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.
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