Center for Railway Engineering

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Browsing Center for Railway Engineering by Author "Abebe, Demissie (Mr.)"

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    Automatic Slip Controller for Railway Vehicles Using Fuzzy Logic
    (Addis Ababa University, 2017-07) Emebet, Gebeyehu; Abebe, Demissie (Mr.)
    The moment applied on the wheel by the traction motor has to be transferred in to tangential force in order to guarantee the longitudinal motion of train vehicle. The force that is transmitted between wheels and rail is called the adhesion force which is the product of normal force and adhesion coefficient. The adhesion coefficient depends on the slip velocity, conditions of rail surface, a train velocity and temperature in the contact area. From parameters that can influence the adhesion coefficient slip velocity the vital one. Therefore slip controlling is important in order to prevent wear of the wheels and the rail and to use the present adhesion effectively. On this thesis the relation between the wheel road adhesion force coefficient and slip ratio is developed based on Burckhardt Static Modell (BSM) which treats the wheel as a circular beam supported by springs. First the longitudinal slip ratio is to be calculated as the speed difference of the driven and non-driven wheels. Where, the non-driven wheel presents speed of the train vehicle and the driven wheel velocity is considered as the wheel-rail contact specimen. Then adhesion torque observer is designed in order to estimate the existing adhesion in real time. This torque is then applied as a load torque for Proportional Integral (PI) controller based speed loop. After developing proper speed loop to control wheel speed, Fuzzy Logic Controller (FLC) based slip controller is installed inside to compensate traction torque fluctuation caused by adhesion load. The FLC has two inputs namely the slip ratio error and the rate of slip ratio error. Using linguistic rules to increase and decrease compensating torque, the FLC generates adhesion coefficient which will be multiplied by the normal force and wheel radius to be added or subtracted from the traction torque. Then the advantage of the controller is evaluated by considering the speed response with and without the slip controller. For the slip ratio varying drastically from 0.7 to 0.01 and vice versa, the wheel speed response shows up to 50% overshoot and undershoot in the early stage of the accelerating speed. However, the FLC slip controller compensates this up and down to 6% which is a remarkable result to smooth the motion of the system.
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    Reactive Power Compensation and Harmonic Mitigation in 25 kV AC Railway System Using Shunt Active Filter
    (Addis Ababa University, 2016-06) Gebremedhin, Hailay; Abebe, Demissie (Mr.)
    The AC railway systems show typically poor power quality due to their inherent electrical load characteristics. The moving and non-linear load characteristics of the locomotives consume the large reactive power and produce high harmonic currents, so that the total power factor becomes adversely affected. These power quality problems in the AC railway system can constrain the amount of power delivered to the locomotive and have a negative effect on themselves as well as on the public grid. As a result, detailed study of reactive power and harmonic mitigation is required in traction substation. The thesis deals on compensating reactive power consumption and harmonic mitigation of 25 kV electrified railway system of Addis Ababa-Djibouti railway line as case study. The active power filters have gained much more attention because of their performance to mitigate the harmonics and reactive power issues. The performance of active filter depends upon the control theory that is employed to formulate the control algorithm of active filter and controller of the active filter is the key and heart of the filter which greatly affects its performance. So, design of shunt active filter to mitigate the harmonics and reactive power problems with controller based on Synchronous (d-q) reference frame theory is the core area of this work. MATLAB/Simpower computer simulation is used as a simulation tool for the thesis. When the Shunt Active Power Filter is connected to the system, we can observe from the simulation result that THD of load current has reduced from 20.5% to 4.6% which is below 5% the harmonics limit imposed by IEEE 519 standard. The reactive power demand is reduced to nearly zero value, the power factor and pantograph voltage of traction system also improved. Thus, the simulation results after implementing the proposed filter shows that shunt active power filter can effectively compensate reactive power and harmonics.