A review of power quality issues in electrified rails

Electrified railways are among of the largest consumers of electric energy which is supplied from the AC power system to the electric supply station (ESS), where it is transformed to the required voltage level and, if necessary, rectified to DC voltage. Electricity is supplied from the ESS to vehicles via the traction network and also to additional consumers through the lines of longitudinal power supply. Traction current returns back from vehicles to ESS via rails and partly via earth that is electrically connected with rails through low value resistance of ballast [1]. It is a common design of directly supplied traction system but modern electrified rails have some variations in design. In general electrified rails forms multiconductor system with longitudinally distributed power supply stations and connected to lines moving loads (vehicles) and fixed (unmovable) electrical loads (consumers), which operate, in general case, in nonlinear pulse modes. Due to the electrical non-linear nature of power stations and loads, current and voltage distortions appear in system, which have a devastating effect on the entire power system and consumers. AC railway traction systems are high power single-phase loads for threephase supply system and they cause a significant amount of Negative Sequence Current (NSC) into utility grid. Harmonics and consumption of reactive power are further power quality problems that the supply network is encountering. The disturbances propagate in electrical supply system and affect both the external power lines that feed the ESSs, and its own traction and non-traction loads. Current harmonics arising in the traction network due to its nonlinearity, impact the train control systems, which ultimately have a direct effect on the safety of train traffic. Electromagnetic compatibility (EMC) between various railway subsystems has been extensively studied in many aspects by many authors (see for example [1–3]), and also by author [4, 5]. The distribution of the harmonics of traction return current in rails for direct AC and DC supply systems and their influence on the track circuits were investigated also in many works [6–12]. The EMC issues are closely related to the electrical power quality (PQ), but they aren't fully similar terms [13–16]. Electromagnetic compatibility is defined in IEC 61000-1-1 as “the ability of a device, equipment or system to function satisfactorily in its electromagnetic compatibility without introducing intolerable electromagnetic disturbances to anything in that environment” [17]. The development of high-speed railways and the use of new types of rolling stock with power electrical equipment and pulse-width modulation lead to problems with EMC and power quality in feeding grids that can adversely impact on other consumers and equipment at points of common coupling [18–22]. Among these problems are the voltages and currents transients (impulsive and oscillatory), power and frequency variations, waveform distortions, unbalanced loading, and etc. These disturbances cause undesirable effects in utility power systems, such as strong shocks and noise in generators, increased heat and losses in transformers and transmission lines, bad impacts on protective systems. As a result of disturbances, adverse interference occurs, which can cause malfunctions in railway signaling systems.