Analysis of modelling electric transportation networks

Transcription

1 Doctoral school of energy- and geo-technology January 5 20, Kuressaare, Estonia Analysis of modelling electric transportation networks Marek Mägi Tallinn University of Technology marek.magi@he.ee Abstract Tallinn University of Technology is taking part in a research program to look into optimization possibilities in supply networks, substations, electrical energy storing, new type of substations with flywheel technology, which would ultimately lead to higher effectiveness of electrical energy consumption, improve power quality and would be a source of uninterruptible power supply. This article analyzes modelling principles of electric transportation networks, which is a sub part of the mentioned research program. With relatively simple modelling it is possible to make general conclusions about Tallinn s transportation networks. Keywords Model, tram, voltage drop, grid, resistance, transportation Introduction Mentioned research program looks into distribution of electrical energy, transmission and improving effectiveness of electrical energy usage by storing electrical energy. Analysis looks into Tallinn s transportation networks, substations and contact rails for electrical vehicles. Due to the vast amount of data, what needs to be dealt with, this article is a small introduction to a bigger work ahead. The analysis will concentrate to creating mathematical models for supply networks (considering the lengths of contact rails and their resistance, studying the distance influence from the substation to a single vehicle, studying the influence of several vehicles in the network at the same time to a single vehicle and influences of different kinds of cycles (acceleration, breaking to a single vehicle. The output of the analysis should give practical guides for future substations with electrical energy storing devices and for onboard storing devices in the vehicles []. Supply network system with electrical energy storing devices allows storing regenerative energy and reusing it for other consumers in the same network. The power losses in the network can be minimized with new type of substations (equipped with bidirectional power flow controllers to return recuperation energy [2], with optimal adjustment and configuration of contact rails, and with optimal control of energy exchange. Supply of the public transportation (trolleybuses, trams, trains in Tallinn is based on contact rails, which efficiency in optimal cases reaches up to 93-96%. On the other hand, there are periods, when the efficiency drops to 60-70%. This leads to bigger power losses in the network, which leads to over heating and extension of overhead s. The current flow in electrical vehicles (for example in trams has a high fluctuation rate (Fig. 2; though the average consumed power is rather small. The power consumption also depends on the motion cycles: aggressive square impulses (city traffic; 2 smooth triangle impulses (fluent curves. Those two motion cycles create different loads for the grid, depending, on whether the vehicle is equipped with traction converter or acceleration rheostat. Traction converters able smoother breaking current and there are not energy losses in the rheostat. Measurements have indicated that the current flow maximum values in case of trams are in the range of -500A +500 A, but the average current is only 30 A (Fig. 2. Power losses in the network are proportional with the square value of the current. The voltage varies in a broad scale because of load peaks. High current peaks cause voltage drops and power losses in the contact grid [3]. The furthest the vehicle is from the substation in the same contact rail the highest are the possibilities for bigger voltage drops in the contact rail. On the other hand, since the vehicle is always moving compared to the substation (Fig., the locations, where the maximum power losses appear, are not that easily determined. For example in the furthest contact rail point (from the substation the vehicle might be moving slowly and the current values, thus the power losses, are small, but in the middle point the vehicle might accelerate and the power losses are the highest in the contact rail under observation. Those maximum values can be found with modelling transportation networks and analyzing how vehicles operate in them. The analysis can provide vital data to determine how powerful 73

2 vehicles could be operated in the network and where should the new type of substations with electrical energy storing devices be situated to provide the best efficiency. Fig.. Voltage stabilization with storing substation In time the public transportation has grown and the old substations are not anymore located optimally. Fig. 2. Example of real measurement values of the speed and current of a tram on the Viaduct of Pärnu oad (only in case of acceleration rheostat. Available data for modelling The electrical part of a tram, train or trolleybus system consists of rectifier-substations, cables, overhead and rails network, which supply them with voltage. For trams and trolleybuses the voltage is 600 V DC in Tallinn. For d.c system Ohm s law can be applied, but it must be reminded that the vehicle is always moving compared to the substation. To construct models all the resistances must be known cable types from the substation to the overhead s, cross sections of the over head s (80, 00 and 20 mm 2 for trams, resistivities, lengths of the overhead s, earthing systems and rails for vehicles. Many substations are common for trams and trolleybuses in Tallinn. For trolleybuses isolated earthing system is used. For future references there are many load cycles recorded from trams in TTU. This information can be used for comparison and for modelling loads in the model. 2. Modelling starting points For modelling, equivalent circuit diagram must be constructed (Fig. 3 and then all the data supporting the model needs to be inserted to a program. There are many different programs with what we could make models. In this article Microsoft Excel is taken under observation for its simplicity and ability to manage different kind of data. The process is later simplified with operator interface sheets and inside macros for processing. For processing, input and output cells must also be created. Supporting data includes: Dividing the general equivalent circuit diagram (Fig. 3 into smaller sections (Fig. 4; 2 Defining traction substations and their data; 3 Defining single traction substation s feeders and their data (Table ; 4 Dividing smaller sections into sections based on their location compared to the feeder s connection point (Table 2. Fig. 3. Example of a general equivalent circuit diagram 74

3 Table. Example of Tallinn s traction substation s supply feeders (to the overhead s Traction Installed Feeder Cable type +/- Length Connection name +/- Consumer substation Power, kwh +/-, m AJ 2400 F PS240 õ 00 ELEKTIJAAMA + TAM F2 ASB-/SB- 60/55 MEE PUIESTEE ± TAM x400/x400 F3 ASB-/SB- 80/85 KESKVÄLJAK ± TAM x800/x800 F4 ASB-, x VIU + TAM ASB-, x VIU2 - TAM ASB-, x VIU3 - TAM F5 SB-, x POSTI + TAM F6 PS VALLI + TAM Table 2. Example for different overhead sections For every single overhead section there should be inserted equations for finding resistances depending on the location on the rails. For finding resistances from the substation, there are two basic equations. First for moving towards to the substation feeder s connection point: where ( = ( overhead resistance from the vehicle to the substation feeder s connection point; overhead resistance in its full length from one connection point to the other; l position of the vehicle; l start_point position of the first connection point; l end_point position of the second connection point. Second for moving away from the substation feeder s connection point: ( = (2 In case of loops the equation is (first connection point is virtually in the centre of the loop: 2 = = = ( + end _ point ( + 2 (3 Fig. 4. Example of an equivalent circuit diagram of a smaller section (from Maneezi St to Narva oad 75

4 3. Visualizing the results With simple Visual Basic programming it is possible to illustrate the resistances in different locations. Without any doubt the resistance should decrease, when the vehicle is moving towards to the substation feeder s connection point, and increase, when the vehicle is moving away from it (Fig. 5. When the resistance equations are determined, it is possible to find voltage drops and power losses in a single point on the overhead under observation (where the vehicle is situated. Fig. 5. Example figures of resistance of overhead in different locations (left without loops, upper with a loop on the left side of the zero point For simple usage, operator interface sheets are recommendable (Fig. 6 with input and output cells. With giving a sample load cycle it is possible to analyze voltage drops on the overhead s also considering with the fact that the vehicle has a different location in time due to its speed (Fig. 7. The loads are looked simplified in this article. Detail load modelling will be included later wards [2]. Fig. 6. Operator interface with input and output cells Fig. 7. Sample load cycle of a vehicle in time (left and voltage drops on the overhead in time (right 76

5 4. Conclusions Main objective is to construct mathematical models to all kinds of electric transportation and analyze their networks to find out optimization possibilities. Models should be usable for analyzing several vehicles on the same contact rail (energy exchange opportunities between multiple vehicles and substations as it usually takes in every day life. Models should also consider different kinds of variables: load cycles, weather conditions etc. Deeper analysis [4] could be made to s, where frequently occurs acceleration and braking, such as stops, crossings and crosswalks. The work should support development of future substations with electrical energy storing devices (lead acid batteries, lithium batteries, supercapacitors, nickel batteries, electrolyser, H 2 -storage and fuel cells, flywheels, vanadium redox flow batteries, pneumatic storage and for onboard storing devices in the vehicles. Energy storage devices provide a prospective measure for the reduction of grid losses and power peaks [3]. eferences. oasto, I., Lehtla, T., osin, A. Control strategies of ultracapacitors * Doctoral School of Energy and Geotechnology. Kuressaare: TTÜ, p. 2. ankis, I., Vitols, A. Analysis of power consumption for tramcar with pulse mode speed regulation * Doctoral School of Energy and Geotechnology. Kuressaare: TTÜ, p. 3. Joller, J., Lehtla, M. Power Analyze and New Loss Minimization Possibilities of a Tram System * EPE-PEMC 2002 Dubrovnik & Cavtat 6 p. 4. Lehtla, M. Control and Protection of a Traction Supply System * Doctoral School of Energy and Geotechnology. Kuressaare: TTÜ, p. 5. Joller, J. Jõuelektroonika. Tallinn: TTÜ Elektriajamite ja jõuelektroonika instituut, p. 6. Modelling program AILSIM v Siemens s SITAS SES energy storage system. n/pub/products/benefits/mt/innovation.htm 77