short-circuits so as to be able to plan the coordination ... · short-circuit by calculating the...

MOTORS: a flexible traffic simulator to

investigate rail and metrorail capability

G. Cosulich, T. Ghiara

E/ecfra &r./.

Abstract

The design and revamping of a metrorail transit system needs a fast and flexiblesimulation tool which allows the electrical plant to be sized thus avoiding heavyoversizing. In complex power supply systems, such as 25 kV AC systems or metrosystems with a large number of junctions, a similar tool is particularly useful. Themain features of this tool must be: modularity, flexibility and electromechanicalintegration. In this paper, the tool code structure is described and somecomparative examples are presented.

1. Introduction

The authors have developed MOTORS [1], MOdular Traffic Object RailwaySimulator, a program running on a standard MS-DOS personal computersupporting Windows graphic interface, a tool conceived as an aid to the design andverification of guided electrified transport systems.

Its main characteristics are essentially modularity, flexibility and ease of use.During the development of the design, particular importance was given to themodularity of the code so as to be able to implement new functions as well as toease of use and interpretation of the results. Therefore, the concept of modularityis to be understood in a wide sense: the structure of the software is modular andmade up of objects; it is divided into modules which, once they have beenassembled, define the system to be examined.

The modular structure makes it possible to carry out electrical, mechanical orelectromechanical verifications of the system under review, supplying as result theprofile of all the electrical and mechanical quantities of interest; moreover, thesimulator contains the modules for the definition of the train timetable, the study of

Transactions on the Built Environment vol 18, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

382 Computers in Railways

short-circuits so as to be able to plan the coordination of the extra fast circuit-breakers and for the analysis of the ground network.

The tool flexibility and ease of use make it possible to analyse the influence ofthe different design choices on the potentiality of the line, on the powerinterchange and on the extent of energy absorbed by the power supply line and toanalyse the system in any situation of equipment failure.

2. MOTORS - MOduIar Traffic Object Railway Simulator

MOTORS is an integrated electromechanical simulator for railway and metrorailtransit systems [2], developed in C++, a objects oriented language which makes itpossible to manage easily large size software projects.MOTORS is a modular tool which enables electrical, mechanical and

electromechanical analyses of electrified transport systems (railway, tram andunderground railways) to be performed.

The simulator consists essentially of an electrical and mechanical module whoseintegration enables integrated electromechanical analyses of guided electrifiedtransport systems to be carried out [3,4].

The mechanical module makes it possible to simulate the running of the trainsand thus to: study the traffic on the line, verify the composition of the convoys inrelation to the performance of the locomotives and determine the train timetablecompatibly with the stops in the stations, overtaking and train crossings.

Through the electrical module it is possible to analyse the behaviour of theelectrical network: verify the potentiality by establishing the arrangement andoptimum size of the ESS, analyse the behaviour of the network in the case of ashort-circuit by calculating the short-circuit currents in the various junctions of thenetwork and thus calibrate correctly the protection system, calculate thelongitudinal voltage on the rail, calculate the step voltage between the track andthe ground when the position of the short-circuit is changed.

Integrated use of the electrical and mechanical modules, moreover, makes itpossible to study the electrified transport system under conditions of real use,bearing in mind the existing mutual interactions between the mechanical variableswhich characterise the convoys and the electrical variables which characterise thefeeder network. By using the simulator in an integrated way, the electrical andmechanical equations are solved simultaneously and this makes it possible to takeinto consideration the traction curve variations of the locomotives in relation to thevalue of the pantograph voltage and current values: the effects of the voltage andcurrent limitations, imposed by traction drive, on the variation of the torqueactually supplied at the wheels are considered. These phenomena affect the runningof the convoys and the profile of the electrical sizes of the supply network.

The use of non integrated tools could lead, following the introduction ofsimplified hypotheses, to an oversizing of the feeder system and to an incorrectcalculation of the potentiality of the line.

The MOTORS main characteristics are briefly described below, tanks to itsmodularity is able to simulate the behaviour of transport systems; the implementedmodels make it possible to characterise all existing supply systems, whether in AC


Computers in Railways 383

Train operation mode determinationI

Train resistance effort calculation1

Network admittance matrixcalculation

Train required power computationCurrent and voltage limits verification

or in DC, and to define all possible topological characteristics of the route so as tobe able to study and design complex rail junctions.

The mechanical module makes it possible to control the trains at fixed headwayor by means of a pre-established timetable; in this case it is possible to define, foreach train, the moment it enters the line, its speed and the point of entry, the pointof exit and the length of the stopsin the various stations.

The route may be defined byspecifying the length and gradientof the slopes, the length of thecurves and the radius, the lengthand position of the tunnels, thelength and position of the speedlimits, position of the stations,position of the crossing pointsbetween the various sections.

The electrical module makes itpossible to define thecharacteristics of different supplysystems, in particular of DC andAC supply systems, characterisedby different voltage and feederfrequency values.

The conductor feeding systemis represented by the matriximpedance model [5] in particularfor AC systems, Carson Clemparameters with ground return forself and mutual effect areevaluated, according togeometrical and physicalconfigurations, for overhead lines(contact wire and feeder), rails andground wires.

The main AC substationcharacteristics are the nominal

Flow chart of the tool core

frequency, the number of transformer groups and the transformer nominal power,short-circuit impedance and transformer ratio.

The main DC substation characteristics are the transformer nominal power,short-circuit impedance and transformer ratio, the number of rectifier groups, thecharacteristics of the rectifier groups (6 pulse or 12 pulse with series or parallelconnection), the filter characteristics (if any) and the characteristics of theregenerative groups (if any).

Autotransformer or booster substations may be considered for AC systems.Each vehicle may be modelled by the traction and braking effort diagrams at the

wheels, the auxiliary power system characteristics, the efficiency diagram, themaximum speed and acceleration, the power factor, the convoy weight, the Davis



equation parameters for dynamic resistance evaluation, the maximum voltage dropallowed, the maximum overvoltage allowed, the regenerative braking capacity andthe type of speed regulation: constant maximum speed or coasting with speed bandlimits.

The choice of substation parameters and substation positioning may beoptimised by means of integrated solutions; conductor section evaluation may beoptimised acording to the headway required, the characteristics of the vehicle and acertain set of route parameters.

Fault and normal condition analysis may be performed in order to calculate allthe electrical parameters. Output data are organised into two kinds, over time andof the integral type. The speed diagram, pantograph voltages and currents areshown for the vehicle and the power, voltage, current and power factor are shownfor the ESS. The recapitulatory data of the integral type give the time taken by thevehicles to cover a distance, the minimum pantograph voltage, the total energysupplied by the system for traction, the energy recovered by the system, the energydispersed in the tunnels, the Max, RMS and average power values, current andvoltage in ESS and in the feeders.

Figure 1 shows the flow chart of the core of the programme relevant to theintegrated solution procedure of the electromechanical equations of the systemwhich outlines the integrated solution of the system.

Assuming that the electrical transient of the traction system is negligible ifcompared with the mechanical time constants of the vehicle movement, MOTORSsolves, at each integration step of the mechanical equations, an electromechanicalload-flow: the tool, starting from the train power requested, function of the trainspeed, the pantograph voltage, the power factor and the efficiency, solves theelectrical load-flow and then updates the mechanical state variables, such as trainspeed and position. The system equation of electrical networks is solved bybuilding the admittance matrix; the currents and voltages at each network node aresupplied in output.

3. APPLICATION

Some applications are described in the following in order to illustrate syntheticallyease of use and the main characteristics of the tool. The examples show how it ispossible with MOTORS to analyse transport systems characterised by differentsupply systems and different types of route, highlighting the usefulness of anintegrated electromechanical simulation tool. In the following paragraphs, thefeeder system of two hypothetical transport systems will be checked: the firstconsisting of a railway line in alternating current and the second consisting of anLRT (Light Rail Transport) system in direct current. The main input data aredescribed for both examples given and the most interesting results are commentedon.

3.1 AC case study

The study concerns a 60 km double track railway line which connects five towns.The purpose Is to see whether It la possible to feed the system with only one



substation at 25 kV in AC situated in the middle of the section. The line is used byvehicles whose power is equal to 5MW with 300 seconds headway. Thecharacteristics of the route, relevant to a planoaltimetric profile, tunnel, speedrange and position of the stations are shown in figure 2.

In order to demonstrate the usefulness of an electromechanical tool, only themechanical simulation was carried out initially, then the results obtained were usedto verify the supply system. This way of proceeding showed that the network isalmost at the limit of operation and thus the tool was used in an integratedelectromechanical way.

Gradient: + 0.2 Gradient: - 0.2 %

Speed limit: 180km/h

(ESS )

| Station 31

30m 15000m 30000m 45000mFigure 2: Track characteristics

59970 m

In figure 3, the speed diagrams obtained using the two different approaches arecompared; the broken line represents the functioning of the simulator in anintegrated way whereas the continuous line is the result of the mechanical

simulation alone.The difference between the different speed ranges is more obvious if one looks

at Table 1 which gives the Run Times (R.T.) obtained using the two different

approaches.

200

150 -

•=* 100-

0 1 2Position [m]

Figure 3: Speed diagram obtained with and without integrated simulation

X104

NameSTAT1STAT2STAT3STAT4STAT5

Position [m]30

15000300004500059970

Mechanical simulationTin [s]21132600308835564021

Table 1

Tout [si21442631311935874052

V [km/hj0

118.2118.2123.6124.2

R.T |s|0456457437434

Electromechanical simulTin s]̂21132715321436854156

Tout [si21442746324537164187

V [km/hi094.4115.4122.7122.5

R.I.IS]0571468440440

: Train up, Run Times Comparison



The mechanical characteristics in electrical drive depend on voltage supply. Inparticular, the rated mechanical characteristics of vehicles used in the metrorailsystem falls when the pantograph voltage is below a fixed value. This involvessolving the electrical and mechanical equations in an integrated way, provided thatthe vehicle model takes into account the above-mentioned dependence. Thus, it ispossible to evaluate the real system capacity.

In the purely mechanical approach, the train is able to fully exploit the nominalpower of its drive whereas in the integrated electromechanical approach, the powerwhich the vehicle can use is limited by both the current limit of the drive and thepantograph voltage drop whose fall is a function of the position and powerabsorbed or supplied by all the vehicles on the line.

Figures 4 and 5 show respectively the pantograph voltage and current profilesof a train in relation to its position; it can be seen how the pantograph voltage (fig.4) goes below the nominal value of 27500V and the effect (fig 5) of theintervention of the current limit of the drive.

300

<L 200

Position [m]Figure 4: Pantograph Voltage

)0 I

in II

0 1

"̂ K

2 7

I \

4

V

5

l_

Position [m]Figure 5: Pantograph Current

3.2 DC case study

This second example of application of the tool intends to verify the sizing of anLRT (Light Rail Transport) system, double track with continuous current at 750V.One objective of the analysis is to check the effect obtained, in terms of energysaving, using vehicles which return energy to the network during the braking phaseand ESS able to return energy to the main feeder network.

The sizing was checked for vehicles with a power equal to 1.2 MW withheadway equal to 120 seconds.



Figure 6: Track characteristics

The characteristics ofthe route are shown infigure 6, the maximumslope is less than 4%. Line1 covers, in bothdirections, the upper partof the Y from point A topoint B; line 2 the lowerpart from point A to pointC in both directions.

The catenary is fed by4 ESS all in parallel.

The system wasanalysed under fourdifferent hypotheses (table2), assuming that the ESSand the trains are not of

830

the regenerative type, that the trains are but the ESS are not, that all or only two ofthe ESS are of the regenerative type.

Figure 7 shows the model, implemented within the simulator, of two quadrant

ESS. The characteristic of thenon regenerative ESS coincideswith the functioning in the firstquadrant.

Table 3 summarises the resultsobtained relevant to the powerconsumed and returned by theESS of the system in the variouscases of application. From thetable it can be seen that thepower supplied in case 1 is 20%higher than for cases 3 and 4, Figure 7: ESS Modelsystem with vehicles and ESS ofthe regenerative type; the datumrelevant to the power consumed is more interesting as in the first case it is about40% higher with respect to the cases with ESS of the regenerative type.

Furthermore, it can be seen that from the point of view of energy recovery, itdoes not seem necessary, from the economic point of view, to include four ESS ofthe regenerative type as two would appear to be sufficient.

Backwards bridge Forwards bridge

VehicleESS 1 BridgeESS 2 BridgeESS 3 BridgeESS 4 Bridge

CASE1Not regenerative

ForwardsForwardsForwardsForwards

CASE 2RegenerativeForwardsForwardsForwardsForwards

CASE 3RegenerativeForwards

Forw. and Back.Forw. and Back.

Forwards

CASE 4Regenerative

Forw. and Back.Forw. and Back.Forw. and Back.Forw. and Back.

Table 2: Cases Studied



Figure 8 shows the speed diagram of the train travelling along line 2 whilefigure 9 shows the profile of the pantograph voltage and current in relation to theposition: it can be seen that the voltage remains in a range of 600-800 Volts andthat during the braking phase, the vehicle returns high current values to thenetwork.

Table 4 gives the integral values calculated by the tool relevant to ESS 2 infigure 6: the table makes it possible to check that the power requested by thevehicles is less than the nominal one and that the peak values, for both the forwardand backward bridge, are within the required range. The tool supplies as result, thecurrent values in the feeders, which makes it possible to check the section.

Traction power [kW]Return power [kW]Power consumption [kW]

CASE1Averagevalue28280

2828

CASE 2Averagevalue22110

2211

CASESAveragevalue21846011583

CASE 4Averagevalue21906201570

Table 3: ESS Power

100

1000 2000 3000 4000 5000 6000

Position [m]Figure 8: Speed diagram, Train line 2 case 3

1000

7000

2000

-1000 -20001000 2000 3000 4000 5000 6000 7000

Position [m]Figure 9: Pantograph Voltage and Current, Train line 2 case 3

Figure 10 shows the current and voltage profiles at the feed clamps of the ESS2: the forward bridge works about 75% compared to the backward bridge.

Further analyses were performed with positive results to verify that in the caseof a failure of each ESS, the system would still be able to meet traffic demands.



ESS 2

Traction power [kW]Return power [kW]Traction current [A]Return current [A]Voltage [V]Feeder 1 Current [A]Feeder 2 Current [A]Feeder 3 Current [A]Feeder 4 Current [A]Minimum voltage [V]

CASE 3Peak value

3178-26014058-3321—2139242420002022

RMS value——13151020785666730689537

Average value633-382808-488785499507328264

718Table 4: ESS 2 recapitulary data

1000

500

-500

10000

5000 2~c

o ji

50 100 150 200 250-5000

Time [s]Figure 10: ESS 2 Voltage and Current, case 3

5. Conclusions

MOTORS is a design and verification tool for electrified transport systems,developed using the most up to date means and according to the qualitative criteriaexpected from this type of tool. As briefly described in the preceding paragraphs,thanks to the characteristics of flexibility, ease of use and clarity in the presentationof the results, quick verifications and analyses of rail, tram and metrorail feedersystems are possible. Its modular structure makes separate or integrated use of thesolution module of the electrical network and of the mechanical solution modulepossible thus enabling rapid partial verifications to be made.

The tool's flexibility and ease of use make it possible to analyse the influence ofthe different design choices on the feeder system: for example, the influence of theposition of the substations and parallel points on the profile of the voltages andpower interchange in the network or the influence of the conductor section on thecurrent distribution in the secondaries of the ESS transformers.

Moreover, the tool makes it possible to correctly size the electrical equipmentwhenever exploitation of the line varies and to analyse the potentiality in anybreakdown situation of the feeder system.

Moreover, it is possible to verify the short-circuit currents and voltage step inrelation to the position of the failure on the line.

The tool has already passed the verification and validation stages: the moduleswhich form the tool have been checked for what concerns both the modellingadopted and the software. The results, for both the mechanical and electrical



modules, have been compared with the results supplied by tools already validated.The verification also involved the functioning of the electromechanical load-flowalgorithm: analytical calculations have been made on the congruence of the energybalance between the energy absorbed by the trains and that supplied by the ESS.

The tool has been used to verify and design feeder systems of italian High Speedlines in both direct and alternating current, as well as of conventional andmetropolitan lines.

References

[1] G.Cosulich, T.Ghiara: "Progettazione e verifica degli impianti perI'aUmentazione del sistemi di trasporto elettrificati", Ingegneria Ferroviaria,Luglio 1995.

[2] G.Cosulich, T.Ghiara: "La simulazione come strumento di progettazione perI'alimentazione di sistemi ferroviari", 96% Riunione Annuale AEI, Roma 24-27 Settembre 1995, pp. 99-106.

[3] G. Guidi Buffarini, A. Luzi, M. Romano, Programmi di CalcoloComputerizzato per la Verifica delle Alimentazioni del Sistemi di TrazioneElettrica a Corrente Continua, Ingegneria Ferroviaria, Aprile 1992.

[4] B. Mellit, J. Allan, Z.Y. Shao, W.B Johnston, C.J. Goodman, Computerbased methods for induced-voltage calculation in AC railways, BEEInternational Conference on Main Line Railway Electrification, No. 312,London 1989.

[5] J. Arrilaga, C.P Arnold, BJ. Marker, Computer modelling of electricalpower systems, J. Wiley & Sons, April 1991.

[6] A. Capasso, S. La Rosa, Elettrificazione monofase 25 kV 50 Hz:Osservazioni sul calcolo del sistema elettrico, Ingegneria Ferroviaria,Maggio 1979.

[7] P.Pozzobon, G.Sciutto: "Advanced computer simulation for analysis anddesign of electrified transportation systems: electrified plants and drives",COMPRAIL '92, Washington DC, USA 18-20 August 1992, pp. 445-473.

[8] P. Cesario, T. Ghiara, O Ventura, "L'elettrificazione in corrente continua edalternata monofase: caratteristica degli impianti nell'ipotesi di uso intensioni elevate ed analisi armonica delle variabili elettriche", CongressoNazionale Sviluppo e prospettive dei trasporti elettrificati: ricerca edinnovazione, Genova, Italy, Novembre 1992.

[9] G.L.Solbiati: "A program for the electric simulation of a traction line"',COMPRAIL '90 Roma, Italy, Marc 27-29 1990.

[10] J.D.Glover, A.Kusko, S.M. Peeran: "Train voltage analysis for ac railroadelettrification"', IEEE Transaction on Industry Application No. 4 July/August1984.

[11] A.Bobbio, G.Cosulich, T.Ghiara: "Simulatore di trqffico per reti di trasportopubblico su gomma", Automazione e Strumentazione Elettronica Industriale,Rivista Anipla edita da Bias, No. 3 Marzo 1994, pp. 57-62.


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