Traction converters
Traction power electronic converters play a key role in modern transportation. Unlike industrial converters they face slightly different and specific challenges with regards to design, operation mode or safety.
In this post we provide a brief look into the power electronics that we usein everyday’s life when using traditional means of public transport such as trams, trolleybuses, subway or train. Similar technology also steadily penetates into the battery based e-mobility sector like e-buses in the public domain and electric cars in the individual transportation.
General differences between industrial and traction converters
Compared to industrial power electronics, traction converters must be designed for wider range of ambient temperatures, typically from -30°C to +40°C (or even more). They must tolerate vibrations and shock as defined e.g. in IEC EN 61373.
The power converters are installed in containers located mostly on the roof of the vehicle (due to low floor vehicle design). There they are exposed to all kind of weather conditions,
such as rain, snow or direct sunshine. In case of bottom location the equipment may also be exposed to dust, sand or salt (particularly in winter time).
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Body of the containers is made of stainless steel or aluminum alloys. The boxes are usually divided into two sections with different level of ingress protection.
Coolers, cooling fans or input smoothing reactors are installed in the first section with lower ingress protection (e.g. IP23) whereas all other equipment (traction converter, auxiliary converters, battery chargers) is housed in the other section with much higher ingress protection (e.g. IP55).
Power converters for trolleybuses
Trolleybus is a vehicle for public transportation. Main body is similar to a bus. The electric power is supplied from overhead traction supply.
Due to its construction (vehicle insulated from the ground), trolleybuses require double insulation that must be continuously monitored. By reduction of insulation resistance or increase of the potential of the body against ground the vehicle must be set out of order (safety critical).
Power converters are supplying either three-phase asynchronous motors or synchronous motors with permanent magnets. Containers are mounted on the roof of the vehicle and include smoothing reactor, input filter, contactors, rectifier and IGBT based inverter with regenerative functionality. There are also converters for auxiliary drives (e.g. on-board compressors), battery charger or air conditioning unit.
Double insulation is realized by inserting an aluminum plate between power modules and corresponding heat sinks. Insulation is realized by thin epoxy layer or similar material.
Power converters for trams and light rail
Traction converters for trams do not require double insulation which makes the design somewhat easier. Containers have again two sections with input reactor, converter cooler and cooling fans in one part and power electronics itself in another part with higher degree of protection. Cooling fans may use step control or to be entirely speed-controlled (EC fan) when the service duty allows it.
Traction converters are modular and so are the power modules. Three modules build a three-phase inverter, additional modules are used for resistive braking (chopper) or regenerative braking. Modern trams often feature distributed drives, i.e. there are multiple motors driving individual axles rather than one or two large drivers.
The containers need a system to vent the overpressure in case of an internal fault. Pressure flaps are commonly used.
Power converters for heavy rail
Railway traction converters have specific designs depending on their installation. In conventional locomotives the traction converters are located in a dedicated machine room. In train sets, the equipment can be placed in a machine room as well. However, most train sets have the traction converters located on the roof. Railway converters feature higher rated power and correspondingly also generate more heat losses.
Another specialty in railway are the different supply systems. In Europe the three most common railway supply systems are 3 kV DC, 25 kV/50 Hz and 15 kV/16.7 Hz.
In case of AC supply the traction vehicle also features a traction transformer. Due to interoperability, many modern traction vehicles are of multi-system type, i.e. they can operate under different traction supply.
It increases the cost of the equipment. At the same time it is very convenient in service when the vehicle operates on a route having different supply systems (especially international trains but also within one country).
The largest challenge is the space restriction and weight limits (max. weight per axle). Liquid cooling is broadly used as it allows more compact design of the converters, lower thermal resistance and absence of air cooling ducts.
The equipment is rated within the medium voltage class and proper clearances must be ensured. Another important factor is the elimination of EMC disturbances due to interaction of power and control circuits. The switching pulses are therefore usually transmitted via fiber optic cables.
Additionally, traction equipment shall be designed to allow access for regular maintenance and repair works.
References
[1] Skoda, https://www.skoda.cz/en/
[2] Traction (series), https://mb-drive-services.com/category/traction/
[3] Research and Innovation Centre for Electrical Engineering, http://rice.zcu.cz/en/
