Electric Railway Systems Research Articles

Extra-Fast DC Distribution System Protection for Future Energy Systems

DC distribution systems are being widely adopted in many industrial, commercial, and military energy systems such as commercial buildings, data centers, railway electrification systems, off-shore wind farms, and shipboard power systems. A dc distribution system can improve energy efficiency and reduce the size or weight of power systems. A typical dc distribution system, like the ones mentioned, includes multiple power electronics converters, distributed capacitors, active loads, and short lines with low impedance. Thus, in case of a short circuit fault, the dc fault current increases extremely fast without zero-crossing. This is a great challenge for dc system protection. In addition, in-line dc series arc faults are also a potential threat that must be detected and localized to reduce fire hazards. This paper presents a comprehensive protection approach leveraging the converter fault current limiting function as well as an advanced fault localization approach to achieve reliable fault detection, localization, and isolation. Numerical simulation and experimental tests are used to validate the effectiveness of the proposed method.

YN/VD connected balance transformer-based hybrid power quality compensator for harmonic suppression and reactive power compensation of electrical railway power systems

In this study, YN/VD connected balance transformer-based hybrid power quality compensator (HPQC) is proposed for harmonic suppression and reactive power compensation in electrified railways. The HPQC is composed of a balance transformer in YN/VD connection, a controllable three-phase voltage source inverter (VSI), as well as fully-tuned passive power filters (PPFs). It is able to integrate the active compensation device with the main transformer, without using auxiliary step-down transformer. This circuit configuration enables one to adopt VSI with low power rating, so that merits such as easy for system integration and low operation losses can be achieved. In this paper, the equivalent circuit model and the corresponding mathematical model of the HPQC in the harmonic domain are established to reveal its mechanism on harmonic suppression. And then, the working principle and control strategy of such a new topology are elaborated in detail. Finally, simulation and experimental results are provided to verify the feasibility and effectiveness of the above-mentioned structure and control strategy.

On the Rising Interdependency between the Power Grid, ICT Network, and E-Mobility: Modeling and Analysis

Boosting critical infrastructures’ (CIs) preparedness to threats, including natural disasters and manmade attacks, is a global imperative. The intrinsic dependencies and interdependencies between CIs hinder their resiliency. Moreover, the evolution of CIs is, in many cases, en routè to tighten those interdependencies. The goal of this paper is to uncover and analyze the rising interdependency between the electric power grid, information and communication technology (ICT) networks, and transportation systems that are heavily reliant on electric-power drivetrains, collectively referred to hereafter as electro-mobility (e-mobility). E-mobility includes electric vehicles (EVs) and electric railway systems. A new influence graph-based model is introduced, as a promising approach to model operational interdependencies between CIs. Each of the links of the influence graph represents the probability of failure of the sink node following a failure of the source node. A futuristic scenario has been analyzed assuming increased dependency of the power grid on ICT for monitoring and control, and high penetration levels of EVs and distributed energy resources (DERs) in an urban region. Inspecting the influence graph shows that the impact of interdependency between the power grid, the ICT network, and the transportation network, for the case study analyzed in this paper, does not lead to failures during normal operation with proper design; however, it is severe during emergency conditions since it leads to failure propagation among the three CIs. This paper sets the stage for more research on this topic, and calls for more attention to interdependency analysis.

Stability Analysis of PV Plant-Tied MVdc Railway Electrification System

As ac electric railways have various problems (e.g., power quality issues, complex catenary networks, limited supply region, and passing neutral section), the medium-voltage direct current (MVdc) railway electrification system (RES) may be a potential alternative electric railway mode and can be freely accessed to the renewable energy systems. However, some instability issues may be aroused due to the interaction between flexible MVdc RES and electronic-based dc–dc equipment. In this paper, a comprehensive small-signal impedance model, combining a photovoltaic (PV) plant and an MVdc RES, is performed, while available electric variables and control parameters are taken into consideration. Then, a novel passivity-based stability criterion (PBSC) is introduced to determine the instability issue about associate influential factors, including proportional and integral parameters of traction substation and PV converters, the power of train converters, and the dc catenary length. A virtual impedance control scheme is designed to stabilize the system. The experimental results demonstrate that the PBSC can identify the instability in the MVdc RES. The results also match well and support the conclusions obtained.

Model analysis and real-time implementation of model predictive control for railway power flow controller

Railway power flow controllers (RPFCs) are known as a power electronics based full-fledged compensator in the field of electric railway systems (ERSs). They have attracted experts attention over the last years due to their outstanding performance in dealing with power quality (PQ) problems. The complexity, randomness and uncertainty characteristic of ERS and traction loads have led the control of RPFC to be difficult. In this paper, a precise modeling method based on space-state model (SSM) and dynamic switching pattern is presented. Applying the extracted equivalent model and mathematical equations, a modified model predictive control (MMPC) method is proposed to improve the compensation and dynamic performance of RPFC. To eliminate the low frequency oscillation (LFO) and other external disturbance effects, a dual-loop modified d-q frame control is implemented with frequency locked capability. Precise simulations have been provided to verify the modeling and proposed control procedure performance. Moreover, a real-time laboratory setup based on RT-LAB and Opal-RT is provided to confirm the simulation results and theoretical analysis according to the desired specifications of the real ERS.

REM-S–Railway Energy Management in Real Rail Operation

This paper presents the prototype implementation of an advanced automation architecture for electrical railway systems, designed to operate them as cyber-physical systems, such as smart grids. This REM-S architecture, from the literature, is a distributed system coordinating rolling stock, electrical substations, sideway resources within the constraints, and objectives (overall energy demand, power consumption, and cost optimization) dictated by the control center. The prototype software suites presented here are the REM-S Offline Suite and the REM-S Online Suite. Among others, they consist of an application for day-ahead optimization, which is performed by the control center for the next day, and another application for minute-ahead optimization, which is continuously performed during rail operation on so-called intelligent substations at time intervals of, e.g. 15 min. The REM-S Offline Suite can be used for dry runs of test scenarios before running the aforementioned applications in combination with others on the participating trains, substations, and so forth, as part of the REM-S Online Suite within a railway system in real time. The REM-S Online Suite, which is an implementation of a distributed optimization, was validated in a field test, performed in a suburban 3-kV dc railway line in Malaga, Spain during real rail operation where the objectives within the constraints given by the control center have been successfully reached. Besides, the software architecture of both software suites also selected algorithms that are presented for a better understanding of the overall communication and distributed real-time optimization processes.

Adaptive droop control for better current-sharing in VSC-based MVDC railway electrification system

The share of voltage source converter (VSC) technology is increasing in conventional power systems, and it is penetrating into specific transportation systems such as electric vehicles, railways, and ships. Researchers are identifying feasible methods to improve the performance of railway electrification systems (RESs) by utilizing VSC-based medium-voltage direct current (MVDC) railways. The continuous motion of electric trains makes the catenary resistance a variable quantity, as compared to the traction substation (TSS), and affects the current-sharing behavior of the system. A modified droop control technique is proposed in this paper for VSC-based MVDC RES to provide more effective current-sharing while maintaining catenary voltages above the minimum allowable limit. The droop coefficient is selected through an exponential function based on the ratio between the concerned TSS current and the system average current. This enables small adjustments of droop values in less concerning marginal current deviations, and provides higher droop adjustments for large current deviations. Meanwhile, the catenary voltages are regulated by considering the voltage data at the midpoint between two TSSs, which experiences the lowest voltages owing to the larger distance from the TSSs. The proposed techniques are validated via simulations and experiments.

Power Quality Control of Smart Hybrid AC/DC Microgrids: An Overview

Today, conventional power systems are evolving to smart grids, which encompass clusters of AC/DC microgrids, interfaced through power electronics converters. In such systems, increasing penetration of the power electronics-based distributed generations, energy storages, and modern loads provide a great opportunity for power quality control. In this paper, an overview of the power quality control of smart hybrid AC/DC microgrids is presented. Different types of power quality issues are studied first, with consideration of real-world hybrid microgrid examples, including data centers, electric railway systems, and electric vehicles charging stations. It shows that compared to traditional centralized power quality compensations, smart interfacing power converters from distributed generations, energy storages, and loads, and the AC and DC subgrids interfacing converters are promising candidates for power quality control. To realize the smart interfacing converters' power quality control, both primary converters control and secondary system coordination are required. In this paper, a thorough review of the primary control of interfacing converters to integrate the power quality compensation are presented, with a focus on the hybrid AC/DC microgrid harmonics compensation and unbalance compensation. For multiple interfacing converters, the secondary control with system-level coordination and optimization for harmonics and unbalance compensation (considering both unbalance and harmonics in single-phase and three-phase systems) are also presented. Challenges like low switching frequency of interfacing converters, parallel interfacing converters operation, and interfacing converters communications are discussed, and typical solutions for primary and secondary controls to deal with them are presented. The paper also includes rich case study results.

Assessing High-Order Harmonic Resonance in Locomotive-Network Based on the Impedance Method

Many new types of locomotives are put to use in electric railway systems, resulting in instability problems such as high-order harmonic resonance occurring as a consequence of the interaction between the electrical locomotive (including electrical multiple units) and the traction network. In order to investigate this problem, this paper adopts the impedance-based approach to analyze the stability properties of the system interaction. The detailed impedance of the traction network is obtained by taking its distributed parameters into account. Then the locomotive converter with single-phase transient current control algorithm (TCCA), which can represent the locomotive when performing the impedance-based stability analysis, is modeled as the dq-frame impedance by single-phase dq decomposition. By plotting bode diagrams of the impedance for the traction network and the locomotive separately, and calculating the phase margin at the intersection point of the magnitude of the two impedances, the stability of the locomotive-network system is evaluated and thus the high-order harmonic resonance can be predicted. Moreover, a time-domain model of locomotive-network systems is established in this paper, which can reproduce the high-order harmonic resonance in the time domain as a first validation step of the analysis. Finally, a field test in a traction substation of the Beijing-Harbin Railway is carried out to finally validate the effectiveness of the proposed method. The stability analysis method in this paper is simple and effective without the need for a complex calculation of some norms or Eigenvalues. As a result, the high-order harmonic resonance can be predicted before a new type of locomotive is put into use.

AC railway electrification systems — An EMC perspective

Railways are electrified in many different ways. In this article, the main options for electrifying a high speed AC railway are reviewed from an electromagnetic compatibility (EMC) perspective. Firstly, the trend of increasing the usage of electrified trains to replace conventional diesel locomotives is pointed out. On this basis, the significance of considering EMC in the railway environment is explained, with a view to preventing the malfunction of the railway system. Secondly, different electrification options are introduced, namely the rail-return, booster-transformer and auto-transformer systems. The benefits and drawbacks of each electrification option are considered based on the interference level to the trackside railway signaling and telecommunication systems. The discussion of each electrification system is verified using electromagnetic simulations. By comparing the different electrification schemes, it is shown that the auto-transformer system has better EMC performance and delivers higher power to the train.

An Impedance Measurement Method at Controlled Frequency Band for Both Traction Power System and Electric Train

Abstract Electric railway systems occur instability and oscillation frequently for the mismatched impedances between the traction power system (TPS) and the voltage-source-converter (VSC)-based electric train. However, the parameters of both TPS and electric train (hereinafter TPS-train) are not fully known so that utilizing the mathematical deduction to quantify the impedances is difficult. This paper gives a method to measure the impedances at controlled frequency band. A chirp-PWM signal is introduced and utilized to drive the insulated gate bipolar transistor (IGBT) modules of disturbance circuit, which can produce a desired broad spectral excitation at controlled frequency band. The impedances can be then calculated with the corresponding response voltages and currents after connecting the disturbance to the tested system. Then, the stability and oscillations can be identified using the measured TPS and electric train impedances. The proposed method is easy to implement with its simple hardware structure and software control, and also it gives a high measurement accuracy. Simulations verify the effectiveness of this method.

A data-driven ground fault detection and isolation method for main circuit in railway electrical traction system

Due to the complex and harsh operation conditions, like corrosion, aging cable and static electricity, of electrical traction drive system, ground fault will generate large short circuit current to harm the key components. Effective fault diagnosis is important, but also challenging. The conventional method used for ground fault detection only takes advantage of voltage measurements of DC-link. Other measurements onboard are also available, which are correlated with the voltage measurements. Taking the correlation into account will improve the detection performance. To this end, this paper presents a data-driven solution, which makes full use of the correlation between the voltage measurements with other measurements onboard. The proposed method consists of two components: (1) a canonical correlation analysis-based fault detection method, which takes into account the correlation within measurements; (2) a fault isolation method by means of the fault direction, which can be obtained with the available faulty data stored in the long-term operation. The developed method is applied to a traction drive system. It is shown that the proposed approach is able to improve the fault detection and isolation performance significantly with respect to three performance indicators, namely fault detection rate, detection delay and correct isolation rate, in comparison with the conventional method, which only uses the voltage measurements of DC-link.

A Practical Approach to Mitigate Low-Frequency Oscillation in Railway Electrification Systems

The low-frequency oscillation (LFO) phenomenon has frequently happened in the railway electrification systems (RES), which often caused the traction atresia of electric trains and protection malfunction of traction network. In this letter, a practical mitigation approach is presented based on impedance analysis, which indicates that changing the operation mode of the electric trains to diode-bridge rectification (DBR) circuit can mitigate the LFO because the impedance of the rectifier is increased. Then, combining the Prony technology, an online identification and mitigation approach, is proposed here for mitigating the LFO phenomenon. Experimental results verify the effectiveness of the proposed method in stabilizing voltage fluctuation of the RES. This method can also identify the oscillatory frequency and the corresponding attenuation factor.

Электромагнитная безопасность тяговых сетей в режимах плавки гололеда

Электромагнитная безопасность тяговых сетей в режимах плавки гололеда

Overview of Harmonic and Resonance in Railway Electrification Systems

Harmonic distortion and harmonic resonance problems have been widely concerned and reported in railway electrification systems (RESs) due to the harmonic injection from the nonlinear electric train, especially high-speed/high-capacity/large-power trains. This paper presents an overview of the harmonic and resonance problems in the RES, including harmonic problem composition, harmonic modeling, available influential factors assessment, harmonic resonance, and associated suppression methods. The harmonic problem mainly consists of the background harmonics brought from the utility system, resonance-region harmonics interacted by capacitive and inductive network elements, and characteristic harmonics generated from the switching process of the onboard power conversion system. The mathematical modeling and analysis methods are introduced, including frequency scanning analysis, S-domain analysis, resonance mode analysis, and modal sensitivity analysis. Available influential factors/parameters have been fully investigated against above-mentioned harmonic problems. Finally, different harmonic suppression methods have been compared and summarized in this paper.

Analytical investigation of railway overhead contact wire dynamics and comparison with experimental results

Abstract In this paper an analytical method for studying the free response of continuous vibrating systems with distributed and possibly non-proportional viscous damping is proposed. The most general case the method refers to is a piece-wise homogeneous Euler-Bernoulli beam, with lumped elastic and inertial elements and subjected to tensile load. The practical application of the method to a contact wire is also presented, aiming at analysing its dynamic response. Contact wires are typically used in the overhead contact line of the railway electrification system but, despite their wide diffusion, their damping properties have not been exhaustively studied. This study aims at experimentally validate the analytical method to define a reliable dynamic model of overhead contact lines. The wire is modelled as an axially loaded homogeneous beam, with lumped elastic and inertial elements (i.e. droppers and clamps). A state-form expansion applied in conjunction with a transfer matrix technique is adopted to extract the eigenvalues and to express the eigenfunctions in analytical form. Experimental measurements have been carried out in the Dynamics & Identification Research Group (DIRG) laboratory of Politecnico di Torino considering two different damping scenarios, and the modal properties of the test bench have been extracted by using a linear subspace identification technique. The damping distribution is finally investigated starting from the experimental data, in order to seek for the most appropriate damping model.

Energy Management of a Smart Railway Station Considering Regenerative Braking and Stochastic Behaviour of ESS and PV Generation

The smart grid paradigm has provided great opportunities to decrease energy consumption and electricity bills of end users. Among a wide variety of end users, electrical railway systems with their huge installed power capacity should be considered as a vital option in order to avoid wasted energy, provided that an energy management system is utilized. In this study, a mixed-integer linear programming model of a railway station energy management (RSEM) system is formulated by a stochastic approach, aiming to utilize the emerged regenerative braking energy (RBE) during the braking mode in order to supply station loads. Furthermore, the proposed RSEM model is composed of an energy storage system (ESS), RBE utilization, photovoltaic (PV) generation units, and an external grid in this paper. The passengers' impact on RBE as well as the stochastic behaviour of the initial state-of-energy of ESS along with uncertainty of PV generation by the RSEM model are also evaluated. The model is tested under a bunch of case studies formed considering several combinations of the cases that an ESS or PV are available or not and using RBE is possible or not.

New Medium-Voltage DC Railway Electrification System

This paper highlights the potential of medium-voltage dc electrification for railway traction. A method is proposed for determining the dc voltage level in order to guarantee, in terms of traffic, equivalent performances to an ac-electrified railway line. Railroad traffic, type of transport service, and railway network topology being given, the analysis method takes into account the following constraints: voltage at the pantograph, heating of overhead line and rail-to-ground voltage. Once the dc voltage level is chosen, the dc electrification system is compared with a standard 25-kV ac electrification in the case of the French high-speed line from Paris to Strasbourg.

Hardware-in-the-Loop Simulation on Fault Current Limiting Operation of RE-123 Coated Conductors Under the Influence of Spatial Inhomogeneity

We have investigated transient behavior of RE-123 coated conductors in fault current limiting operation based on experimental hardware-in-the-loop simulation (HILS). Fault current limiting function of superconducting electric power application is attractive to maximize the potential of power system; such a function can be realized by the transition to the resistive state of superconducting wire. However, the phenomena is relatively complicated: nonlinear current transport properties varying with time-dependent temperature rise. Furthermore, from practical point of view, the influence of spatially homogeneity of cooling and local critical current properties in RE-123 coated conductors must be clarified for reliable operation. To demonstrate this, we developed HILS system using real-time digital simulator. Fault current limiting operation was successfully demonstrated for dc electric railway system. This method enabled us to evaluate the real-time responses of RE-123 coated conductors such as nonlinear properties and thermal behaviors under a power grid network. In particular, we applied RE-123 coated conductors with spatially inhomogeneous cooling and local critical currents to the experiment to understand the influences of them on the fault current limiting and recovery functions. Furthermore, such transient phenomena were successfully described by an electrically and thermally coupled analysis taking into account of temperature dependent nonlinear transport properties of the RE-123 coated conductors based on the percolation transition model. The method and findings will become crucial for effective and reliable designs of superconducting fault current limiters.

Estimation of current distribution in the electric railway system in the EMTP-RV

Current distribution in the electric traction system is an important factor regarding the electromagnetic influences on the surrounding metallic structures. The current value directly determines the magnetic field and consequently coupling of the systems. A single rail traction system can be equipped with return conductor. The return current flowing through return conductor has an opposite direction of supply current and decreases coupling and interferences.This paper presents the AC current distribution for both designs, with and without return conductor. Also, the soil resistivity is varied. The higher the soil resistivity, the higher is the amount of return current in rails and return conductor. The different construction of return conductor results in different impedance of return path. The relations of return conductor construction and return current value are analysed.Lightning can directly impact different traction system elements such as contact network, tower or return wire. The current distributions in several cases of lightning impacting different traction system elements are presented.