1a matrix converter with space vector control enabling overmodulation

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A Matrix Converter with Space Vector Control Enabling OvermodulationJ. Mahlein

A Matrix Converter with Space Vector Control Enabling Overmodulation

J. Mahlein, O. Simon, M. Braun

Universitt Karlsruhe, Elektrotechnisches Institut

Kaiserstr. 12

D-76128 Karlsruhe, Germany

Tel. :++49-721-2783, Fax :++49-721-358854

mahlein@eti.etec.uni-karlsruhe.de

Keywords : Matrix converter, Vector control, Modulation strategy

Abstract :

In this paper the design and testing of a matrix converter using a new method of vector overmodulation is described. The selection of the bi-directional switches as a main part of the converter is discussed and an overview of admissible commutation switching sequences is given. The theoretical output voltage limit which is 86.6% of the input voltage on sinusoidal operation can be increased up to 105%. Using overmodulation means to stress the grid and the load with non sinusoidal currents for higher voltage transfer ratio.

Introduction

The matrix converter is a simple 3 to 3 phase converter as shown in figure 1. By using 9 bi-directional switches the matrix converter is able to create a variable output voltage system of a desired frequency and magnitude [1], [2]. If an ordinary LC-filter is added the grid is loaded nearly by sinusoidal current. This can only be done if the desired maximum output voltage is restricted to 86.6% of the input voltage which is seen as a disadvantage of this special type of converter.

The matrix converter is often compared with a DC voltage link converter. If the line side of the voltage link converter is realised by a simple 3 phase diode rectifier the maximum DC link voltage is 100% of the phase to phase input voltage. Stressing the converter with a load the DC link voltage will decrease to a minimum of 86.6% of the phase to phase voltage depending on the size of the link capacitor and the inductivity of the net and filter coils. The available sinusoidal output voltage ratio is between 100% and 86.6% of the input voltage. A higher voltage transfer ratio compared to the matrix converter has to be paid by costs for a large link capacitor.

M atrix converter

BDS11BDS12BDS13

LC -filter

ElectricBDS21BDS22BDS23

grid

BDS31BDS32BDS33

load

Figure 1: Schematic of the matrix converter

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A Matrix Converter with Space Vector Control Enabling OvermodulationJ. Mahlein

Since all diode rectifiers do not enable sinusoidal load or power recovery to the grid, neither a variable power factor like the matrix converter does. Considering that the requirements for converters connected to the grid will increase with respect to power quality, the matrix converter has to compete with the self commutated line side voltage converter. This type of converter enables a greater output voltage than the line voltage by charging up the DC link. It has the need of 12 transistors and a large DC-link capacitor. The matrix converter consists of 9 bi-directional switches (BDS), 18 transistors if Insulated Gate Bipolar Transistors (IGBTs) and a proper commutation sequence is chosen. Both converter types have the need of filter elements and proper signal proceeding. Comparing this facts the matrix converter is a topology which will take place in several electrical drive applications.

Bi-directional switch configurations

To build up a matrix converter it is necessary to have a switch which is able to conduct current and to block voltage in both of its directions. For a small filter design and a low current ripple it is favourable to chose a high switching frequency of the converter. IGBTs offer these demands from small up to high power levels making them a perfect device for this application. Unfortunately, a bi-directional conducting IGBT is not available at this time and for proper commutation it will be necessary to have a bi-directional device which can be controlled in both current flow directions. This BDS device has to be built up from discrete components. There are three possible configurations to construct a BDS.

Rectifier bridge with IGBT

The configuration in figure 2 has the advantage that only one transistor is needed to build up a BDS but there are 5 semiconductors needed at least to build up a whole switch in which 3 semiconductors will lead a current. This will increase the conduction losses of the BDS. Neither is this configuration able to control the current direction making it useless for proper commutation which will be described later.

Figure 2: Rectifier bridge with IGBT

Emitter connected double configuration

The BDS is realised by two IGBTs with anti parallel diodes in which two semiconductors will lead a current at a certain time instant.

Figure 3: IGBTs emitter connected

The diodes have to be added to ensure a reverse blocking and a bi-directional conduction capability of the switch due to figure 3. With this configuration it is possible to build up a single output phase arrangement of the converter. For example a modular output phase arrangement can be built with the IGBTs of BDS 11, 21, 31 in figure 1 mounted together on a heat sink. The 3 phase converter can be simply built by adding two identical arrangements. Each BDS has to get its own galvanic isolated

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A Matrix Converter with Space Vector Control Enabling OvermodulationJ. Mahlein

gate unit. There are 9 isolated gate power supplies needed altogether. With this configuration the current direction can be controlled in both directions.

Collector connected double configuration

The collector connected double configuration (figure 4) has the same characteristic as the emitter configuration. Input phase arrangements (e.g. BDS 11, 12, 13 in figure 1) can be built which allows to save 3 galvanic isolated gate power supplies for the 3 phase converter because three IGBT emitters are connected to the same input phase and three belong to the same output phase. This circumstance makes the collector configuration more interesting for industrial applications. The disadvantage is a non modular signal structure of the converter.

Figure 4: IGBTs collector connected

Commutation sequence

The basic rule for matrix converter commutation is a switching sequence which does not interrupt the output currents to the inductive load or short-circuit the input voltage sources. Therefore, the demand of a BDS which can control the current in both directions becomes clear. If no clamp circuit is desired because of its extra power losses a step-by-step commutation strategy has to be chosen. The commutation strategy can be voltage controlled as described in [3] or current controlled as given in [4]. A mixture of both is conceivable but not necessary. All possible switching sequences have been investigated. In figure 6 a switching sequence is described by four numbers. Each number represents a switching instance of a transistor as numbered in figure 5 over the time of the commutation sequence. Starting situation is a current il with the transistor 1 and 2 switched on.

12

Vc

34

il

Figure 5: Single output phase

For example a switching sequence with a positive current il is 2314 which means that in a first step transistor 2 is switched off. Second step transistor 3 is switched on, third step transistor 1 is off and fourth transistor 4 on. After the sequence has passed the current il has commutated on transistor 3. For backward commutation the numbers of the transistors have to be exchanged and a sequence has to be chosen from figure 6. In order to compensate the delay times of the IGBTs a time interval has to be added after every switch command.

Theoretically, there are 24 combinations existing of which 12 are not suitable because they will short-circuit the voltage sources or interrupt the load current. Figure 6 shows the remaining sequences in their valid quadrants. Non marked sequences will lead to a proper commutation but the knowledge of

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A Matrix Converter with Space Vector Control Enabling OvermodulationJ. Mahlein

voltage polarity and current direction is needed making the signal processing more complicate than required.i l

2 3 1 42 3 1 4

2 3 4 13 2 1 4

2 4 3 13 1 2 4

4 2 3 13 1 4 2

4 2 3 13 1 4 2Vc

4 2 1 31 3 4 2

4 1 2 31 4 3 2

1 4 2 31 4 2 3

Figure 6: Legal commutation sequences

Current conditioned commutation

The two elliptically marked sequences 2314 and 1423 appear in two voltage quadrants which means that these sequences are independent from voltage polarity. The load current has to be measured in each phase and the current direction has to be evaluated for every switching cycle. A wrong detection of the direction which may appear near zero crossing of the current will interrupt the load current, which means that the blocking voltage at the transistor will rise rapidly with Lload d il d t destroying the voltage sensitive semiconductors if no protection devices are added.

Voltage conditioned commutation

Two rectangularly marked sequences (4231, 3142) can be found which are free of current direction knowledge. Measuring the voltage polarity between the input phases is necessary for this kind of commutation. A polarity detection error which may occur at small voltage values will short circuit the input phases during the commutation process. Especially, the filter capacitors have to be connected with a low impedance commutation circuit to the switches which allows a short circuit current to rise fast. Fortunately, the commutation circuit has 4 semiconductors in series which means that a short

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