (DC) and then inverted to 50 or 60 hertz (Hz) for commercial use. The components of a single shaft microturbine system are well defined in the this sections.

Fig 2.2: Block diagram

2.1 Turbo CompressorThe basic components of a microturbine are the compressor, turbine generator, and recuperator (see Figure 2.1). The heart of the microturbine is the compressor-turbine package, which is commonly mounted on a single shaft along with the electric generator. Two bearings support the single shaft. The single moving part of the one-shaft design has the potential for reducing maintenance needs and enhancing overall reliability.In microturbines, the turbo compressor shaft generally turns at high rotational speed, about 96,000 rpm in the case of a 30 kW machine and about 80,000 rpm in a 75 kW machine. One 45 kW model on the market turns at 116,000 rpm [2]. There is no single rotational speed power size rule, as the specific turbine and compressor design characteristics strongly influence the physical size of components and consequently rotational speed. For a specific aerodynamic design, as the power rating decreases, the shaft speed increases, hence the high shaft speed of the small microturbines. Recuperators are heat exchangers that use the hot turbine exhaust gas (typically around 1,200F) to preheat the compressed air (typically around 300F) going into the combustor, thereby reducing the fuel needed to heat the compressed air to turbine inlet temperature. Depending on microturbine operating parameters, recuperators can more than double machine efficiency.The controllers of the gas turbine implements three major control loops: start up, speed and temperature. For the purpose of these modeling tests, the speed control, receives the most attention. The reason for this is that during start up, the unit is not on-line, and in temperature control mode, the governor will not respond to system frequency changes.The primary valve demand control signal is selected by a low value select gate from the outputs of these control loops [3].2.2 GeneratorThe microturbine produces electrical power via a high speed generator turning on the single turbo-compressor shaft. The high-speed generator of the single-shaft design employs a permanent magnet (typically Samarium- Cobalt) alternator, and requires that the high frequency AC output (about 1,600 Hz for a 30 kW machine) be converted to 50 or 60 Hz for general use. This power conditioning involves rectifying the high frequency AC to DC, and then inverting the DC to 50 or 60 Hz AC. Power conversion comes with an efficiency penalty (approximately five percent).

2.3 Power Conditioning UnitAs discussed, single-shaft microturbines feature digital power controllers to convert the high frequency AC power produced by the generator into usable electricity. The high frequency AC is rectified to DC, inverted back to 60 or 50 Hz AC, and then filtered to reduce harmonic distortion. This is a critical component in the single-shaft microturbine design and represents significant design challenges, specifically in matching turbine output to the required load. To allow for transients and voltage spikes, power electronics designs are generally able to handle seven times the nominal voltage. Most microturbine power electronics are generating three phase electricity.2.4 MODELIn this paper, the model proposed in [3,4] is considered for microturbine. The modeling of microturbine has been done in Matlab/Simulink (Fig. 2.3). As can be seen in Fig. 2.3, the model is made up of speed controller, acceleration controller, temperature controller and fuel system (including valve positioner and actuator). The exhaust temperature function f1 and torque function f2 is given by:F1=TR-700(1-WF1)+550(1-W)..(1)F2=1.3(WF2-0.23)+0.5(1-W)(2)where w denotes turbine speed, Wf1 and Wf2 are fuel flows signals, and TR denotes rated exhaust temperature

Fig 2.3: micro turbine model

CHAPTER-3MATRIX CONVERTER (MC)This chapter aims to give a general description of the basic features of a three phase to three phase matrix converter in terms of performance and of technological issues. This chapter does not require to the reader a special knowledge of the matrix converter technology. It is worth noting that the three phase to three phase configuration is just one of the possible direct AC-AC converter topologies [1], which are not in the scope of the present report.The matrix converter has several advantages over traditional rectifier-inverter type power frequency converters. It provides sinusoidal input and output waveforms, with minimal higher order harmonics and no sub harmonics; it has inherent bi-directional energy flow capability; the input power factor can be fully controlled. Last but not least, it has minimal energy storage requirements, which allows to get rid of bulky and lifetime- limited energy-storing capacitors.

Fig 3.1: matrix converterBut the matrix converter has also some disadvantages. First of all it has a maximum input output voltage transfer ratio limited to @ 87 % for sinusoidal input and output waveforms. It requires more semiconductor devices than a conventional AC-AC indirect power frequency converter, since no monolithic bi-directional switches exist and consequently discrete unidirectional devices, variously arranged, have to be used for each bi-directional switch.Finally, it is particularly sensitive to the disturbances of the input voltage system .The comments and remarks made in this chapter are somewhere supported by simulation results obtained from a simplified simulation model.3.1 The topologyThe matrix converter consists of 9 bi-directional switches that allow any output phase to be connected to any input phase. The circuit scheme is shown in Fig.3.2.The input terminals of the converter are connected to a three phase voltage-fed system, usually the grid, while the output terminal are connected to a three phase current- fed system, like an induction motor might be. The capacitive filter on the voltage- fed side and the inductive filter on the current- fed side represented in the scheme of Fig.3.2 are intrinsically necessary. Their size is inversely proportional to the matrix converter switching frequency. It is worth noting that due to its inherent bi-directionality and symmetry a dual connection might be also feasible for the matrix converter: a current- fed system at the input and a voltage- fed system at the output

Fig 3.2: TopologyWith nine bi-directional switches the matrix converter can theoretically assume 512 (29) different switching states combinations. But not all of them can be usefully employed. Regardless to the control method used, the choice of the matrix converter switching states combinations (from now on simply matrix converter configurations) to be used must comply with two basic rules. Taking into account that the converter is supplied by a voltage source and usually feeds an inductive load, the input phases should never be short-circuited and the output currents should not be interrupted. From a practical point of view these rules imply that one and only one bi-directional switch per output phase must be switched on at any instant. By this constraint, in a three phase to three phase matrix converter 27 are the permitted switching combinations.3.2 The performanceThis section gives a short description of what are the performances of a matrix converter. A qualitative analysis of some performance parameters is carried out. Some numerical resultsbased on a simplified model of a matrix converter system are also shown.

3.3 The output voltageSince no energy storage components are present between the input and output side of the matrix converter, the output voltages has to be generated directly from the input voltages. Each output voltage waveform is synthesized by sequential piecewise sampling of the input voltage waveforms. The sampling rate has to be set much higher than both input and output frequencies, and the duration of each sample is controlled in such a way that the average value of the output waveform within each sample period tracks the desired output waveform [2]. As consequence of the input-output direct connection, at any instant, the output voltages have to fit within the enveloping curve of the input voltage system. Under this constraint, the maximum output voltage the matrix converter can generate without entering the over- modulation range is equal to v3/2 of the maximum input voltage: this is an intrinsic limit of matrix converter and it holds for any control law [2],[4].Entering in the over- modulation range, thus accepting a certain amount of distortion in the output voltages and input currents, it is possible to reach higher voltage transfer ratio [5]-[7]. In Fig.3.3 the output voltage waveform of a matrix converter is shown and compared to the output waveform of a traditional voltage source inverter (VSI). The output voltage of a VSI can assume only two discrete fixed potential values, those of the positive and negative DC-bus. In the case of the matrix converter the output voltages can assume either input voltage a, b or c and their value is not time-invariant: the effect is a reduction of the switching harmonics

Fig 3.3: output voltage waveforms generated by a VSI & a MC

3.4 The input currentLikewise to the output voltages, the input currents are directly generated by the output currents, synthesized by sequential piecewise sampling of the output current waveforms. If the switching frequency of the matrix converter is set to a value that is much higher than the input and output frequency, the input currents drawn by the converter are sinusoidal: their harmonic spectrum consists only of the fundamental desired component plus a harmonic content around the switching frequency.In Fig.3.4 the input current drawn by a matrix converter for a 2 kHz switching frequency is shown. It can be noted that the amplitude of the switching harmonic components is comparable to the fundamental amplitude. It is then obvious that an input filter is needed in order to reduce the harmonic distortion of the input line current to an acceptable level. It follows that care should be used in speaking about matrix converters as an all silicon solution for direct AC/AC power conversion, since some reactive components are needed.

The matrix converter performance in terms of input currents represent a significant improvement with respect to the input currents drawn by a traditional VSI converters with a diode bridge rectifier, whose harmonic spectrum shows a high content of low-order harmonics. By the light of the standards related to power quality and harmonic distortion of the power supply this is a very attractive feature of matrix converter.

Fig 3.4: matrix converter input current and harmonic spectrum switching frequency 2khz3.5 The input power factor controlThe input power factor control capability is another attractive feature of matrix converters, which holds for most of the control algorithms proposed in literature [2], [3], [8]-[11]. Despite of this common capability it is worth noting that a basic difference exists with respect to the load displacement angle dependency. For instance, the algorithm proposed in [2] does not require the knowledge of the load displacement angle in order to fully control the input power factor. On the contrary, the algorithm in [3] does require the knowledge of the load displacement angle whenever the reference input power factor is different from unity. From an algorithm computational burden point of view this is a drawback, since it implies additional quite heavy calculations.

Fig 3.5: Matrix converter input line-to-neutral voltage. Instantaneous input current and its average value. switching frequency 2kHz3.6 Implementation of the Matrix ConverterLooking at the basic features of the matrix converter that have been briefly described in the previous sections it might be surprising to establish that this converter topology, today, has not found a wide utilization yet. The reasons have to be sought in a number of practical implementation problems that have slowed down the development of this technology.

3.7 The bi-directional switch realization and commutationA first key problem is related to the bi-directional switches realization. By definition, a bidirectional switch is capable of conducting currents and blocking voltages of both polarities, depending on control actual signal. But at present time a true bi-directional switch is still not available on the market and thus it must be realized by the combination of conventional unidirectional semiconductor devices. Fig 3.1. shows different bi-directional switch configurations which have been used in prototype and/or proposed in literature.Another problem, tightly related to the bi-directional switches implementation, which has represented a main obstacle to the industrial success of the matrix converter, is the commutation problem. The commutation issue basically rises from the absence, in the matrix converters, of static freewheeling paths. As consequence it becomes a difficult task to safely commutate the current from one bi-directional switch to another, since a particular care is required in the timing and synchronization of the switches command signals.

Fig 3.6: possible discrete implementations of a bi-directional switchThe problem of the bi-directional switches implementation and the relevant commutation issue will be surveyed more in detail.3.8 The input filter issueAlthough the matrix converter is sometimes presented as an all silicon solution, due to the lack of the bulky and expensive DC-link capacitors of traditional indirect frequency converter, it also requires a minimum of reactive components, represented by the input filter. The input filter acts as an interface between the matrix converter and the AC mains (Fig.3.7). Its basic feature is to avoid significant changes of the input voltage of the converter during each PWM cycle, and to prevent unwanted harmonic currents from flowing into AC mains [2],[19]. As matter of fact, due to the discontinuous input currents, the matrix converter behaves as a source of current harmonics, which are injected back into the AC mains [16]. Since these current harmonics result in voltage distortions that affect the overall operation of the AC system, they have to be reduced.The principal method of reducing the harmonics generated by static converters is provided by input filter using reactive storage elements

Fig 3.7: schematic representation of a matrix converter adjustable speed driveThe problem of the input filter design for a matrix converter has been addressed in quite few papers and looking at the literature, different configurations have been proposed for the matrix converter input filter. Such differences are a consequence of different design criteria, or at least differently weighted, different switching frequencies and different modulation strategies. In Fig.3.8 three input filter configurations usedin matrix converter prototype are shown.

Fig 3.8: Basic input filter configurations used in matrix converter prototypesIn general, the design of an input filter for static power converters operating from an ac power system has to meet three main requirements:1) carrying out the required switching noise attenuation;2) having a low input displacement angle between filter input voltage and current;3) guaranteeing overall system stability.In addition to these requirements, a set of considerations related to cost, voltage attenuation, system efficiency and filter parameter variation have to be made for an optimized input filter designThe first requirement is usually dictated by the EMI control standards: the input filter has to reduce the input current and output voltage total harmonic distortion below given values. In order to achieve this result, the resonant frequency of the filter has to be positioned accordingly to the converter switching frequency and its PWM pattern. Whe n the input current harmonic spectrum generated by the converter is known, the filter resonance frequency is positioned where no unwanted harmonic components exist, which is usually the frequency range comprised between the fundamental and the switching frequency. In practice, due to the presence of imperfections and asymmetry in gating signals as well as implementation inaccuracies, some unwanted or uncharacteristic harmonics with small amplitude might exist in this region. If no damping is provided, these unwanted harmonics can be amplified by the filter to unacceptable level. On the other hand, a highly damped filter could not meet the harmonics attenuation requirementsWith regard to the matrix converter, Fig.3.8 shows that single stage filter configurations have been basically used to provide harmonic attenuation, but in the light of the new and future EMI standards such configurations are not expected either to meet the regulations or to be economically convenient With regard to the second requirement, it follows by the presence in the filter of reactive storage elements. As it can be clearly seen from Fig.3.8, a phase displacement of the filter input current with respect to the line-to-neutral voltage proportional to the filter capacitance value is always present. Thus, in order to maintain high input power factor the capacitor size has to be minimized. This typically translates into an upper limit for filter capacitor value [4], [23].Yet, the capacitor size limitation has several implications on the filter design. In order to meet the required attenuation specifications, the filter inductor size increases, which results in the overall filter size increase. Moreover, the input filter output impedance, related to the total filter capacitance, is more difficult to control, potentially resulting in converter instability [21].As far as the matrix converter is concerned, a high displacement angle of the input line current due to the input filter capacitance component might be compensated by the matrix converter, setting as reference for the input current a lagging displacement angle. But in this way the maximum voltage transfer ratio for the converter would be significantly reduced. Therefore, even for the matrix converter, the upper limit of the input filter capacitance is set by the minimum acceptable AC mains power factor.The last but not least requirement refers to the control of the impedance interaction between the input filter and the converter. In general, the filter output impedance should be as low as possible when compared to the converter input impedance [23], [24]. The filter output impedance can be reduced by increasing the filter capacitor size. Practically the impedance interaction constraint determines the lower bound on the filter capacitor value. Additionally, proper filter pole damping is extremely important for achieving low filter output impedance for all frequencies and, thus, overall system stability.With regard to the matrix converter, although the stability issue did not appear in the relevant literature, it is not immune from this phenomenon. In conclusion, an optimised design of the matrix converter input filter is a quite difficult task, since relies on a system level approach and in the light of the new coming harmonic and EMI reduction standards it can be somehow considered an outstanding issue.3.9 The protection issueLikewise any other static converter, the matrix converter needs to be protected against the overvoltages and the overcurrents that might be destructive for its semiconductor devices. An effective and robust protection scheme plays a important role in the implementation of a stable and reliable power converter.With respect to an AC drive application of the matrix converters, over voltages can originate externally, as voltage surge existing onto the AC mains, or internally as consequence of a switch commutation error or timing inaccuracies that cause the interruption of an output motor current. This commutation-dependent risk is peculiar to the matrix converter which does not have, differently from traditional DC link converter, any automatic static freewheeling path for the output motor currents. As it will be better explained, the commutation strategies for bi-directional switches today available do neither require, in normal operating conditions, free wheeling paths to safely commutate the output currents nor snubber circuit. The only operating condition in which a free wheeling path is needed is when the motor is disconnected due to an emergency shut-down of the converter. In this case, to prevent destructive over voltages from appearing onto the matrix switches a free wheeling path to the motor currents has to be provided. As far as the overcurrents are concerned, they can rise either from a short circuit through the converter of two input voltages or from an output line-to- line or line-to-earth short circuit. In both cases the protection strategy usually adopted consists in turning all the switches off, using the fact that the currents are monitored and power semiconductors can both withstand and switch considerable overcurrent on a non-repetitive basis [29]. It is obvious that such simply protection strategy can be used only if a free wheeling path is provided to the motor currents. Therefore, the overcurrent protection can be considered as somehow included in the overvoltage protection scheme.

Fig 3.9: Clamp circuit as common protection for all matrix converter bi-directional switchesThe first protection scheme proposed in [2] and [11] is a clamp circuit made up of one or two capacitors connected to all input and all output lines through two diodes bridges (Fig.3.9). This clamp circuit is operative for all nine bi-directional switches. It protects the switches from the surge coming from the input AC line as well as from the surge on the output side that would be otherwise produced whenever an emergency shut-down of the converter is required. As a matter of fact, in the latter case, when the inductive currents of the motor are interrupted, the energy stored in the load is transferred to the clamp capacitor and no critical overvoltage is caused if the capacitor is large enough. Furthermore, the clamp circuit prevents output voltage spikes caused during switches commutation by the parasitic inductance of the power switch matrix and by the unavoidable timing inaccuracies. Since the capacitor voltage increases at each switching operation, some means to discharge the capacitor is required. An efficient energy removal method is to use the clamp energy to power system auxiliaries [11], even though a back up power supply would be probably needed due to the short term ride-through capability of the matrix converter [26].This protection scheme has the advantages of being very simple; it has small hardware requirements and it is safe in all operating conditions. But it has also some drawbacks: it increases the number of the required semiconductor devices by 12 fast-recovery diodes, that might be reduced at 6 using some diodes of the power bi-directional switches [27]; it increases the amount of reactive components needed; and last but not least the optimum design of the clamp capacitor requires the knowledge of the equivalent circuit parameters of the motor [4]. A second recently proposed [28] passive protection scheme for low power applications relies on the use of three varistors, in triangle configuration, added at the input and output side of the converter, as shown in Fig.2.9.

Fig 3.10: Matrix converter with varistor protectionThe input triangle has to protect the converter switches from the voltage surges coming from the AC mains. With regard to the output side, the risk of overvoltages originates, once more, from a hard converter shut-down due to an emergency stop or a converter error. In order to avoid that the output voltages rise to destructive level, the energy stored in the motor leakage inductances has to be managed, providing a free wheeling path to the motor currents. Since this stored energy is rather small, the varistors can be the devices which provide the free wheeling path to the motor currents and absorb the relevant energy. During normal operations, the losses caused by the varistors are not worth mentioning. But the varistors triangles, by themselves, are not sufficient to guarantee, during a converter shut-down, a reliable protection of the matrix IGBTs: a problem occurs when a turning-off bi-directional switch reaches its blocking capability with a certain delay with respect to the others. In this case, the already turned off switches may experience the full overvoltage and being destroyed. In order to protect the single IGBT, a simple circuit made up with a suppressor diode is added to any IGBTs. The basic scheme of the added circuit is shown in Fig.3.11.

Fig 3.11: Gate driver with suppressor protectionThe inserted diode has the characteristic of a Zener diode with a high breakdown voltage. When the collector emitter voltage of the IGBT rises above the breakdown voltage of the suppressor diode , the IGBT is charged again and becomes conductive in its non-saturated region. This operation causes high losses in the IGBT, but it lasts only until all IGBTs are off and so it does not harm the chip.Compared to the clamp circuit solution the varistor/suppressor diode protection scheme demands for some hardware modifications but it has the advantage of not requiring additional power semiconductor devices and reactive storage component, yielding a more compact and costly effective solution. As for the clamp circuit, the equivalent circuit parameters of the motor have to be known in order to select the suitable varistor.An interesting and elegant protection scheme which might be used to prevent output side overvoltages due to hard shut-down of the converter was firstly proposed in [29] and more recently implemented in [30]. The method simply consists in a proper control strategy of the matrix unidirectional switches to be carried out after the emergency stop command has been set and before shutting-down the converter. The control strategy basically aims to create the same operating conditions of a traditional DC-link voltage converter at the shutdown. In traditional DC-link voltage converter (Fig.3.11), when all the switches are turned off, a static free-wheeling path to the motor currents is provided by the free-wheeling diodes. Through these paths the magnetic energy stored in the motor can be automatically transferred to the DC- link energy storage elements without any overvoltages and overcurrents risk. For the matrix converter, since no static free-wheeling paths are available, such operating condition must be actively imposed [30]. The positive and negative DC rails are respectively substituted by the most positive and most negative input line-to-neutral voltage. For each output line current, the unidirectional switches of the matrix that provide a flowing path direct to and coming from the positive and negative rail respectively, have to be turned on.

Fig 3.12: Conventional topology of a diode rectifier voltage source inverterCompared to the previous protection schemes, this solution does not require additional hardware or reactive components; it is efficient and elegant. But it does not protect the converter from input voltage surge and dangerous problems could rise if a temporary input power interruption occurs during freewheeling operations. A possible solution to these drawbacks might be the use of three star connected varistors at the input of the matrix switches.

CHAPTER-4MODELMC is an array of controlled semiconductor switches that connects directly the three-phase source to the threephase load. In the other words, MC performs a direct AC/AC conversion. While, AC/AC conversion is conventionally achieved by a rectifier stage, a dc link and an inverter stage. Since, in the MC the switching is performed on sinusoidal waveforms, the output voltage quality can be better than the conventional rectifierinverter structure. Also, there is no dc-link (large energy storage element) in MC. So, the MC is more compact compared to conventional AC/AC converters [5,6]. A common matrix converter structure consisting of 3x3 switches is shown in Fig. 3. As can be seen, it connects a three-phase voltage source to a three-phase load [6].The matrix converter requires a bidirectional switch capable of blocking voltage and conducting current in both directions. Unfortunately, there are no such devices currently available, so discrete devices need to be used to construct suitable switch cells. In this paper, the common-emitter back to back structure is used as bidirectional switch. The Simulink model of this switch is shown in Fig. 4.1.Normally, the matrix converter is fed by a voltage source and, for this reason; the input terminals should not be short circuited. On the other hand, the load has typically an inductive nature and, for this reason, an output phase must never be opened [5]. Considering Fig. 3.12 and defining the switching function of a single switch as [5]:

K = {A, B,C} _ j={a,b, c}The constraints discussed above can be expressed by:

The load and source voltages of Fig. 3 with reference to supply neutral are considered as follows:

So, it can be written that:

where T is the instantaneous transfer matrix.In order to derive modulation rules, it is also necessary to consider the switching pattern that is employed. This typically follows a form similar to that shown in Fig. 4.2.

Fig 4.1: Simulink model of bi-directional switch

Fig 4.2: switching patternBy considering that the bidirectional power switches work with high switching frequency, a low-frequency output voltage of variable amplitude and frequency can be generated by modulating the duty cycle of theswitches using their respective switching functions.Let mkj(t) be the duty cycle of switch Skj, defined as mkj(t)=tkj/Tseq, which can have the following values

The low-frequency transfer matrix is defined by:

The low-frequency component of the output phase voltage is given by:

Some modulation techniques have been presented for MC control. The most popular of them are Venturini, Scalar, and Space Vector Modulation (SVM) methods [5].In this paper, the Venturini method is applied for MC control. In this method, switching timing can be expressed in terms of the input voltages and the target output voltages, as follows:

where, im V is the amplitude of source voltages

CHAPTER - 5MATLABMatlab is a high-performance language for technical computing. It integrates computation, visualization, and programming in an easy-to-use environment where problems and solutions are expressed in familiar mathematical notation. Typical uses include Math and computation Algorithm development Data acquisition Modeling, simulation, and prototyping Data analysis, exploration, and visualization Scientific and engineering graphics Application development, including graphical user interface building. Matlab is an interactive system whose basic data element is an array that does not require dimensioning. This allows you to solve many technical computing problems, especially those with matrix and vector formulations, in a fraction of the time it would take to write a program in a scalar no interactive language such as C or Fortran. The name matlab stands for matrix laboratory. Matlab was originally written to provide easy access to matrix software developed by the linpack and eispack projects. Today, matlab engines incorporate the lapack and blas libraries, embedding the state of the art in software for matrix computation. Matlab has evolved over a period of years with input from many users. In university environments, it is the standard instructional tool for introductory and advanced courses in mathematics, engineering, and science. In industry, matlab is the tool of choice for high-productivity research, development, and analysis. Matlab features a family of add-on application-specific solutions called toolboxes. Very important to most users of matlab, toolboxes allow you to learn and apply specialized technology. Toolboxes are comprehensive collections of matlab functions (M-files) that extend the matlab environment to solve particular classes of problems. Areas in which toolboxes are available include signal processing, control systems, neural networks, fuzzy logic, wavelets, simulation, and many others. The matlab system consists of five main parts: Development Environment. This is the set of tools and facilities that help you use matlab functions and files. Many of these tools are graphical user interfaces. It includes the matlab desktop and Command Window, a command history, an editor and debugger, and browsers for viewing help, the workspace, files, and the search path. The matlab Mathematical Function Library. This is a vast collection of computational algorithms ranging from elementary functions, like sum, sine, cosine, and complex arithmetic, to more sophisticated functions like matrix inverse, matrix eigenvalues, Bessel functions, and fast Fourier transforms. The matlab Language. This is a high-level matrix/array language with control flow statements, functions, data structures, input/output, and object-oriented programming features. It allows both "programming in the small" to rapidly create quick and dirty throw-away programs, and "programming in the large" to create large and complex application programs.Matlab has extensive facilities for displaying vectors and matrices as graphs, as well as annotating and printing these graphs. It includes high-level functions for two-dimensional and three-dimensional data visualization, image processing, animation, and presentation graphics. It also includes low-level functions that allow you to fully customize the appearance of graphics as well as to build complete graphical user interfaces on your matlab applications. The matlab Application Program Interface (API). This is a library that allows you to write C and Fortran programs that interact with matlab. It includes facilities for calling routines from matlab (dynamic linking), calling matlab as a computational engine, and for reading and writing MAT-files. SIMULINK:Simulink is a software add-on to matlab which is a mathematical tool developed by The Math works,(http://www.mathworks.com) a company based in Natick. Matlab is powered by extensive numerical analysis capability. Simulink is a tool used to visually program a dynamic system (those governed by Differential equations) and look at results. Any logic circuit, or control system for a dynamic system can be built by using standard building blocks available in Simulink Libraries. Various toolboxes for different techniques, such as Fuzzy Logic, Neural Networks, dsp, Statistics etc. are available with Simulink, which enhance the processing power of the tool. The main advantage is the availability of templates / building blocks, which avoid the necessity of typing code for small mathematical processes.Concept of signal and logic flow:In Simulink, data/information from various blocks are sent to another block by lines connecting the relevant blocks. Signals can be generated and fed into blocks dynamic / static).Data can be fed into functions. Data can then be dumped into sinks, which could be scopes, displays or could be saved to a file. Data can be connected from one block to another, can be branched, multiplexed etc. In simulation, data is processed and transferred only at Discrete times, since all computers are discrete systems. Thus, a simulation time step (otherwise called an integration time step) is essential, and the selection of that step is determined by the fastest dynamics in the simulated system.

CHAPTER-6 SIMULATION RESULTSIn this section, the MTG is simulated in Matlab/Simulink. The model of PMSG available at Simulink library is used for generator simulation. This PMSG has 8 poles and its rated power is 30Kw.In simulations, the focus will be on comparison of output voltage quality of two MTG interface converters (matrix converter and conventional rectifier-inverter structure). In order to perform a true comparison, switching frequency of both converters is set to 5kHz and output LC filters parameters are chosen to be the same. The block diagram of the simulated system is shown in Fig. 6.1.The reference speed of the MTG is set to 45000 rpm. At first, The RLC load is 0.2 pu. Then, at t=14sec, the load has a step increase to 0.8 pu. The torque response of the microturbine is compared with the load demand in Fig. 6.2. It can be seen that the torque has a good convergence. Also, speed of MTG is shown in Fig.6.3. As it can be observed, the speed converges to its reference value, too.

Fig 6.1: Simuated systemAt this speed, the output frequency of the PMSG is 3000Hz and must be converted to power system frequency (60Hz). As it is mentioned earlier, it can be achieved using matrix converter or conventional rectifier-inverter structure. In Figs 6.4(a) and 6.4(b), PMSG output phase-a voltages at 0.2 pu and 0.8 pu loads are shown.

Fig 6.2: Mechanical Torque of MTG

Fig 6.3: Speed of MTGAt this speed, the output frequency of the PMSG is 3000Hz and must be converted to power system frequency (60Hz). As it is mentlioned earlier, it can be achieved using matrix converter or conventional rectifier-inverter structure.In Figs 6.4(a) and 6.4(b), PMSG output phase-a voltages at 0.2 pu and 0.8 pu loads are shown.

Fig 6.4:PMSG output voltage:(a) load= 0.2 pu (b) load=0.8 puMatrix and conventional converters operate on these load voltages to construct a 60Hz, 440V(p-p) output voltage. Output waveforms of these converters before filtering are shown in Figs. 6.4 and 6.5, respectively.

Fig 6.5:MC output voltage: load=0.2 pu (top) load =0.8 pu (buttom)As it can be seen, the voltage THD values (5.5% and 4.5% for 0.2 and 0.8 pu loads) using MC are less than the ones in the case of conventional rectifier-inverter structure (7.2% and 6.5% for 0.2 and 0.8 pu loads). Fig 6.6: load terminal voltage using MC: load=0.2 pu (top) load=0.8 pu(bottom)

CHAPTER - 7SIMULATION OUTPUT WAVEFORMS

Fig 7.1: PMSG output

Fig 7.2: Matrix converter output

Fig 7.3: LC filter output

Fig 7.4: Speed characteristics

Fig 7.5: Torque characteristics

CHAPTER-8CONCLUSIONIn this paper, application of the matrix converter as output frequency converter in microturbine-generator is addressed. Comparison of simulation results of MTG using matrix and conventional interface converters demonstrated the ability of MC to deliver a higher quality voltage to the load.Also, it is worthy to be noted that through application of MC the large dc link capacitor which is common in the rectifier-inverter structure is omitted. So, the interface converter can be more compact and less expensive.

REFERENCES[1] E. F. Pavinatto, M. B. Peres, P. D. Reis, L. S. Pereira, and F. P. Salles, Use of microturbines in remote isolated oil and gas facilities in Brazil, IEEE Ind. Appl. Mag., pp. 62-68, Nov./Dec. 2008.

[2] A. K. Saha, S. Chowdhury, S. P. Chowdhury, and P. A. Crossley, Modeling and performance analysis of a microturbine as a distributed energy resource, IEEE Trans. Energy Conv., vol. 24, no. 2, pp. 529-538, Jun. 2009.

[3] T. Yu, J. Tong, and K. W. Chan, Study on microturbine as a back-up power supply for power grid black-start, IEEE Int. Conf. 2009.[4] R. Noroozian, M. Abedi, G. B. Gharehpetian, and S. H. Hosseini, Modelling and simulation of microturbine generation system for on-grid and off-grid operation modes, Int.l Conf. on Renewable Energies and Power Quality(ICREPQ09), Apr. 2009.

[5] P. W. Wheeler, J. Rodriguez, J. C. Clare, L. Empringham, and A. Weinstein, Matrix Converters: A Technology Review, IEEE Trans. Ind. Elec., vol. 49, no. 2, pp. 276-288, Apr. 2002.

[6] P. W. Wheeler, J. C. Clare, L. Empringham, M. Bland, and K. G. Kerris, A vector-controlled MCT matrix converter induction motor drive with minimized commutation time and enhanced waveform quality, IEEE Ind. Appl. Mag., pp. 59-65,Jan./Feb. 2004.1