matrix converter _ report

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INTRODUCTION

As demands for energy savings have increased in recent years, inverters are being used in a wider range of applications. Demands for lower cost, smaller size and higher efficiency will continue to further expand the range of 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. This matrix converter has higher efficiency, smaller size, longer lifespan and fewer input current harmonics than inverters and has high potential 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 equivalent conventional AC-AC matrix converter (CMC) 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 into the desired AC voltage. A voltage smoothing capacitor is required in the DC link circuit, and an electrolytic capacitor is typically used for this 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 converters avoid multi step commutation procedure of the conventional matrix converter, improving system reliability in industrial operations. Its 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 is not converted to a DC voltage, there is no need for an energy storage device such as an electrolytic capacitor. Bi directional switches are needed as the semiconductor switches, since an AC voltage 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. This matrix converter has higher efficiency, smaller 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 will be generated and therefore, a DC reactor (DCL) is inserted to reduce the current harmonics in the input current. In a conventional inverter, it is necessary to connect a braking unit to the DC link circuit in order to dissipate the regenerated power from the motor. A PWM rectifier was often used to reduce the input current harmonics and to realize 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 converter unit is able to provide performance equivalent to that of a PWM rectifier and an inverter. Additionally, the charge-up circuit is unnecessary since the large electrolytic capacitor is not needed for the matrix converter. As a result, smaller size and longer lifespan can be achieved. In Fig. 2, a matrix converter system is compared to a conventional system that uses a PWM rectifier and an inverter. The conventional system needs a filter capacitor, a filter reactor and a boost-up reactor in addition to a main unit. The matrix converter system, however, only needs a main unit and a 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 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.

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 all silicon solution for direct AC/AC power conversion, since some reactive 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 hi

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