High-Power Modular Multilevel Converters with SiC JFETs

Converter (M2C) using Silicon Carbide (SiC) switches. The main focus is on a theoretical investigation of the conduction losses of such a converter an...

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High-Power Modular Multilevel Converters with SiC JFETs Dimosthenis Peftitsis, Georg Tolstoy, Antonios Antonopoulos, Jacek Rabkowski, Jang-Kwon Lim, Mietek Bakowski, Lennart Ängquist, Hans-Peter Nee Introduction

•Silicon Carbide •Modular Multilevel Converter •SiC JFETs •Diode-less operation •HVDC

SiC JFETs seem to be an excellent candidate for future high-power applications due to the high current density and the very low on-state resistance which have been recently reported. In order to evaluate the performance of the existing SiC JFETs on a Modular Multilevel Converter (M2C) (Figure 1), a silicon-based sub-module (SM) has been replaced by a SiC one. An open-loop control method is utilized in order to balance the SM voltages. It is experimentally shown that the operation of the SiC SM is identical to the Si-based, while power losses and efficiency calculations show that a future, high-power SiC M2C for HVDC application would approximately have an efficiency that exceeds 99.8%.

ABSTRACT

Figure 1: Outline of an M2C. (a)

Sub-module design with SiC JFETs

(b

SM SM

 The SiC-based SM which uses JFETs is identical in size

Module Voltage Measurement

and appearance to the Si-based ones which use Si MOSFETs(Figure 2).  The diode-less operation of the SiC JFETs is utilized for the SiC SM, and thus no external anti-parallel diodes are needed (Figure 3). A standard drive-unit for the SiC JFETS has been implemented on the PCB providing a negative voltage for turning-off. The blanking time for the SiC SM has been set to 100ns, thus the reverse current is shortly flowing through the body-diode and then through the channel. This results in the reduction of the conduction losses, because of the lower on-state voltage across the channel than the bodydiode.

(a)

SiC JFETs

Figure 2: Phase-leg of the lab prototype showing the SiC and the Si submodules.

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Power losses for various output powers for 3.3 kV SiC JFETs

Power losses for various output powers for 4.5 kV SiC JFETs

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Email info: [email protected] [email protected] [email protected] [email protected] [email protected]

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future M2C assuming two different SiC JFET switches: 3.3KV/1.2KA and 4.5KV/1.2KA. 167 and 125 SM per arm have been assumed for each case respectively, while the DC input voltage is equal to 300KV. According to the experiments the worst-case, average switching frequency equals approximately to 200Hz. 3.3KV/1.2KA device shows better performance from the power losses point of view and in this case the total efficiency of the converter would be approximately 99.8% at 350MW.

Teknikringen 33, 10044, Stockholm, Sweden

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The power losses and the efficiency as well have been estimated for a 350MW

KTH – Royal Institute of Technology School of Electrical Engineering Laboratory of Electrical Machines and Power Electronics

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Si SM Si SM Si SM Si SM SiC SM 0.5

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Corresponding Authors: Dimosthenis Peftitsis Georg Tolstoy Antonios Antonopoulos Lennart Ängquist Hans-Peter Nee

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Upper and lower arm currents and circulating current

Power losses estimation for a future high-power HVDC M2C with SiC JFETs.

CONTACT INFORMATION

Driver

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Figure 3: (a) Sub-module design with Si MOSFETs, (b) sub-module design with SiC JFETs.

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V dc link V line-line

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phase, was implemented in the lab. The DC input voltage is 500V, which means 100V DC across each SM, while the peak of the load current is 5A and the output frequency equals to 50Hz. A variety of experimental results verify the feasibility of using SiC JFETs in such a multilevel converter.

C

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Three-phase output currents

 A down-scale 10KVA prototype, consisting of 10 SM per

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Experimental results

IlowJFET [A]

This paper studies the possibility of building a Modular Multilevel Converter (M2C) using Silicon Carbide (SiC) switches. The main focus is on a theoretical investigation of the conduction losses of such a converter and a comparison to a corresponding converter with silicon insulated gate bipolar transistors. Both SiC BJTs and JFETs are considered and compared in order to choose the most suitable technology. One of the sub-modules of a down-scaled 10 kVA prototype M2C is replaced with a submodule with SiC JFETs without anti-parallel diodes. It is shown that diode-less operation is possible with the JFETs conducting in the negative direction, leaving the possibility to use the body diode during the switching transients. Experimental waveforms for the SiC sub-module verify the feasibility during normal steady-state operation. The loss estimation shows that a 300 MW M2C for high-voltage direct current transmission would potentially have an efficiency of approximately 99,8 % if equipped with future 3.3 kV 1.2 kA SiC JFETs.

Uc

N Sub-modules

KEYWORDS

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Conclusions The

experimental waveforms show that the SiC SM operates identically to the Si counterpart. The high-temperature operation of SiC devices leads to the reduction of the cooling requirements. It is also shown that the diodeless operation of the SiC JFETs on the SM is feasible. An efficiency that exceeds 99.8% has been estimated for a future 350MW M2C with SiC JFETs used for HVDC transmission.