Electric Traction has been a popular traortation system for years and nowadays it is expanding more and more in Iran . These systems in comparison with other traortation systems, introduce advantages such as low cost, fast speed and low pollution on environment. Although electric traction systems are spread around the world for years, in Iran it is not widespread and several new projects are under development. Due to this reason, the importance of electric traction systems is evident. Development of electric traction systems cause some disadvantages. Electric traction load is one of the most polluting loads for electric distribution system and decreases the power quality due to high power electric drives, fast changing loads and proximity to domestic loads in the distribution system. Due to these problems, it is necessary to study the power quality effects of electric traction system and design compensation methods to reduce the power quality effects. The first step for studying the electric traction systems and their power quality effects is to know these loads characteristics. By knowing characteristics of these loads it is possible to calculate needed power profile according to movement algorithms, which is defined based on traffic rules. After calculation of power profile of electric traction load, it is necessary to connect this calculated load to electric power distribution system and simulate the overall system that contains traction system drives, electric substations and electric power distribution system. The connection point between load and utility that is named PCC is the point that in it power quality effects should be investigated. To do this, power quality analyzers should be used to measure the power quality phenomena according to power quality standards. Power quality standards introduce limitations for power quality phenomena in the PCC and also introduce some mitigation methods. It is necessary to restrict power quality effects to limits that defined in power quality standards. Usual power quality effects in distribution feeders that are feeding the electric traction systems are different for different electric traction systems (DC or AC). These are high variable current and voltage harmonics, voltage flicker, unbalance in 3-phase systems, low power factor and voltage sag. Usual power quality effects due to connection of DC electric traction systems to electric power distribution system are high variable voltage and current harmonics and over allowed voltage flicker. For AC electric traction systems, Keywords: Electric Traction System, Power Quality phenomena, Power Quality Standards, Mitigation methods
