railway electrical smart grids

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N THE LAST DECADE, THE DEVELOPMENT OF NEXT-generation electrical smart grids (ESGs) has been one of the pri-orities in the field of electrical engineering, both for most of theresearch centers and for the industry. In short, an ESG consistsof the integration of information technologies into the electrical

system to improve its controllability. In traditional power systems, the controlhas been carried out only by the power plants and some elements of the grid(transformer tap-changers, compensation capacitors, and reactances), whereas,in the next- generation smart grids, most of the elements can respond to controlorders from the system operator, which allows, for instance, for the integrationof the distributed generation and the demand into the control schemes of thepower system. Because of this improved controllability, ESG technologies prom-ise a significant improvement in the capacity utilization, the reliability of the sys-tem, and the energy efficiency of the grid.

Although rail power systems (RPSs) are a special case of electrical powersystem, they are operated in a very different way. While the goal of the power

By Eduardo Pilo de la Fuente,Sudip K. Mazumder, andIgnacio González Franco

An introduction to

next-generationrailway power systems

and their operation.

Digital Object Identifier 10.1109/MELE.2014.2338411Date of publication: 29 September 2014

SUBSTATION IMAGE: COURTESY OFEDUARDO PILO DE LA FUENTE,

BINARY NUMBERS— © MICROSOFT



IEEE Electr i f icat ion Magazine / SEPTEMBER 20140

system is to provide electrical power with the agreed charac-teristics, the final objective of the railway system is to trans-port passengers and goods according to a schedule. For thatreason, the operation of a railway ESG (RESG) has to be differ-ent from conventional ESGs.

Most of the RPSs share some characteristics that makethe development of specific ESG technologies adapted to

railways particularly important.1) Most electrical loads are trains, which are spatiotempo-

rally varying loads. The consumption of the trains isrelated to the way each train is driven, which allows itto even become a generator for a limited amount oftime when braking. Just by stopping acceleration andstarting braking, the load can vary from 10 to −8 MW ina few seconds. The RPS has to be able to deal with thesechanges, which occur very often along a given journey.

2) Electrified railways are normally considered one of themost energy-efficient modes of transport, especiallyover economically viable operating distances. Its poten-tial for energy savings is largely due to regenerativebraking, whose efficiency depends largely on when andwhere it is carried out.

3) Railway lines normally cross wide areas and, there-fore, are often interconnected to several electricalgrids, which are normally heterogeneous, as stronggrids coexist in the field with weaker grids. A smartcontrol that takes into account the specificities ofeach network is crucial to improve the overall reliabili-ty and capacity utilization.

4) An RESG relies heavily on good bidirectionalcommunications between trains and the infrastruc-ture, which is sometimes difficult to achieve, forinstance, in tunnels or in remote areas.

This article describes railway power systems and theiroperation. In addition, the main control actions that can beperformed by an RESG are introduced, explaining how theycan improve the performance of traditional RPSs, e.g., reducingcosts, increasing energy efficiency, and enhancing reliability.

Railway Power System Grids

System DescriptionAs shown in Figure 1, RPSs normally take the electricityfrom other power systems, which, in turn, have their owngeneration plants and electrical grids (transmission grid forbulk power transfer, and distribution grid for retail powersupply) and whose characteristics may vary significantly(strong grids coexist in the field with much weaker grids).

In liberalized electricity sectors, the transmission, dis-tribution, and generation activities are typically carriedout by different companies. The electrical system opera-tors (ESOs) are the companies in charge of balancing gen-eration and demand and operating the transmission gridin such a way that the reliability of the system is guaran-teed. The distribution system operators are companiesthat operate the distribution grids in such a way to ensurethat electricity is supplied to every customer with therequired quality. Finally, the generation companies areresponsible for producing the energy that has been pro-grammed in each power plant. The energy produced can besold by means of contractual agreements or in organizedelectricity markets (generally spot markets, including day-ahead and intraday sessions), operated by an electricity mar-ket operator. However, final corrections to the program areintroduced by the ESO to solve technical restrictions and torespond to the unexpected variations that occur in real time.

ESS

ESO 1 EMO 1

T&D Grid1

Power Plants

TractionSubstation

RailwayPower System

T&D Grid 2

Power Plants

TractionSubstation

TractionSubstation

RDG

ESO 2 EMO 2

LegendEMO: Electricity Market OperatorRDG: Railway -Side Distributed Generation

Power SystemSide Traction

SubstationTraction

Substation

Figure 1. The interconnections of railway power systems to other power systems.



IEEE Electr i f icat ion Magazine / SEPTEMBER 2014 51

In some countries, some transmission and distribution(T&D) grids and power plants, owned by railway companies,are used specifically for traction purposes. Because of theirusage, these grids have particular characteristics (e.g., two-wire, single-phase lines using a low frequency of 16.6 Hz).

Regarding the railways side, when it is not verticallyintegrated, the infrastructure manager [referred to as the

railway system operator (RSO) as an analogy with ESOs inpower systems] plays a dual role: 1) regulating the trafficto ensure safety and an adequate flow of trains and2) controlling the railway power system. The train opera-tors are independent companies whose activity is totransport loads or passengers with trains. As users of therailway infrastructure, they must pay for the services theyuse, including power supply, to the RSO.

As shown in Figure 1, RPSs are connected to transmis-sion or distribution grids by means of traction substations(TSSs). It should be noted that not all TSSs are fed directlyfrom the T&D grid: sometimes railway-side electrical linesconnect several TSS. In the case of dc-fed railways, substa-tions include transformers and rectifiers. In the case of ac-fed railways, substations include mainly transformersand, when the frequency of the T&D grid and railways isdifferent, frequency converters (static or rotary).

Although they are not widely used, energy storage sys-tems (ESSs) allow the temporal excesses of power to bestored and used later in a deferred way.

Finally, the railway power plants (referred to as railway-side distributed generation (RDG) in Figure 1) are generators(typically distributed sources of energy) controlled directlyby the RSO, which allows for railway-oriented operation,and connected directly to the railway grid.

The Operation of Electrified Railways

Control of the SystemThe operation of an electrified railway includes two differ-ent facets that have to be controlled in a compatible way:1) the traffic flow operation (which refers to the way thetrains move) and 2) the electrification operation (whichrefers to the way the power is supplied). Therefore, thecontrol centers of the railway are charged with supervis-ing and operating both the traffic and the electrification.

To ensure the safe and efficient operation of the railway,a signaling system is typically in place to manage the traf-fic flow. The traffic control is typically structured in layers.First, a protection layer is responsible for the safety of thetrain movements and is in charge of giving the orders toensure that no train leaves its safe operation conditions(for instance, by getting too close to another train or byexceeding its maximum speed in a specific section).Depending on the specific technology used in the signalingsystem, the degree of automation of the control may bevery different: from manual control (based on visual sig-

nals and relying on a person taking the right actions) tofully automated control (based on communications and

relying on a control unit to ensure that the system is alwaysin a safe state). Additional layers, always subordinated to theprotection layer, are commonly used to improve the quali-ty of the traffic flow according to different criteria (such aspunctuality and regularity). The control related to energyconsumption optimization would correspond to theseoperational layers.

The electrification has a similar architecture. A firstlayer, in charge of protecting the electrical equipment andinfrastructure, continuously checks if all of the electricalquantities (voltages, currents, etc.) are within the allowablerange and, otherwise, isolates the failure to avoid furtherdamages. Additional layers are responsible for optimizingthe operation of the railway grid by reconfiguring its topolo-gy, operating the tap changers of the transformers, etc.Unfortunately, these control actions are quite limited andare often too slow for launching them frequently. Thus, thegrid is normally designed to supply power in the worst-casescenario with very few control actions, which leads to quiteoversized infrastructures.

Although the traffic and electrification are two facetsphysically coupled in railways (the electrical loads dependon the way each train is driven and the way a train is drivendepends on the voltages and, therefore, on the electricalloads), even the upper layers of these two control systemsare usually completely uncoupled.

The Operation of Train ServicesAn important concept for understanding RPS operations is theinterrelation between the train movement and its power con-sumption, which can be used to accelerate the train, to com-pensate the losses due to running resistance forces—red curvesin Figure 2—and/or to feed the onboard equipment (air condi-tioning, pumps, compressors, lighting, etc.). Similarly, when atrain equipped with electrical braking systems brakes, thekinetic energy is converted into electrical power and used tofeed onboard equipment, to feed other electrical loads by inject-ing this power back into the catenary (regenerative brake), or, ifnone of the previous options is possible, to heat up the onboardresistors installed for that purpose (rheostatic brake).

The power usage related to train movement dependsessentially on how the train is driven. The driver, which can be

a person or an automatic driving system, decides which forceis required to move the train as wanted within the operatinglimits of the train (see Figure 2)—this depends on the voltageand the speed at which it is operating. Four types of drivingactions are normally performed: 1) accelerating, where thetraction equipment exerts a force to increase the speed, 2)braking, by exerting a force to reduce the speed, 3) cruising, byexerting only the force required to compensate the runningresistance (which maintains the speed), and 4) coasting, whenthe train does not exert any force at all.

For a given journey duration, a train can be driven inmany different ways: accelerating, braking, and coasting dif-

ferently (in different locations and with different intensities).One driving strategy commonly used for analysis is the




minimum time driving (MTD), whichcorresponds to the fastest way of driv-ing while satisfying the limits of the roll-ing stock and the infrastructure. Thecommercial driving is normallydesigned by adding some time marginsto the MTD to allow the trains torespond to the small perturbations thatoccur in real operation (typically delays).

It is important to highlight thateach driving style can lead to a verydifferent spatiotemporal distributionof the power consumptions (see Figure3) and, consequently, to significantlydifferent requirements for the RPS.This flexibility is the key for conceiv-ing smart strategies for driving thetrains for many different purposes,

such as saving energy and augment-ing the traffic capacity.

To manage the traffic flow, each train has to fulfill aschedule, which means that it must reach the next station[or the next regulation point (RP)] in the specified time(see Figure 4), within a tolerance (represented in green).The time in these intermediate RPs is checked to allow fora better adjustment of the train driving: if the train arrivestoo early, it can slow down or, conversely, drive faster if it

is delayed.In the last decade, many researchers have worked inten-

sively to design ecodriving strategies, i.e., driving strategiesthat minimize the energy consumption of the train. Impor-tant energy savings have been achieved by smartly adjustingthe coasting sections, which avoids consuming energy thatwill be given back to the catenary (or wasted in rheostats)later. However, coasting typically augments the trip duration,and, therefore, a tradeoff between energy savings and tripduration has to be found by using optimization techniques.

The design of schedules optimized to enhance opera-tion has also been a very active research topic. Forinstance, in dc railways, such as commuter trains andmetros, the synchronization of departures, arrivals, anddriving strategies has been used to improve the receptivityof the contact line (catenary or active rail) when regenera-tive braking is used, leading to significant energy savings.There are also experiences in which similar techniqueshave been used to improve the traffic capacity of a line.

Both techniques, ecodriving and the smart scheduling,have been very successful in improving the performanceof electrical railways, especially in terms of energy effi-ciency, when the operation is planned. As they require ahuge computational effort, their application to real-timecontrol is very unusual today.

The Next-Generation Railway ESGsAlthough improving energy efficiency has often beenclaimed as the main change vector toward the future

railway ESGs, it is important to high-light the enhancement of the control-lability of the electrified railways thatcan be achieved with RESG technolo-gies. By managing the traffic and elec-trification in an integrated way, theRESG can efficiently solve differentoperation problems (capacity limita-tions, changes in the planned opera-tion, etc.), including many issues thatcannot be addressed within the tradi-tional control schemes. To allow this,the RESG has to integrate the mis-sions of the railway (to move trains totransport goods and persons) and theelectrification (to supply the electrici-ty required by the trains), as repre-sented in Figure 5.

To explain why the RESG canimprove the operation of electrified

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–50

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kN

Rheostatic Brake

RegenerativeBrake withMaximumPower(8,800 kW)

50 100 150

Horizontal Drag

Ramping Drag 25 mm

Traction 50%(4,400 kW)



200km/h

250 300 350

SERIE 103 Tare Weight: 425 t; Power: 8,800 kW;Maximum Speed: 350 km/h

Figure 2. The maximum traction and braking forces for series 103trains from Renfe, Spain. (Figure courtesy of Luis E. Mesa.)

By managing thetraffic andelectrification in an

integrated way, theRESG can efficiently

solve differentoperation problemsthat cannot beaddressed within thetraditional control

schemes.




railways, it is important to underlinethat RPSs are generally not infinitegrids. RPSs are normally designed to beable to supply electrical power in theworst-case conditions defined in therequirements, both for normal opera-tion (with all of the elements of the

system working properly) and fordegraded operation (for instance,assuming the loss of one or two sub-stations). In the design process, a spe-cific operation is assumed, includingrolling stock characteristics, train fre-quencies, and driving strategy (typical-ly MTD). Once the electrification is in service, the operationconditions change as time goes on: the transport demandtends to grow (following the economy growth) and so do theelectrical requirements. As long as the operation is lessdemanding than planned, the electrification can provide thepower the trains request. But once the limits of the electrifi-cation start being reached (e.g., some line sections are tem-porally overloaded, the voltage drops become too large inspecific points, etc.), the electrification starts creating bottle-necks to the operation at specific peak moments in specificlocations. When this occurs, it is, of course, possible toupgrade the electrification by adding some reinforcements(additional conductors to the catenary, new substations, etc.).A different approach would be possible by using RESG tech-nologies: adapting the operation so that the rated limits ofthe electrification are not exceeded. This is the goal of severalongoing research projects, which are exploring these controlmechanisms and developing different RESG technologies. Anexample is the project MERLIN, a European initiative that isexpected to deliver the final results by the end of 2015 (see“The MERLIN Project”).

This section describes some of the features that will bepossible in future RESGs, grouped into three categories:1) smart train operation, 2) smart operation of the RPS, and3) smart interaction with other power systems.

Smart Train OperationAs discussed in the “Railway Power System Grids” section,train driving provides a flexible tool to the RSO to adjust thepower consumption profiles to the needs of the system. Asthe traction energy consumption represents an importantpart of the operation cost, railway companies have devotedmuch effort to optimize the driving strategies to minimizethe energy consumption. This is normally an offlineprocess where the results are a reduced set of driving strat-egies, each for a different journey duration.

Smart train driving (STD), i.e., controlling online theway the trains are driven, is the most direct mechanismfor performing an active management of the demand(AMD), a key feature in most ESGs, in RPSs. Because of the

computational load it involves, optimizing the train driv-ing in a short time is a major challenge the RESG will have

to address, especially if a representation of the electrifica-tion is included in the optimization model. It should benoted that the main objective of the STD can be verydiverse: minimizing the energy consumption, adaptingthe train consumption to the capacity of the infrastructurein a specific area, and reducing the cost of the electricity.

In Figure 6, in addition to minimizing the energy con-sumption (case A), which has been taken as the base case,two other types of driving changes are introduced. Case Bcorresponds to a limitation of the power peak supplied bythe electrical grid 2, e.g., due to temporary capacity limita-tions. Naturally, depending on their type (current or volt-age capacity limitations), the power consumption shouldbe modulated differently for better results. If these limita-tions were at a TSS level (instead of at an electrical gridlevel), the adjustments would be similar, but covering adifferent area. Finally, case C corresponds to a transfer ofpart of the energy consumption from electrical grid 2 toelectrical grids 1 and 3, which could be advantageous, forinstance, if the prices of the energy were higher in theelectrical grid 2 [price-oriented driving (POD)].

It should be noted that, in general, modifying the powerprofile normally implies modifying the speed profiles and,therefore, the arrival times to the RP. Consequently, in addi-tion to a spatial shifting of the power consumption, a tem-poral shifting is also performed. When performing POD, thiscan be useful as electricity prices often vary, not only withthe location of the supply but also with the time.

In addition to driving the trains, another important aspectof the smart train operation is the management of all of the

Speed

Time

Power

Maximum Speed

Speed

Time

Power

Maximum Speed

Figure 3. Two examples of different driving strategies for the same trip and duration (singletrain), leading to different power profiles.

Time

Position

RP 3

RP 2

RP 1(Origin)

RP 4(Destination)

Train 1 Train 2

Figure 4. The traffic mesh for two consecutive identical trains.




auxiliary loads onboard (e.g., the cooling system of thetraction equipment, air conditioning, lighting, and entertain-ment equipment). Some of these loads can be managed insmart way, modulating the power consumption according tothe needs of the system (and the train itself).

Smart Operation of the RPSRPSs normally have a limited set of controllable devices,including the switching devices (breakers, disconnectors,etc.), on-load tap changers of the transformers, and con-verters used to connect the RPS to the T&D grid (in 16.6-Hzsystems and similar). Because of this limited controllabili-ty, the conception of AMD strategies has been, so far, themost common approach in RESG research.

However, this tendency is very likely to change in thefuture thanks to the advances in power electronics that

make it possible to reach higher voltages, transferring morepower more efficiently and more affordably. With thesetechnologies, a smart control of the power flows within theRPS would be possible.

Figure 7 compares two different concepts for control-ling the power flows. In solution A [Figure 7(a)], the powerflow is controlled at every coupling point to the supplyinggrids (in the TSS) by the controlling devices labeled PFR.Alternatively, in solution B [Figure 7(b)], the controllingdevices (PFE in the figure) set the power flows betweenadjacent sections, which indirectly control the power flowsupplied by each TSS. (The ESS and RDG are controlled inthe same way as in solution A.)

Regardless of which solution is adopted, controlling thepower flows within the RPS can significantly help toachieve the purposes of the RESGs, such as minimizing the

ESS

TractionSubstationRailway

Power System(Controlled by

RSO)

RDG

RailwaySystem

Operator

RSO

Controls theRailways Power

the Traffic Flow( )

Flows ( ) and

TractionSubstation

TractionSubstation

TractionSubstation

Figure 5. The control of future railway ESGs.

The MERLIN Project

The MERLIN project (http://www.

merlin-rail.eu) is an important initiative

in the European Union (EU) context that

comes up as a response to the fifth call

issued by the European Commission as

part of Seven Framework Programme.

The par tnership gathered to achieve theMERLIN objectives is composed of 20

partners from eight EU member states

(Czech Republic, Germany, France,

Belgium, Italy, Spain, United Kingdom,

and Sweden), comprising different

European railway systems integrators and

equipment suppliers (ALSTOM, AnsaldoSTS,

AnsaldoBreda, MerMec, SIEMENS, CAF, and

Oltis Group), along with railway operators

(RENFE), infrastructure managers (ADIF, RFF,

Network Rail, and Trafikverket), supported

by consulting companies (D´Appolonia),

universities (Newcastle University and

RWTH-Aachen University), and research

centers and professional associations [the

Association of European Railway Industries

(UNIFE), the Union Internationale desChemins de Fer (UIC), and the Spanish

Railways Foundation (FFE)].

As mentioned on its official site, the aim

of the MERLIN project is to investigate and

demonstrate the viability of an integrated

management system to achieve a more

sustainable and optimized energy usage in

European electric mainline railway systems

and to provide an integrated and optimized

approach to support operational decisions,

leading to a cost-effective, intelligent

management of energy and resources through: ■ improved design of railway distribution

networks and electrical systems and

their interfaces ■ better understanding of the influence of

railway operations and procedures onenergy demand

■ identification of energy usage optimizing

technologies ■ improved traction energy supply ■ understanding of the cross-dependencies

between technological solutions ■ improving cost-effectiveness of the

overall railway system ■ contribution to European standardization

(TecRec).




losses, reducing the cost of the electricity, and smootheningthe voltage profiles. But, in addition, as the power flows canbe readjusted to come from different TSSs, the reliability ofthe system is significantly improved. In some cases, thismay allow for a reduction of the overrating of the infra-structure elements. (Transformers, converters, and lines aredesigned to work in conditions that could be avoided ormitigated by using RESG.)

Smart Interaction with Other Power SystemsProbably the most important aspect of the RESGs is theimprovement of controllability that can be achieved, whichmakes it possible to adapt the operation in real time to theoncoming events originated inside or outside the domainof the RPS. Because of their relative size, RPSs have animpact on the T&D grids to which they are connected,which has to be carefully analyzed to avoid nuisances toother customers. But, for the same reason, they can also

efficiently help the ESOs and the T&D grid operator performan appropriate operation.

Here are just two examples of the richer interactionbetween heterogeneous smart grids (RESGs and ESGs) thatwill be possible with the adoption of RESG technologies:

x When an incident occurs in the T&D grid and itscapacity has been reduced temporarily, the T&D gridoperators can prioritize other customers and ask therailway to reduce its consumption from a specific setof substations: the RESG allows it.

x With RESG technologies, in the future, railways couldalso provide ancillary services (e.g., secondary bandregulation) to help balance the generation and thedemand in an electrical system.

BiographiesEduardo Pilo de la Fuente ([email protected]) iswith EPRail Research and Consulting, Spain.

Sudip K. Mazumder ([email protected]) is with theUniversity of Illinois at Chicago.

Ignacio González Franco ([email protected]) is with theSpanish Railways Foundation (FFE).

Figure 7. The different strategies to control the power flows in an RPS: (a) case A: control at the power supply points and (b) case B: power equalization.

Electrical PowerSystems

Railway PowerSystem

TractionSubstation

PFR PFR PFR PFR

Electrical PowerSystems

Railway PowerSystem

LegendPFR: Power Flow RegulatorPFE: Power Flow Equalizer

: Controlling Device

PFE PFE PFE

TractionSubstation

TractionSubstation

TractionSubstation

TractionSubstation

TractionSubstation

TractionSubstation

TractionSubstation

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OriginalModified

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Original Modified

Figure 6. The different driving changes executed to manage the power demand in an RPS: (a) base case, (b) power peak reduction, and (c) energy transfer.

railway electrical smart grids

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