Abstract. 1 Introduction

Transcription

1 Short circuit analysis for traction power supply system of new concept guided busway A. Del Naja, V. Galdi, L. Ippolito & A. Piccolo Diparimento diingegneria dell'informazione ed Ingegneria Elettrica - Universitd di Salerno - Via Ponte don Melillo, 1- Abstract Today, environmental quality of cities is an emerging issue. Even if electrical propulsion systems can solve the problem of pollution they show limits due to the limited maximum range. To solve the problem, in this 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 equipped with a pantograph installed under the deck which permits to deliver the traction power, moreover the vehicles are equipped with batteries which supply the required energy to the motors both in presence of a power cut off or in presence of a bus run deviation. In this designing stage many problems must be solved, in particular in the paper an outline methodology for the calculation of short-circuit and the evaluation of the effects of the fault currents is reported. Moreover, some simulation are carried on to determine the adequate sections of the power system conductor. 1 Introduction Today's ever increasing traffic volumes have produced high levels of pollution in the major cities of the world, causing heavy consequences to communities. From some years, in the biggest cities governors had to

2 490 Computers in Railways impose traffic restriction to avoid the overcoming of the pollution danger levels. This policy can produce only temporary results, but to achieve steady and lasting benefits electric public transport must be encouraged. The electric propulsion systems can contribute significantly to the lowering of the current pollution levels. On the other hand, the limited maximum range represents the main problem when using such a kind of systems. A possible answer to the problem can be represented by a new concept guided busway, called STREAM, implemented by the Ansaldo Trasporti S.p.A.. Owing to its originality, some design problems must to be solved before its large scale use. Amongst the major problems the evaluation of the short-circuit currents magnitude represents a crucial element in the definition of the protections, of their insertion in the power schema and of their operation. The paper analyses these aspects aiming to provide some useful consideration for the design of the power supply 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.

3 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 with the 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 the traditional traction power supply networks. So* large attention has to be paid 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 line presents 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 modules embedded in the roadway at the surface level. These modules can be connected in different ways to form the contact line. The easiest solution is to connect all the modules in series through junction conductors, with the first module connected to the substation and the last one shorted at the end of line. Although this topology of the power network is cost-effective, some experimental tests have proved that a not negligible contact resistance can be ascribed to the junction conductors between the modules, producing a considerable thermal stress and a voltage drop when the line current flows through it. This represents a reduction of the system efficiency in heavy traffic condition and can cause operating problems with very long line due to the heavy voltage drop. Some studies and experiences have suggested to migrate towards a new kind of topology for the power network. A possible solution is to adopt an arborescent power network topology, which is more usual in power system instead of traction lines. With the arborescent topology each branch (or link) is supplied from the trunk (or main line) and only the 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

4 492 Computers in Railways segments realised grouping some STREAM modules. Each segment represents a subsection of the contact line and has the first module connected to the feeder wire and the last shorted at the end, as schematically reported in fig. 1. This means that the vehicle traction power is transferred through the feeder to the insertion point of the segment on which the vehicle is moving and beyond this point it is transferred through the positive circuit of the STREAM modules making part of the segment. Figure 1 - Proposed power network configuration As far as the return circuit concerns, it is composed of the negative circuit of the STREAM modules of the considered segment and of a negative feeder wire which is connected to the ground of the substation. The segment length will depend on many parameters as the number of vehicles running on the STREAM system, the geometry of the modules, the equivalent section of the positive and negative conductors of the modules, etc.. Just with relation to the equivalent sections of the conductors, it must be treated as one of the main design parameters for the STREAM line. In fact, any variation of the sections can produce large changes of the remaining parameters and influence significantly the line protection system. Adopting the previous power schema, each segment delivers only a small part of the total current in normal operation, corresponding to the traction current of the vehicles moving on it. This means that in normal operation the current flowing into each segment is very small with respect 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 in substation, these means that the system is equivalent to an electrically insulated system, while in the conventional traction system the rail is connected to earth continuously. This occurrence represents a qualifying element for the system, because even if it produces an higher resistance of the return circuit which makes worse the protection level of the system

5 Computers in Railways 493 in presence of a remote line-to-ground fault [1], it makes the system fit with respect to the future international standards oriented to limit the ground currents. Even if the proposed power network configuration solves the problem of the power losses along the line, improving the system efficiency, it complicates the design of the protection system due to 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 instantaneously the short circuit in order to avoid that the energy delivered to the single module 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 the STREAM power module. The analysis of these fault conditions reveals that through a good co-ordination of the sections of components of the contact line, the distance between IM and IN can be reduced, minimising the probability to have hazardous short-circuit. So, in this first design stage, during which a parametric analysis is completing, it seems to be useful to adopt a simplified approach to the short-circuit analysis, neglecting the unidirectional component of the fault current. Positive conductor I ^ 1 \ 1 \ 1 \ Negative Feeder wire Negative conductor Figure 2 - Positive conductor to negative conductor fault

6 494 Computers in Railways Because the system must be considered electrically insulated only positive-to-negative faults are analysed for the proposed traction power distribution system. The faults are studied with or without a pre-fixed fault impedance. This fault is shown schematically in fig. 2. In order to evaluate the short circuit currents, the modelling of the electric component of the system has been made. 4.1 Power Source (Substation) The power source is modelled with the Thevenin equivalent circuit, with a resistance (Rsse) of the substation which takes into account also the reactance of the power transformer and of the rectifier and the overlapping rectifier. In detail RSSE can be expressed as follows: 3V3 4-V3' ^ "^ 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 the power transformer. 4.2 Contact line System Resistance In first approximation, the contact line and feeders can be modelled as pure resistances. Length_of_segment Feeder wire I Length_of_line -STREAM conductor Figure 3 - Contact line equivalent circuit These resistances are function of the section and of the distance of the fault. In particular, the range of variation for the distance of the fault is

7 Computers in Railways 495 [0, length_of_line] on the feeders and [0, length_of_segment] on the generic 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 the power network are considered. In the first a single-side fed contact line is used for a double-way STREAM line, while in second one a double-side fed 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 to evaluate the short-circuit current in presence of a fault at any distance from the starting point of the line. By means of the calculation code a parametric analysis of the short-circuit current, with or without a pre-fixed fault impedance, can be made. Making reference to the power network topology of fig. 4a, the qualitative curve of the short-circuit current normalised to the DC current (loc) of the substation is reported in fig Distance [m] Figure 5 - Short-circuit current vs. distance

8 496 Computers in Railways In order to estimate the thermal effects of the short-circuit current and to evaluate the weight of the section of the equivalent conductors which form the STREAM module, some preliminary analysis can be made to determine the temperature of the different components of the STREAM contact line. In the simplest way the global temperature of the conductor n during the short-circuit current may be determined by using a thermodynamic calculation in adiabatic environment. Making this assumption, from the thermal balance between the heat storage of the conductor and the heat produced by the short-circuit current, it results Integrating and simplifying, the final expression is " '<* di - obtaining the short-circuit current as I J l + q(0,-0o)' where K = In and «Po Ll + a(0,.-0o)j /= 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-mm^] $ = Reference temperature [ C] fy&o= Temperature rise [ C] 8^= Conductor section [mm^] i- Conductor lenght [m] From the previous equations, it can be analysed the increase in temperature versus the short-circuit current for various value of fault duration, as reported in fig. 6, where the reduced increase in temperature for the negative conductor is due to its larger section.

9 Computers in Railways 497 I* OOO Short-circuit current [A] a) positive conductor b)negative conductor Figure 6 - Increase in temperature vs. intensity and short-circuit time Particularly interesting it results the analysis of short-circuit with a fault 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 negligible if its section is much greater than the section of the positive one, in the worst case the thermal behaviour of the positive conductor can be reproduced in adiabatic environment with a set of short-circuit currents and 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 of about 1 hour, it results that to protect the stream modules it is necessary to co-ordinate the section of their positive conductor with the rated current of the breaker to limit the maximum temperature 0^of the conductor at the end of the short circuit, as depicted in Fig. 8.

10 498 Computers in Railways S=4(X) miit.,,--""...---""" S=500iW Short-circuit duration [s] Figure 8 - Increase in temperature vs. short-circuit time The previous methodology permits to identify the rated current of the high speed circuit breaker and, in correspondence of the maximum fault current, to size the STREAM module. 5 Short-circuit Numerical Analysis To test the adequacy and accuracy of the calculation code and for verifying the correctness of the choices about the sizing of the catenary system conductors and of the high speed circuit breaker in substation, a test power network has been considered. The main characteristics of the power network are: F>pwer Network Characteristics Network topology single-side fed contact line length of the line: m length of the segment: 60 m Substation Power rating: 800 kva Rated DC Voltage: 750V Vcc% Transformer: 5% Contact line Positive feeder section: 300 mm Negative feeder section: 300 Positive conductor section: 334 Negative conductor section: 2692 mnf Fault characteristics Fault impedance 0 Q Maximum temperature Occ 65 C

11 Computers in Railways 499 With the calculation code the line-to-line fault current versus distance from the substation is determined and plotted in fig. 9. g (J u ' Distance [m] Figure 9 - Short-circuit current vs. distance Depending on these results the breaking capacity of the high speed circuit 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 line at 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 the n 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 constrains in presence of a fault current. 6 Conclusions When the pollution become intolerable, electrical propulsion systems are the real chance of succeeding. STREAM is a new concept guided busway for public transport which can help to give an answer to the problem. Its natural characteristics give it the flexibility of a conventional bus. In the paper, the analysis of the power network for the future STREAM line has been presented, with reference to the short-circuit, and

12 500 Computers in Railways the differences with the networks of the conventional traction systems are pointed out. The developed methodology for the short-circuit analysis has been applied to a test network to the purpose of an outline sizing of the power conductors. In the next future, the research will be oriented to the detailed study of the system operation and to determine the thermal profile of the STREAM modules both in operational and fault conditions. Bibliography [1] Fusco, G., Gagliardi, F., Ippolito, L., Piccolo, A.: Rappresentazione del guasto remoto in impianti di trazione in c.c., Proceedings of 'Giornate di Lavoro della Associazione Italiana di Ricerca Operativa (AIRO '93)', Capri (Naples), Italy, September, 1993, pp