AC Railway Systems Research Articles

Smart AC-DC Coupled Hybrid Railway Microgrids Integrated with Renewable Energy Sources: Current and Next Generation Architectures

In recent years, there has been increasing interest in integrating the smart grid concept into railway networks, which has been driven by the need to enhance energy efficiency and reduce air pollution in such energy-intensive systems. Consequently, experts have actively sought innovative solutions with which to tackle these challenges. One promising strategy involves integrating renewable energy sources (RESs), energy storage systems (ESSs), and electric vehicle charging stations (EVCSs) into current electric railway systems (ERSs). This study begins by examining the concept of implementing smart grids in railway systems through bibliometric analysis. It then delves into the realization of a hybrid railway microgrid (H-RMG) designed to enhance power flow capacities, improve energy efficiency, and address power quality issues in traditional AC railway networks. This paper introduces various future AC–DC-coupled hybrid railway microgrid (ADH-RMG) architectures centered around a shared DC bus acting as a DC hub for upgrading conventional AC railway systems utilizing interfacing static converters. Through an exploration of different possible ADH-RMG configurations, this research aims to offer valuable insights and a roadmap for the modernization and reconstruction of existing railway networks using smart grid technologies. The integration of RESs and EV charging infrastructures within the ADH-RMG concept presents a promising pathway toward establishing more sustainable and environmentally friendly railway systems.

Power Quality Measurement and Active Harmonic Power in 25 kV 50 Hz AC Railway Systems

Railway electrical networks rated at 25 kV 50 Hz are characterised by significant levels of voltage and current harmonics. These frequency components are also time varying in nature due to the movement of trains and changing operational modes. Processing techniques used to evaluate harmonic results, although standardised, are not explicitly designed for railway applications, and the smoothing effect that the standard aggregation algorithms have on the measured results is significant. This paper analyses the application accuracy of standardised power quality (PQ) measurement algorithms, when used to measure and evaluate harmonics in railway electrical networks. A shorter aggregation time interval is proposed for railway power quality measurement instruments, which offers more accurate estimated results and improved tracking of time varying phenomena. Harmonic active power present in railway electrical networks is also evaluated in order to quantify the impact it has on the energy accumulated by electrical energy meters installed on-board trains. Analysis performed on 12 train journeys shows significant levels of non-fundamental active power developed for short periods of time. As an energy meter will inadvertently absorb the financial cost of non-fundamental energy produced by other trains or other external power flows, results are provided to support recommendations for future standards to measure only fundamental frequency energy within train energy measurement meters.

Real-Time Hierarchical Neural Network Based Fault Detection and Isolation for High-Speed Railway System Under Hybrid AC/DC Grid

Reliable and comfortable high-speed railway (HSR) has skyrocketed in popularity as a transportation medium for traveling around the world. High-voltage direct current (HVDC) electrification system has been introduced to the HSR gradually. However, the coexistence of AC and DC systems will last for a long time because AC railway systems are still in the dominant position. A detailed HSR traction system transient model operating under the hybrid AC/DC grid was established in PSCAD/EMTDC. We proposed a real-time fault detection and isolation (FDI) method for the simulated model using neural network (NN). Hierarchical structure of the monitoring system has been employed. Low-level sub-monitors supervised the conditions of their local regions and the top-level monitor collected all the feedback from sub-monitors making the final evaluation of the entire HSR system based on a voting strategy. Both off-line and real-time experiments were conducted to validate the effectiveness of the proposed method. In the experiments, the sub-monitors were designed based on Gated Recurrent Unit (GRU) algorithm and implemented on the Xilinx VCU128 FPGA board. For the off-line experiment, the sub-monitors used the training and testing dataset both from PSCAD/EMTDC to construct the architecture of their individual GRU networks and to verify how great the networks can be. For the real-time task, the sub-monitors interfaced with a real-time HSR system emulator running on the Xilinx VCU118 FPGA board to test the performance in the real-time application. The results proved that our proposed FDI method has the capability of real-time detection and can achieve better accuracy within reasonable time and resource consumption than other NN-based methods. Moreover, the method was capable of standing against noises from measured signals to some extent.

Comparative Study on the Position of Shunt Active Power Filters in 25 kV AC Railway Systems

AC electrified railway systems show power quality problems such as harmonic current pollution, feeder voltage perturbation, and reactive power demand. These power quality problems have a bad effect on the AC railway system itself as well as other electric systems connected together. This paper investigates comparatively two configurations of a shunt active power filter (SAPF) for improving power quality in the AC railway systems. Different control algorithms corresponding to the installation position of the active power filter at the substation or at the sectioning post are proposed respectively. A comparative case study on the two positions of SAPF has been performed comprehensively.

Optimal area control of AC railway systems via PWM traction drives

The latest generation of AC-fed traction drives, employing high-speed switching devices, is able to control the reactive power drawn by each locomotive or train from the overhead line. If the conditions at each train could be fed back to a central control point, it would be possible for a centrally located controller to calculate optimal values for the reactive power in each drive and to send these commands back to the individual equipments. A scheme is proposed whereby the AC power flows are analysed in real time and the results used to optimise some particular system objective via control of the PWM equipments as mobile reactive power compensators. The system voltage profile can be improved and the overall power factor at the feeder substation made nearer to unity. Computational verification is presented with results obtained from a multi-train simulator with the power network optimisation operating in several test case scenarios. The test results are compared with those obtained without compensation and with classical trackside Var compensation methods. The scheme is shown to give the best improvement where an existing fleet of diode or thyristor equipments is partially replaced with PWM front-end equipments. From these results, the potential for PWM locomotives to improve overall system performance is clearly confirmed.

Network switching and voltage evaluation using an expert system in ac railway systems

The development and implementation of a Prolog expert system using computer graphics is reported for the on-line monitoring and control of the 50 Hz single-phase overhead current collection catenary system currently in operation in Hong Kong. Running as an on-line consultant on a personal computer, this expert system allows operating personnel to perform intelligent network switching on a graphic picture representing the whole of the catenary line system. Users are able to find out effects of their own choice of switching sequence by examining graphic displays. Valid switching brings about changes in display configurations and colours to record changes in the on/off status of circuit-breakers or isolators, or the relocation of supply points. The expert system also checks against any invalid switching which causes a violation of interlocking rules, an isolation of overhead-line sections from the supply, or a phase-to-phase short-circuit of the utility system. Work to develop a methodology of evaluating voltage across voltages across crossovers which run between parallel overhead lines is also reported. A crossover with excessive voltages across it should not be used to divert a train owing to the risk of damaging insulators across the crossover. An accurate evaluation of crossover voltages usually involves the solution of a large set of simultaneous differential equations governing train timetables, controls, movements and power consumptions. It is shown how train consumptions may be referred to their nearest static busbars and the resultant admittance matrix does not include any moving busbars (trains). This ‘reduced’ matrix occupies only a very manageable memory for either a full power flow solution or any other approximate method of voltage evaluation

Simplified equation for the shielding factor of multiple shielding conductors

Electromagnetic interference problems have become increasingly important due to the development of high-voltage bulk power transmission lines and the expansion of ac railway systems. The shielding factor of the shielding conductor is very important in calculating the electromagnetically induced interference voltage. In conventional methods of calculation, the resultant shielding factor of multiple shielding conductors is represented by the product of shielding factors of individual conductors and the effect of mutual interaction between shielding conductors are neglected. As a result, the calculated induced voltage does not always agree with the actual one and the calculated results are often too optimistic. On the other hand, the exact expressions of induced voltage which are derived taking into account the mutual interaction between shielding conductors are too complicated and impractical. This paper proposes a simplified expression for the shielding factor of multiple shielding conductors. A feature of the proposed method is that it always gives a result on the safe side. The proposed method can be applied to a railway system as well as to a transmission system.