REAL TIME TRACTION POWER SYSTEM SIMULATOR G. Strand Systems Engineering Department Fixed Installation Division Adtranz Sweden e-mail:gunnar.strand@adtranz.se A. Palesjö Power Systems Analysis Division ABB Powers System AB, Sweden e-mail:ann.palesjo@sepow.mail.abb.se Abstract: This paper describes how the traction power system analysis tool SIMTRAC can be modified to be used as a real time simulator for SCADA in Traffic Control Centers. The main difficulty with making a real time traction power system simulator is the fact that the loads (trains), which have varying power demand, are moving in the system. The power demands depends on factors such as the topography of the track, the speed and weight of the train. The already available part of SIMTRAC provides sophisticated modeling and simulation features. SIMTRAC is able to simulate all components in an electric traction power system. The kernel used for electrical calculations are SIMPOW, by ABB Power Systems, Sweden. To simulate an electric traction power system models of trains, train movements and electric components (for example power substations) are needed. All SIMTRAC models are described in the Dynamic Simulation Language (DSL) which is a part of SIMPOW. The challenge with this project is to reach real time performance on large systems with heavy traffic. Optimization of SIMTRAC models is the way to solve the problem. Keywords: SIMTRAC, SIMPOW, railway, train, simulation, traffic, control, system 1. Introduction To control a traction power system, there are nowadays quite often advanced computerized controlling and surveillance systems (in this paper they are called traffic control centers). The staff, who are going to use these systems would be much more efficient if they could train their skills in a training center off line the real system. The traction power system analysis tool SIMTRAC is to be further developed for handling real time simulations. This means that it will be possible to use SIMTRAC as the simulator-core in these training centers. Now SIMTRAC has difficulties to reach real time performance for traction power systems that are heavily loaded. The reason for this is that the trains, which are described as moving loads with dynamically changing size, make the simulation very heavy from an electrical point of view. Therefore the train models must be optimized and the communication between SIMTRAC and SIMPOW, the calculation core in SIMTRAC, must be looked over. A new module, which controls the simulation time and the communication between SIMTRAC and the training system, also needs to be developed. 2. Traffic Control Centers The traffic control centers are a main part of today s rail system. In these centers it is possible to control the signalling system as well as the traction power supply system (SCADA). In the real center it is difficult to practise what happens when an incident occurs in the traction power system. Therefore it is important to have a training facility so the staff can practise under organized conditions. TRAFFIC CONTROL CENTER Training Centre Real Centre Operators interface SIMTRAC ~ ~ Electrical input Figure 1 Difference between Traffic Control Center and Traffic Control Training Center
In the training center the staff will be placed in a Traffic Control Center identical to the real one, with the difference that the traction power system is replaced by SIMTRAC Real Time. (Figure 1). 3. User interface for trainer The trainer, who controls the training session for the trainees, has a user interface from which it is possible to control events in the simulation. The trainer will have the possibility to change the speed of the simulation so trainees, who are new with the system may train in a slower speed. 4. Traction Power System The difference between traction power systems and ordinary power systems is of course the trains, which make it difficult to simulate as they are moving and have a varying power demand. Another obvious difference is that there is only one phase. The frequency might also differ from the frequency in an ordinary power system. Symmetrical components are usually used in simulations of power systems. This means that a single phase system is represented with positive sequence data only, in the same way as a symmetrical three phase system is represented. 5. Description of SIMPOW SIMPOW is used as the calculation kernel in SIMTRAC. SIMPOW is a powerful simulation-tool, which can simulate anything in an electrical power system. SIMPOW is used for load flow calculations, dynamic simulations, short circuit calculations and linear analysis. Standard components of different kinds of regulators, machines, transformers etc. are available in SIMPOW. SIMPOW also has a Dynamic Simulation Language (DSL), which makes it possible for the user to make new models. The user can make models both of regulators and primary components, such as a train or a converter station. The possibility to include special components in the simulation makes SIMPOW very flexible and suitable for many different applications. In SIMPOW the time step is variable. If there are no events in the simulation the time step can be large, but when an event occurs the time step will be truncated to reach the time of the event. After the event a small time step is used until SIMPOW finds it possible to increase the time step again. There is also a possibility for the user to set a preferred time step. There might be need for a number of iterations until an accurate solution is reached. 6. Description of SIMTRAC SIMTRAC is a simulation tool, developed by Adtranz Sweden, which is used for all kind of simulations and analysis involving a traction power system, both AC and DC, with moving vehicles. The kernel used for the electrical calculations is SIMPOW. All SIMTRAC models are made in the Dynamic Simulation Language (DSL). DSL makes it possible to modify models easily and to make new models of unique components. The interface to the SIMTRAC-user is built to make it easy to describe the topography of tracks, electrical data for the traction power system, characteristics of trains and the time table for the trains. The topography, data of the track and the catenary are built up by lines, which have the following properties: Start position End position Stations Name Traffic Connection Name Slopes Gradient Curves Radius Tunnels Tunnel constant Catenary Impedance Resistance Inductance Speed Profiles Speed Note: A Traffic Connection is the place on the line where other lines can connect. The electrical node also need a Traffic Connection to be connected to the system. (Figure 2) In each time step the system of equations, which describes the electrical power system and the DSL models, is solved.
The train characteristics are based on the Speed-tractive effort curve (Figure 5) [1], which shows the maximum available force at the wheels, for propulsion. = Traffic Connection = Line Tractive effort vs. Speed 2 Figure 2. The figure shows the use of Traffic Connections in lines. 15 1 5 When the simulation is started the trains are running according to a time table. What the simulation tool SIMPOW sees in the traction power system during the simulation, are moving loads (trains) asking for enough power (enough current for the voltage given) and electrical system components such as generators, converter stations etc. (Figure 3) V Catenary Trains = moving loads with varying size Figure 3 The traction power system at a certain time, when broken down to electrical components The trains are running according to the time table, which either uses the departure time or the dwell time at a station. The time table also describes the trains paths. Speed control is one of the key features in a traction power system simulator. Speed restriction profiles are given as input to the simulations (Figure 4). It is possible to use more than one profile for a line, as for example freight trains and high speed trains usually have different speed restrictions. This is mostly due to the difference in braking distance. 1 2 3 4 5 6 7 8 9 1 11 12 13 Figure 5. Speed - tractive effort curve When simulating, the train model calculates the force needed to accelerate, decelerate or keep current speed. From the force it calculates the electric power needed to supply the motors. Usually there are limitations in power consumption from the catenary depending on the voltage. The possible acceleration/deceleration is, besides the Speed - Tractive effort curve combined with dynamic mass, also influenced by: Adhesion (Figure 6) Curve resistance Running resistance Tunnel resistance The model, which controls the train movement is called the Node Mover. It checks the speed of the trains with the train model and then calculates the position of the trains on the track in order to allow SIMPOW to do the necessary electrical calculations. It also generate requests, treated by the SIMPOW core, for necessary topological changes, such as when a train passes a power substation. 2 15 1 Limitation by Adhesion Tractive effort vs. Speed 9 8 Train speed Speed Restriction Profile 5 7 6 5 4 3 2 1 Accelerating Braking 2 4 6 8 1 12 Figure 6 The tractive effort-curve with the adhesion limitation included. 2 4 6 8 1 12 Travelled length (m) Figure 4 A train adjusts its speed to a speed restriction profile. SIMTRAC also includes several different electrical components used as power supply for traction power systems, for example rectifiers and rotating converter stations.
7. Optimization of simulation speed With SIMTRAC it is not possible to simulate in real time for big systems with heavy traffic. Therefore SIMTRAC needs to be faster if it is going to be used as a real time simulator. SIMTRAC is of course very dependent of the simulation kernel SIMPOW. The current SIMPOW version has enough capacity for the real time simulations, and do not need to be modified for that use. 27 268 266 264 262 26 258 256 254 252 Tractive effort vs. Speed (SPLINE) Knee All input tables from SIMTRAC is read by SIMPOW as a piecewise linear function. This may cause some problem for SIMPOW. If a breakpoint is passed while solving the system of equations, SIMPOW still solves the system with the function used before that breakpoint (Figure 7). If a solution is found, SIMPOW will truncate the step and continue from the breakpoint. If the system of equations can not converge, SIMPOW starts solving the system again from the last step but with a shorter time step. Since this happens a lot of times in a big traction power system simulation with SIMTRAC the time loss is substantial. Another thing that causes problem when using linear interpolation between points in a table is that the solution might be so close to the breakpoint, that together with other events in the system, it changes between the two functions and makes it difficult to find a solution with ordinary Newton iteration. 4 5 6 7 8 Figure 8. Continues curve, with use of SPLINE-functions, for the same data as in fig 7, but with more values near the knee. The parts of SIMTRAC, which need to be modified are mainly the train model and some converter station models. In the train model, as well as in the converter station models, there are several places where the SPLINE function would speed up the calculation. For example when reading the Speed - tractive effort curve (Figure 5) in the train model. The train model used today in SIMTRAC, describes the train very accurate, because it is made for system analysis. In such an analysis it is very important to have accurate results. 27 268 266 264 262 26 258 256 254 252 Function 1 of piecewise linear curve Tractive effort vs. Speed Solution before truncating step Breakpoint Function 2 of piecewise linear curve 4 5 6 7 8 Figure 7 Piecewise linear curve. SIMPOW try to solve equation working on wrong function To make the simulation go faster, a possible way is to model the input curves as SPLINE-functions [2] instead of piecewise linear functions. Then the curve is continues and SIMPOW can solve the system of equations at once. One problem with the use of SPLINE is that SIMPOW has problem finding the correct step length at a knee (breakpoint) (Figure 8), if variable step length are used. This can be solved by making an simulation event at every knee which tells SIMPOW to truncate the current step at the passe of the knee and restart the simulation from that point with a minimum time step. At the Traffic Control Training Centers it is not necessary to get result with the same accuracy as in the system analysis made with SIMTRAC. Therefore parts of the train model can be simplified: It is possible to neglect the Curve resistance in most systems. It is possible to neglect Tunnel resistance in most systems. In simulations SIMTRAC assumes an all-out - condition for the trains. This means that the train uses maximum allowed acceleration, if possible all the time. The all-out -condition is used to get the worst possible case for the traction power system, which often is the dimensioning case. If the train model is modified to run as soft as a real train driver (motorman), a lot of simulation speed will be gained in the simulation. The reason is that the peak load from several trains at once makes the system heavier loaded and therefore makes it more difficult to solve the system of equations. If it is possible to assume that it is the same adhesion coefficient * for the whole traction system, the Speed - tractive effort curve can be given as input with the adhesion influence included. * Adhesion limitation is calculated with the formula adhesion = (track adhesion coefficient) + ADH1/(speed+ADH2) where ADH1 and ADH2 are train specific constants
One more possible way to make the simulation faster is to calculate the braking curve before each change of speed restriction and before stops. The braking curve will then be included in the speed restriction profile for the train. This calculation is now performed in the simulation. 8. Use of SIMTRAC as simulator for training system If SIMTRAC shall be used as a real time simulator for a external system such as a Network Control Center, it must be able to transfer output data while the simulation runs. The simulation kernel SIMPOW is now writing all output data to a file. It has to be modified to send output data to an I/O interface instead. The external control of simulation events and simulation time is already possible to control in SIMTRAC/SIMPOW, but parts of it need to be automated to be easier to use, today it is done manually by user through line commands or via a batch file. 9. Conclusion SIMTRAC has possibilities to be a real time traction power simulator, even for large traction power systems. To reach that performance some models need to be modified. Most simulation speed is gained if SPLINE-functions are implemented when reading input tables from SIMTRAC to SIMPOW. 1. References [1].William W. Hay, Railroad Engineering, John Wiley & Sons, 1982. [2] Bronstein, Semendjajew, Tashenbush der mathematik, B. G. Teubner Verlagsgesellschaft, 1979