Traction power systems for electrified railways:evolution,state of the art,and future trends

Traction power systems(TPSs)play a vital role in the operation of electrified railways.The transformation of conventional railway TPSs to novel structures is not only a trend to promote the development of electrified railways toward high-efficiency and resilience but also an inevitable requirement to achieve carbon neutrality target.On the basis of sorting out the power supply structures of conventional AC and DC modes,this paper first reviews the characteristics of the existing TPSs,such as weak power supply flexibility and low-energy efficiency.Furthermore,the power supply structures of various TPSs for future electrified railways are described in detail,which satisfy longer distance,low-carbon,high-efficiency,high-reliability and high-quality power supply requirements.Meanwhile,the application prospects of different traction modes are discussed from both technical and economic aspects.Eventually,this paper introduces the research progress of mixed-system electrified railways and traction power supply technologies without catenary system,speculates on the future development trends and challenges of TPSs and predicts that TPSs will be based on the continuous power supply mode,employing power electronic equipment and intelligent information technology to construct a railway comprehensive energy system with renewable energy.

Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

High-voltage direct current(HVDC) transmission is a crucial way to solve the reverse distribution of clean energy and loads. The line commutated converter-based HVDC(LCCHVDC) has become a vital structure for HVDC due to its high technological maturity and economic advantages. During the DC fault of LCC-HVDC, such as commutation failure, the reactive power regulation of the AC grid always lags the DC control process, causing overvoltage in the AC sending grid, which brings off-grid risk to the wind power generation based on power electronic devices. Nevertheless, considering that wind turbine generators have fast and flexible reactive power control capability, optimizing the reactive power control of wind turbines to participate in the transient overvoltage suppression of the sending grid not only improves the operational safety at the equipment level but also enhances the voltage stability of the system. This paper firstly analyses the impact of wind turbine's reactive power on AC transient overvoltage. Then, it proposes an improved voltage-reactive power control strategy, which contains a reactive power control delay compensation and a power command optimization based on the voltage time series prediction. The delay compensation is used to reduce the contribution of the untimely reactive power of wind turbines on transient overvoltage, and the power command optimization enables wind turbines to have the ability to regulate transient overvoltage, leading to the variation of AC voltage, thus suppressing the transient overvoltage. Finally, the effectiveness and feasibility of the proposed method are verified in a ±800kV/5000MW LCC-HVDC sending grid model based on MATLAB/Simulink.

Review of the Analysis and Suppression for High-frequency Oscillations of the Grid-connected Wind Power Generation System

High-frequency oscillation(HFO)of gridconnected wind power generation systems(WPGS)is one of the most critical issues in recent years that threaten the safe access of WPGS to the grid.Ensuring the WPGS can damp HFO is becoming more and more vital for the development of wind power.The HFO phenomenon of wind turbines under different scenarios usually has different mechanisms.Hence,engineers need to acquire the working mechanisms of the different HFO damping technologies and select the appropriate one to ensure the effective implementation of oscillation damping in practical engineering.This paper introduces the general assumptions of WPGS when analyzing HFO,systematically summarizes the reasons for the occurrence of HFO in different scenarios,deeply analyses the key points and difficulties of HFO damping under different scenarios,and then compares the technical performances of various types of HFO suppression methods to provide adequate references for engineers in the application of technology.Finally,this paper discusses possible future research difficulties in the problem of HFO,as well as the possible future trends in the demand for HFO damping.

Analysis of the Electric Locomotives Neutral-section Passing Harmonic Resonance

For the further analysis and suppression of the electric locomotives neutral-section passing overvoltage, on the basis of theoretical analysis of the neutral-section passing harmonic resonance conditions, this paper establishes simplified harmonic resonance simulation models of the electric locomotives neutral-section passing using MATLAB/Simulink, and makes the overvoltage simulation analysis of the existing electric locomotives neutral-section passing system in the event of a harmonic resonance. Results show that when the system harmonic resonance occurs, the operating overvoltage of the neutral-section passing is serious intensified by the overvoltage of the harmonic resonance, which will make the voltage of the pantograph collector head exceeding 100kV. This amplitude of the overvoltage will breakdown the air gap, which will be a serious threat to the safety operation of the electric railway. However, this kind of neutral-section passing overvoltage hasn’t cause the attention in the field and theoretical studies, which need more analysis and verification in the further study.

Antioxidant Activity of 50 Traditional Chinese Medicinal Materials Varies with Total Phenolics

This study was designed to determine the total phenolic content of 50 herbs and to examine their antioxidant potential. In the sample preparation, 60% ethanol was chosen as the extraction solvent for the subsequent experiments. Folin-Cicolteau phenol reagent and a colorimetric method were used to determine the total phenolic content of the selected herbs. The result showed that total phenolic content of those herbs ranged from 2 to 185 mg/g. In antioxidant assay, the ferric reducing/antioxidant power (FRAP) values ranged from 2 to 134 mg GAE/g;the IC50 values of DPPH?·, ·OH and ?scavenging were in the range of 0.06 - 5.50 mg/mL, 0.017 - 0.636 mg/mL and 0.050 - 0.681 mg/mL respectively. Flos caryophylli was the exceptant in the ?scavenging assay because there was no linear relation between the concentration and the scavenging percentage. Compared to gallic acid, ascorbic acid and butylated hydroxytoluene (BHT) in antioxidant assay as positive control, the most potential antioxidant herbs were Cacumen platycladi, Radix et Rhizoma rhei, Rhizoma rhodiolae crenulatae, and Rhizoma sanguisorbae with considerable content of phenolics. Especially, a positive and significant correlation was found between the total phenolic content and FRAP value or DPPH· scavenging percentage.

Analysis of rail potential and stray current in MVDC railway electrification system

In contrast to the conventional direct current railway electrification system(DC-RES),the medium voltage direct current(MVDC)-RES is considered promising for long-distance high-speed corridors.In the MVDC-RES,traction substations(TSSs)are placed much farther and train loads are much heavier than in the conventional DC-RES.Hence,the MVDC-RES brings a drastic change in catenary voltage,TSS spacing,and train loading,which affects rail potential and stray current.In this connection,this work performs some significant quantitative analysis of rail potential and stray current in the MVDC-RES environment.An MVDC simulation model is proposed and different grounding schemes are analyzed for a single-train and two TSSs scenario as well as for a multi-train multi-TSS scenario.According to the simulation and analysis,the maximum values of rail potential and stray current at MVDC-RES distances and the maximum safe distance between adjacent TSSs are determined.

Busbar Differential Relaying Method Based on Combined Amplitude and Phase Information of High Frequency Transient Currents

Busbar differential relaying method based on combined amplitude and phase information of high frequency transient currents is put forward in this paper for the speed and reliability problems of busbar protection based on fundamental frequency. Under the analysis of features of bus high frequency differential currents, complex wavelet analysis is used to extract the amplitude and phase features of 1/4 period high frequency differential currents, and amplitude and phase information are used to form the polar coordinates. Bus fault is identified intuitively and precisely according to polar locus differences. This polar coordinates represented busbar differential protection scheme based on high frequency transient signals can not only avoid TA saturation, realizing quick protection, lots of PSCAD/EMTDC simulations also show that this busbar differential protection scheme works well under different fault conditions.

Study on Distribution Coefficient of Traction Return Current in High-Speed Railway

The distribution coefficient of return current network is an important method to decrease the rail potential. In order to resolve the problem of high rail potential in high-speed railway based on EN50122-1 and Pr EN50170 the distribution coefficient of longitudinal traction return current conductors is calculated through changing the interval of transverse connection. Based on field data and theoretical analysis, the parameters of longitudinal traction return current conductors are calculated. Results indicate that the best distance of the transverse connection is 400 m – 600 m.

Small Signal Stability Analysis for a DFIG-Based Offshore Wind Farms Collected Through VSC-HVDC Transmission System

This paper modeled a doubly fed induction generator (DFIG) - based offshore wind farm integrated through a voltage source converter –based high voltage direct current (VSC-HVDC) transmission system, which is collected with infinite bus for small signal stability analysis. The control system of HVDC system is considered for the stability analysis. The impact of the VSC control parameters on the network stability is studied. The lineared dynamic model is employed to do small signal stability analysis by the eigenvalue analysis. The locus of the eigenvalue, which is corresponding to the oscillation model is studied. Time domain simulations conducted in Matlab/Simulink are used to validate the small signal stability analysis.

Small-Signal Stability Analysis of Wind Power System Based on DFIG

This paper focuses on the small-signal stability of power system integrated with DFIG-based wind farm. The model of DFIG for small-signal stability analysis has built;the 3-generator 9-bus WECC test system is modified to investigate the impacts of large scale integration of wind power on power system small-signal stability. Different oscillatory modes are obtained with their eigenvalue, frequency and damping ratio, the results from eigenvalue analysis are presented to demonstrate the small-signal stability of power system is enhanced with the increasing output of the wind farm.

Transmission Lines Distance Protection Using Differential Equation Algorithm and Hilbert-Huang Transform

This paper proposed the scheme of transmission lines distance protection based on differential equation algorithms (DEA) and Hilbert-Huang transform (HHT). The measured impedance based on EDA is affected by various factors, such as the distributed capacitance, the transient response characteristics of current transformer and voltage transformer, etc. In order to overcome this problem, the proposed scheme applies HHT to improve the apparent impedance estimated by DEA. Empirical mode decomposition (EMD) is used to decompose the data set from DEA into the intrinsic mode functions (IMF) and the residue. This residue has monotonic trend and is used to evaluate the impedance of faulty line. Simulation results show that the proposed scheme improves significantly the accuracy of the estimated impedance.

A power-quality monitoring and assessment system for high-speed railways based on train-network-data center integration

In recent years,with the rapid development of high-speed railways(HSRs),power interruptions or disturbances in traction power supply systems have become increasingly dangerous.However,it is often impossible to detect these faults immediately through single-point monitoring or collecting data after accidents.To coordinate the power quality data of both traction power supply systems(TPSSs)and high-speed trains(HSTs),a monitoring and assessing system is proposed to access the power quality issues on HSRs.By integrating train monitoring,traction substation monitoring and data center,this monitoring system not only realizes the real-time monitoring of operational behaviors for both TPSSs and HSTs,but also conducts a comprehensive assessment of operational quality for train-network systems.Based on a large number of monitoring data,the field measurements show that this real-time monitoring system is effective for monitoring and evaluating a traction-network system.

Evaluation of Voltage Quality and Energy Saving Benefits of Urban Rail Transit Power Supply Systems Considering the Access of Photovoltaics

Along the route of urban rail transit(URT),the subway depots,parking lots,and elevated subway stations are equipped with sufficient photovoltaic(PV)generation conditions.The access of PVs in the URT power supply system should not only make full use of nearby renewable energy,but also improve the economic operation of the system.In order to quantify the impact of PV access on the URT power supply system,this paper proposes a method to evaluate the voltage quality and energy saving benefits of the URT power supply system,considering PV access.First,by fully considering the random fluctuations of PV output power,as well as the intermittency and impact of traction load,the URT power supply system with PV access is modeled.Then,such values as system voltage and power are calculated through the hybrid AC/DC load flow calculation method.Combined with the results of the load flow calculation,the indicators describing voltage quality and power saving benefits of the system are proposed to quantify the influence of PV access on the system.Finally,a case study concerning a certain section of a subway line is presented to verify the proposed method.It shows that the proposed method can effectively quantify the voltage quality and power saving benefits of the system after PV access,providing valuable insights for design and maintenance personnel to comprehend system state and operating characteristics.

A Self-determined Evaluation Method for Science Popularization Based on IOWA Operator and Particle Swarm Optimization

With the increase of science popularization, evaluation of science popularization has become an urgent demand. Considering science popularization bases as independent agents, a self-determined evaluation approach for science popularization using induced ordered weighted averaging (IOWA) operator and particle swarm optimization (PSO) is proposed in this paper.Firstly, six factors including science popularization personnel, space, fund,media, activity and influence are selected to construct an index system for science popularization evaluation. On this basis, the absolute dominance and relative dominance of evaluation indexes are used as induced components, and the prior order of the evaluation indexes is determined. Besides, the optimization model of index weighted vectors is established by IOWA operator, index weighted vectors are calculated by particle swarm optimization algorithm, and index weighted vectors and evaluation value vectors are obtain. Finally, the optimal evaluation vectors and evaluation results are given according to the Perron-Frobenius decision eigenvalve theorem .

Gene Coding Sequence Identification Using Kernel Fuzzy C-Mean Clustering and Takagi-Sugeno Fuzzy Model

Sequence analysis technology under big data provides unprecedented opportunities for modern life science. A novel gene coding sequence identification method is proposed in this paper. Firstly, an improved short-time Fourier transform algorithm based on Morlet wavelet is applied to extract the power spectrum of DNA sequence. Then, threshold value determination method based on kernel fuzzy C-mean clustering is used to combine Signal to Noise Ratio (SNR) data of exon and intron into a sequence, classify the sequence into two types, calculate the weighted sum of two SNR clustering centers obtained and the discrimination threshold value. Finally, exon interval endpoint identification algorithm based on Takagi-Sugeno fuzzy identification model is presented to train Takagi-Sugeno model, optimize model parameters with Levenberg-Marquardt least square method, complete model and determine fuzzy rule. To verify the effectiveness of the proposed method, example tests are conducted on typical gene sequence sample data.

Harmonic Pollution Level Assessment in Distribution System Using Extended Cloud Similarity Measurement Method

The purpose of harmonic pollution level assessment is to judge whether the harmonic pollution of a bus or a region is beyond the limit. Considering the fuzziness of the index criteria values caused by harmonic variation,this paper proposes an assessment method for harmonic pollution level based on the extended cloud similarity measurement. Firstly, according to the main characteristics of harmonic pollution, a set of evaluation index of harmonic pollution level is formed, including the total harmonic distortion rate of voltage,the fifth harmonic current RMS value, the seventh harmonic current RMS value,the eleventh harmonic current RMS value, and thirteenth harmonic current RMS value. Secondly, considering the group decision-making behavior of the harmonic pollution level assessment, the group eigenvalue method is utilized to integrate the weight of multiple operators and to calculate the comprehensive weight of the assessment index. On this basis, the harmonic pollution are divided into four levels, and the index criteria values are expressed as triangular fuzzy numbers. Besides, the fuzzy numbers are unified by the extended cloud.Finally, the extended cloud similarity and the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is utilized to get the evaluation results.The monitoring data of seven 10 kV buses in Guangzhou is adopted to verify the validity and rationality of the evaluation method.

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 currentsharing 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.

Integration of PV and Battery Storage for Catenary Voltage Regulation and Stray Current Mitigation in MVDC Railways

Innovative advancement in power electronics is reshaping the conventional high-voltage transmission systems and has also opened a new paradigm for researchers to consider its benefits in the railway electrification system(RES).In this regard,the medium-voltage direct current RES(MVDC-RES)is a key area of interest nowadays.In this paper,a secondary energy source(SES)consisting of renewable energies(REs)and energy storage systems(ESSs)is proposed to solve the issues of catenary voltage regulation,rail potential,and stray current in the MVDC-RES.Some of the major integration topologies of the SES are analyzed for MVDC-RES and the most effective one is proposed and implemented.The voltage at the point of connection(PoC)of the SES is used as a reference for controlling different operation modes of REs and ESSs.Moreover,feedforward control is used at the ESS converter to attain the quick response from the batteries for the desired operation.The proposed scheme improves the catenary voltage,and reduces the rail potential and stray current.Besides,the scheme provides higher energy density and reduces line losses.Simulation results are provided to validate the operation modes and advantages of the proposed scheme.

A cumulative standard deviation sum based method for high resistance fault identification and classification in power transmission lines

High resistance fault poses an enormous challenge to the existing algorithms of fault detection and fault classification.In this paper,the standard deviation and accumulation method are employed to perform the fault detection and classification.It is primarily built in two stages.Firstly,the standard deviations for the measured current’s signals of the local and remote terminals is computed to extract the fault feature.Secondly,the cumulative approach is used to enlarge the fault feature to perform the high resistance fault.The proposed scheme is known as Standard Deviation Index(SDI),and it is obtained for the three phases and zero sequence.The proposed algorithm has been tested through different fault circumstances such as multiple faults locations,fault resistances,and fault inception time.Moreover,far-end faults with high-resistance,faults happened nearby the terminal,faults considering variable loading angle,sudden load change,different sampling frequency,bad signaling and a fault occurred in the presence of series compensation are also discussed.The results show that the proposed scheme performed remarkably well regarding the fault with resistance up to 1.5kΩand can be detected within a millisecond after the fault inception.Additionally,the computational simplicity that characterizes the processes makes it more efficient and suitable for domain applications.

Faulty Phase Identification for Transmission Line with Metal Oxide Varistor-protected Series Compensator

The nonlinear operation of metal oxide varistor(MOV)-protected series compensator in transmission lines introduces complications into fault detection approaches. The accuracy of a conventional fault detection schemes is adversely affected by continuous change of the system impedance and load current at the point of a series compensation unit. Thus, this study suggests a method for detecting the faulted phase in MOV-protected series-compensated transmission lines. Primarily, the fault feature is identified using the covariance coefficients of the current samples during the fault period and the current samples during the pre-fault period. Furthermore, a convenience fault detection index is established by applying the cumulative sum technique. Extensive validation through different fault circumstances is accomplished, including different fault positions,resistances, and inception times. The experimental results show that the proposed method performs well with high resistance or impedance faults, faults in noisy conditions, and close-in and far-end faults. The proposed method is simple and efficient for faulty phase detection in MOV-protected series-compensated transmission lines.