Light rail transit (LRT) is a form of urban passenger rail transit that combines the features of a tram and rapid transit. While its rolling stock is more similar to a tram than a traditional railway, an LRT operates at a higher capacity and speed than a tram, and often on an exclusive right-of-way.
light rail network
In the United States of America (USA), light rail operates primarily along exclusive rights-of-way and uses either individual tramcars or multiple units coupled to form a train that offers more than a tram but is lower capacity and lower speed than a long heavy-rail passenger train or rapid transit system.
In many cities, LRT systems resemble traditional underground or at-grade subways and heavy-rail metros. In other systems, a light rail network may operate on city streets, sharing space with automotive traffic.
Light Rail Power Supply Basics
Typical voltages range from 600V–750V DC, with more recent installations tending towards higher voltages
Most light rail or tram systems get their power from overhead catenary systems. Typical voltages range from 600V–750V DC, with more recent installations tending towards higher voltages.
These voltages are lower than those used by traditional electrified railways, which use much higher AC voltages up to 25 kV.
Overhead power lines
A network of overhead power lines can be unsightly, especially in historic cities, so some systems get their power from a ground-level power supply or APS (Alimentation Par le Sol). The most common variant of an APS utilizes a buried third rail line, placed in the ground between the running rails, to power the trains.
For additional safety, when an approaching LRT is detected, the segment below the train is automatically energized. The technology was first introduced in Bordeaux, France, in 2003. More recently, APS has been adopted everywhere else in France, plus in locations in Brazil, Ecuador, Australia, and Dubai.
Alternative methods
Many cities have adopted APS or supercapacitor-powered inner-city LRT systems
Alternative methods of power transfer include inductive power transfer using coils buried under the rails, and trams with onboard energy storage systems; these systems can contain supercapacitors or batteries of different types and recharge during stops, such as the MetroCentro in Seville, Spain.
Many cities have adopted APS or supercapacitor-powered inner-city LRT systems to avoid unsightly overhead wiring in historical city centers, from Angers in France to Zaragoza in Spain (the A–Z in the title).
- Why use DC power instead of AC?
A DC supply eliminates the need for a bulky, heavy transformer, making the tram much lighter and allowing for a low floor, which is a big advantage when passengers need to be able to step on and off from street level. The DC voltage from the overhead pantograph can just go straight into the main bus line to power the motors, ancillaries, etc.
DC is typically provided by a series of electrical substations spaced along the route that supplies power ranging between 1.5–5.5 MW. These substations are fed by grid AC power with a typical voltage in the range of 15kV–25kV and they convert this power to DC using trackside transformer and rectifier systems.
Power supply solution
Two options for reuse are returning the power to the DC high-voltage source or capturing the excess energy
There are considerable challenges in creating a stable light rail power supply solution. The dynamic power consumption of each train varies based on its operating status, and a typical light rail power supply system must accommodate numerous trains.
A train can also act as a power source: just like an automotive electric vehicle (EV), the train’s traction motor can be used to generate energy when the train is slowing down or stopping, which can then be transformed into electricity for use elsewhere.
Two options for reuse are returning the power to the DC high-voltage source or capturing the excess energy in a wayside energy storage system (ESS). The ESS can store energy in batteries, supercapacitors, or even a flywheel.
Performance evaluation
The overhead wire has a resistance, which, although small, is not zero, and so, the DC voltage at the catenary depends on the distance to the substation and the current flowing in the wire. The power system designer must ensure that the system has enough power generation capacity under worst-case conditions.
Optimal locations for substations are typically determined using a computer model that simulates proposed light rail operations along an accurate description of the planned route. The simulation also evaluates performance in case the power distribution system is degraded—if a substation goes offline, for example.
LRT Power Designs: Call RECOM!
EN50155 lays down the minimum requirements regarding the electrical and mechanical operating environment
Like their heavy rail counterparts, both wayside and onboard light rail power systems must meet strict technical and performance requirements, as well as railway standards such as EN 51055 and EN 50163.
EN50155 lays down the minimum requirements regarding the electrical and mechanical operating environment, as well as equipment reliability and expected useful life, while EN50163 specifies the main characteristics of the supply voltages of traction systems, including LRT.
For example, for a nominal catenary voltage of 750 V DC, all systems should withstand 500–900 V continuously, 1000 V for 5 min, and 1270–1000 V on a straight line for durations of 20 ms to 1s.
EN51055 and EN50163
The EN51055 and EN50163 requirements far exceed the capabilities of industrial-grade “off-the-shelf” modular DC/DC converters. However, help is at hand, RECOM specializes in custom solutions for onboard and trackside applications that meet EN50155 and EN50163 requirements for light rail power supplies.
Its heavy rail and LRT power supply conversion products include DC-DC power converters, AC-DC power supplies, bidirectional converters, and DC-AC inverters.
RECOM’s SD3206
SD3206 integrates features required by EN50155: reverse polarity protection, EMC, and surge filtering
For example, RECOM’s SD3206 is a semi-custom “plug and play” 3.2kW digital battery charger designed to operate at a 480–630V DC input voltage to fast charge a 24V–28V DC battery pack or supercapacitor at up to 100A.
The highly efficient converter (95–96%) requires a space of only 470 x 213 x 80mm, so it can be easily mounted under the floor or in the roof space of the train.
The SD3206 integrates features required by EN50155: reverse polarity protection, EMC and surge filtering, inrush current limiting, and output current sharing in one unit. These features would otherwise be accomplished by adding external components. Control can be analog (Vout programming, remote ON/OFF) or digital via RS485.
Rail environmental standards
The power supply meets all relevant rail environmental standards for conducted and radiated EMI, mechanical shock and vibration, thermal shock, temperature cycling, and fire protection.
For custom designs, RECOM’s sister company, PCS, has a long history of experience in developing power supplies for railway and LRT applications that meet EN51055 requirements, including onboard battery chargers with the capacity of delivering up to 18kW to fast-charge large battery packs.
