Protection circuit for the matrix converter

Dipl.-Ing. M. Pfeifer, Prof. Dr.-Ing. G. Schröder University of Siegen, Institute for Power Electronics and Electrical Drives

E-mail: markus.pfeifer@uni-siegen.de, guenter.schroeder@uni-siegen.de

Abstract- In this paper a protection circuit is presented by which the matrix converter is protected using a free wheeling path available any time. This conducts the load current if necessary, so that the parts of the inverter can not be damaged. With this feature, no complicated commutation strategy and control method are necessary to operate the matrix converter safely. Other advantages arise because a buffer capacitor is not needed for protection. So the size, the weight and the costs of the inverter are reduced.

I. INTRODUCTION

The matrix converter belongs to the group of direct converters. A special feature of the direct converter is that it does not need a dc voltage link. Many advantages, but also some disadvantages, are the result of that fact. The largest disadvantage is that it is easily possible to destroy the components of the matrix converter. In the literature some protective measures are mentioned, which can be divided in two main areas:

One method is based on not to allow a current interruption in the inductive load. In this process intermediate steps are added into the switching operation. This creates multi step commutation [5]-[18] which can be more or less complicated to be implemented into the control. This method needs a certain effort in the measuring systems. They have to work faultlessly to control the converter. A power interruption or emergency off are difficult to control with this method.

Another method is based on the mitigation of the conse- quences of a current interruption. The objective is to keep the voltage peaks that are caused by the current interruption smooth, so that the semiconductors of the matrix converter will not be damaged [1]-[4]. During testing of the capacitor based circuit it turned out that this can povide protection against over voltage only under special premises. One condition is that the capacitor has to be matched exactly to the connected machine. This is difficult if different machines shall be controlled with the converter. It can also be detected that the voltage peaks are limited in their height, but they do not disappear completely. This can be improved with a high-quality capacitor with a small ESL (Equivalent Series Inductance), but the costs rise enormously. Also the size and weight of the converter rise. This is crucial in some applications.

D1 D3 D5

D2 D4 D6

D7 D9

D8 D10 D12

C

M3~

S2

S1 S4 S7

S5 S8

S3 S9S6

D11

supply3~

Fig. 2. Matrix converter with capacitor based protection circuit

iL

Ac

1 2

Ac

3 4

0

uK

iL

Ac

1 2

Ac

3 4

1

uK

iL

Ac

1 2

Ac

3 4

2

uK

In the beginning state the current flows through IGBT 2, whereas the IGBT’s 1 and 4 are also activated.

In the intermediate step 1 IGBT 2 is switched off. The result is that the current is now commutated to the bottom path. Current is not interrupted.

Step 2 shows the target state. Like in the beginning state, a state has been found that allows the current to flow in both directions.

Fig. 1. Principle of the two step commutaion

II. DESCRIPTION OF THE SWITCHABLE FREE WHEELING PATH

The solution of the problem explained above is a free wheeling path available at any time, that independently of the matrix converter switching gives the load current the opportunity to continue to flow [19]. This solution has already been simulated with MATLAB/Simulink®. The simulation supports the previously taken approaches. The free wheeling path for the matrix converter has to be a special one, because the matrix converter is a direct converter, that works with three oscillating input voltages.

Six diodes are switched by six IGBT’s (Insulated Gate Bipolar Transistor) in a way that they provide a free wheeling path parallel to the matrix converter topology. In several critical situations, it can offer a path to the current and protect the switch matrix in this way. This will be made clear in some examples below.

III. DEFINITIONS AND EXPLANATIONS TO THE FOLLOWING

EXAMPLES

- The matrix converter is shown in a possible switching state, that is defined to be switching state (a). This means that L1 is connected to the output terminals V an W, and L3 is connected to output terminal U. - An RB-IGBT (Reverse Blocking Insulated Gate Bipolar Transistor) or an IGBT is to be regarded as switched-on when it is shown in red color in the drawing. - The designations S1.1, S1.2, T1, D1, L1,… in the explanatory texts are related to the designations in the illustrations. - The current flow assumed is shown as a thick green line in the background. The load current flow direction is indicated

by red arrows. - In the examples, the frequency of the three oscillating input voltages is 50 Hz. - In all examples, the section from 16,66 ms to 20,00 ms of the three oscillating input voltages is regarded. It is shown enlarged in figure 3. - The properties of the selected section are (see figure 3):

Phase L1 has, compared to the other two phases, the most positive potential. Phase L3 has, compared the other two phases, the most negative potential. Phase L2 has, compared to the other two phases, a potential between the most positive and the most negative potential. In the enlarged section, the potential of the phases changes with time.

- The switchable free wheeling path is marked purple in figure 4.

IV. EXPLANATION OF THE SWITCHABLE FREE

WHEELING PATH

The switched-on IGBT’s T1 and T6 do not cause a short-circuit because the connected diodes in the portrayed section (16.66 ms to 20.00 ms) are reverse biased.

The switching state of the matrix converter does not have any meaning to the statement above. That means that the matrix converter is able to occupy all eligible switch positions without causing a short circuit. As an example, the matrix converter is shown in the positions a and b in figures 4 and 5. None of these switching states causes a short circuit in conjunction with the switchable free wheeling path.

L1

L2

L3

U V W

S1.2S1.1

S2.1

S3.1

S2.2

S3.2 S9.2

S8.2

S7.2S4.2

S5.2

S6.2

S4.1

S5.1

S6.1

S7.1

S8.1

S9.1

Because the diodes are reverse biased, the current can continue to flow after the RB-IGBT’s are switched off. As an example, the RB-IGBT’s 7.1 and 7.2 are switched off in switching state a in order to reach another possible switching state. The short moment of switching-off is displayed in figures 6 and 7.

0 0.005 0.01 0.015 0.02-300-200-100

0100200300

Netz L1-N(blau) L2-N(rot) L3-N(grün)

Zeit in sec.

Spa

nnun

g in

Vol

t

0.017 0.018 0.019 0.02-300-200-100

0100200300

Netz L1-N(blau) L2-N(rot) L3-N(grün)

Zeit in sec.

Spa

nnun

g in

Vol

t

Time [s]

Vol

tage

[V

]

Time [s]

Volta

ge

[V]

L1-N(blue) L2-N(red) L3-N(green)

L1-N(blue) L2-N(red) L3-N(green)

Fig. 3. selected section 16.66 ms to 20.00 ms

T1 T2 T3 T4 T5 T6

D2D1 D3 D4 D5 D6

i i iwu v

Fig. 4. Switching state a

W

S4.2

S5.2

S6.2

S7.1

S8.1

S9.1 S9.2

S8.2

S7.2

iw

D2D1 D3

T1 T2 T3 T4 T5 T6

D4 D5 D6

V

S4.1

S5.1

S6.1

iu vi

U

L1

L2

L3

Ac

Ac

Ac

S1.2S1.1

S2.1

S3.1

S2.2

S3.2

Fig. 5. Switching state b

It is obvious that the current previously been lead by the RB-IGBT’s S7.1 and S7.2 it is now taken over by the switchable free wheeling path. The load current can now continue to flow through the path D6, T6, S3.2 or D6, T6, source, S4.1 (case 1 in figure 6). For the current direction assumed reversely, the current existing after switching off can be taken over by path D3, T1, S4.1 or the path D3, T1, source, S3.1 and so continue to flow (case 2 in figure 7). So the danger of destruction of one or both RB-IGBT’s (S7.1 and S7.2) is eliminated. The direction of the electric current is irrelevant for the function of the switchable free wheeling path.

Also in case of two or three switches being switched off together, there is a free wheeling path available at every time.

The interruption of one or two phases leading current does not cause any damage, because the switchable free wheeling path offers the opportunity to the electric current to continue to flow.

V. SIMULATION RESULTS AND THEIR EVALUATION

In this section simulation results are presented. They show the effect of the switchable free wheeling path on the matrix converter. The simulation result in figure 8 shows the three output voltage. The corresponding currents through the matrix onverter with switchable free wheeling path, which

where produced using the space vector modulation are shown in figure 9. Here no multi step commutation was implemented which normally would have had caused a current interruption. Because of the switchable free wheeling path no current interruption is caused and so there are no destructive voltage peaks. In a single commutation sequence, which includes the time for switch-off the IGBT, the time that the free wheeling diode needs to reach the conducting state and the switch on time of the following IGBT, it can be stated when watching the complete current flow timing that by “bypassing” of the current flow by the free wheeling path no distinct manipulation of the space vector modulation is generated, because these component determined times are negligible. In figure 10 and 11 is shown that a change of the output frequency also has no effect on the protection function of the switchable free wheeling path. The power of the connected machine should not be ignored, but it can be handled more tolerantly than in case of the capacitor based protection. An overdimensioning does not have any appreciable negative influence on the protection properties.

L1

L2

L3

U V W

Ac

Ac

Ac

S1.2S1.1

S2.1

S3.1

S2.2

S3.2 S9.2

S8.2

S7.2S4.2

S5.2

S6.2

S4.1

S5.1

S6.1

S7.1

S8.1

S9.1

iu iv

D2D1 D3 D4 D5 D6

T1 T2 T3 T4 T5 T6

iw case 1

Fig. 6. Current flow direction in case 1

L1

L2

L3

U V W

Ac

Ac

Ac

S1.2S1.1

S2.1

S3.1

S2.2

S3.2 S9.2

S8.2

S7.2S4.2

S5.2

S6.2

S4.1

S5.1

S6.1

S7.1

S8.1

S9.1

iu vi

D2D1 D3 D4 D5 D6

T1 T2 T3 T4 T5 T6

0.02 0.025 0.03 0.035 0.04-500

-250

0

250

500

Time [s]

Vol

tage

[V]

Fig. 8. The three output voltages of the matrix converter with switchable free wheeling path

iw case 2

Fig. 7. Current flow direction in case 2

0.02 0.025 0.03 0.035 0.04-50-40-30-20-10

01020304050

Time [s]

Cur

rent

[A]

Fig. 9. The corresponding current through the matrix converter with switchable free wheeling path

VI. CONCLUSION

Due to the distinctive features of the switchable free wheeling path no complicated strategy of commutation is necessary. The results of this are lower effort of calculation, less measurements, faster reaction on certain events. A break down of power supply is no problem as well.

Furthermore, an over voltage protection circuit can be omitted and the problems connected with it, because the switchable free wheeling path bypasses the energy if necessary in a way that no damage is caused to the RB-IGBT’s.

Many advantages arise by the switchable free wheeling path for the improved matrix converter in comparison to solutions suggested at present:

clearly higher safety in operation small losses in operation no buffer capacitor no unsafe commutation strategies lower computing power needed lower costs lower weight small size

VII. REFERENCES

0.02 0.025 0.03 0.035 0.04-500

-250

0

250

500

Time [s]

Vol

tage

[V]

Fig. 10. The three output voltages with a frequency of 150 Hz

[1] M. Marcks: “Direkte Regelung eines Matrixumrichters sowie die Möglichkeit zum stromlosen Schalten“, Dissertation, Technische Universität Darmstadt, 1998

[2] P. Nielsen; F. Blaabjerg; J.K. Pedersen: “Novel Solutions for Protection of Matrix Converter to Three Phase Induction Machine”, Proceedings of IEEE-IAS '97, 1997, pages 1447-1454

[3] J. Mahlein; M. Braun: “A matrix converter without diode clamped over-voltage protection”, IPEMC 2000, Peking

[4] P. Nielsen, F. Blaabjerg, K. Pedersen: “New Protection Issues of a Matrix Converter: Design Considerations for Adjustable-Speed Drives”, IEEE TRANSACTIONS ON INDUSTRIAL APPLICATIONS, 1999

[5] N. Burany: “Safe control of 4 Quadrant Switches”, IEEE-Industry Application Society, 1989, pages 1190-1194.

[6] A. Alesina; M. Venturini: “Analysis and design of optiumum amplitude nine-switch direct AC-AC converters”, IEEE Transactions on Power Electronics, Vol. 4, No.1, 1989, pages 101-112.

[7] M. Braun: “Eine dreiphasiger Direktumrichter mit Pulsbreitenmodulation zur getrennten Steuerung der Ausgangsspannung und der Eingangsblindleistung“, Dissertation, TH Darmstadt, 1983.

0.02 0.025 0.03 0.035 0.04

-20

-10

0

10

20

Time [s]

Cur

rent

[A]

Fig. 11. Output currents with a frequency of 150 Hz

[8] W. Hofmann; M. Ziegler: “Multi-step commutation and control policies for matrix converters”, ICPE’01, Seul, Korea, 2001

[9] M. Ziegler; W. Hofmann: “Der Matrixumrichter - Kommutierung in nur zwei Schritten“, SPS/PC/Drives, 1999, pages 521-530

[10] B. Klug: “Untersuchung der Steuerung für Matrixumrichter und Entwicklung eines neuen Verfahrens zur Reduzierung der Gleichkomponente mit Raumzeigermodulation“, Dissertation. Uni Cottbus, 2005

[11] R.R. Beasant; W.C. Beattie; A. Refsum: “An approach to the realization of a high power Venturini converter”, IEEE PESC, 1990

[12] M. Ziegler; W. Hofmann: “A New Two-Step Commutation Policy for Low Cost Matrix Converters”, PCIM 2000 Power Conversion, 2000, pages 445-450

[13] L. Empringham; P.W. Wheeler; J. C. Clare: “Intelligent Commutation of Matrix Converter Bi-directional Switch Cells using Novel Gate Drive Techniques”, PESC, Japan, 1998

[14] L. Wei; T. A. Lipo; H. Chan: “Robust Voltage Commutation of Conventional Matrix Converter”, Proceedings of VPEC/CPES, 2003, section 9.2

[15] M. Ziegler; W. Hofmann: “Knowledge Based Fast Commutation in Converters”, PCIM 2003 Power Conversion, 2003, pages 143-146

[16] B. H. Kwon; B. D. Min; J. H. Kim: “Novel commutation technique of AC-AC converters”, IEE Proc. Electric Power Aplications, Vol. 145, No 4, July 1998, pages 295-300.

[17] L. Empringham; P.W. Wheeler; J. C. Clare: “Bi-directional switch commutation for matrix converters”, EPE Lausanne CD paper 409; 1999

[18] M. Ziegler; W. Hofmann: “Implementation of a two steps commutated matrix converter”, IEEE PESC, Charleston/USA, 1999, pages 175-180.

[19] M. Pfeifer; G. Schröder: “Schaltbarer Freilaufkreis für den Matrix-umrichter”, Patentanmeldung: DE102008016840.8.