Electronica

Chapter 2

Fundamentals of Matrix Converter
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________________ 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 nosubharmonics; 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. But the matrix converter has also some disadvantages. First of all it has a maximum inputoutput voltage transfer ratio limited to ≅ 87 % forsinusoidal 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 [2],[3].The comments and remarks made in this chapter are somewhere supported by simulation results obtained from a simplified simulation model described in chapter 4.

2.1 The topology The 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.2.1. The input terminals of the converter are connected to athree 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.2.1 are intrinsically necessary. Their size is inversely proportional to the matrix converter switchingfrequency [2].

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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. SAa SAb
Bi-directional switch

A

SAc SBa SBb SBc SCa SCb SCc C B

a b c

Fig.2.1 Circuit scheme of a three phase to three phase matrix converter. a,b,care at the input terminals. A,B,C are at the output terminals.

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.

2.2 The performance This section gives a short description of what are the performance of a matrix converter. A qualitative analysis of some performance parameters is carried out. Some numerical results based on a simplified model of a matrix converter system are also shown....