Railway Traction Power System Research Articles

Measurement-Based Wideband Model and Electric Parameter Extraction of Railway Traction Power System

Harmonic resonance and power instability issues plague the operation of the traction power system (TPS). Establishing effective models with the correct parameters is widely accepted for analyzing such problems. However, it is still a challenging task for gaps between models and actual scenes. This article proposes an adaptively intrinsic parameter extraction methodology for TPSs based on wideband impedance measurement. An adaptive equivalent parameter model representing the TPSs is established after determining the impedance characteristics of each frequency region. The sensitivity index to the measuring impedance from different schemes is estimated through test data. Also, the measurement scheme is determined, which only needs to inject twice wideband excitation signal into the system at two connection points and measure three pairs of voltage and current sampling signals. The intrinsic parameters extracted are calculated from the adaptive equivalent model and the wideband impedance measurement results. The proposed method can be added to the existing measurement algorithms. It can provide accurate equivalent parameters and guidance for efficient model building. RTLab-PHIL experimental results show that the proposed method yields good parameter extraction and that it can also adapt to changes for various railway TPSs.

Low Frequency Stability of AC Railway Traction Power Systems: Analysis of the Influence of Traction Unit Parameters

Dynamic interactions between AC railway electrification systems and traction unit power converters can result in low frequency oscillation (LFO) of the contact-line voltage amplitude, which can lead to a power outage of the traction substation and the shutdown of train traffic. Several system parameters can influence the low frequency stability of the railway traction power system, including contact-line length and traction unit parameters such as transformer leakage inductance, DC-link capacitance, control bandwidths and synchronization systems. This paper focuses on the influence of these parameters on the LFO. The methodology is based on a frequency-domain analysis. Nyquist and Bode diagrams are used to determine the stability limit. The validation of the method is performed through the use of time-domain simulations.

Design and application of a single‐tuned passive harmonic filter to suppress harmonic distortion and resonance for railway traction power systems—A case study

The railway traction loads produce massive harmonics that are injected directly into the traction power system. This causes serious waveform distortion in the supply voltage and even invokes a harmonic parallel resonance, which may lead to significant harmonic current and voltage amplification. Consequently, severe electrical equipment failures and feeder tripping problems occurred frequently in the traction substation. A clear harmonic parallel resonance at a 27.5-kV feeder can be observed from the field measurement result of the traction substation without filter banks. This paper proposes a high-performance single-tuned 3rd passive filter for harmonic mitigation and harmonic resonance suppression in the traction power system. In the ST filter design process, the impact of the quality factor (Qf) design of the filter reactor on the filtering performance is considered. Furthermore, the driving point impedance as seen from the traction load is also studied. The design and the engineering consideration of the harmonic filter are also presented, which comply with the recommendations of the IEEE and IEC standards. Besides, the unbalance-current protection of a capacitor bank is introduced in this paper. Finally, the proposed filter is implemented in an actual traction substation, from the field measurement results; the proposed harmonic filter delivers very good performance.

MVDC Railway Traction Power Systems; State-of-the Art, Opportunities, and Challenges

Advances in voltage-source converters (VSCs), as well as their successful application in VSC-HVDC systems, have motivated growing interests and research in medium-voltage direct current (MVDC) traction power systems (TPSs) for high-speed rail (HSR) applications. As an emerging power-converter-based infrastructure, this study reviewed developments that shape two key evolving pieces of equipment—namely, high-power traction substation (TSS) converters, and power electronic transformers (PETs)—for MVDC TPS as well as prospects for smart grid (SG) applications in the future. It can be deduced that cost-effective and robust high-power TSS converters are available from hybrid modular multilevel converters (MMCs) for enhanced performance and fault-tolerance capability. In addition, silicon carbide (SiC) MMC-based PETs with input-series-output-parallel (ISOP) configuration are present for greater weight/size reduction and efficiency for MVDC rolling stock design. Finally, the implementation of a smart MVDC TPS incorporating a sophisticated railway energy management system (REM-S) based on the smart grid principles is feasible in the future, with numerous benefits. However, there are related challenges, like knowledge gaps on these technologies, the high costs involved, and lack of standardization to overcome to realize widespread future commercial deployment.

Optimal operations of transformers in railway systems with different transformer operation modes and different headway intervals

This paper presents an approach to the optimal operations of transformers in railway systems. Railway systems are supplied from bulk supply substations that consist of two transformers that are connected in the main-tie-main configuration. There are three operation modes for these transformers, namely non-parallel mode, parallel mode, and single transformer mode. Due to the fact that the traction loads are inherently dynamic, the optimal operation mode for the transformers requires a thorough simulation by including the train dynamics and train schedules. This paper aims to identify the optimal operation mode of the transformers under different headway intervals. In this study, the railway traction power system of the Mass Rapid Transit 2 (MRT2) in Malaysia is modeled and simulated in ETAP-Etrax Software with the dynamic load flow analysis to achieve an accurate estimation of transformer losses under the three transformer operation modes and different headway intervals. The results show that the parallel mode has lower transformer loss than that of non-parallel mode for all the headway intervals. It is also found that the single transformer mode has the lowest transformer losses for headway intervals of 5 min 48 s and above. Although the percentage of transformer loss reduction and the headway interval margin for optimal operation of single transformer mode may differ, this approach can also be applied to other DC railway systems.

An Improved Controlled-Frequency-Band Impedance Measurement Scheme for Railway Traction Power System

Railway traction power system (TPS) impedance affects the stability and operation performance of connected power electronics devices (i.e., electric trains) with the closed-loop control system. In order to identify the detailed TPS impedance, the wideband-response-based impedance measurement technique has become attractive for its fast execution time and accuracy, especially the chirp-based technique for its bilateral controllability of injected wideband disturbance. However, for the reason that the disturbance device is directly powered by the measured system, the fundamental component separates some harmonic clusters in the frequency domain and leads to unsatisfactory uniformity of the injected wideband disturbance. In this article, a segmented-chirp-based technique is proposed to eliminate this phenomenon caused by the fundamental component, where the chirp signal is segmented and carried on the fundamental only when the amplitude is relatively smooth on top (or bottom), which facilitates a uniform wideband distribution to keep a good measurement accuracy. Meanwhile, the presented modular disturbance circuit with a flexible topology can better adapt to the various industrial engineering conditions. Finally, the detailed measurement results based on the simulation and downscaled experimental system validate the effectiveness of the proposed measurement scheme.

Modelling of high-speed railway traction power supply systems based on subspace identification

In this paper, reduced mathematical models for high-speed railway traction power systems based on subspace identification method (SIM) are proposed. SIM applies reliable algebra tools to estimate state variables, and then provides an analytic representation for the identified systems, especially those of large dimension. Accuracy for identification results of different orders is discussed, under both normal operation and fault cases. With appropriate accuracy-simplicity trade-off, a uniform order decision is deduced. Reduced models derived from identification could be used for further studies of network-side impact on train-network coupling problems.

Analysis and Adaptive Mitigation Scheme of Low-Frequency Oscillations in AC Railway Traction Power Systems

The diffusion of modern electric trains equipped with high-power voltage-source converters has introduced new dynamic interactions with the ac traction power system that may create unexpected low-frequency oscillations (LFOs). This paper presents for the first time a detailed analysis of the operating conditions of electric trains and a traction network, revealing that specific changes in the operating conditions of the train-network system are responsible for the oscillations and may lead to instability. The study is carried out with an analytical dynamic model considering the transient direct current control and focuses on evaluating how the mismatch between the parameters of the controller and the controlled object of the train-network system affects the system dynamics and stability. It is shown that the retuning of controller parameters improves the dynamic performance and the stability of the train-network system, and hence an adaptive mitigation method is proposed to mitigate LFOs adopting a new design of the controller with minimal change and cost to the original system, which provides a simple solution to guide engineering practice in railway traffic. Simulations and experimental results are presented to fully validate the proposed method.

A Novel Controlled Frequency Band Impedance Measurement Approach for Single-Phase Railway Traction Power System

The accurate information of the wide-bandwidth impedance versus the frequency is urgently needed for evaluating the system resonances, instabilities, and operations of the railway traction power system (TPS), and to avoid/control the harmonic resonance and oscillation issues. As the system topology and detailed parameters of the TPS are not fully known even timely varying, we have to obtain the detailed wide-bandwidth impedance information through exciting the harmonic disturbance into the system, and then, calculating the response information. Therefore, a controlled wide-bandwidth impedance measurement approach is presented in this paper, in which, a butterfly-type disturbance circuit and chirp pulsewidth modulation signal model are incorporated to generate the desired controlled-bandwidth harmonics with a high aggregation as well as the average amplitude. Impedance measurement results of the proposed approach have been validated through both simulation and experiment. Considering the measured errors, the proposed method is efficient in testing the wide-bandwidth impedance of the single-phase railway traction system.

Modeling and Detecting False Data Injection Attacks against Railway Traction Power Systems

Modern urban railways extensively use computerized sensing and control technologies to achieve safe, reliable, and well-timed operations. However, the use of these technologies may provide a convenient leverage to cyber-attackers who have bypassed the air gaps and aim at causing safety incidents and service disruptions. In this article, we study False Data Injection (FDI) attacks against railway Traction Power Systems (TPSes). Specifically, we analyze two types of FDI attacks on the train-borne voltage, current, and position sensor measurements—which we call efficiency attack and safety attack— that (i) maximize the system’s total power consumption and (ii) mislead trains’ local voltages to exceed given safety-critical thresholds, respectively. To counteract, we develop a Global Attack Detection (GAD) system that serializes a bad data detector and a novel secondary attack detector designed based on unique TPS characteristics. With intact position data of trains, our detection system can effectively detect FDI attacks on trains’ voltage and current measurements even if the attacker has full and accurate knowledge of the TPS, attack detection, and real-time system state. In particular, the GAD system features an adaptive mechanism that ensures low false-positive and negative rates in detecting the attacks under noisy system measurements. Extensive simulations driven by realistic running profiles of trains verify that a TPS setup is vulnerable to FDI attacks, but these attacks can be detected effectively by the proposed GAD while ensuring a low false-positive rate.

A Negative Sequence Compensation Method Based on a Two-Phase Three-Wire Converter for a High-Speed Railway Traction Power Supply System

This paper presents a negative sequence compensation system based on a novel two-phase three-wire converter to eliminate a negative sequence current for the high-speed railway traction power system with a three-phase V/V traction transformer. In this compensation system, the proposed two-phase three-wire converter is fed by two single-phase power sources formed with two step-down transformers connecting to the two feeder sections. The proposed converter contains three switch legs, one of which is connected to the common ground wire of two single-phase voltages. Hence, a switch leg is saved compared with two conventional single-phase converters, and the ratings of the power switches are not increased. In order to enhance the dynamic and steady-state performances of the compensation system, a compound control method composed of hysteresis control and dividing frequency control is presented. Simulation and experimental results are presented to demonstrate that the proposed compensator and its control strategy are very effective.

Research on Harmonic Detection Methods of Electrified Railway

This paper introduces the characteristics of the traction power system of electric railway, and explores the harms to the power system caused by electric railway and the necessity of harmonic suppression of traction power supply system. In order to achieve real-time detection and compensation for harmonic waves of electric railway traction power system, this paper provides an improved harmonic detection method with initial compensating angle of delay time. Detection method is simple, and has better real-time, it can be used to detect selective harmonic for electrified railway.

Research on Synthesis Control of Power Quality for Electrified Railway

This paper analyzes necessity of synthesis control of power quality aiming at characteristics of traction power supply system for electrified railway. According to the fact that the existing power quality control, the measure and means were introduced, analyzed and compared in detailed. From reliability, technology and economy point, the project of synthesis Control was put forward on power quality of electrified railway in the paper. In order to achieve realtime detection and compensation for power quality of electrified railway traction power system, this paper provides a detection method of selective harmonic current, so vertiginous harmonic current of traction load is detected. Synchronously vertiginous passive power is gained basing on hilbert digital phase-shifting filter. And imbalance load current is balanced through regulating magnetron static VAR compensator (SVC) device. The kind new injectiontype hybrid active power filter (HAPF) combined with magnetron SVC make to compensate reactive power effectively, eliminate harmonic drastically and balance load imbalance current availably, and improve reliability and security of electrified railway greatly.

Evaluation of Maintenance Schedules on Railway Traction Power Systems

In electrified railways, the traction power systems carry power to trains and their reliability is vital to the quality of train services. There are many components in the traction power system, from interface with utility distribution network to contacts with trains, and they are physically located along the rail line. Subject to usage, environment, and ageing, conditions of components deteriorate with time. Regular maintenance has to be carried out to restore their conditions and prevent them from failure. However, the decisions on the suitable length of maintenance intervals often lead railway operators to the dilemma of minimizing both risk of failure and operation cost. On the basis of a stochastic lifetime model, this article presents a generic software evaluation tool that enables the operators to manage risk of failure and cost quantitatively in order to match their preferred levels of service quality. The lifetime model includes the effects of condition restoration because of maintenance of regular intervals and ageing acceleration because of electrical stress from traffic demands. Examples of simulation results are given to illustrate the applicability of this lifetime model.

Electric load estimation techniques for high-speed railway (HSR) traction power systems

As modern (e.g., high-speed railway (HSR)) traction power systems (TPS) become more and more comparable in size to grid capacity, dynamic load estimation (DLE) has become not just an important tool for TPS planning, but also an indispensable tool for utility companies to evaluate traction system's accurate unbalance impact on the grid. Without a good DLE algorithm, unbalance impact can easily be underestimated and causes power system instabilities. A good DLE must be carried out with a power engineering perspective while incorporating real railway operating principles and practices. However, due to the lack of well-documented literature on this subject and the interdisciplinary nature of DLE, it usually presents a difficult task for the system planner. As such, this paper presents an accurate DLE algorithm capable of achieving these goals, while providing a complete coverage of all the principles and parameters used during the derivation. The methodology developed here is applicable to HSR TPS and to conventional railways as well with minor modifications. Unbalance impact evaluation of the new Taiwan HSR is presented in the last part of the paper, while further application of the proposed DLE algorithm is also proposed.

A frequency domain model for 3 kV DC traction DC-side resonance identification

Frequency-dependent effects in railway traction power systems arise from the impedance of substation and locomotive line filters and the traction line. Harmonic noise from traction drives and substations can excite resonances and produce overcurrent or overvoltage conditions at critical points in the network. In this paper, the harmonic feeding impedances of a 3 kV DC traction system seen from the rectifier substation, locomotive drive converter and pantograph terminals are presented. Several substation and locomotive filters are considered with a frequency-dependent traction line. Resonances attributed to the substation filter, locomotive filter and traction line are separate and distinct, the line introducing poles and zeros in the audio frequency (AF) range which vary in frequency and magnitude with locomotive position. >