impose traffic restriction to avoid the overcoming of the ......Short circuit analysis for traction...

Short circuit analysis for traction power supply

system of new concept guided busway

A. Del Naja, V. Galdi, L. Ippolito & A. Piccolo

Diparimento di Ingegneria dell'Informazione ed Ingegneria

Elettrica - Universitd di Salerno - Via Ponte don Melillo, 1-

Abstract

Today, environmental quality of cities is an emerging issue. Even ifelectrical propulsion systems can solve the problem of pollution theyshow limits due to the limited maximum range. To solve the problem, inthis paper a new concept guided busway for public transport is reported.

It is composed of a series of power modules, installed on the roadway,feeding electric vehicles. The vehicles, public bus or cars, are equippedwith a pantograph installed under the deck which permits to deliver thetraction power, moreover the vehicles are equipped with batteries whichsupply the required energy to the motors both in presence of a power cutoff or in presence of a bus run deviation.

In this designing stage many problems must be solved, in particular inthe paper an outline methodology for the calculation of short-circuit andthe evaluation of the effects of the fault currents is reported. Moreover,some simulation are carried on to determine the adequate sections of thepower system conductor.

1 Introduction

Today's ever increasing traffic volumes have produced high levels ofpollution in the major cities of the world, causing heavy consequences tocommunities. From some years, in the biggest cities governors had to

Transactions on the Built Environment vol 34, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509

490 Computers in Railways

impose traffic restriction to avoid the overcoming of the pollution dangerlevels. This policy can produce only temporary results, but to achievesteady and lasting benefits electric public transport must be encouraged.The electric propulsion systems can contribute significantly to thelowering of the current pollution levels. On the other hand, the limitedmaximum range represents the main problem when using such a kind ofsystems.

A possible answer to the problem can be represented by a newconcept guided busway, called STREAM, implemented by the AnsaldoTrasporti S.p.A.. Owing to its originality, some design problems must tobe solved before its large scale use. Amongst the major problems theevaluation of the short-circuit currents magnitude represents a crucialelement in the definition of the protections, of their insertion in thepower schema and of their operation. The paper analyses these aspectsaiming to provide some useful consideration for the design of the powersupply system of this new concept guided busway.

2 System description

STREAM is composed of a power supply system, a contact line and

electric propulsion vehicles. The originality of the system is that the

contact line is embedded in the roadway along the main routes of the

vehicles. The contact line is realised with rigid and water-proof modules,

which contain a flexible power conductor inside. Each module, 3 or 6

meters long, has insulated metal segments, about 48 centimetres long, on

one half of its surface. When vehicles do not run on the considered

module, the power conductor is on the bottom of the module and the

segments are insulated, making the line harmless and hazard free. When

a moving or stationary vehicle is over the module, a magnetic pick-up,

positioned under the vehicle, exerts an attraction force on the power

conductor which raises on the top of the module, connecting the

segments to the power supply voltage. In this way, the vehicle current is

absorbed through a positive sliding-shoe on the energised segment, and

returns through a second sliding-shoe on the negative terminal of the

module, represented by a metal segment on the other half of module

surface. This means that the contact line is live only in presence of a

stationary or moving vehicle and the considered segment is connected to

the traction supply only when a vehicle run over it.


Computers in Railways 491

Because of the vehicle has equipped with a moving current collector

under its deck, it can collect traction power, lowering the collector, when

motoring or charge its on board services when stopping. But, anytime, it

can disconnect, raising the collector, from the contact line, running withthe on-board electric power source. This feature gives to the STREAM

vehicle the same operating flexibility as a bus.In the STREAM system the traction power can be delivered by AC/DC

current converter or more effectively from electrical substations at

750 VDC.Today, in the developing of the STREAM system the problems are on

the power supply design. It presents substantial differences with thetraditional traction power supply networks. So* large attention has to bepaid to the sizing of the supply system and of the protection system.

3 Power Supply Network Analysis

With respect to a conventional DC traction line, the STREAM linepresents some important differences, which impose to re-consider the

topology of the power supply network.The main difference amongst the DC traction line and new system is

that the conventional contact line is now replaced by rigid modulesembedded in the roadway at the surface level. These modules can beconnected in different ways to form the contact line. The easiest solutionis to connect all the modules in series through junction conductors, withthe first module connected to the substation and the last one shorted atthe end of line. Although this topology of the power network iscost-effective, some experimental tests have proved that a not negligiblecontact resistance can be ascribed to the junction conductors between themodules, producing a considerable thermal stress and a voltage dropwhen the line current flows through it. This represents a reduction of thesystem efficiency in heavy traffic condition and can cause operatingproblems with very long line due to the heavy voltage drop.

Some studies and experiences have suggested to migrate towards anew kind of topology for the power network. A possible solution is toadopt an arborescent power network topology, which is more usual inpower system instead of traction lines. With the arborescent topologyeach branch (or link) is supplied from the trunk (or main line) and onlythe traction currents of the vehicles moving on the branch flow through

it.Following the previous approach, the new catenary system can be

constituted by a positive feeder which feed energy into some insulated



segments realised grouping some STREAM modules. Each segmentrepresents a subsection of the contact line and has the first moduleconnected to the feeder wire and the last shorted at the end, asschematically reported in fig. 1. This means that the vehicle tractionpower is transferred through the feeder to the insertion point of thesegment on which the vehicle is moving and beyond this point it istransferred through the positive circuit of the STREAM modules makingpart of the segment.

Figure 1 - Proposed power network configuration

As far as the return circuit concerns, it is composed of the negativecircuit of the STREAM modules of the considered segment and of anegative feeder wire which is connected to the ground of the substation.

The segment length will depend on many parameters as the number ofvehicles running on the STREAM system, the geometry of the modules,the equivalent section of the positive and negative conductors of themodules, etc.. Just with relation to the equivalent sections of theconductors, it must be treated as one of the main design parameters forthe STREAM line. In fact, any variation of the sections can produce largechanges of the remaining parameters and influence significantly the lineprotection system.

Adopting the previous power schema, each segment delivers only asmall part of the total current in normal operation, corresponding to thetraction current of the vehicles moving on it. This means that in normaloperation the current flowing into each segment is very small withrespect to that one flowing into the positive feeder.

A further discussing point is the ground connection of the system.Actually, the STREAM contact line is connected to ground only insubstation, these means that the system is equivalent to an electricallyinsulated system, while in the conventional traction system the rail isconnected to earth continuously. This occurrence represents a qualifyingelement for the system, because even if it produces an higher resistanceof the return circuit which makes worse the protection level of the system



in presence of a remote line-to-ground fault [1], it makes the system fitwith respect to the future international standards oriented to limit theground currents. Even if the proposed power network configuration

solves the problem of the power losses along the line, improving thesystem efficiency, it complicates the design of the protection system dueto the varying section of the power conductors (feeders and modules).This imposes a deep investigation on the protection philosophy through

the analysis of the short circuit.

4 Short circuit analysis

To protect each single segment it is necessary to detect instantaneouslythe short circuit in order to avoid that the energy delivered to the singlemodule of the contact line can overcome the maximum energy that the

modules are able to dissipate.The possible fault scenarios for the considered system can be

summarised as follows:• short-circuit without fault impedance or with an impedance to which

corresponds a fault current (Ip) greater than the rated current (7/v) of

the high speed circuit breaker located in substation;• short-circuit with an impedance to which corresponds a fault current

in the range [/M, /yvL where IM represents the rated current of theSTREAM power module.The analysis of these fault conditions reveals that through a good

co-ordination of the sections of components of the contact line, thedistance between IM and IN can be reduced, minimising the probability tohave hazardous short-circuit. So, in this first design stage, during which aparametric analysis is completing, it seems to be useful to adopt asimplified approach to the short-circuit analysis, neglecting theunidirectional component of the fault current.

Positive conductor

Negative conductorI ^ 1 \ 1 \ 1 \

Negative Feeder wire

Figure 2 - Positive conductor to negative conductor fault



Because the system must be considered electrically insulated onlypositive-to-negative faults are analysed for the proposed traction powerdistribution system. The faults are studied with or without a pre-fixedfault impedance. This fault is shown schematically in fig. 2.

In order to evaluate the short circuit currents, the modelling of theelectric component of the system has been made.

4.1 Power Source (Substation)

The power source is modelled with the Thevenin equivalent circuit, witha resistance (Rsse) of the substation which takes into account also thereactance of the power transformer and of the rectifier and theoverlapping rectifier.

In detail RSSE can be expressed as follows:

3V3

4-V3' 6-100-^ "̂ 4 ̂ '" ^ "̂

where P^ is the rated electrical input power of the rectifier,

VDC represents the rated DC voltage, V^ % is the short circuit voltage,

in per cent, of the power transformer and 1^ ^^ is the maximum value

of the three phase short-circuit current at the secondary windings of thepower transformer.

4.2 Contact line System Resistance

In first approximation, the contact line and feeders can be modelled aspure resistances.

Length_of_segment

-STREAM conductorFeeder wire

ILength_of_line

Figure 3 - Contact line equivalent circuit

These resistances are function of the section and of the distance of thefault. In particular, the range of variation for the distance of the fault is



[0, length_of_line] on the feeders and [0, length_of_segment] on thegeneric segment of the contact line. The equivalent circuit of the contact

line is schematically reported in fig. 3, where the line is represented

through a series of resistances in parallel and the fault point is evidenced.

4.3 Calculation Methodology

On the basis previous considerations two different configurations for thepower network are considered. In the first a single-side fed contact line isused for a double-way STREAM line, while in second one a double-sidefed contact line is considered, as shown in fig. 4.

a) single-side fed contact line b) double-side fed contact line

Figure 4 - Power network topology

For these configurations a calculation code has been implemented toevaluate the short-circuit current in presence of a fault at any distancefrom the starting point of the line. By means of the calculation code aparametric analysis of the short-circuit current, with or without apre-fixed fault impedance, can be made. Making reference to the powernetwork topology of fig. 4a, the qualitative curve of the short-circuitcurrent normalised to the DC current (loc) of the substation is reported infig. 5.

1000 1200 1400 1600Distance [m]

Figure 5 - Short-circuit current vs. distance



In order to estimate the thermal effects of the short-circuit current andto evaluate the weight of the section of the equivalent conductors whichform the STREAM module, some preliminary analysis can be made todetermine the temperature of the different components of the STREAMcontact line.

In the simplest way the global temperature of the conductor n duringthe short-circuit current may be determined by using a thermodynamiccalculation in adiabatic environment. Making this assumption, from thethermal balance between the heat storage of the conductor and the heatproduced by the short-circuit current, it results

Integrating and simplifying, the final expression is

" ' < *di -

obtaining the short-circuit current as I — J —

where K = In«Po

l + q(0,-0o)'

Ll + a(0,.-0o)Jand

/= Short-circuit current [A]a= Temperature coefficient of resistance [1/°C]

po= Conductor resistivity at t% [H-mm]

p- Conductor resistivity at i) [Q-mm]At- Duration time [s]

Qc= Mean volumetric heat capacity [J/°C-mm̂ ]$„= Reference temperature [°C]

fy&o= Temperature rise [°C]

8̂ = Conductor section [mm̂ ]i- Conductor lenght [m]

From the previous equations, it can be analysed the increase intemperature versus the short-circuit current for various value of faultduration, as reported in fig. 6, where the reduced increase in temperaturefor the negative conductor is due to its larger section.



I*.

10000 15000 20000 25000 30000 5000 10000 15000 20000 25OOO 30000Short-circuit current [A]

a) positive conductor b)negative conductorFigure 6 - Increase in temperature vs. intensity and short-circuit time

Particularly interesting it results the analysis of short-circuit with afault impedance which limits the fault current below the rated current of

the high speed circuit breaker in substation.Carrying on this outline analysis and considering that the increase in

temperature inside the module due to the negative conductor is negligibleif its section is much greater than the section of the positive one, in theworst case the thermal behaviour of the positive conductor can bereproduced in adiabatic environment with a set of short-circuit currentsand assuming different initial temperature 0j, as shown in fig. 7.

I. =1000 A .,---'

Short-circuit duration [min Short-circuit duration [min]

Figure 7 - Increase in temperature vs. intensity and short-circuit time

From the curves, obtained considering a short circuit duration ofabout 1 hour, it results that to protect the stream modules it is necessaryto co-ordinate the section of their positive conductor with the ratedcurrent of the breaker to limit the maximum temperature 0̂ of the

conductor at the end of the short circuit, as depicted in Fig. 8.



S=4(X) miiT.,,--""...---""" S=500iW..

10 20 30 40 50Short-circuit duration [s]

Figure 8 - Increase in temperature vs. short-circuit time

The previous methodology permits to identify the rated current of thehigh speed circuit breaker and, in correspondence of the maximum faultcurrent, to size the STREAM module.

5 Short-circuit Numerical Analysis

To test the adequacy and accuracy of the calculation code and forverifying the correctness of the choices about the sizing of the catenarysystem conductors and of the high speed circuit breaker in substation, atest power network has been considered. The main characteristics of thepower network are:

F>pwer Network Characteristics

Network topologysingle-side fed contact linelength of the line: 1.500 mlength of the segment: 60 m

SubstationPower rating:Rated DC Voltage:Vcc% Transformer:

800 kVA750V5%

Contact linePositive feeder section: 300 mmNegative feeder section: 300Positive conductor section: 334Negative conductor section: 2692 mnf

Fault characteristicsFault impedance 0 QMaximum temperature Occ 65 °C



With the calculation code the line-to-line fault current versus distancefrom the substation is determined and plotted in fig. 9.

g 15000-3(Ju 10000 -'

800 1000 1200 1400Distance [m]

Figure 9 - Short-circuit current vs. distance

Depending on these results the breaking capacity of the high speedcircuit breaker results 25 kA. To choose the rated current of the breaker,it must to be considered the total number of vehicles running on the lineat the same time, a suitable criterion is to satisfy the relation

IK = 1,3 • 2^1 vh , where 7^ is the traction current absorbed from then

vehicle.

Moreover, the analysis permits to verify that the 1^ is lower than the

short-circuit current at the end of the line (1^ ^̂ ), which results for the

considered network equal to 3780 kA.Applying the aforementioned methodology also the sizing of the

positive conductor can verified in order to satisfy the thermal constrainsin presence of a fault current.

6 Conclusions

When the pollution become intolerable, electrical propulsion systems arethe real chance of succeeding. STREAM is a new concept guided buswayfor public transport which can help to give an answer to the problem. Itsnatural characteristics give it the flexibility of a conventional bus.

In the paper, the analysis of the power network for the futureSTREAM line has been presented, with reference to the short-circuit, and



the differences with the networks of the conventional traction systemsare pointed out.

The developed methodology for the short-circuit analysis has beenapplied to a test network to the purpose of an outline sizing of the powerconductors.

In the next future, the research will be oriented to the detailed study ofthe system operation and to determine the thermal profile of the STREAMmodules both in operational and fault conditions.

Bibliography

[1] Fusco, G., Gagliardi, F., Ippolito, L., Piccolo, A.: Rappresentazionedel guasto remoto in impianti di trazione in c.c., Proceedings of'Giornate di Lavoro della Associazione Italiana di RicercaOperativa (AIRO '93)', Capri (Naples), Italy, September, 1993,pp. 158-165.


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