Proceedings of 8th Transport Research Arena TRA 2020, April 27-30, 2020, Helsinki, Finland Rail Voltage 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 voltages 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 Traction substation RPCD Rail Potential Control Device SI Section Insulator 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, 1 Söylemez and Açıkbaş / TRA2020, Helsinki, Finland, April 27-30, 2020 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 transformers, 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. 2 Söylemez and Açıkbaş / TRA2020, Helsinki, Finland, April 27-30, 2020 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.