**Marek Adamowicz 1,\* and Janusz Szewczyk <sup>2</sup>**


Received: 17 September 2020; Accepted: 22 October 2020; Published: 24 October 2020

**Abstract:** The design of rolling stock plays a key role in the attractiveness of the rail transport. Train design must strictly meet the requirements of rail operators to ensure high quality and cost-effective services. Semiconductor power devices made from silicon carbide (SiC) have reached a level of technology enabling their widespread use in traction power converters. SiC transistors offering energy savings, quieter operation, improved reliability and reduced maintenance costs have become the choice for the next-generation railway power converters and are quickly replacing the IGBT technology which has been used for decades. The paper describes the design and development of a novel SiC-based DC power electronic traction transformer (PETT) intended for electric multiple units (EMUs) operated in 3 kV DC rail traction. The details related to the 0.5 MVA peak power medium voltage prototype, including the electrical design of the main building blocks are presented in the first part of the paper. The second part deals with the implementation of the developed SiC-based DC PETT into a regional train operating on a 3 kV DC traction system. The experimental results obtained during the testing are presented to demonstrate the performance of the developed 3 kV DC PETT prototype.

**Keywords:** silicon carbide; dual active bridge dc-dc converter; power electronic traction transformer; 3 kV DC railway traction; electric multiple unit

#### **1. Introduction**

Rail is among the most efficient and lowest emission modes of transport. It is under constant pressure to increase the accessibility of connections and quality of passenger services to enhance their competitiveness with other transport modes. For rail to compete more effectively with other modes of transport and attract more passengers, it needs the next generation of passenger trains that will be lighter, more energy-efficient and cost-effective. Traditional passenger trains consisting of a locomotive and several carriages penalize performance in terms of acceleration since they have a limited number of available drive wheelsets. Modern electric multiple units (EMUs) do not require a locomotive, as hey consist of several self-propelled units in a fixed assembly. Traction is distributed along the length of the train and the motors are housed by bogies of different carriages [1]. The demand for EMUs used for local, regional and intercity transport, grows globally every year. Thanks to the use of common bogies for the central wagons, EMUs are shorter and lighter than trains pulled by locomotives. This translates into lower movement resistance, better acceleration and lower energy consumption [2]. Moreover, EMUs better implement the specificity of passenger traffic and station service as they have a greater number of doors and provide better exchange of travelers.

Quantitatively, the most important part of energy drawn from the overhead traction line by an EMU is consumed by the traction propulsion systems. Traction drive is crucial for the EMU's efficiency, reliability and availability. The on-board propulsion converters, which for years have been traditionally installed in separate technical compartments, are currently mounted on the roof [3] or under the floor [4,5] of the EMU train. This corresponds to the growing requirements for the number of passenger seats due to the limited length of the passenger train.

The most common R&D works carried out by the manufacturers of traction drives are focused on improving efficiency and reducing the size of the on-board propulsion and power systems [3–6]. Another important topic is the reduction of noise emission [3]. Noise from mechanical and electrical components is troublesome to the human ear and significantly reduces travel comfort. A noticeable source is the working traction motor, whose noise depends on the frequency of switching transistors and the control method used in the drive inverter [7,8].

In the case of a drive inverter for rolling stock for 3 kV DC traction, it would be difficult to find any significant changes in the construction and control technology in the last decades [2,9,10]. The 3 kV dc-line voltage is a challenging task for power electronics. Due to a significant voltage drop on the overhead line, the traction power supply works at a higher voltage, usually 3.6 kV or even higher. This results in a requirement where the operation of a vehicle is often expected from 2.4 kV up to more than 4 kV, with the nominal power available from 2.8 kV or 3 kV DC [9]. A power conversion chain operating under direct current catenary is reduced to a bulky input filter and a voltage source inverter (VSI). Nowadays, EMUs powered by 3 kV DC voltage are equipped with two-level VSIs constructed of six 6.5 kV insulated gate bipolar transistors (IGBTs) generating pulse width modulated (PWM) voltage for supplying medium voltage asynchronous traction motors.

A small number of EMUs are equipped with three-level neutral point clamped (NPC) VSIs using 3.3 kV IGBTs [11]. A typical traction drive with a two-level VSI is shown in Figure 1a. In such a system, the LC filter capacitor also takes on the role of energy storage and is cyclically charged and discharged with impulse currents. The line filter choke, which is the most massive element of the inverter and weighs several hundred kgs, serves to limit high frequency, high di/dt pulse currents flowing between the catenary and the traction vehicle when the capacitor is recharged.

The power supply of the inverter has a large impact on both noise and the electromagnetic and thermal properties of traction motors. Table 1 shows the switching frequencies of HV IGBTs used in propulsion inverters for 3kV DC traction.

The harmonic voltages caused by PWM inverters operated with frequency ranging from individual hundreds of Hz to 2 kHz are the cause of significant current heat losses in the traction motor winding resistances [7,8]. The additional losses due to inverter supply can amount in the range of a few percent of the rated motor power and also occur in the laminated core of the traction motor. The winding and core losses most notably entail thermal problems, increasing the traction motor temperature by an average 30–50 K compared with a sinusoidal supply [8], but also lead to reductions in efficiency. Moreover, the negative effects of the low frequency PWM operation of the inverter feeding the traction motor are increased mechanical loads and increased noise emission. Typical voltage and current transients during the switching of the 6.5 kV IGBT are shown in Figure 1b. The main reason for low IGBT switching frequency are the relatively long switching times of the order of several microseconds and a characteristic current tail during turning off. The increase in the switching frequency of IGBTs would be at the expense of an unacceptable increase in the switching losses. Due to the above-mentioned reasons, current research and development conducted by manufacturers of the traction drives for rolling stock are focused on the utilization of new generation power transistors made of silicon carbide (SiC). Thanks to the exceptional properties, such as: significant achievable energy savings, quieter operation, improved reliability, and reduction in maintenance costs, SiC MOSFETs are ideal for traction power converters new designs instead IGBTs—which were in use in the rolling stock industry for decades [3–6,10]. The most obvious benefit of SiC metal-oxide semiconductor field-effect transistors (MOSFETs) compared to IGBTs is a significant reduction in switching losses, up to 55%, and total power losses, up to 80% [6].

**Figure 1.** Conventional electric drive of 3 kV DC traction rolling stock with asynchronous traction motor and 6.5 kV IGBT-based two-level inverter: Lf, Cf—inductance and capacitance of the traction line filter; TM&G—traction motor and gear, w—wheels (**a**); Typical collector-emitter voltage (1) and collector current (2) transients during switching of the 6.5 kV IGBT, 4μs/div (**b**); Typical drain-source voltage (1) and drain current (2) transients during switching of the 10 kV SiC MOSFET, 500 ns/div (**c**) (comparison in the text).

**Table 1.** Switching frequencies of HV IGBTs used in rail inverters for 3kV DC traction.


Typical voltage and current transients during switching of the 10 kV SiC MOSFET are shown in Figure 1b. As it can be seen from Figure 1, in the case of an SiC-MOSFET there is in principle no tail current, and so without this contribution, clearly the switching loss is extremely smaller. Table 2 shows the overall losses distribution between the switching and conductive losses for HV SiC MOSFET and HV IGBT technology [12].


**Table 2.** Loss comparison between IGBT and SiC MOSFET: 5 kV, 33A/cm2, 50% duty [12].

\* two 6.5 kV IGBTs in series have been selected as device closest to HV SiC IGBT and SiC MOSFET.

The IGBT switching frequency is limited by the power dissipation, which increases the IGBT junction temperature. To preserve the reliability of operation, the total power dissipation density is limited at the value of 200 W/cm2 which maintains the chip temperature of 125 ◦C. The packaging technology further reduces the switching losses of the IGBT to 100 W/cm2, which is the reason for limiting the operating frequency of the 6.5 kV IGBT-based two-level inverters to below 1000 Hz.

Lower switching losses of HV SiC MOSFETs give the possibility of increasing the PWM frequency of the propulsion converters towards tens of kilohertz. This, in turn, makes it possible to reduce the size of passive components of the traction inverter-primarily the bulky traction line filter. The weight of the filter inductor for the conventional traction inverter shown in Figure 1a is about 500 kg. The unique properties of SiC transistors allow to reduce the weight of the input filter inductor proportionally to the increase in the switching frequency. The issue supporting the reduction of dimensions of the input filter is the use of an additional high-efficiency active-front end (AFE) stage at the input of the traction inverter, which provides regulation of the current drawn from the grid. Due to the lower losses of SiC devices, the need for cooling is also substantially reduced [5]. Reducing the size of the cooling system leads to a reduction in the volume, weight and costs of the entire traction drive [6,8–10].

The higher resolution of the PWM generation in SiC-based traction inverters has a positive effect on reducing harmonic losses of the traction motors-making the whole traction systems more efficient. As stated in [4], the use of SiC power modules combined with expanding the control region outputting regeneration torque have made it possible to reduce the energy consumption rate of the railbound vehicle operated on suburban line by more than 37% compared to conventional systems. By increasing the PWM frequency above 20 kHz the acoustic noise level on the platform and in the cabin may be pushed beyond the audible range. In the case of railway inverters, the switching frequency of high voltage (3.3 kV and 10 kV) MOSFET SiC transistors is several kHz [4–6]. In order to obtain noiseless operation of MV drives, multi-level topologies are used with the use of low-voltage (1.2 kV or 1.7 kV) SiC transistors. Among others, power electronic traction transformers (PETTs), built with power electronic building blocks (PEBB) with built-in input/output isolation, realized by small size and high power density medium frequency transformers (MFT) are of particular interest [13–19].

So far, the primary purpose of SiC-based PETTs were primarily traction drives powered from the AC lines: 15 kV/16.7 Hz [13,14,18,19] and 25 kV/50 Hz [15–17]. AC powered locomotives and EMUs have a complex and voluminous conversion chain including a step-down transformer, rectifier, low-frequency filter and traction inverter. Conventional line frequency transformers (LFTs) used in AC EMUs occupy up to 12% of the weight of rail vehicle and a corresponding part of the train space [2]. Lightweight PETTs using MFTs, if they replaced bulky LFTs, could be easily installed on the roof, resulting in more space for passengers inside the train. AC PETTs offer not only a way to reduce the weight of the on-board electric equipment, but also to add additional functionalities and improve energy efficiency [19].

Strictly speaking, the AC PETT replaces the system of an LFT and an active rectifier and the AC PETT topology does not include a propulsion inverter, which stays the same as in a classical propulsion system and is connected to DC output terminals of the PETT [3,13–19].

As it has been highlighted in [13], the presently most promising medium frequency MF topologies for traction feature on their input-side a large number of series connected four-quadrant converters, with an overall switching frequency in the range of 5–8 kHz, modulated with Phase-Shifted PWM. The results presented in [13] have shown that the use of a passively-damped LCL filter for the input-side results in a significantly lighter solution, compared to a single-pole type (single inductor) reported in most cases. The work [14] shows the benefits of using the originally 15 kV/16.7 Hz-type PETT in a traction drive supplied from a 3kV DC network. The main innovation aspect of the PETT proposed in [14] is the possibility to be operated with a HV-AC electric system, as well as after reconfiguration on a MV-DC catenary. A three-stage technology have been studied. Four-quadrant converters (4QC) connected in cascade and forming the PETT input stage provide, via insulated dual active bridge (DAB) DC-DC converters, voltage stabilization at the output DC terminals of the PETT, while limiting the ripple of current drawn from the DC traction network. Two solutions have been presented: the first solution using hard switching techniques, using a three-times silicon-conversion, and a second solution based on ZVT/ZCS switching of silicon semiconductor devices. The DC PETT described in [14] provides a higher quality of drive operation during DC voltage disturbances which normally occur in 3 kV DC traction, than conventional propulsion systems with two-level and three-level inverters. Prototype modules have been realized with association of diodes and IGBTs in order to provide reverse-blocking devices. However, the concept was verified only on a small scale test rig. The works [15] and [16] present the method of optimal sizing of 25 kV/50 Hz-type PETTs using 3.3 kV SiC MOSFETs that leads to the best efficiency at rated power in a given volume. The authors emphasize that the efficiency cannot be the only parameter to make a clear choice of an isolated DC-DC converter included in a PETT. An important parameter should be also the acoustic noise, even if it is difficult to take it into account in preliminary studies. The results obtained in [16] for 3.3 kV SiC MOSFET-based resonant DC-DC converters show that designing converters with a switching frequency in the range of 20 kHz would not lead to a dramatic reduction of the PETT efficiency. In [19] the design and development of the 1.2 MVA medium voltage PETT prototype for 15 kV/16.7 Hz traction applications have been presented. The use of MFTs developed in [19] allows for weight reduction and power density improvement (0.5–0.75 kVA/kg) compared to conventional traction chains (0.2–0.35 kVA/kg). The authors highlight the new possibilities in terms of management of the railway networks due improved power quality and grid compliance due to the multilevel input voltage waveform and high apparent switching frequency seen from the grid side. The application of PETT to 3 kV DC rail traction means that the entire modular converter operates at much lower maximum voltage than in the case of the AC PETT, which encourages the integration of the propulsion inverter within the structure of the DC PETT with the use of unified low-voltage cells. The use of low-voltage (1.2 kV or 1.7 kV) SiC devices for this purpose seems to be an excellent solution.

The presence of built-in galvanic isolation in low voltage cells, realized utilizing MFTs, allows for series connection of the cells on the catenary side to obtain DC rail traction voltage and, independently, the configuration of the output terminals of the cells as a three-phase medium voltage multi-level inverter on the traction motor side. However, the use of low voltage power devices and electronics in combination with high insulation requirements, imposed by the application to 3 kV DC rail traction, is a challenging research topic regarding the insulation strength of the cells, in particular SiC power devices and MFTs.

The world's first and only traction unit equipped with the roof-mounted SiC-based 3 kV DC PETT is shown in Figure 2. It is the PESA 308 EN81 series electric passenger railcar (construction number 308B-007) with an originally rated power of 560 kW that operates in Polish regional passenger rail transport since 2007. The work presented in this paper covers some of the recent research and development efforts to develop, design, test, commission, and install of the 335 kVA (500 kVA peak) SiC-based 3kV DC PETT on the PESA 308 test traction unit for a field trial.

**Figure 2.** The EN81 series 3 kV DC electric passenger railcar used for the field trial with the roof-mounted SiC-based DC power electronic traction transformer (PETT) prototype.
