**2. Configuration of the 3 kV DC PETT Topology**

The general description of the 3kV DC traction propulsion system with the SiC-based DC PETT is shown in Figure 3.

The key parts of the system include: nine power electronic cells (1) each consisting of eight commercially available 1.2 kV SiC MOSFET power modules (2) and medium frequency transformers (3) placed in separate air-cooled chambers; a compact input traction LCL filter of a small size has an area of a small chamber (4); a medium voltage asynchronous traction motor integrated with the gear unit (7). The DC PETT has, essentially, a three-stage construction: (1) DC input conversion stage; (2) AC output conversion stage and (3) AC-link, which is an especially designed medium frequency power transformer. The DC input conversion stage is used to adapt constant frequency DC traction voltage to a medium frequency required for conversion, while the AC output stage adapts medium frequency of the AC link to the output AC voltage useful for controlling the traction motor. The input terminals of the nine power electronic cells are connected in series which means that the DC PETT input stage is configured as a 19-level cascaded H-bridge four-quadrant converter (4QC) supporting the 3 kV DC railway traction network. The output terminals are connected in series, three pairs of terminals per phase, creating a three-phase output stage, which means that the traction motor is supplied by a three-phase seven-level cascaded H-bridge (CHB) traction inverter. Table 3 lists the overall system requirements and specifications.

The proposed SiC-based DC PETT takes full advantage of the SiC technology while meeting the requirements of train manufacturers. During DC PETT operation, the energy drawn from the 3kV overhead contact line is processed in discrete portions, with full DC voltage divided into 19 levels, which helps meet stringent electromagnetic compatibility (EMC) standards, including EN 50121-2, EN 50121-3-1, EN 50121-3-2 and EN 50238. The use of a cascade system of series-connected 1.2kV SiC MOSFETs to switch the full voltage of the railway traction enables a much higher operating frequency of the power converter than using HV counterparts. At the same time, the selection of low-voltage transistors with high switching frequency can provide noiseless operation with comparable or even smaller total switching losses of the power converter [19]. The internal terminals of the power electronic cells are configured as nine DAB DC-DC converters connected in series and forming the insulation

stage of the DC PETT. Each of the nine 4QC-DAB-DC/AC power electronic cells assembled in this configuration is rated for 38 kVA (56 kVA peak). Each cell, in addition to the galvanic isolation realized with the MFT, contains two energy storage elements, which is a characteristic feature of PET devices and enables the independent implementation of various control tasks at the input and output of the device.

**Figure 3.** General scheme of the SiC-based 3 kV DC PETT: (1) Basic 4QC-DAB-DC/AC power electronic cell with two energy storage elements; (2) 1.2 kV SiC MOSFET power modules assembled in a 9 kV insulation frame; (3) medium frequency transformers; (4) compact input LCL traction filter; (5) output stage configured in the form of seven-level CHB inverter; (6) medium voltage traction motor integrated with the gear; (7) distributed air cooling system; U, V, W—traction motor phases.



As the heat generation is concentrated around the MFTs and SiC power modules, a distributed air cooling system (6), when uses PWM controlled fans and eighteen cooling ducts is provided in order to deal with the heat. The use of low voltage SiC MOSFETs makes the construction of the DC PETT more competitive in price than two-level traction inverters using HV SiC MOSFETS due to very high costs of high voltage semiconductor power devices, high voltage capacitors and other high voltage electronic components. The price of CAS120M12BM2-type 1.2 kV/193 A SiC MOSFET power module starts at about \$300, the price of CAB450M12XM3-type 1.2 kV/450 A SiC MOSFET starts at about \$700, while for the 3.3 kV nHPD2-type SiC MOSFET modules prices start at about \$9500. The 10 kV and 6.5 kV SiC MOSFET power modules are not available on the market yet.

The modular construction of the DC PETT ensures even weight distribution on the roof of the EMU train. The center of gravity of the device is distributed symmetrically on both sides of the longitudinal axis of the vehicle. The distribution of the basic cells of the PETT on the EMU roof surface is shown in Figure 4. The operation of current-controlled nine-level four-quadrant converter connected to the 3 kV DC traction enables the minimization of the integrated input LCL traction filter and the mitigation of the resonances coming from the railway grid at a level that cannot be obtained in a conventional two-level inverter. As it can be seen from Table 3, for the investigated EN81 railcar, the weight of the traction filter chokes was reduced by nearly 10 times.

**Figure 4.** Assembly of power electronic cells of the DC PETT on the roof surface: (**a**) overall view of the whole device from the CAD program; detailed view of the 4QC-DAB-DC/AC power electronic cells and the LCL filter in the middle at the bottom (**b**); photos of the prototype (**c**) and (**d**).

Moreover, modular construction enables quick replacement of individual cells in case of failure. Therefore, the minimization of the MTTR (Mean Time to Repair), which is one of the basic indicators of reliability required in the railway industry [20,21] can be provided.

#### **3. Low Voltage SiC Power Modules**

According to numerous recent publications, e.g., [20,21], cascaded cells converter systems, such as the CHB topology, are a very attractive solution to interface power electronic systems to MVA-scale medium voltage applications. For the systems connected to medium voltage grids the choice of fast switching and low voltage drop 1.2 kV or 1.7 kV devices (giving up to 15 cells per phase stack) can be the most suitable trade-off between efficiency and power density, with efficiencies above 99% at a power density of about 5 kW/dm3 [22].

Higher switching frequency obtainable at lower blocking voltages allows to reduce loss and volume contributions of the grid filter inductances and the cooling system.

With the wide availability of low voltage SiC MOSFET power modules on the market their performance in the range of maximum working currents offered is constantly increasing along with the progressive heat dissipation capacity. Figure 5 shows a comparison of the dimensions of the 1.2 kV SiC MOSFET power module, part no. CAS120M12BM2, used in this project, with a rated drain current of 138 A (TC = 90 ◦C), RDS(on) = 23 mΩ, junction to case thermal resistance RthJC = 0.125 ◦C/W, maximum dissipated power PD = 450 W (TC = 90 ◦C) characterizing commutation circuit series inductance of 15nH and a base surface of 65.4 cm2, and new generation 1.2 kV SiC MOSFET power module, part no. CAB450M12XM3, available on the market from mid-2019, with 3 times higher rated drain current ID = 409 A (TC = 90 ◦C), RDS(on) = 4.6 mΩ, RthJC = 0.11 ◦C/W, PD = 750 W (TC = 90 ◦C), and characterizing commutation circuit series inductance more than twice as low, Lstray = 6.7 nH and 36% less footprint.

**Figure 5.** The 1.2 kV SiC MOSFET power module, part no. CAS120M12BM2, used in the project, with a rated current of 138 A (left) and a 36% smaller 1.2 kV SiC MOSFET power module, part no. CAB450M12XM3, with a rated current of 409A, available on the market from mid-2019.

The use of SiC MOSFET 1.2 kV power modules in the design of the proposed 3 kV DC PETT requires higher insulation strength than the original housing offers to withstand the full operating voltage of the converter. To increase the voltage strength, an additional frame made of insulating material has been developed and mounted between each transistor module and the heat sink, which is shown in Figure 6. A flexible, thermally conductive silicone film with thermal conductivity of 5 W/mK and a breakdown voltage of 9 kV AC was used between the base of the power modules and the heat sink.

There are several design challenges associated with switching performance of the SiC power modules which need to be carefully addressed including the minimization of ringing and overshoots caused by the parasitic loop inductance. These loop inductances together with SiC MOSFET output capacitance create a resonant tank, which is the source of unwanted EMI emissions [23,24]. To overcome the ringing, parasitic parameters converter should be minimized by a careful converter design.

**Figure 6.** General view and cross-section of the frame of insulating material, ensuring increased voltage strength of the SiC MOSFET transistor module housing and heat sink insulation: (**a**) overall view from the CAD program; (**b**) photo of the prototype.

Figure 7 shows the drain-source voltage (UDS) of the SiC MOSFET and the primary side transformer current of the developed DAB DC-DC converter measured in the developed SiC-based 3 kV DC PETT during commutation of 700 V DC voltage within 80 ns. A single standard PPA2124150-type, 1.5 uF snubber capacitor was attached to the DC bus of each SiC MOSFET power module. As can be seen in Figure 7, thanks to the obtained minimization of the switching loop inductance the measured magnitude of the voltage ringing is about 40 V (5.7%), which is an acceptable level.

**Figure 7.** SiC MOSFET drain-source voltage (1), 100 V/div and the primary MFT current (2), 25 A/div of the developed DAB DC-DC converter during commutation of 700V DC voltage, 400 ns/div.
