INTRODUCTION
As demands for energy savings have increased in recent years, inverters are being used in a wider range of applications. applications. Demands for lower cost, smaller smaller size and higher efficiency will continue to further expand the range of inverter inverter applications. However, as a trend towards eco-friendly products increases, some sort of measure is necessary to suppress the harmonics contained in the inverter input current.
A matrix converter is capable of converting an input voltage directly into an arbitrary AC voltage, instead of converting that voltage into a DC voltage as inverters. inverters. This matrix converter has higher efficiency, smaller size, longer lifespan and fewer input current harmonics than inverters and has high potential po tential for realizing the above mentioned demands.
A three phase AC ± DC ± AC sparse matrix converter (SMC) having no energy storage elements in the DC link and employing only 15 IGBTs as opposed to 18 IGBTs of a functionally functionally equivalent conventional AC-AC AC-AC matrix converter (CMC) is is proposed. The realization effort could be further reduced to only 9 IGBTs (Ultra Sparse Matrix Converter, USMC) in case the phase displacement of the fundamentals of voltage and current at the input and at the output is limited to
±n/6.
The dependency of the voltage and current transfer ratios of
the systems on the operating parameters is analyzed and a space vector modulation scheme is described in combination with a zero current commutation procedure.
EVOLUTION
OF MATRIX CONVERTER
The inverter is a well known device that converts an input AC voltage into a DC voltage by a rectifier, and then controls the semiconductor switch of a PWM inverter to convert the DC voltage voltage into the desired desired AC voltage. A voltage smoot smoothing hing capacitor capacitor is required in the DC link circuit, and an a n electrolytic capacitor is typically used for t his purpose.
The Sparse Matrix Converter is an AC/AC converter which offers a reduced number of components, a low complexity modulation scheme, and low realization effort. Invented in 2001 by Prof Johann W Kolar, sparse matrix matrix converters avoid multi multi step commutation procedure of the conventional matrix converter, improving system reliability in industrial operations. Its principle principle applications are in highly compact integrated AC drives.
The matrix converter arranges semiconductor switches into a matrix configuration and controls them to convert an input AC voltage directly into the desired AC voltage. Since the input AC voltage voltage is not converted to a DC voltage, there is no need for an energy storage device such as an electrolytic electrolytic capacitor. Bi directional switches switches are needed as the semiconductor switches, since an AC voltage vo ltage is impressed on it.
Fuji Electric is developing a matrix converter capable of converting an input voltage directly into an arbitrary AC voltage, instead of converting that voltage into a DC voltage as inverters. inverters. This matrix converter has higher efficiency, smaller size, size, longer lifespan and fewer input current harmonics than inverters and has high potential for realizing the above mentioned demands.
EVOLUTION
OF MATRIX CONVERTER
The inverter is a well known device that converts an input AC voltage into a DC voltage by a rectifier, and then controls the semiconductor switch of a PWM inverter to convert the DC voltage voltage into the desired desired AC voltage. A voltage smoot smoothing hing capacitor capacitor is required in the DC link circuit, and an a n electrolytic capacitor is typically used for t his purpose.
The Sparse Matrix Converter is an AC/AC converter which offers a reduced number of components, a low complexity modulation scheme, and low realization effort. Invented in 2001 by Prof Johann W Kolar, sparse matrix matrix converters avoid multi multi step commutation procedure of the conventional matrix converter, improving system reliability in industrial operations. Its principle principle applications are in highly compact integrated AC drives.
The matrix converter arranges semiconductor switches into a matrix configuration and controls them to convert an input AC voltage directly into the desired AC voltage. Since the input AC voltage voltage is not converted to a DC voltage, there is no need for an energy storage device such as an electrolytic electrolytic capacitor. Bi directional switches switches are needed as the semiconductor switches, since an AC voltage vo ltage is impressed on it.
Fuji Electric is developing a matrix converter capable of converting an input voltage directly into an arbitrary AC voltage, instead of converting that voltage into a DC voltage as inverters. inverters. This matrix converter has higher efficiency, smaller size, size, longer lifespan and fewer input current harmonics than inverters and has high potential for realizing the above mentioned demands.
Fig. 1
Inverter and matrix converter
As can be seen in Fig. 1 (a), the inverter require charge-up circuit to suppress the inrush current that flows to the electrolytic capacitor connected to the DC link circuit. If a diode rectifier is used as the rectifier, a large amount of input current harmonics harmonics will be generated and therefore, a DC reactor (DCL) is inserted to reduce the current harmonics in the input current. In a conventional conventional inverter, it is necessary necessary to connect a braking unit unit to the DC link circuit in order to dissipate the regenerated power from the motor. motor. A PWM rectifier rectifier was often used to reduce the input current current harmonics and to realize motor motor regeneration.
The matrix
converter, on the other hand, is able to realize motor regeneration with almost no input current harmonics. In other words, a single single converter unit is is able to provide provide performance equivalent to that of a PWM rectifier and an inverter. Additionally, Additionally, the charge-up circuit is unnecessary since since the large electrolytic electrolytic capacitor is not needed for the matrix converter. As a result, result, smaller size and longer lifespan lifespan can be achieved. In Fig. 2, a matrix converter system is compared to a conventional system that uses a PWM rectifier and and an inverter. The conventional system needs needs a filter filter capacitor, a filter reactor and a boost-up reactor in in addition to a main main unit. The matrix converter system, system, however, only needs a main main unit and a filter filter reactor.
Therefore, the
configuration becomes simple and a panel size of the system can be reduced by half or more. In addition, since the matrix converter uses one-stage AC-AC direct conversion, a low loss system can be realized, achieving at least 1/3 lower loss than in the conventional system.
Fig. 2 Comparison of matrix converter with the conventional system
PRINCIPLE
The matrix converter consists of 9 bi-directional switches that allow any output phase to b connected to any input phase. The circuit scheme is shown in Fig. 3.
Fig. 3 Circuit scheme of a three phase to three phase matrix converter a, b, c are at the input terminals. A, B, C are at the output terminals.
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 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, i.e. a current fed system at the input and a voltage fed system at the output.
With nine bi-directional switches, the matrix converter can theoretically 9
assume 512 (2 ) 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.
The Output voltage
Since no energy storage components are present between the input and output sides of the matrix converter, the output voltages have 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. 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 v312 of the maximum input voltage. This is an intrinsic limit of matrix converter and it holds for any control law.
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.
In fig. 4 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 either input voltage a, b or c and their value are not time invariant and the effect is a reduction of the switching harmonics.
Fig. 4 Output voltage waveforms generated by a VSI and a matrix converter
The Input Current
Likewise 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. 5 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 thee input line current to an acceptable level. It follows that care should be taken in speaking about matrix converters as an ³all silicon´ solution for direct AC/AC power conversion, since some react ive components are needed.
Fig. 5 Matrix converter input current and harmonic spectrum. Switching frequency 2 kHz.
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.
The Input Power Factor Control
The input power factor control capability is another attractive feature of matrix converters, which holds for most of the control algorithms proposed in literature. Despite of this common capability, it is worth noting that a basic difference exists with respect to t he load displacement angle dependency.
Fig. 6 matrix converter input line-to-neutral voltage, instantaneous input current and its average value. Switching frequency 2 kHz
For instance, the algorithm proposed does not require the knowledge of the load displacement angle in order to fully control the input power factor. On the contrary, the algorithm does require the knowledge of thee 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.
NEW TECHNOLOGIES FOR THE PRACTICAL APPLICATION OF MATRIX CONVERTER S
The circuit configuration and operating principles of the matrix converter have been known for some time, but there are many problems in achieving practical application. The new technologies that solved these problems are introduced below.
Technology for realizing a reverse blocking IGBT
Table 1 shows the bi-directional switches that are used in matrix converter. An AC voltage is impressed on the bi-directional switches. Because conventional semiconductor switch such as IGBTs do not have reverse blocking capability, diodes for reverse blocking are needed as shown in Table 1 (a). The problem with this diode, however, was that it increased on-state loss and decreased efficiency.
Table 1 Bi-directional switches
In order to solve this problem, Fuji Electric is developing a new IGBT having reverse blocking capability (RB-IGBT). Under a reverse bias, the conventional IGBT generates a large leakage current because its depletion region extends to the dicing surface at the chip side, where severe strain exists after the mechanical dicing process. In the newly developed RB-IGBT, a deep isolation region is formed in the dicing area to prevent expansion of the side surface of the depletion region and to ensure the reverse blocking capability. Recent advance in IGBT manufacturing technology have enabled the realization of the device. The RB-IGBT has the same basic structure as the conventional IGBT, and thus their characteristics are also similar. Moreover, the reverse recovery characteristics are also similar. .Moreover, the reverse recovery characteristic of the RB-IGBT is approximately the same as t hat of the conventional diode.
Fig. 7 Comparison of the matrix converter losses
Fig. 7 compares the loss of matrix converters with each of bi-directional switches shown in table 1(a) and 1 (b). By using the RB-IGBT, the on state loss of a series connected diode is eliminated and although the switching loss remains nearly the same, on state loss can be reduced by approximately 30%.
y
Protection
Technology
Fig. 8 shows the commutation and protection circuit of the matrix converter. Commutation is the process wherein the current flowing to a switch Sa for example, is transferred by turning on a switch Sb and turning off a switch Sa so as to transfer that current to switch Sb. The switch must be controlled, so that there is no short circuit and the load current is not interrupted. If the load current is interrupted, a large surge voltage is impressed upon the semiconductor switch and the switch is damaged.
Therefore, similar to conventional PWM inverter, dead time is provided to prevent a short circuit condition and surge voltage generated during this dead time interval is absorbed by a protection circuit. As a result, loss increases and the protection circuit grow in size, as it requires a large electrolytic capacitor to absorb energy. This reduced advantage of the matrix converter.
Fig. 8 Commutation and protection circuit
The commutation problem is solved by controlling the two RB-IGBTs that compose a bi-directional switch independently. In other words, by keeping a reverse biased switch constantly in its on state, the device is made to behave the same as the freewheeling diode in the conventional PWM inverter, and the load current is not interrupted. The forward biased switch is turned on and off with dead time and controlled similar to a conventional PWM inverter to prevent a short circuit condition. For example, in Fig. 8, if VRS > 0, San and Sbp are reverse biased and therefore are always turned on, while Sap and Sbn are turned on and off with dead time. As a result, while short circuit conditions are being prevented, interruption of the load current is also prevented and the current is commutated safely. In addition, a protection circuit is necessary to protect the device from over current and/or overvoltage. An electrolytic capacitor is generally used in the protection circuit to absorb energy stored in the load. However, using the electrolytic capacitor for the protection circuit reduces the advantage of the matrix converter. To overcome the problem, a new protection circuit is developed. The new protection circuit dissipates the load energy quickly without absorbing the energy to the capacitor. As a result, the electrolytic capacitor is not necessary.
y
Control Technology
With the matrix converter, simultaneous control of the output voltage and input current is possible, but simultaneous and independent control is not easy to implement. The control method becomes complicated because switching one bi-directional switch in order to output a certain voltage causes the change of the input current condition. The higher speed, higher performance and lower cost of control devices in recent years, however, have made it possible to realize even complicated control with ease. In the conventional control method for a matrix converter, the pulse pattern for each bi-directional switch is calculated directly from the condition for obtaining the desired AC output voltage and the condition in which the input current becomes a sinusoidal wave. The control method is unique to the matrix converter and is capable of outputting various pulse patterns. However, since the pulse pattern is calculated directly, it is difficult to control the input current and the output voltage independently.
Fig. 9 Control method for the matrix converter
Then, a new control method was developed, and is shown in Fig. 9. This method is based on the virtual indirect control of a virtual PWM rectifier and a virtual PWM inverter. The matrix converter pulse pattern is obtained by synthesizing the pulse patterns of the virtual PWM inverter and the virtual PWM rectifier. This method enables the input current and output voltage to be controlled independently. In addition, since this control method can be implemented as a direct extension of the control of the conventional PWM inverter, techniques developed in the past can be applied largely without change.
The virtual indirect method
controls the input current and output voltage, and as shown in Fig. 10, assumes a virtual converter comprised of a virtual PWM rectifier and a virtual PM inverter.
Fig. 10 Principle of virtual indirect control method
The virtual indirect control method is based upon the principle that states, ³In a three phase power converter, if the final input and output connections relations are made equal, then the input and output waveforms will not depend on circuit topologies.´ In Fig. 10, for example, if there exist intervals during which the virtual rectifier turns on S rp and Stu, and the virtual PWM inverter turns on switches Sup, Svp and Swn, then the input and output connection relations will be such that R-phase is connected to U-phase and V-phase, and T-phase is connected to W-phase. Consequently, the matrix converter similarly turns on switches Sru, Srv and Stw'. As a result, R-phase is con nected to U-phase and V-pha se, and T-pha se is connected to W-phase, and the operation of the matrix converter becomes same a s that of the conventional PW M system.
Fig. 11 Input and output waveforms
Fig. 11 shows waveforms of the matrix converter with the virtual indirect control method. The load is an induction motor. Unity power factor of the input is observed, and good sinusoidal waveforms were obtained for both the input and output currents.
Fig. 12 Input power factor and THD vs. load torque
Fig. 12 show s the input po wer factor and total harmonic distortion (THD) o f the input current versus load torque. The input power factor is more than 99% at 50% load torque or higher. THD of the input current is also less than 10% at 50% load torque or higher.
Fig. 13 Acceleration and deceleration characteristics (100 r/min 1,200 r/min 100 r/min)
Fig. 14 Impact load torque characteristic (0% 100% 0%)
Figs 13 and 14 show waveform of the acceleration deceleration characteristic and impact load torque characteristic, respectively, in the case of using the vector control method for the induction moto r control. The magnet izing current remains constant even when the torque current changes and it can be verified that vector control achieves good results, similar to those of the conventional motor control. Moreo ver, during deceleration it can b seen that input current increases and power is regenerated.
BLOCK DIAGRAM
BLOCK DIAGRAM D E S CRI P TION
The decade counter is one of the most important components in the matrix con verter configuration. The decade count er used here is CD 4017 . It has sixteen pins. The switching signals are provided by the decade counter. The signals to the decade counter are provided by the 555 IC used as an astable multivibrator.
The optical isolator is used to isolate the DC from the AC stage. The switches used are TIP127 transistors. A 12V, 50Hz AC supply is converted to a higher frequency by the switches.
CIRCUIT DIAGRAM
D E S CR I P TION
The circuit diagram of the matrix converter has 3 main functional sections. The three functional sections are: `
Switching frequency source
`
Decade Counter
`
Switches
S witching
Frequency
S ource
The switching frequenc y sourc e is the 555 timer IC. It provides the switching frequenc y. The frequenc y range of the timer IC is from 50Hz to 3.5 kHz . The output from pin 3 of the 555 timer IC is fed to the counter IC. Decade Counter
CD4 017 is used as the decade counter. It is a 16 pin IC. It has 10 outp uts and a carry. The output from the decade counter is configured to a 3 pin out system and to the fourth output pin RE SET is applied. Any three of the output pins can be used for the output configuration required for the matrix converter. The pins 3, 2 and 4 have been used as the output in the circuit diagra m. The pin numb er 7 is set as the RESET. Each of the three pins provides an output frequency with the time period of fraction of a millisecond. This output frequency is fed to the switching transistors via an optical isolator. IS4N 35 is the optical isolator used. The optical isolator is used o nly to provide isolation betwee n the input and the out put sides. S witches
TIP 127 is used as switches. TIP 127 is a pnp configuration transistor. The AC from the supply is fed directly to the switches. The switches turn ON and O FF depending upon the output from the decade counter. Depending on the ON and O FF cond ition of the switches, the AC current from the supply splits. Th is split in the AC gives a high frequency characteristic to the output wave.
A WAVEFORM FILE
P CB
LAYOUT
C OM P ON E N T S
details ADVANTAGES
`
Sinusoidal i/p and o/p waves with min harmonics.
`
Input power can be fully controlled.
`
Loss is one third of that in conventional converters.
`
Minimum energy storage requirements, no more bulky, limited energy storage capacitors hence there is possibility of a more compact design
`
Adjustable (including unity) power factor
`
Bidirectional power flow
`
High quality waveform
DISADVANTAGES
`
Higher complexity in modulation and analysis effort
`
Requires more semiconductor devices
`
Sensitive to disturbances of the i/p voltage system
APPLICATIONS
`
Implementation as power supplies
`
Realization of highly compact AC drives
`
High potential in industrial, military, marine and avionics
`
Employed in Wind Energy Conversion S ystems(WECS)
FUTUR E SCOPE
`
Matrix converters can be seen as a future replacement concept for variable speed drives technology
`
Future applications in fields that now use PWM rectifiers and inverters.
CONCLUSION
As proposed, the full functionality of a 3 phase AC-AC converter can be achieved by using a few number of switches, and the absence of a dc link with high efficiency.
