Proceedings CEEM' 2012/Shang 'hai

EMI disturbance in Double Galvanic Insulation Transformer for high voltage insulation- Application on multilevel converter for

lOkV SiC Mosfet drivers

Lilia Galai 1, Fran�ois Costa 1,2, Bertrand Revol 1 t SATIE, ENS Cachan, CNRS, 61, av du Pro Wilson, 94230 Cachan

2 UPEC, Place du 8 mai 1945,93000 st Denis

Lilia.galai@satie.ens-cachan.fr

Topic: EMC in Power Engineering

Abstract: Actually, in locomotives operating at

50Hz or 162/3Hz, the power train is supplied by a heavy 25kV/3kV iron transformer. To reduce the weigh and volume of this system, a multilevel converter directly linked to the catenary, is intended to be developed based on multilevel techniques at medium frequency. So, each component of the power electronic system has to be insulated for a maximal voltage of 60kV (catenary transient maximal voltage). Particularly, the drivers' system supply of the semiconductors must be insulated for this voltage too. Each driver need to be insulated for another voltage level: 10kV (Voltage level of mosfet SiC used for this application). To deal with this specific constraint, the "Double Galvanic Insulation Transformer" (DGIT) and an adapted supply were

developed. This system was developed to supply SiC Mosfet drivers which particularity is the speed of these switch commutation. The result of this particularity is a high dv/dt. The goal of this study is to see with simulation and measurement the effect of all conducted disturbances acting on the DGIT and its power supply.

I. INTRODUCTION Each level of the multilevel converter is

composed by one current inverter and one voltage inverter (fig. 1) which switches (Mosfet SiC) are supplied by the DGIT system. These two converters are insulated by a transformer for a 60k V dielectric strength. Thanks to this multilevel converter, the catenary voltage is divided by the number of steps [1], [2], [3], [4], [5].

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Figure 1: Multi level converter

The study was made for one level of a multilevel converter presented in figure 1. The structure of one level is shown in figure 2.

Drivers wpply

Current Inverter Voltage inverter

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L ___________________________ J

Figure 2: One level o/the multilevel converter and

localization o/the DG1T

On one level, switches have to be insulated from the others because their reference voltages are different. They need to be insulated for the voltage due to the direct connection to the catenary (fig. 1). This voltage can be three times higher during transients. It implies that the supply of each driver needs a high level insulation for 60kV and 10kV (fig. 2).

In this case, the main difficulty is to elaborate galvanic insulation for two high voltage levels and in the same time, supplying many systems, at different potential, with the same in-voltage.

To fulfill to all requirement of this application, it has been decided to conceive a Double Galvanic Insulation Transformer (DGIT) [6], [7]. Firstly, the sizing of the DGIT was developed. Secondly, the supply adapted to the DGIT was dimensioned and validated

[8].

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Proceedings

II. TRANSFORMER FREQUENCY STUDY A. Modelization of the DGIT

The determination of two transformers in cascade (representing a DGIT with one or several secondary cores) was developed (the model in figure 3 is equivalent to the DGIT in figure 2).

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yp� ;. L ...

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Figure 3: Simplified electrical model of a DGIT

The cascade association of transformers is made thanks to a transformation of admittance to transmittance matrix.

Figure 4: Succession of transformers

In order to calculate the transfer function (ratio Vs2Nel) of two transformers in cascade (figure 4 and 4) we can make a block diagram with an association of transmittance blocks which permit to make

this operation [14]: [J;J= ['z;] * [�]. (EgA)

B. Validation of the DGITmodel

The input impedance of a transformer is composed by a resistive part not depending on the frequency, an inductive part in low frequencies and a capacitive part in high frequencies. Thanks to this information a validation of the model developed can be made with the measurement of the DGIT input impedance in open and short-circuit

[8] and with the measurement of the transfer function (fig.6).

Figure 5: Measurement method for DGITI

978-1-4673-0029-2/12/$26.00 ©2011 IEEE

CEEM' 2012/Shang 'hai

DGIT transfer function . . " ;

-10

........... -20

'-.... r-....

-30

ill � -40 c

'" 'm (9 -50

-60

-70

-80 -- Simulated, �

measured

10 10 10 10 Frequence (Hz)

Figure 6: Transfer fimction for a one secondary DGIT

Figure 6 shows that the model is quite similar to the measurement.

The behaviour of the DGIT IS interesting for high frequencies. It can be observed on the transfer function curve, that the attenuation of the signal increases with the frequency. So the high frequencies signals are naturally filtered by the DGIT.

III. TRANSFORMER POWER SUPPLY A. Power supply development method

The power supply system upstream of DGIT is a voltage inverter at fixed frequency and phase controlled by modulation functions of each leg.

The DGIT is mainly inductive, the reactive power consumption is important. To limit this power, it was selected to operate with a resonant inverter by placing a capacitor in series with the DGIT (fig. 7). The double isolation does not allow an easily measurement of the load and maintaining a high level of isolation, it was chosen to separate the control input and output.

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Figure 7: DGIT power supply

Based on the electrical model, the behavior of the input and output voltages and currents were simulated using PSIM® and compared to measurements.

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In order to verify the results of the model, a mock up was realized (fig. 8).

Figure 8: Inverter, DGITs and MOSFET SiC drivers

Thanks to the validation of the waveforms, the EMI disturbance can be added (in order to simulate a switch disturbance) and its influence can be verified with measurements [10].

B. The conducted EMI re action of the DGIT

Measurements was made on a DGIT8 (DGIT with one primary and 8 secondaries) and a DGIT4 on parallel. This structure is used for supplying all the drivers of one converter level.

A signal at a frequency higher than the power supply frequency was added at the DGIT structure like it can be seen in figure 9.

Equivalent DGIT (res I - - - - - - -

- , IsOGIT

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Figure 9: Adding of the perturbation to the DGIT system

The frequency of perturbation source is

50kHz (fig.lO) and the voltage about 100V. This frequency is the double of the frequency supply. In addition, for this frequency, the attenuation is about 18dB. Beyond this frequency the attenuation is more than 20dB: the signal is insignificant.

Figure 10: Voltage signal of the perturbation source

CEEM' 2012/Shang 'hai

In figure 11, the measurement of the DGIT input voltage, the input and the output DGIT currents, without disturbance, are represented.

Figure 11: Voltage and currents signals without disturbance

In figure 12, the measurement of the DGIT input voltage, the input and the output DGIT currents, with disturbance, are represented.

Figure 12: Voltage and currents signals with

disturbance

In figure 12, it can be seen that the perturbation is a 50kHz signal acting on the output current of the DGIT. All signals at the DGIT input are not disturbed. It can be deduced that the DGIT acts like a filter and that the power supply of the DGIT system don't need a protection for the high frequency disturbances coming from drivers supplied by the DGIT.

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Proceedings

Figure 13: Common mode current caused by the

perturbation source at the DGIT output

With the measurement of the common mode current at the DGIT output (fig. 13), it can be seen that the perturbation IS essentially from common mode.

IV. CONCLUSION In this paper, the application of the

Double Galvanic Isolation Transformer (DGIT) is introduced: It permits to supply the driver of a 10kV SiC Mosfet with a double galvanic insulation for 10k V and 60kV. The SiC technology permits a speed switching of converter, so it results a high dV/dt.

Thanks to the addition of a conducted disturbance at the DGIT system, the reaction of the DGIT is shown: a common mode current appears and acts on the DGIT output current and not on the DGIT input signals. It's due to the frequency properties of the DGIT which acts like a low pass filter. So the DGIT power supply is not a victim of the high voltage switching and don't need a specifically filter.

The next step of the study is to minimize the action of the common mode current on the driver card with a reflection on different solutions [11] and to verify the results with a realization of the model including the 10kV SiC Mosfet.

V. REFERENCES [1] F. lturriz, P. Ladoux; Phase-Controlled

multilevel converters based on dual structure associations, IEEE Transactions on Power Electronics, Volume: 15 , Issue: I, 2000.

[2] J. Martin, P. Ladoux; B. Chauchat, J. Casarin, S. Nicolau, .Medium frequency transformer for

CEEM' 2012/Shang 'hai

railway traction: Soft switching converter with high voltage semi-conductors, SPEEDAM 2008.

[3] F. Iturriz, P. Ladoux, Soft switching DC-DC converter for high power applications, 21st International Conference on Power Electronics Automation, Motion, Drives & Control Power Quality, Nuremberg (Germany), 26-28 May 1998

[4] F. lturriz,.; P. Ladoux, Phase Controlled Multilevel Converters Based On Dual Structure Association, IEEE Transactions on Power Electronics, vol. 15, nOl, January 2000, pp. 92-102.

[5] J. Martin, P. Ladoux, B. Chauchat, Characterisation of !GBTs in Soft Commutation Mode for Medium Frequency Transformer Application in Railway Traction, PCIM'09, Nuremberg (Germany), 12-14 May 2009.

[6] P. Poulichet, F Costa, E Laboure, High­Frequency Modeling of a Current Transformer by Finite-Element Simulation, IEEE Transactions on Magnetics, Volume: 39 , Issue: 2 , Part: 2, 2003.

[7] S.C. Tang, S.Y. Ron Hui, H. Shu-Hung

Chung, A Low-Profile Power Converter Using Printed-Circuit Board (PCB) Power Transformer with Ferrite Polymer Composite, IEEE Transactions on Power Electronics, Volume: 16 , NO: 4, Juillet 2001.

[8] L. Galar, F. Costa, B. Revol, High insulation Power supply for high-power !GBT gate drivers in a multilevel converter, PCIM' 11, Nuremberg, 17-19 May 2011

[9] J-R. Sibue, J-P. Ferrieux, G. Meunier, R. Periot, Generalized average model of series -parallel resonant converter with capacitive outputfilter for high power application, ISlE 2010 IEEE International Symposium on Industrial Electronics

[10] P. Musznicki, JL. Schanen, P. Granjon, P.Chrzan, Better Understanding EM! Generation of Power Converters, IEEE 36th Power Electronics Specialists Conference, 2005. PESC 'OS, June 2005

[11]A. Majid, J. Saleem, K. Bertilsson, EM! filter design for high frequency power converters, lIth International Conference on Environment and Electrical Engineering (EEEIC), 18-25 May 2012

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