Proceedings of 8th Transport Research Arena TRA 2020, April 27-30, 2020, Helsinki, Rail Control by System Weakening in Electrified Railways

Mehmet Turan SÖYLEMEZ a*, Süleyman AÇIKBAŞ b

a Istanbul Technical University,Control and Automation Engineering Department Maslak, Istanbul, Turkey b HI-SIM Technology & Engineering, Istanbul Technical University Technopolis, Maslak, Istanbul, Turkey

Abstract

Controlling rail voltage especially in DC electrified railways is an important issue for safety and economic reasons. A new possibility has started to arise in controlling the rail with the increased usage of current limiting function in the railway vehicles. In some cases, it could actually be possible to isolate (weaken) catenary/3 rd rail of a problematic area in order to reduce the rail voltages in that area. The vehicles in the weakened area would withdraw less current, resulting in less rail voltages. This idea forms the departure point of this paper. The paper covers the advantages and disadvantages of different techniques that can be used in controlling rail voltages and uses a realistic case study to illustrate the results with the help of a rail system simulation tool.

Keywords: traction simulation, rail voltage reduction, power system sectioning, system weakening.

* M. Turan SÖYLEMEZ, Tel.: +90-533-514-1730; E-mail address: [email protected] Söylemez and Açıkbaş / TRA2020, Helsinki, Finland, April 27-30, 2020

Nomenclature

TSS RPCD Rail Potential Control Device SI Section VLD Voltage Limiting Device

1. Introduction

Most of the electrified railways use rails as the return current conductor since the rails form an excellent media for conducting electrical energy. However, using rails as return current conductor might cause high stray currents, which might harm metallic utilities in urban areas, especially in DC electrified lines, where the traction currents are considerably high as explained in Paul (2016). In order to reduce the stray currents, a floating earth strategy is used in most of the modern DC electrified railways as discussed by Söylemez and Açıkbaş (2005), Lee and Lu (2006), and Zaboli et al (2017). In this strategy, rails and traction substations are isolated from the earth as much as possible.

A particular problem related with the floating earth strategy is that the rail voltages can increase up to undesired levels. Several thresholds for rail voltage and the durations allowed over these thresholds are given in EN 50122- 1 standard. When a new system is planned or a major modification is to be implemented in an existing system, the compliance of the system to this (or a similar) standard is usually sought. Noncompliance to the standard would impose a risk on human life and might also cause voltage limiting devices (VLDs) or so called rail potential control devices (RPCDs), which are installed along the line in order to control rail voltages, to short circuit the rails to the earth causing excessive stray currents.

Traction system simulations are extensively used for ensuring the rail voltage levels are below the acceptable limits along with many other things. There exist several factors that affect the rail voltages in a given system. Among these factors, the most obvious ones are the voltage level to be used in the traction system (Söylemez and Açıkbaş (2005)), the distance between the traction substations, the resistance, inductance and capacitance of the network and rail-to-ground resistance (Gu et al. (2018) and Xie (2006)), the parameters of the VLDs (Söylemez and Açıkbaş (2006)) and the characteristics of the vehicles that are running on the line. It is also possible to reduce the rail voltages by paralleling the running rails between adjacent lines or shifting the connection point of negative feeders of traction substations in a problematic area.

A new operational possibility has started to arise in controlling the rail voltages with the increased usage of current limiting function in the railway vehicles. In extreme cases, such as traction substation (TSS) outages at the end of the line, it could actually be possible to isolate (weaken) catenary/3 rd rail of a problematic area in order to reduce the rail voltages in that area. The vehicles in the weakened area would withdraw less current, resulting in less rail voltages. This idea forms the departure point of this paper. The paper covers the advantages and disadvantages of different techniques that can be used in controlling rail voltages and uses a realistic case study to illustrate the results with the help of a rail system simulation tool first described in Söylemez and Açıkbaş (2004).

The rest of the paper is organized as follows. The rail system simulation tool and the model used in simulations are described in the next section. Several methods to control rail voltage together with the proposed method are discussed with their advantages and disadvantages in Section 3. A realistic case study is considered in Section 4, where the proposed method is illustrated and the effects of the length of the weakened area on rail voltages and some other critical parameters such as energy consumption and train voltages are examined. Section 5 contains the conclusive remarks and ideas for possible future work.

2. Model and the Simulation Tool

It is possible to use an analytical approach to analyze the stray currents and rail voltages for a railway traction power system as it is done by Çolak and Hocaoğlu (2003), and Lee and Wang (2001). Although analytical approaches improve our understanding on how several parameters are affecting rail potentials and stray currents,

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they are insufficient to have conclusive results for a given real system, since in such systems, usually there exist many nonlinearities which are very difficult to track by analytical approaches. Therefore, rail traction power simulation tools are usually used in order to take nonlinear and complex behavior of power substations and trains into account.

A realistic simulation program takes all kinds of details into account including line alignment data (such as gradients and curves), passenger stations, the characteristics of the power lines, rails, trains and , and constraints due to speed limits and other signaling conditions. A multi – line and multi – train traction power simulation software called SimuX first described in Söylemez and Açıkbaş (2004) is utilized in this paper. SimuX provides a user-friendly environment to simulate rail traction systems taking into account all the aforementioned details including the regenerative braking and under-voltage behavior of the vehicles. The program has been used in preparation of more than 50 academic articles and industrial projects. The methodology employed by SimuX is explained in the following.

A traction power system simulation consists of several components as depicted in Fig. 1. It should be noted that the signalling system is usually assumed to work in order not to affect the flow of the trains, and the environmental conditions as well as the social component given in Fig. 1 are assumed to provide the worst case working conditions (i.e. fully loaded trains, max temperatures etc) in dimensioning of the traction power system. Two of the components namely mechanical component (train movement simulation) and electrical component (solution of the power network) are the most critical components among the components shown in Fig. 1. These two parts are isolated in some simulation tools, such that first, a train movement simulation is done, and then, the results of this simulation are fed into power network solution. However, in order to be able to properly examine the performance limits of the system it is necessary simulate these parts concurrently since they are coupled and provide feedback to each other as stated by Goodman et al (1998).

Fig. 1 The main components of a railway traction power simulation program

The trains are assumed to move along the track with no slipping or sliding in train movement simulation. Due to Newton’s third law the following equation can be given:

= − − − (1) where M and a are the mass and acceleration of the train, respectively. Here, F is the tractive effort applied by the motor, FG is the gradient force, and FC is the curve force that depends on the radius of the track (see Ku et al (2000)). In equation (1), FR is the resistance force that is usually calculated by the help of Davis Equation (Davis (1926)) as follows:

= + + (2)

Here, V represents the velocity of the train, A, B and C are constants that depend on the number of axles, the mass and the frontal area of the train.

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It is possible to use the following basic algorithm to simulate train movements: 1. Determine the target acceleration by the help of characteristics of the train, and considering signalling constraints. 2. Calculate the corresponding tractive effort for the target acceleration using equation (1). 3. Find the maximum possible tractive effort (or maximum braking tractive effort in case of deceleration) considering the speed and the motor characteristics of the train as well as the train voltage. 4. Calculate the maximum achievable acceleration corresponding to maximum possible tractive effort. 5. Determine the actual (achieved) acceleration as the minimum of target and maximum achievable accelerations. 6. Calculate the velocity and the position of the train depending on the actual acceleration

The electrical component of railway traction simulation programs employs different approaches to solve the traction power network equations. Modified load flow approach and direct matrix method are among the most commonly used methods in this framework. SimuX employs a direct matrix approach, where nodal analysis is used to formulate the network matrix. The power network is assumed to consist of resistances and pure voltage or current sources at a given time in this approach. This usually results in a high dimension matrix equation, which is dynamically changing and subject to some nonlinear constraints. Goodman and Siu (1994) discuss several difficulties that arise in the solution of this equation and propose a diakoptics approach. SimuX uses a modified version of SparseLib (see http://www.netlib.org/sparse/index.html and Kundert (1986)), which utilizes a set of procedures to efficiently pre-order and solve a sparse matrix using an LU factorisation.

A simple model of a section of the electrical system between two TSS that would help calculation of stray currents and rail voltages is shown in Fig. 2. The conductivity between the rail and the ground is modelled by dividing the track into small segments called as cells and assuming that each cell is connected to the earth (with a resistance RRG ) at a single point as discussed in Pham et al (2001), and Yu and Goodman (1990). In Fig 2, RL1 and R L2 are the resistance of the catenary line, which change with the position of the train. Similarly, R r is the resistance of a rail cell with length L. Finally, RNG is the resistance between the negative bus of the TSS and the ground. Obviously, the smaller values of L yield better approximations. Nevertheless, a cell length of 100m is found adequate to obtain accurate enough results in this paper.

Fig. 2 A simplified model used for computer simulations of railway traction system.

3. Rail Voltage Control Methods

There are many factors that can affect the rail voltage. Among these factors, arguably the most important one is the earthing strategy used along the line. Rail voltage is usually not an issue for directly connected earth systems since there is a deliberate connection between running rails and the earth. However, considerable rail voltages can be encountered in floating earth or diode earth systems as discussed in Söylemez and Açıkbaş (2005). Most of the modern metro lines and light rail transit systems use floating earth strategy in order to mitigate stray currents.

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VLDs might help in reducing the rail voltage, if the rail voltage exceeds a predefined threshold. The parameters that are most important for such a case are examined in Söylemez and Açıkbaş (2006). However, VLDs are considered as protection devices and should not be triggered frequently in order to keep the stray currents within the limits defined by the standards. Moreover, in many cases frequent triggering of VLDs might result in tripping the power supply system and hence cause significant delays in railway service.

Assuming that the isolation of the rails from the earth are done in a standard way, it is possible to argue that rail voltage is almost directly proportional to the current that flows through the rails and the overall resistance of the rails. The current that flows through the rails depend on many factors. For instance, the gradient of the line will have an important effect on the power demand of the trains, and in turn affect the current values and hence the rail voltage. The location and the number of passenger stations as well as frequent change of speed limits/profiles also affect rail voltages. As for more passenger stations and/or more changes in the speed profile of the vehicles means more power demand from the system and hence more currents and more rail voltages. The headway between the consecutive trains plays a major role in the magnitude of the rail voltages. As a rule of thumb, usually the more the headway the less the rail voltages are, since the overall power demand from the system decreases with increased headway. The characteristics of the railway vehicles used in the line also affect the rail voltages. For instance, the weight, motor power, efficiency, auxiliary power of the vehicles play an important role in determination of the power demand from the system and in turn rail voltages. In addition to these, the current limiting with respect to line voltage functions of modern vehicles can be effective in determination of the power demand of a vehicle. Such vehicles reduce their power demand by limiting currents in lower voltage to ensure the stability of the overall system. Unfortunately, most of these factors are predetermined before the electromechanical design phase, and therefore, cannot be used for controlling the levels of rail voltages in many practical cases.

The distance between the TSSs is also important. More distance between TSSs means a longer rail section between the TSS and a train, which in turn means more rail resistance and hence more rail voltage. Therefore, in general, the less the distance between the TSSs the less the rail voltages are. Unfortunately, increasing the number of TSS is very costly and it might not be possible to build a TSS anywhere required along the line (consider metro lines or highly populated urban areas). An alternative to this approach could be shifting the negative feeder connection point of TSSs in such a way that the rail is divided more evenly from an electrical point of view. This approach, however, has two shortcomings: First, while the distance between negative feeder connection points shortens in one area, it will increase in another. Hence, there might be a waterbed effect. Second, using big cross-section and long negative feeder cables can be forbiddingly expensive.

In order to reduce the overall rail resistance, two rails of each line are connected with certain intervals, and more importantly four rails of two parallel lines can also be connected so that rail voltages in parallel lines are balanced. However, this approach has its disadvantages as well. First of all it might not be possible to connect the rails of two lines as frequently as required due to structural (e.g. two lines built in two separate tunnels) or technical (e.g. use of track circuits by the signaling system) constraints. Moreover, the effect of paralleling rails might not be as much as expected, since lowered rail resistance might mean more currents to flow through the system resulting in less than expected rail voltage drops. In some cases, using extra cables in addition to rails can be considered in order to reduce the rail resistance and hence the rail voltages. However, similar problems to rail paralleling occur for this solution as well.

In many modern urban railway systems, all of the TSSs are connected in such a way that they supply the whole line. Thus, in most areas trains are fed with double-end feeding, and it is possible that a train demanding power can also be supplied by a TSS quite far away from the demand point. This approach, in contrast to sectioning the line into power zones by isolating points causing the power demand is fed by single end, is particularly useful for operational purposes and improving energy efficiency of the line since it allows more opportunities for the usage of regenerative energy. Details of these two feeding schemes’ advantages and disadvantages are outside of the scope of this paper.

An interesting approach called system weakening is proposed in this paper in order to reduce the rail voltages. The approach is particularly useful when a problematic area exists in terms of rail voltages mainly supplied by many TSSs. The basic idea behind system weakening is to isolate the problematic area such that only one or two TSSs supply power for this area. Hence, currents coming from further away TSSs are blocked. Weakening the power system results in more voltage drop at trains. As discussed above, most of the modern railway vehicles reduce their power demand by limiting the current withdrawn from the line when the train voltage drops below a certain 4

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threshold as described in the European standard EN 50388. Therefore, less current flows through the weakened area resulting in less rail voltages. In many cases, system weakening can be easily achieved by simple operational manipulations, and do not require extra investments. A particular disadvantage, however, is that it might result in less regenerative energy usage and hence less energy efficiency, under the temporary operation condition in which it is applied.

4. Case Study

A 1500 VDC metro line with realistic data is used as a case study in this paper. The length of the line is approximately 15 km. There exist 13 passenger stations numbered as S01…S13 and 7 traction substations named as TSS1…TSS7. Traction substations are used in every other station area along the line as can be seen in Fig 3.

Fig. 3 General view of the line used in the case study

The line is assumed to be on a hilly area with almost constant gradient around 2% (up to 4% locally). The conductivity between the rails of each track and the earth is assumed to be 0.0067 S/km. The system is designed to work without any problems with 90 seconds headway under normal and n-1 conditions. Normal condition means that all TSSs are healthy and feeding the line. n-1 condition means one TSS is out.

It is also assumed that the operator wants to achieve 150 seconds headway under of n-2 conditions. n-2 condition means that two TSSs are out. The worst cases among these scenarios are the ones when 2 neighboring TSSs are out.

The examined case study in this paper, considers the extreme case of n-2, which is the outage of the first two traction substations (TSS1 and TSS2). Therefore, a long area (appx. 5 km) at the beginning of the line is fed single end and mainly by TSS3. In order to compensate for a worst case occurring in the operation and to be able to compare the results, 140 seconds headway is used in all simulations while the trains are assumed to wait in all stations for 25 seconds.

There exists a section insulator (SI or also called as DILA-S) at the entrance of each station area in relation to the train movement direction on each line. The SIs in TSS areas are normally open and the other SIs are normally closed as indicated in Fig. 1. Four feeders are used in each of the TSSs in order to feed the lines. Two feeders used for each line: one feeder is connected to one side of the SI (which is normally open) and the other is connected to the other side. As a result, the line is interconnected so that all TSSs can feed a train that demands power.

Simulation results show that the rail voltage can be as high as 172 V and can stay over 160 V for more than a second for the described operating conditions (called as Normal scenario hereafter). The EN 50122-1 standard defines thresholds for allowed rail voltages. The standard allows permanent rail voltages less than 120V, and rail voltages up to 150V for duration less than 300 sec. However, allowed time for rail voltages over 160V drops rapidly. For 160V rail voltage, the maximum allowed time is 1 sec, and for 170V rail voltage it is 0.8 sec. Therefore, it is not possible to endorse the system under given operating conditions due to EN 50122-1 standard.

Nonetheless, if the system is weakened by opening the SIs in S06 station area for both lines then the area from the beginning of the line to S06 station will only be fed by TSS3. It is shown by simulation that the rail voltage drops to 148 V for this case (called as SI_Open scenario) and the system is in accordance with the EN 50122-1 standard. The change of rail voltage observed for the trains with maximum rail voltage with respect to time are given in Fig 4.

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Fig. 4 Change of rail voltage for the trains with max rail voltage in scenarios “Normal” and “SI_Open”

Actually, it is possible to weaken the system even more by cutting off the west feeders of TSS4 instead of opening the SIs in S06. This means that an even longer area (up to TSS4) is fed only by TSS3. Simulation results show that the maximum rail voltage is lowered to 135V in this case (called as TM4_WestOff scenario).

As far as TSS loads are concerned it is possible to show that TSS3 is loaded up to 67% of its nominal rating as for 1 hour RMS powers are concerned for the Normal scenario. The load of TSS3 increases up to 81% for SI_Open and to 99% for TM4_WestOff. As for the comparison of energy consumption in different scenarios, it is observed that 2.78 kWh is consumed per vehicle kilometer under Normal scenario. Energy consumption rate increases to 2,85 kWh/(vhc*km) for SI_Open and 2,94 kWh/(vhc km) for TM4_WestOff scenarios. In other words, there is an increase in energy consumption between 3% and 6% as a result of system weakening. This is mainly due to less regenerative energy usage and increased energy loss. Therefore, it is usually not recommendable to use system weakening as a permanent method to decrease rail voltages in a system. Nevertheless, it is possible use the proposed technique in some extraordinary (temporary) situations as in this case study.

As an academic curiosity, the location of the SIs around S06 are changed between TSS3 and TSS4 with 200 m intervals in order to examine the change of rail voltages, TSS loads and energy consumption depending on the location of SIs. These scenarios have been named as SI_5500, SI_5700, …, SI_7300 corresponding to the cases where SI’s are located at 5500m, 5700m, …, 7300m, respectively. In contrast to TM4_WestOff scenario, an additional scenario where east feeders of TM3 are cut off is also considered. In this last scenario, which is named as TM3_EastOff, the smallest type of weakening is achieved since TM3 is not required to feed any train on its east side.

The maximum rail voltages observed throughout simulation are given for each scenario in Fig 5. As it can be observed from this figure, system weakening does not have much effect on the rail voltages up to a certain point, where SI’s are placed at 6300m. Actually, it is observed that the rail voltages increase slightly for the scenarios where isolation takes place before 6300m. When the SI positions are more than or equal to 6300m, rail voltage levels suddenly decrease to acceptable levels. This is due to the fact that the trains accelerating and decelerating in S06 passenger station area are fed by TM3 for these scenarios. A further analysis reveals the fact that the rail voltage levels do not change considerably for SI positions between 6500m and 6900m, and further improvement in terms of rail voltages is only possible as the location of isolation is over 7000m, where the accelerating and decelerating trains in S07 passenger station area are started to be fed by TM3.

The power load of TM3 with respect to its nominal rated power is given for each scenario in Fig 6. The power demand from TM3 increases with the increased area for isolation as it can be seen from this figure.

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175 170 165 160 155 150 145 140

Max.Rail [V] Voltage 135 130 125

Scenario

Fig. 5 Change of maximum rail voltage for different scenarios

100 95 90 85 80 75 70 65 60 Max. 1 hourRMS Power [%] 55 50

Scenario

Fig. 6 Change of maximum power load of TM3 for different scenarios

Energy efficiency of the system for each scenario is examined using Fig 7. In order to make a meaningful comparison between different scenarios, energy consumption per vehicle*km values are given in this figure. As it can be expected, isolating the first part of the system to allow system weakening results in more energy consumption between 3% and 6%. An interesting observation is that the best case among weakened system scenarios occurs when the SI’s are around 6300m, ie in SS06 passenger station area. The reason for this is considered due to the fact that the regenerative energy produced by braking trains for SS06 station are used more by the system when SI’s are placed in this region. Actually, it is possible to observe this fact in Fig 8. Receptivity rate of the system, which is defined as the percentage of the regenerative braking energy used by the system, is given for different scenarios in this figure. As it can be observed, receptivity rate is at its best under normal scenario, and reduces with the application of system weakening. The best receptivity rates among weakened scenarios are obtained when the SI’s are around station S06.

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2,95

2,9

2,85

2,8

2,75

2,7 Energy per Vehicle per km [kWh/vhc/km]

Scenario

Fig. 7 Change of energy consumption for different scenarios

100,0 99,0 98,0 97,0 96,0 95,0 94,0 93,0 Receptivity Rate [%] 92,0 91,0 90,0

Scenario

Fig. 8 Receptivity rate for different scenarios

Another side effect of system weakening is related with minimum train voltages. A weakened system usually implies less train voltages. As it can be observed in Fig 9, minimum train voltages could be 10 to 50 V less in weakened scenarios. According to EN 50163 standard the minimum allowed train voltage is 1000 V in a 1500 V DC fed system. Therefore, it is possible to claim that the minimum train voltages are still within acceptable limits for the case study considered.

1.200 1.190 1.180 1.170 1.160 1.150 1.140 1.130

Min. Train Voltage [V] 1.120 1.110 1.100

Scenario

Fig. 9 Change of min. train voltage for different scenarios

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5. Conclusion

Several factors that affect the rail voltages in an electrified railway have been examined in this paper. It is shown in particular that it is possible to reduce the maximum rail voltage in a given system by weakening the power supply system in the area of interest. This is achieved by sectioning the traction system by opening some of the section insulators that are already available in the system or cutting off some feeders in neighboring TSSs. Application of this idea can easily be done by simple operational arrangements. Since extra installments are usually not required, this is considered as a very inexpensive solution in comparison to its alternatives.

A possible disadvantage of the method is an increase in energy consumption of the line as a result of sectioning. Therefore, it is usually not recommendable to use system weakening as a permanent method to decrease rail voltages in a system. Nevertheless, it is possible to use the proposed technique in some extraordinary (temporary) situations as in the case study considered in the paper.

Making a more comparative work on different rail voltage control methods and determining the effects of system sectioning in a more detailed way is considered as part of the future research.

References

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