Railway power supply system models for static calculations in a modular design implementation Usability illustrated by case-studies of northern Malmbanan RONNY SKOGBERG Master’s Degree Project Stockholm, Sweden 2013 XR-EE-ES 2013:006 Railway power supply system models for static calculations in a modular design implementation Usability illustrated by case-studies of northern Malmbanan RONNY SKOGBERG Master of Science Thesis Royal Institute of Technology School of Electrical Engineering Electric Power Systems Stockholm, Sweden, 2013 Supervisors: Lars Abrahamsson, KTH Mario Lagos, Transrail AB Examiner: Lennart Söder XR-EE-ES 2013:006 Abstract Several previous theses and reports have shown that voltage variations, and other types of supply changes, can influence the performance and movements of trains. As part of a modular software package for railway focused calculations, the need to take into account for the electrical behavior of the system was needed, to be used for both planning and operational uses. In this thesis, different static models are presented and used for train related power flow calculations. A previous model used for converter stations is also extended to handle different configurations of multiple converters. A special interest in the train type IORE, which is used for iron ore transports along Malmbanan, and the power systems influence to its performance, as available modules, for mechanical calculations, in the software uses the same train type. A part of this project was to examine changes in the power systems performance if the control of the train converters were changed, both during motoring and regenerative braking. A proposed node model, for the static parts of a railway power system, has been used to simplify the building of the power system model and implementation of the simulation environment. From the results it can be concluded that under normal conditions, for the used train schedule, the voltage variation should not restrict the trains traction performance. It can also be seen from the results that a more optimized power factor control with a higher regenerative brake power or generation of reactive power could be used to limit the need for investments in infrastructure or to increase the traffic for a given system layout. i Sammanfattning I ett flertal tidigare undersökningar och rapporter har konstaterats att spän- ningsvariationer, och andra förändringar, hos strömförsörjningen till tåg kan påverka dess prestanda och dess färd längs rälsen. Som en del av ett modu- lärt programpaket för tågrelaterade beräkningar uppstod därför ett behov av elkraftsberäkningar, både för planering och operativ drift. I denna rapport sammanställs och används ett antal olika statiska modeller för tågrelaterade effektflödesberäkningar. Modellen för omformarstationer har även utökats för att hantera konfigurationer då olika typer av omformare används. Ett särskilt intresse för tågtypen IORE, som används för malmtransporter längs Malmbanan, och dess påverkan av en förändrad strömförsörjning, har funnits då olika typer av mekaniska beräkningar för denna tågtyp utförs i andra befintliga moduler. En del av projektet bestod i att undersöka förändringar i elförsörjningen, på grund av en ändrad styrning av tågens omformare, både vid återmatning och motordrift. En föreslagen nodmodell för den statiska delen av elnätet har använts för att förenkla elsystemsmodellen och uppbyggnaden av simuleringsmiljön. Av resultaten från simuleringarna kan man anta att under normala förhållanden, och med det använda körschemat, bör ej spänningen vara en begränsande faktor för tågens drift. Övriga simuleringar visar också att en mer optimerad effektfaktor för högre återmatad bromseffekt eller för generering av reaktiv effekt kan användas för att slippa investeringar i infrastrukturen, eller för att utöka trafikmängden för ett givet system. ii Contents 1 Introduction 1 1.1 Background . 1 1.2 Aim . 2 1.3 Limitations . 3 1.4 Structure of thesis . 3 1.5 Previous related work . 4 2 Power flow analysis 5 2.1 Introduction . 5 2.2 Power flow . 6 2.3 Losses . 7 2.4 Admittance matrix . 8 3 Railway power systems 10 3.1 Overview . 10 3.2 Power supply . 11 3.3 Rail current return systems . 12 3.3.1 Booster transformer . 13 3.3.2 Auto transformer . 13 3.4 High-voltage transmission system . 15 3.4.1 Transformers . 16 3.5 Converter stations . 18 3.5.1 Rotary converters . 18 3.5.2 Static converters . 21 3.6 Trains . 22 3.6.1 Asynchronous trains . 23 3.6.2 Thyristor based trains . 25 3.6.3 Regulation for motoring . 27 3.6.4 Regulation for regenerative braking . 28 4 Computer model implementation and calculations 29 4.1 Program layout . 29 4.2 A modular standard node . 30 4.3 Converters . 31 4.3.1 Converter losses . 31 4.3.2 Parallel converters . 33 iii 4.4 Trains . 34 4.4.1 IORE locomotives . 34 4.4.2 Thyristor based locomotives . 38 4.5 Mathematical model . 39 4.5.1 Solver . 39 4.5.2 Equations and constraints . 40 4.5.3 Optimization . 45 4.6 Java-GAMS interaction . 47 5 Case-study and simulation 48 5.1 The iron ore line . 48 5.2 Railway line model . 49 5.3 Train models . 50 5.4 Simulations . 52 5.4.1 Normal operation . 53 5.4.2 Converter station outages . 55 5.4.3 Effects of alternative power factor control . 59 6 Conclusions and future work 63 6.1 Conclusions . 63 6.2 Future work . 64 A Numerical data used 65 A.1 Per-unit system . 65 A.2 Converters and grid-connection . 66 A.3 Catenaries . 66 A.4 High-Voltage transmission lines . 67 A.5 Trains . 69 A.6 Electrical layout . 70 Bibliography 72 iv 1| Introduction 1.1 Background Electric power systems are spread throughout the society and they are an important part of every day life. A special area within electric power systems is electric railways, which is used to power trains for transportation of both freight and people. Some of the differences compared to an ordinary power system is the large voltage variation allowed and that the loads are not stationary, which makes the power system layout change over time. The railway power supply system also uses a single-phase design to transfer power to the trains compared to a more common three-phase transmission line used for other power systems. One important element of an electric railway system is the availability of electric power as needed for the intended traffic situation. The infrastructure itself is limited as to the amount of installed capacity and strength of the grid, and over sizing the system could be costly. The tractive power available for the trains could also be limited because of the voltage level of the power system, either by design or from a standard [1]. Both a too low or a too high voltage level could impair the power transfer to, or from, the trains. Modern trains with a power electronic based traction system, where the power factor can be controlled in software, could be used to improve the power supply systems performance. One solution to keep the catenary voltage high, and with less losses, is if the locomotives were given the possibility to compensate the reactive power in the network by operating with a leading power factor [2]. As this is not always allowed for certain railway systems [3], other methods needs to be considered for better system performance such as changes in the timetable or limiting the trains power demand. As part of a bigger software suite, TRAINS, developed by the company Transrail Sweden AB, an implementation of the power flow calculations of an AC railway power supply system, named TRAINS AC Supply, is needed to give information about the power system. Electrical models for a railway power system is not available in the existing software, and is to be addressed in this thesis to be able to combine the electrical and mechanical calculations of 1 a railway system. During the infrastructure planning, time table construction and train operation, the information from the calculations could be used to alter the design or suggest operational changes. The train operations software, TRAINS Performance, keeps track of all trains and the status of the power system after obtaining the results from TRAINS AC Supply. The mechanical aspects of train movements is already implemented in other modules of TRAINS, and can be used to evaluate travel time and power demand. TRAINS AC Supply can be used to calculate if the intended tractive power is feasible or if there is any system violations that needs to be addressed, and for a given system situation give a recommended power demand for the different trains. One of Transrail’s other products CATO, Computer Aided Train Operation, is used nowadays to send information about optimum speed and tractive power to the driver. This speed recommendation is calculated with the intent to minimize the number of stops and speed limits due to nearby trains on the same track. The speed calculated for each train assumes that the maximum tractive power is always available. Together with information about the power system, tractive power limits occurring in the system could also be taken account for. Information about an unfeasible tractive power could be useful during both planning of a train schedule and during realtime operation. For a degraded state of the power supply system, as in the case with a power outage, the maximum tractive power recommended by CATO for each train could be changed to try to mitigate the effects on the time table. 1.2 Aim The aim of this thesis is to make it possible to integrate power flow calculations into the TRAINS software suite, considering the consequences of voltage levels in the power system for an AC railway. An additional part of this thesis is to evaluate if it is possible to increase the system performance by optimization of the reactive power.