Power converters in Railway Applications

Mar 9, 2020
Schematic of a power converter in railway applications
Round the world, rail networks are expanding, driven by increasing affluence and mobility, tourism and asset upgrades for better sustainable and environmentally-friendly transport. China and India particularly are investing heavily into a market with a predicted value of 180 billion euros by 2020 spanning 1.38 million route-kilometers [1]. Provision of new equipment is lagging demand however so there is also a program of upgrades to existing infrastructure and rolling stock to improve efficiency to keep transport moving.

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Utilising assets efficiently is vital

New and existing rolling stock can be utilised more efficiently with ‘smart’ systems, tracking idle and active stock to enable increased traffic density. Operational condition can also be monitored using the techniques of Remote Condition Monitoring (RCM) and Condition Based Maintenance (CBM). The techniques identify repair and maintenance just when needed, saving costs and improving reliability and availability. Condition monitoring can collect other data such as occupancy rates and distance travelled which can be analysed to achieve further efficiency savings. The rail sector is expected to spend around $27 billion in the next 12 years on this IoT technology [2].

Sensing Remote Condition

Asset status and condition can be determined by sensors measuring axle count, bearing temperature, supply voltage fluctuations, acoustic noise, shock/vibration, door operation cycles, occupancy, air quality, light levels and more. Although a sensor can be as simple as a thermistor to measure temperature, increasingly ‘intelligence’ is added, with perhaps a digital signal processor (DSP), data logger and a wired or wireless interface, possibly using long range (LoRa) or WLAN radio. Rolling stock might also be tracked by GPS. With the sensor self-contained and communicating wirelessly, upgrades for legacy assets are easier and in new stock, cabling is reduced and flexibility improved with the possibility of remote updates and customisation.

Powering sensors

A temperature sensor in a rail application
Fig. 1: A temperature sensor in a rail application
A sensor typically needs just a few watts to operate at a low voltage, down-converted from the main system rail using a DC/DC converter. The system rail nominal is often 110VDC but sometimes is as low as 24V. Power requirements can be higher though in multi-channel sensor arrangements or when actuators have to be driven, perhaps up to about 40W.

Individual isolated DC/DCs are normally employed for each sensor to keep output voltage drops low and to avoid ground loops which might cause EMC problems such as sensor cross-interference. The system supply rail however is not ‘clean’. Standard EN 50155-2017 allows a variation of +25%/-30% in normal operation with dips to 60% and surges to 140% of nominal for 100ms with no ‘deviation of function’ allowed and 125% to 140% of nominal for one second with performance degradation. To cover these variations, DC/DC converters for 110V systems must typically operate from 43 to 160VDC.

Fast transient overvoltages are also present on the system rail as defined in the EN 61000-4-x series of standards, however suppressors and simple LC filters can attenuate these. Complete loss of supply can also occur though; EN 50155 defines interruptions in two classes S1 and S2 with the worst being 10ms loss of supply from nominal input with no degradation in performance. For some equipment the interruption can be 30ms on supply change-over (Class C2). To cover this, hold-up capacitance external to the converter is normally added after a series isolating diode, which can also provide reverse polarity protection – another requirement from the standard.

An example application is shown in Figure 1 where a fan is activated by a carriage temperature sensor, with temperature and status signalled via a WLAN connection. A RECOM 8W DC/DC converter is shown with an input range of 43 to 160V providing a regulated, isolated, 5V power rail for the sensor circuitry. The DC/DC is very compact at just 32 x 20 x 10mm. Extended hold up and reverse polarity protection is shown along with an optional EMI filter for high immunity to transients and low conducted emissions although the RECOM DC/DC already has high compliance levels to rolling stock EMC standard EN50121-3-2.

Compliance with RIA 12

Specifications for rail DC/DC converters sometimes include compliance with the UK RIA 12 standard which defines higher energy surges up to 385V for 20ms in 110V systems (Vin, nox x 3.5). However, with a source impedance of just 0.2 ohms, clamping the input with a transient voltage suppressor dissipates excessive power. A solution from RECOM is their RSPxx-168 pre-regulator (Figure 2) which drops the surge voltage to a maximum value within the input range of their DC/DC converters. Three different modules are offered for DC/DC converters with 20W, 150W or 300W ratings.

RECOM surge protector operation for RIA 12 applications

Fig. 2: RECOM surge protector operation for RIA 12 applications

Conversion at higher power levels

Centralised rolling stock systems and trackside installations also require power converters at higher power levels. ‘Brick’ format DC/DC converters are popular such as the RPxxx-RW and RPxxxRUW series from RECOM which include parts operating over a 12:1 input range with nominal values covering all the rail standards from 24 to 110VDC including surges and brownouts. The DIN rail format is also common with an example being the 120W ruggedised SQ120 part with its 110VDC input, compliant with EN 50155. The portfolio also includes rack-mounted AC/DC and DC/DC converters for rolling stock and trackside applications from 8W up to 4kW with DC, single-phase or three-phase AC inputs. An EN 50155-compliant 300W AC/AC inverter is also available with adjustable output frequency between 30 and 600Hz.

The rail environment is stressful

Standard EN 61373 defines the levels of shock, vibration, temperature and humidity levels in the rail environment, dependent on the installation category, which ranges from most severe on the bogies to more benign in areas such as body-mounted inside enclosures (Category 1, Class B). DC/DC converters for sensors are likely to be fitted in this less stressful environment but will often still require encapsulation and ruggedisation. Service life expectation is typically 20 years so products such as those from RECOM are qualified with tests including full performance characterisation, highly accelerated life testing (HALT), high temperature soak and thermal cycling.

Solutions are available off-the-shelf

RECOM [3] has a wide portfolio of rugged EN 50155-compliant DC/DC converter products and complete turn-key solutions for rail applications from low power (8W-240W) modules up to 10kW power supplies, with customisation available.

The company has long experience of rail applications offering comprehensive engineering support, detailed environmental compliance reports and EMC evaluation. Reference designs are available which include the necessary EMI filtering for EN 50121-3-2 compliance for 24 - 48V or 72 - 110V DC nominal input voltage converters. (R-REF04-RIA12-1 and R-REF04-RIA12-2 respectively).

Off-the-shelf DC/DC converters and power supplies qualified to railway standards from RECOM offer a cost-efficient and easy route to provision of power for sensors and centralised systems from watts to kilowatts. RECOM Parts are available from their distributor partner Arrow Electronics [4].
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