Wide bandgap (WBG) powers trains/traction systems

Train locomotives can be powered by diesel internal combustion engine or steam or by electric power. In this article we will discuss electric power advantages. Electrically powered trains are able to pull a mile-long array of railroad cars because of a high-torque electric traction motor.

Electric power works by switching the electrical current frequency and voltage through manipulation of the magnetic fields. Electric traction systems are categorized as DC or AC, depending on the type of motors used and both have very high starting torque.

DC vs. AC line voltages for motors

Direct current (DC) popular line voltages for overhead wire supply systems have been at 1,500V and 3,000V levels. Third-rail systems are predominantly in the 600–750V range. Some advantages of DC are space and weight considerations, rapid acceleration and braking of DC electric motors, less cost as compared to AC systems, and lower energy consumption. The disadvantages for using direct current: expensive substations are needed at fairly close intervals and the overhead wire or third rail will be large and heavy. See Figure 1.

Figure 1: DC power system for railway traction (Image from GaN Systems)

Third rail systems present a hazard of electric shock, with higher system currents than AC line voltages (DC rails are above 1500 v), and are not considered safe. Very high currents are therefore used, resulting in considerable power loss in the system, thus requiring relatively closely-spaced feed points (sub-stations).

Alternating current (AC) can have high overhead-wire voltages (10,000 volts or above), but advantages here are that fewer substations are required due to lower currents leading to lower voltage drop along the power distribution network between substations. Lighter overhead current supply wire can be used which reduces the weight of any structures needed to support it. As a result, electrification can reduce capital expenditures. See Figure 2.

Figure 2: AC power system for railway traction. An Electric Multiple unit or EMU is a multiple unit train powered by electricity

AC traction for locomotives enables a big improvement over the older DC systems. The primary advantages of AC traction are adhesion levels up to 100% greater than DC traction with higher reliability and reduced maintenance requirements of AC traction motors. See why AC adhesion levels are greater than DC adhesion levels here.

Both AC and DC motors are in use today, depending upon the railway system. AC traction systems have become very popular on the rails in modern times. AC is more often used in most of the traction systems due to several advantages, such as quick availability and generation of AC that can be easily stepped up or down, easy controlling of AC motors, less number of substations requirement, and the presence of light overhead catenaries that transfer low currents at high voltages. Figure 3 shows data on the exact number of AC vs. DC traction systems on the rails.

At the end of 2018, 493 cities in 72 countries and regions had opened urban rail transit, with a total mileage of more than 26,100 km1. This was double the 2014 numbers and will continue to increase in 2020 and beyond.

Reportlinker.com announced that the Global Railway Traction Motor Market is expected to grow by $1.66B during 2020-2024 timeframe progressing at a CAGR of 3% during the forecast period. Market Watch reported that the global electric traction motors market size, share valuation is expected to reach $31.5B by 2026.

Figure 3

Figure 3: Breakdown of AC and DC traction systems worldwide (Image from Reference 5)

SiC and GaN FET EV traction systems compared to Si-IGBT-based traction systems4

The rectifier portion of the rail traction system usually converts 480V, 60Hz AC power to DC power around 650VDC.

The faster switching speeds of SiC power components provide the tools for designers to create railcar power systems that are as much as 50% smaller and 30% lighter than existing systems using other power transistor devices like Si IGBTs. This reduces the weight of the entire motor system by as much as 15%, mostly due to smaller magnetics with SiC higher frequencies. The maximum voltage and current ratings of SiC devices are significantly higher than theoretical capability of Si. SiC has larger margins from failures as well.

Lighter trains are not only more efficient but can be safer as well because lower weight enables the train to stop faster. As a bonus, when SiC and/or GaN devices are also designed into the system’s auxiliary power supplies for passenger comfort devices, such as A/C, Wi-Fi, as well as interior lighting, these systems will also consume far less energy.

SiC and GaN higher operational speeds enable designers to create railcar power systems which are up to 50% smaller and 30% lighter than existing systems, reducing the weight of the entire motor system by as much as 15%. Lighter trains are not only more efficient, but can be safer, too – less weight means they can stop faster.

GaN devices have advantages of low Figure of Merit (FOM) (where the definition of FOM is ‘ON resistance x Gate charge’), and zero reverse-recovery charges (Qrr). GaN transistors are far better than Si because their switching frequency, magnetic design, and switching losses will be significantly reduced in the system.

Si MOSFETs have a typical reverse-recovery charge in the 50- to 60-nC range, depending on their size and characteristics. When the MOSFET turns off, the Qrr in the body diode produces losses that add to the total system’s switching losses. These losses rise proportionally with switching frequency, and make MOSFETs impractical for use at higher frequencies in many applications like traction converter systems.

Traction conversion systems

There are many different types of power conversion systems in AC and DC traction. Here are a few types:

Shinkansen Bullet trains2

The conversion system of Shinkansen trains consists of a PWM converter and a PWM inverter is shown in Figure 4.

The key concept of this high-speed train traction system was the combination of SiC power devices applied to the traction conversion system. The addition of train draft-cooling system and 6-pole induction motors contributed to additional weight-reduction, compactness, and higher reliability. Running tests of the prototype of the developed traction system were performed and confirmed its sound performances. This SiC traction application is a first time for high speed trains.

Table 1

Table 1: Improvement of the Traction System in Tokaido Shinkansen (Image from Reference 2)

Figure 4: A block diagram of a Shinkansen train system (Image from Reference 2)

Traction converter examples3

Silicon IGBTs had been widely used in railway traction converters until SiC power transistors came along. SiC power devices provide higher blocking voltages, higher switching speeds, and higher operating temperatures than Si IGBTs.

A basic standard traction converter design architecture is shown in Figure 5, at right.

SiC is better suited for railway traction applications, for the most part, than GaN due to the higher voltage ratings of the devices aligning with popular DC line voltages for overhead wire supply systems of 1,500V and 3,000V and third-rail DC systems predominantly in the 600V–750V DC range; AC railway systems can have high overhead-wire voltages (10,000 volts or above).

Wolfspeed offers CAS300M17BM2 a 1700V SiC 8.0 mΩ in standard 62mm packaging, this module can be connected in parallel and in series to fit the proper driver voltage and current requirement of traction inverter applications. See Figure 6, at right.

Higher voltage modules rated at 3.3kV, 6.5kV and 10kV are also in development but not commercially available at this time. Expect implementation of these higher voltage module as they become available.

GaN power ICs may also be used in some cases of railway traction at lower voltages up to 800V4 or if properly stacked to handle those higher voltages.

Figure 5

Figure 5: A basic railway traction converter design
(Image from Reference 3)

CAS300M17BM2

Figure 6: CAS300M17BM2 module from Wolfspeed

Conclusion

High torque AC and DC electric traction motors are at the heart of train traction systems. To power these motors, high speed, high voltage power transistors are key to optimum performance within a challenging rectifier design that is key to optimum performance in successfully moving trains across the many miles of track carrying heavy cargo to their destinations keeping economies running.

WBG semiconductors like SiC, GaN, as well as HVIGBTs are the power workhorses in traction systems. Both SiC and GaN are typically used in these power architectures where SiC may be the first choice by designers due to the higher DC line overhead system voltages and third rail DC needs in a railway traction system. SiC handles the higher voltage levels and GaN handles the lower level of the high voltage needs. Maximum voltage and current ratings of SiC devices are significantly higher than theoretical capability of Si. WBG power ICs enable smaller and lighter weight systems because of their high speed and high voltage performance levels. HVIGBTs will fit in where commercially available ratings above 1700V for SiC are more difficult to find.

References
  1. UITP, “World metro figures – Statistics brief,” Singapore, Rep. 2018
  2. Development of SiC Applied Traction System for Shinkansen High-speed Train, Kenji Sato1, Hirokazu Kato, and Takafumi Fukushima, The 2018 International Power Electronics Conference, IEEE 2018
  3. Outlook for SiC devices in Traction Converters, P. Ladoux, M. Mermet, J. Casarin, J. Fabre, ALSTOM Transport – Innovation and Research –Traction Components Engineering, IEEE 2012.
  4. GaN enables efficient, cost-effective, 800V EV traction inverters
  5. Reliability Analysis of DC Traction Power Supply System for Electric Railway, Hitoshi Hayashiya, Masayoshi Masuda, Yukihisa Noda, Koichiro Suzuki, Takashi Suzuki, Tokyo Branch Office, East Japan Railway Company, EPE’17 ECCE Europe, 2017