H. Hõimoja, D. Vinnikov, M. Lehtla, A. Rosin, J. Zakis Tallinn University of Technology, Department of Electrical Drives and Power Electronics, Ehitajate tee 5, 19086 Tallinn (Estonia) Abstract--This paper analyzes energy flows, conversions and losses in tram systems. Proposals for improvement of the efficiency of tram systems by the help of energy storage modules are described. In addition, such advanced methods of energy efficiency improvement as load sharing, real-time traffic management and Ecodriving are discussed. Index Terms--Capacitive energy storage, DC power systems, Substations, Traction motor drives. I. INTRODUCTION The requirements for modern public transportation vehicles combine energy efficiency, environment saving, safety, reliability and comfort. The modern IGBT-based converters enable improved controllability and accuracy of acceleration and braking processes, thus reducing losses, energy consumption, pollution, noise, size, weight of equipment and providing improved reliability. A number of KD Tatra KT4 type trams with 160 kW DC drive in Tallinn were modernized between 2000-2006 by replacement of rheostat control with IGBT choppers and installation of static converters and passenger information systems. These modernized trams show higher efficiency, resulting in up to 48 % lower energy consumption from the catenary. By the use of energy storages, such as supercapacitor stacks, the energy efficiency of trams could be further improved. New traction converters also enable higher acceleration and braking efforts, thus additional low-floor sections can be added to tramcars without affecting dynamic properties. The tram infrastructure, including energy conversion and distribution should also be considered as a topic of interest in terms of loss minimization. The supply is mostly based on the use of rectifier-substations, fed by medium-voltage distribution networks. Due to the lower DC voltage level of 600 V and to the use of powerful traction drives, this type of electrical systems often becomes critical regarding voltage drops [1]. This can cause difficulties when operated with modern variable- frequency AC traction drives, for which the DC-bus at the input of the frequency converters needs to be within a certain tolerance level. Additionally, the possibility of regenerative braking at low speeds is a common property of new vehicles, but it is often inefficient with low traffic and non-reversible traction substations. It finally leads to the use of resistive dissipaters during braking. In that context, considerable energy savings can be achieved by the help of energy storage systems, with a corresponding impact on operation costs. II. POWER ANALYSIS AND LOSS MINIMIZATION POSSIBILITIES IN TRAM SYSTEMS Generally, the electrical part of the tram system consists of traction substations, cables, overhead line and rails network, which supply trams with DC voltage. The energy consumed by an electric vehicle is converted into useful energy with losses. In terms of consumption, the changes are substantial; however, the average consumed power is low. Current peaks cause voltage drops and power losses in the catenary. The energy from substations is consumed according to the following balance [2]: ( )  = + + + = n i i move i loss dr i dev aux i loss gr grid W W W W W 1 , , . ., . , . , (1) where W gr.loss is the losses in the overhead line grid, W aux.dev is the energy consumption of auxiliary devices of a tram, W dr.loss is the drive losses, W move is the energy converted to tram's movement, and n is the number of trams in the grid. The power balance of catenary is ( ) i i i loss dr i dev aux i grid i tram n i g v F P P R I P + + + =  = , . ., . , 2 , 1 , (2) where I tram is the input current of the tram, R grid is the resistance of the overhead line, P aux.dev is the power of auxiliary devices of a tram, P dr.loss is the power loss of the motor, F is the traction force, v is the tram’s velocity [3]. The consumed energy of the tram traction drive is converted to kinetic and potential energy and useful work to overcome the friction and other resistances. ( ) , ) ' ( ' 1 1 2 2 1 0 0 0 0         + + + + = = + + =    ds v c v c g m c dv v m dh mg W W W W s s v v h h t res kin pot t tr η η (3) where η t is the combined efficiency of the drive system of a tram, m and m' are the normal and the effective mass, which consider additionally the inertia of rotating masses, h is the height of the rails from the sea level, v is the speed of movement, c 0 is the resistance coefficient independent of the speed, c 1 is the resistance coefficient proportional to the first exponent of the speed (suspension), and c 2 is the resistance coefficient proportional to the second exponent of the speed (drag resistance). The value of the power equation (Eq. 3) depends on the parameters of the tram, railway profile, operation modes and driver skills. The processes can be optimized according to minimal energy consumption as vehicle and Survey of Loss Minimization Methods in Tram Systems 978-1-4244-4987-3/10/$25.00 ©2010 IEEE SPEEDAM 2010 International Symposium on Power Electronics, Electrical Drives, Automation and Motion 1356