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The flexible traction power supply system (FTPSS), integrating power flow controller (PFC), energy storage system, and photovoltaic (PV) system, is able to cancel the neutral section, realize the collaborative operation of multiple traction substations (MTSs), and enhance the efficient utilization of regenerative braking energy (RBE) and renewable energy in electrified railways. However, co‐dispatching energy among MTSs influences the power flow distribution within the traction network, impacting the traction network voltage (TNV). Maintaining optimal energy dispatch while ensuring compliance with TNV standards is critical. Therefore, a two‐stage energy optimal dispatch and control strategy is proposed for FTPSS here. The first stage develops a synergistic energy optimal dispatch model for MTSs considering TNV constraint and power flow constraints, aimed at maximizing the utilization of RBE and PV while minimizing the electricity cost of FTPSS. In the second stage, a control strategy for multiple converters is designed to ensure the dynamic response of FTPSS. Finally, the effectiveness of the proposed strategy and model is substantiated through field load data and experimental validation, and the electricity cost of the FTPSS is reduced by about 21.4%, meanwhile the TNV is maintained in the range of 22.5–29 kV. [ABSTRACT FROM AUTHOR]

To solve the problem of negative‐sequence and reactive power in electrified railway, this paper proposes a comprehensive compensation method based on V/v transformer and electromagnetic single‐phase var compensator (ESVC) for co‐phase power supply. First, based on the principles of static var generator and single‐phase rotary phase shifting transformer, the topology and model of ESVC have been established, and the compensation mechanism has been analysed. Second, the topological structure and mathematical model of co‐phase power comprehensive compensation device (CPCD) are put forward based on the principles of V/v transformer and ESVC, and the operation mode of CPCD in multiple scenarios is designed. Then the strategy of CPCD with double closed loop control is analysed: corresponding compensation mode is selected according to negative‐sequence unbalance degree and expected power factor value. In this strategy, the compensated current output by ESVC is taken as the quantity of external loop control, and the rotation angle of ESVC is taken as the quantity of internal loop control to realize double‐closed loop control of CPCD. Finally, the comprehensive CPCD compensation model is built on the simulation platform and validated the accuracy of the mathematical model and the effectiveness of the control strategy. [ABSTRACT FROM AUTHOR]

Abstract To solve the problem of negative‐sequence and reactive power in electrified railway, this paper proposes a comprehensive compensation method based on V/v transformer and electromagnetic single‐phase var compensator (ESVC) for co‐phase power supply. First, based on the principles of static var generator and single‐phase rotary phase shifting transformer, the topology and model of ESVC have been established, and the compensation mechanism has been analysed. Second, the topological structure and mathematical model of co‐phase power comprehensive compensation device (CPCD) are put forward based on the principles of V/v transformer and ESVC, and the operation mode of CPCD in multiple scenarios is designed. Then the strategy of CPCD with double closed loop control is analysed: corresponding compensation mode is selected according to negative‐sequence unbalance degree and expected power factor value. In this strategy, the compensated current output by ESVC is taken as the quantity of external loop control, and the rotation angle of ESVC is taken as the quantity of internal loop control to realize double‐closed loop control of CPCD. Finally, the comprehensive CPCD compensation model is built on the simulation platform and validated the accuracy of the mathematical model and the effectiveness of the control strategy.

Abstract The flexible traction power supply system (FTPSS), integrating power flow controller (PFC), energy storage system, and photovoltaic (PV) system, is able to cancel the neutral section, realize the collaborative operation of multiple traction substations (MTSs), and enhance the efficient utilization of regenerative braking energy (RBE) and renewable energy in electrified railways. However, co‐dispatching energy among MTSs influences the power flow distribution within the traction network, impacting the traction network voltage (TNV). Maintaining optimal energy dispatch while ensuring compliance with TNV standards is critical. Therefore, a two‐stage energy optimal dispatch and control strategy is proposed for FTPSS here. The first stage develops a synergistic energy optimal dispatch model for MTSs considering TNV constraint and power flow constraints, aimed at maximizing the utilization of RBE and PV while minimizing the electricity cost of FTPSS. In the second stage, a control strategy for multiple converters is designed to ensure the dynamic response of FTPSS. Finally, the effectiveness of the proposed strategy and model is substantiated through field load data and experimental validation, and the electricity cost of the FTPSS is reduced by about 21.4%, meanwhile the TNV is maintained in the range of 22.5–29 kV.

Abstract The characteristics of traction network (TN) of the three‐phase 3‐kV traction power supply system (TPSS) are: three‐phase conductors are flat layout (layout‐1, layout‐2); the material and cross‐section of conductors are ferromagnetic and irregular, respectively. So the phase impedance matrix (PIM) is unbalanced and the closed‐form formula for conductor impedance is hard to get. To address these issues, firstly, the PIM is derived, and the cause of unbalance is revealed. From the perspective of structure optimization, a new flat layout (layout‐3) and an equilateral triangle layout (layout‐4) is proposed. Then, to calculate the wideband frequency self‐ and mutual impedances of the special phase conductors, a frequency‐ and current‐dependent closed‐form formula based on 4‐parameters equivalent tubular conductor (ETC) model is proposed. Furthermore, an equivalent TN circuit model at 50 Hz is established to calculate the receiving‐end voltage and its negative sequence voltage unbalance factor (NSVUF). Finally, using these two indexes, the performances of the layout‐1 to 4 are evaluated and compared. The results show that the conductor spacing and power supply frequency affect the two indexes much while the dimension of the power supply rail (PSR) not; layout‐4 can completely balance the receiving‐end voltage and improve its magnitude which can enhance the power supply capability.

The characteristics of traction network (TN) of the three‐phase 3‐kV traction power supply system (TPSS) are: three‐phase conductors are flat layout (layout‐1, layout‐2); the material and cross‐section of conductors are ferromagnetic and irregular, respectively. So the phase impedance matrix (PIM) is unbalanced and the closed‐form formula for conductor impedance is hard to get. To address these issues, firstly, the PIM is derived, and the cause of unbalance is revealed. From the perspective of structure optimization, a new flat layout (layout‐3) and an equilateral triangle layout (layout‐4) is proposed. Then, to calculate the wideband frequency self‐ and mutual impedances of the special phase conductors, a frequency‐ and current‐dependent closed‐form formula based on 4‐parameters equivalent tubular conductor (ETC) model is proposed. Furthermore, an equivalent TN circuit model at 50 Hz is established to calculate the receiving‐end voltage and its negative sequence voltage unbalance factor (NSVUF). Finally, using these two indexes, the performances of the layout‐1 to 4 are evaluated and compared. The results show that the conductor spacing and power supply frequency affect the two indexes much while the dimension of the power supply rail (PSR) not; layout‐4 can completely balance the receiving‐end voltage and improve its magnitude which can enhance the power supply capability. [ABSTRACT FROM AUTHOR]

Abstract 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.

The cophase traction power supply system (CTPSS) is a promising solution in the smart electrified railways to effectively eliminate the neutral section, improve regeneration braking energy (RBE) utilization and mitigate voltage unbalance (VU). It works by coordinating the power flow distribution (PFD) among the energy storage (ES), power flow controller, and traction transformer. However, the conventional dispatch considering VU mitigation may result in increasing extra operation costs and accelerating ES aging. Therefore, driven by the day-ahead forecasted information, a dynamic VU regulation strategy is proposed to draw up the VU compensation schedule (VU-CS), which can satisfy the probabilistic limitations of power quality standards. Consequently, a Copula-based VU dependence model is developed to transform the VU-CS into the PFD constraint set of CTPSS. Meanwhile, an ES degradation model is introduced to quantify the system operation cost within different scenarios. Taking the PFD as a VU regulation constraint, an economic dispatch scheme of CTPSS is established to obtain a minimum operation cost and maximum RBE utilization. Finally, case study tests verify the efficiency of the proposed approach. The results show that scheme can improve ES cycle life and decrease cost under the probabilistic limitations of VU. [ABSTRACT FROM AUTHOR]

In remote areas, weak power grids, fewer grid access points, and power quality (PQ) problems dominated by excessive voltage unbalance (VU) and lower power factor (PF) have been prominent obstacles in single-phase 25 kV AC traction power supply systems. To achieve a long-distance power supply with a high PQ, a traction substation group connected to a weak remote power grid is introduced, where the electrical structure is composed of multiple slave traction substations using single-phase traction transformers (TTs), a master traction substation (MTS) using a combination of TTs and a power flow controller (PFC) adopting a modular multilevel converter (MMC). Then, a unified mathematical model of the proposed system suitable for different TT wiring types revealing the effect of asymmetrical grids on the PFC is established. The negative sequence current (NSC) caused by multiple traction stations at the point of common coupling (PCC) and the PF can be improved effectively based on power conservation theory. In addition, to reduce the compensation capacity, an optimization compensation strategy is proposed. Finally, the effectiveness of the proposed algorithm is verified by simulation and a case study. The installed capacity of the PFC is 18.5% lower than that of traditional compensation. [ABSTRACT FROM AUTHOR]

To achieve the UK net‐zero emissions target by 2050, transformations to decarbonise the energy system will be essential. A transition from generating electricity by burning fossil fuel to renewable energy sources (RESs) is one of the most effective ways, in addition to customer transformation, in which people are willing to participate to reduce energy consumption. The railway sector accounts for 0.5% of carbon emissions in the United Kingdom. In order to meet the net‐zero goal, more railway routes must be electrified, as only 38% of railway routes are electrified at present. Furthermore, clean energy is necessary. Therefore, schemes for integrating RESs into the AC high‐speed railway power supply system are proposed and simulated in a case study. Power losses in the power system are comprehensively studied and compared. Owing to the fluctuation of traction load, the power quality issue is inevitable. Thus, the voltage unbalance factor is adopted to measure the severity of the imbalance. The results from the case study demonstrate that the proposed scheme in which the RES is connected to a three‐phase railway power network generates the smallest power losses among all proposed schemes. Moreover, it also requires lower investment expenditure and provides the most significant cost saving in the long run. The results also reveal that the VUFs of all schemes based on a 400 kV transmission system are below the United Kingdom's stringent limit of 1.5%. Furthermore, the CO2 emissions are reduced significantly with RES integration by half for the case study with the 120 MW RES. Although the other proposed schemes had higher losses and lifetime costs, the differences are not significant. Each scheme has its advantages and disadvantages. Several factors need to be considered to choose the suitable scheme, such as the size of land available etc. [ABSTRACT FROM AUTHOR]

All state-of-the-art Power Electronic Traction transformers (PETT) were developed for the existent Medium Voltage (MV) AC Electric Railway Systems (ERS). This work, however, presents a PETT for a novel MVDC-ERS. We studied and evaluated various state-of-the-art PETT topologies in two previous articles to determine which is best for this application, and we presented an 8-module Input Series Output Parallel (ISOP) MVDC PETT with a total power exceeding 1.2 MW. The converter topology used in the modules is the Dual Active Bridge (DAB). In this paper, the complete mathematical model of the converter, the deduction of controller parameters and the decoupling method, and the simulation model are presented in detail. Simulations show how the system works and interacts with a traction motor, as well as its response to input voltage variation and load steps. The results and theoretical notions obtained in this project will lay the foundation of a novel smart MVDC-ERS, meanwhile an experimental prototype is under development.

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. [ABSTRACT FROM AUTHOR]

Railway power conditioner (RPC) can provide comprehensive reactive power, unbalanced current, and harmonics compensation in the traction power supply system. Under full compensation (FC), RPC can increase the three-phase power factor (PF) to unity but leads to high current rating and operation loss. In previous studies, partial compensation (PC) for the reduction of RPC current rating was proposed. An analytical solution of PC coefficients was used, which limits design freedom since identical PF is required at three-phase. This uniform power factor partial compensation (UPFPC) limits RPC in achieving a further reduction in its current rating. This article proposes a nonuniform power factor partial compensation (NPFPC), which can further reduce RPC current rating within the acceptable PF region and under the voltage unbalance threshold. However, determining an analytical solution under NPFPC is complicated, considering three degrees of freedom. This is achieved in this article by defining the lower boundary of grid side PFs and optimizing by particle swarm optimization. It is found through investigations that NPFPC can effectively reduce the compensating current rating by 56% compared to FC mode and 53% compared to UPFPC. Simulation and experimental results verify the validity of the proposed NPFPC. [ABSTRACT FROM AUTHOR]

Simulation tools are essential to design the infrastructure and plan the trains operations of electrical railway systems. Traditionally, the train model (that estimates the electrical, mechanical and kinematical behaviors of the vehicles) and electrical network model (that estimates the electrical behavior of the energy supply network) are developed separately. Then, they are simulated together to estimate the interactions between both subsystems. The paper objective is to compare different models to highlight the impacts on the interactions between the vehicles and the railway electrical network, which are crucial to have accurate estimations of the system behavior. For this purpose, a new dynamical model, which is based on a systemic approach and a causality analysis, is compared to a conventional static model, which is based on a cartesian approach and a power flow analysis. The dynamical model is accurate and has been experimentally validated but requires a long computational time. The static model is fast to compute and give a good estimation of the energy consumption for conventional railway systems. However, it is not always able to estimate the power flows within the DC network, and especially when all traction power substations are blocked. [ABSTRACT FROM AUTHOR]

For a selected train journey with multiple trains in the same traction power system, the given driving strategies of other trains can be abstracted as available external power flow. Aims at reducing the energy consumption of the multi‐train system, this paper presents an analytical solution for optimising the selected train trajectory to maximise the utilisation of the external power flow. Specifically, a spatial‐temporal area which combine the track grade along space and the external power over time is constructed and the problem is converted into the energy‐efficient control of a train travels in the spatial‐temporal area. Classical methods of constrained optimisation is used to show that the set of the optimal control modes and the optimal driving speed for the proposed problem are in a state of change. In order to obtain the optimal driving strategy, the spatial‐temporal area is divided into grids, and the evolution of a key adjoint variable when the external power changes is discussed. The authors illustrate our results by finding optimal control sequences and associated speed profile for the freight train in some elementary but realistic examples. [ABSTRACT FROM AUTHOR]

Actual condition-based maintenance for traction power supply equipment (TPSE) is almost based on completely observable equipment state. However, it is unpractical to accurately reveal the equipment state due to the inescapably uncertainty of state assessment. In order to optimize the maintenance of TPSE, a maintenance model based on partially observable Markov decision process is proposed in this paper. Firstly, the degradation process of the TPSE is described by a four-state Markov process, and the state residence time and its transition probability of the equipment are obtained by equaling fault times in the statistical period. Then, the imperfect maintenance is considered in this paper. And the failure risk of the TPSE after maintenance is quantified for optimizing both the economic cost and the reliability of maintenance strategy. Finally, the practical fault record data of 27.5 kV vacuum circuit breakers for a traction power supply system (TPSS) are used to verify the proposed model. The results show that the maintenance model can provide guidance on decision-making for the maintenance under uncertainty, and the determination of maintenance schemes to optimize both TPSE reliability and operational cost. [ABSTRACT FROM AUTHOR]

The quest for electrification of railway tracks is very high at developing countries, with a very high ratio of population per kilometre length of railway track. India has electrified almost 50% of its track length and aiming 3000 km track electrification every year. Presently in Indian traction system, all the fault locating methods and power factor correction methods for AC high voltage line are based on the conventional model of railway Overhead Equipment (OHE). The conventional OHE model does not include the effect of self-generated reactive power at the geographically challenged area. This paper explains, how the variations in parasite capacitance of OHE adversely affect the prevailing conventional power factor improving methods; after monitoring the active power, apparent power, leading reactive power injected to improve the power factor of AC high voltage traction system for one calendar year, at two different Traction Sub Stations (TSS). Further simulation of 61 km track with actual mixed geographical terrain to validate the experimental results and to develop a new power factor correction algorithm has been done. The proposed algorithm has been implemented on one TSS of Southern Indian Railways division, and it turns out in reducing penalty around 2.5 million Indian rupees per year, paid by sampling region TSS.

24-hour traction current based on Wuhan Metro Line 6 are reported. The fluctuation features, which corresponding to the interval of vehicle operation, indicate that there are four types of inrush current periodically appearing throughout the all-day waveforms. The first type consists of two positive sharp waves. The second type is saddle shaped, and consists of two positive peaks and an anti-peak. The third type consists of two double peaks followed by a sharp peak about 1 minute later. The fourth type consists of two hump shaped waveforms. The whole operation time can be divided into the peak and off-peak hours according to these repetitive segments. The interval of any two adjacent vehicles is 9 minutes and 11 minutes, respectively, in peak and off-peak hours. The distribution and the positive average value of feeder current per hour from 6:00-24:00 are counted and analyzed. Results show that the current distribution in off-peak hours is more concentrated in a narrower range than that in peak hours. The differences in averages between various 15-minute periods in an hour can be as high as tens of amperes, due to the occurrence time of repetitive segments. The rail potential has relatively consistent variation with feeder currents, but it matched best with the sum of up-line and down-line in a same section instead of one of the two lines.

The inconvenient nature of non‐ideal charging characteristics is demonstrated from a power system point of view. A new adaptive charging algorithm that accounts for non‐ideal charging characteristics is introduced. The proposed algorithm increases the local network capacity utilization rate and reduces charging times. The first unique element of the charging algorithm is exploitation of the measured charging currents instead of ideal or predefined values. The second novelty is the introduction of a short‐term memory called expected charging currents. This makes the algorithm capable of adapting to the unique charging characteristics of each vehicle individually without the necessity to obtain any information from the vehicle or the user. The proposed algorithm caters to various non‐idealities, such as phase unbalances or the offset between the current set point and the real charging current but is still relatively simple and computationally light. The algorithm is compatible with charging standard IEC 61851 and is validated under different test cases with commercial electric vehicles. [ABSTRACT FROM AUTHOR]

Addis Ababa, the capital of Ethiopia, inaugurated the first electrified light rail transitsystem 4 years ago in September 2015, which makes it the first of its kind in sub‐Saharan and eastern African countries. The railway line has a total length of 34.25 km with 39 passenger stations and 19 traction substations. Currently, the performance of the traction power supply system degraded severely due to the existence of harmonics in the traction network. Realizing the above problem, the harmonic analysis of Addis Ababa light rail transit electrified power supply system is presented. The analysis is performed on one of the substations called 'Lideta' rectifier substation. To this end, mathematical modelling and analysis of the traction main components such as the catenary, the traction transformer, the traction rectifier and the load have been done. Finally, MATLAB simulation has been performed, and the result comes with a significant reduction in harmonic distortion, which is far below the 5% limits of IEEE standard. [ABSTRACT FROM AUTHOR]

A typical 2 × 25 kV traction power supply system consists of both primary and secondary equipment housed in control rooms, and the traction overhead wiring (OHW) that runs between the substations. The substation secondary equipment consists of the protection and control intelligent electronic devices (IEDs) known as relays. Each section of the OHW between the substations is protected by a set of two relays at both ends of a line for impedance protection and their backup relays. At substations facing the sections at the end of the line, where there is no other substation on the other end, usually the overcurrent and earth fault protection is used. At the substations, there are disturbance fault recorders installed or relays equipped with a capability to record and store disturbance fault records. An analysis of these fault records makes a significant contribution to the efficient running of a traction system such as the Gautrain rapid rail system. A combination of the analysis of the faults that occur in the OHW as recorded and stored in the relays as well as the performance of the IEDs and the protection settings using the on-site tests are a subject of this article. [ABSTRACT FROM AUTHOR]

Hybrid power quality conditioner (HPQC) has been utilised in a co‐phase electrified railway system with the aim of improving power quality indices. A C‐type filter is designed as an auxiliary device alongside the HPQC for high‐order harmonics mitigation in this paper. Alternatively, the C‐type filter compensates a selected portion of load reactive power, while the HPQC compensates for the left required power. Furthermore, the HPQC mitigates grid side unbalance and other harmonics in a co‐phase supplied system simultaneously. In addition, a design procedure of LCL filters is proposed to suppress high‐order harmonic contents induced by the HPQC converters to achieve lower grid‐side total harmonic distortion (THD). Moreover, a flexible partial compensation (PC) method considering the C‐type filter effect on the HPQC performance is adapted to maintain the grid‐side power quality indices in an acceptable range. The C‐type filter and PC method provide VA rating reduction of the HPQC without applying any complication to the compensation control system. Finally, simulation and experimental results are presented to validate the feasibility and functionality of the proposed configuration. [ABSTRACT FROM AUTHOR]

The ‘train-rail’ integrated grounding system for high-speed trains plays the critical role in providing the unique path for power feedback, which mainly consists of two parts - the on-board dynamic grounding sub-system and the fixed terrestrial grounding sub-system. The on-board dynamic grounding sub-system can be classified into two types of grounding modes – working grounding provides the discharging path for transferring the traction power; while the protective grounding maintains reference ground potential for the train body – both of which share the rail as the common path for discharging current. Thus, the traction current discharged from the working grounding wheel sets flows through the neighboring protective grounding wheel sets back to the train body, forming as the ‘train-rail’ current reflux. The ‘train-rail’ current reflux may result in the appearance of a ‘train body’ (TB) current, which may rise the potential of the train body. The TB current and voltage may cause electromagnetic interference to the on-board equipment, electrochemical corrosion on grounding wheel bearing or even on-board partial temperature surging. In this paper, a ‘train-rail’ coupling integrated grounding model is launched to evaluate both the TB current and voltage. Based on this model, different grounding distribution schemes have been tried and compared, considering the impact brought from TB current and voltage. The external grounding resistors are installed in series at the protective grounding wheel sets implemented, with the aim of restricting the TB current and voltage. Ultimately, the best-performed grounding scheme of installing external resistors is found out. [ABSTRACT FROM AUTHOR]

This paper proposes a potential distributed reactive power compensation method in dc traction power systems with reversible substations. It utilizes the entire capacity of reversible substations during night hours and the reminding capacity except for regenerative braking energy feedback during day hours. A unified dispatch scheme is developed to allocate reactive power compensation capacity for each reversible substation. During night hours the entire capacity is used to accomplish the optimal power flow objective. During day hours the excess capacity after active power production is used to improve the system reactive power factor dynamically. The different objectives throughout the day are implemented in a unified formula which avoids manually change. In addition the outage of reversible substations and the low dc voltage conditions are considered and an over-modulation suppression strategy is included in the unified dispatch scheme to adjust the reactive power allocation among reversible substations. This novel utilization method of reversible substations is able to provide significant improvements in the power factor of dc traction power systems which would otherwise require expensive additional equipment such as static synchronous compensator. Finally the effectiveness of the proposed method is verified with MATLAB/SIMULINK. [ABSTRACT FROM AUTHOR]

Energy feedback systems (EFSs) have been widely applied in urban rail transit, and a power flow algorithm and energy-saving evaluation of urban rail power supply systems with EFSs are studied in this paper. First, trains, the reflux system, and traction substations with EFSs are modeled, and the multimode transitions are analyzed, including the maximum power state of the EFS. Then, a modified AC/DC unified power flow algorithm for the DC power supply system, which simplified the DC reflux system is proposed, and a field case study based on a multiple train scenario is presented to validate the algorithm. Furthermore, system-level energy-saving evaluation indexes are applied according to system power flow analysis. When the headway time decreases, the proposed indexes decrease in general mainly as there is more regenerative braking energy absorbed by adjacent trains. The higher the load rates of the step-down substations are, the more daily feedback energy is utilized. When the start voltage of the EFS ranges from 1650 V to 1770 V, the maximum energy-saving rate reaches 11.9%, which occurs when the start voltage is 1670 V. When the no-load voltage is relatively high, the start voltage should not be higher than 1750 V to keep the system running in high efficiency. These results can provide advice for the design and operation of traction power supply systems with EFSs. [ABSTRACT FROM AUTHOR]

The stationary supercapacitor energy storage system (SCESS) is one of effective approaches for the utilization of train's regenerative braking energy in urban rail systems. In this paper, the capacity configuration of SCESSs, the no-load voltage of substation, the control of onboard braking resistors and train operation diagrams are considered comprehensively. Based on the equivalent circuit model, the effects of traction power system parameters on the energy transmission between powering trains, braking trains and SCESSs are analyzed, and the relationships between the train operating parameters and system energy consumptions are revealed. A multi-variable synthetic optimization method is proposed to optimize the SCESS capacity, train operation diagrams and traction power system parameters collaboratively. In order to reduce the search space and improve the efficiency of the optimization algorithm, a hierarchical multi-objective optimization model is established, based on which the design variables and the control variables are optimized iteratively. And as the traffic density has a great impact on the regenerative braking energy distribution of the system, the optimization objective function fully considers the frequency distribution of headways throughout the day. Combined the Elitist Non-dominated Sorting Genetic Algorithm (NSGA-II) with the traction power flow simulator, the flowchart of the two-stage optimization algorithm is designed, and the pareto set of the multi-objective problem is obtained. The advantages of reducing system energy consumptions and configuration cost under the proposed algorithm are verified based on case studies of Beijing Batong Line. [ABSTRACT FROM AUTHOR]

In DC metro systems, the stray current may cause many challenges for the nearby metal structures and the AC power systems, such as the corrosion of the buried pipes and the grounding grids of substations. Therefore, calculating the stray current and analyzing the distribution characteristics of the stray current are necessary for the design of the buried pipelines and substations. To obtain the stray current, considering the through-type structure characteristics of the return current system and the traction power supply system, a multiple power supply sections simulation model (multiple-section model) of stray current is proposed. In the proposed model, the train is equivalent to two dc current sources, and the dynamic results can be completed. Through comparing the rail potentials of simulation results and the field test data of the Shenzhen metro line, the proposed model is verified. And based on the proposed model of the Shenzhen metro line, the stray current distributions are obtained and analyzed. The results indicate that the stray current distribution has a certain periodicity which is approximately equal to the headway time of the train. [ABSTRACT FROM AUTHOR]

Electric traction trains are huge non-linear single-phase loads influencing adversely on power quality parameters on the grid side. Hybrid power quality conditioner (HPQC) has been utilized to compensate current unbalance, harmonics, and low power factor in the co-phase traction system simultaneously. By incrementing the traction load, the rating of the HPQC increases and may constraints its application. In this paper, a C-type filter is designed to compensate for some part of the load reactive power while the HPQC compensates the remaining part of the load reactive power. Hence, the capacity of the HPQC is reduced in full compensation (FC) mode compared to the conventional configuration. The satisfactory performance of the HPQC is associated with its DC-link operating voltage. Therefore, the Genetic algorithm (G.A) is adopted to optimize the DClink voltage performance. Simulation verifications are performed to illustrate the usefulness of the proposed configuration. The simulation results show an 18.86% reduction in the rating of the HPQC with optimized DC-link voltage. [ABSTRACT FROM AUTHOR]

Currently the circuit technology of the DC traction power supply system cannot provide the necessary requirements for introduction of high-speed traffic. Numbers of measures and tools have been developed to improve the traction lines that no longer meet current requirements. One of the most promising means for strengthening the traction DC lines is transition to the distributed power supply of the rolling stock. In this article, a comparative analysis was carried out of energy indicators of the classic centralized power system and distributed power systems with use of one aggregate traction substation and with use of the solar generators. That comparative analysis of these systems was performed on a simulation model with the same parameters of the traction line and rolling stock. [ABSTRACT FROM AUTHOR]

With the rapid development of high-speed and heavy-load electrified railway, the peak impact and the regenerative braking energy content of traction load become increasingly significant, which has become an important problem affecting the construction and operation benefits of electrified railway. On the basis of comprehensively solving the power quality problems of electrified railway, a multi-application strategy based on the railway static power conditioner (RPC) with the energy storage system (ESS) is proposed in this paper to improve the economic benefits of the traction system. Through the implementation of the peak clipping and the recuperation and utilization of regenerative braking energy of traction load with the guidance of such strategy, the comprehensive economic benefits of the traction system can be improved. By analyzing the mathematical relationship between the comprehensive economic benefits and the parameters such as peak clipping rate and regenerative braking energy utilization rate of the system, an optimization model with the goal of maximizing the comprehensive economic benefits is established. Therefore, the optimal configuration capacity of the ESS is obtained. Finally, the experiments are carried out in MATLAB. The results show that, with the multi-application strategy, the peak load clipping and the recuperation and utilization of regenerative braking energy can be simultaneously realized. It is also demonstrated that the comprehensive economic benefits and environmental benefits can be improved while improving energy efficiency. [ABSTRACT FROM AUTHOR]

The full controllable electronic device based railway feeder station offers better power quality and more flexible configurations than conventional transformer based stations. This study investigates a modular multilevel converter (MMC)‐based static frequency converter station with renewable energy access. Wind power generation is coupled into the station via DC link of the back to back converter. The dynamic single‐phase traction load and intermittent renewable generation bring double frequency oscillation and large deviation problems to the DC link voltage. Special design considerations and control schemes are proposed for the MMC to stabilise DC link voltage by controlling the total number of total inserted modules. The proposed control scheme resolves the voltage oscillation issue caused by single‐phase load and reduces the DC link voltage deviation under 10 MW step change. A series of device‐based simulations validate the control scheme which realises a reliable coupling interface for connecting the renewable generation to the DC bus. [ABSTRACT FROM AUTHOR]

In railway traction power supply, co‐phase system with hybrid power quality conditioner (HPQC) is capable of tackling the power quality issues caused by single‐phase traction loads. To further reduce the overall carbon emissions in railway systems, this paper considers to integrate the renewable energy with railway power supply, which however leads to a more complicated system to model, design and control. This paper first investigates its modelling aspect. To reduce the operating capacity of HPQC while addressing the power unbalance, optimal design of the compensation scheme for co‐phase system is formulated as a multi‐objective optimization problem which is then solved by the nondominated sorting genetic algorithm‐II (NSGA‐II). To eliminate the impact of errors arising from imperfect predictions of the loads and renewable power, a hybrid optimal compensation control is proposed, yielding full and optimal compensations. Comprehensive simulation studies, considering three operation modes covering variable traction loads, renewable and regenerative braking power, are conducted. The simulation results confirm the validity of the proposed optimal compensation scheme, achieving an average of more than 20% reduction in HPQC capacity compared to the full compensation scheme. Meanwhile, the power quality requirement is satisfied, even in the presence of real‐time prediction errors. [ABSTRACT FROM AUTHOR]

The access of distributed generations (DGs) in the traction power supply system (TPSS) can not only make use of the renewable energy near the railway route, but also improve the condition of weak external power supply. In order to quantify the effect when DGs access TPSS, this study proposes a method for evaluating the power supply capability and quality of TPSS which considers the access of DGs. First, the method fully considers the randomness and uncertainty of the power provided by DGs and that consumed by electrical locomotives. A comprehensive model of TPSS with the access of DGs is built by means of the stochastic model, and then the index system for evaluating the power supply capability and quality of TPSS is formulated. Finally, the Monte Carlo simulation is applied to calculate the probabilistic load flow to obtain the evaluation indices. The proposed method is verified by simulating a typical TPSS in a remote mountain area. The results show that the method can quantify the power supply capability and quality of the system after the access of DGs. It can provide an intuitive conception of the power supply condition for design and operation personnel. [ABSTRACT FROM AUTHOR]

Back‐to‐back converter based railway traction power supply system (TPSS) can eliminate neutral sections in the traction side and improve power quality in the grid side, but it still has some drawbacks such as low reliability, difficulty in accepting large‐capacity renewable energy, and power mismatches. In this study, a double‐layer DC/AC TPSS with renewable integration is proposed to address these challenges and to improve system performance. The proposed topology breaks the limit of back‐to‐back structure and enables more flexible free energy flow. A top‐down system design method is proposed in this study. Firstly, the characteristics of the proposed TPSS for integration with renewable power are described and compared with the traditional back‐to‐back topology. Secondly, a DC droop controller and a AC droop controller are designed for DC layer grid and AC layer grid, respectively, to control the power flow in each layer. The traditional AC droop control is based on the inductive transmission impedance, but the resistance of traction transmission line cannot be ignored. Thus, a modified droop control strategy with the consideration of line resistance is also proposed in this study. Subsequently, the voltage control strategy for the single modular multilevel converter is designed to track the reference signal from the upper droop controller. Finally, a general double‐layer DC/AC TPSS is designed from bottom to top, and the simulation results confirm that the proposed TPSS with renewable integration is capable of delivering desirable performance. [ABSTRACT FROM AUTHOR]

Electrically powered rail vehicles are considered one of the most energy‐efficient means of transport. Transport activity is responsible for a considerable part of global energy consumption. The transport industry is therefore obliged to develop strategies for more efficient energy use. Railway electrification systems transmit the electrical power supplied by substations to moving trains. However, part of the power supplied is consumed by the electrification systems and does not reach the trains. In this study, transmission losses in the 2 × 25 kV electrification system are evaluated. The 2 × 25 kV system is widely used in high‐speed and high‐capacity railway lines. Assessment of the performance of the 2 × 25 kV system is carried out by computer simulation of the power flows between the elements involved in train movement by means of simplified physical models. The results show that the reduction of transmission losses is a feasible way to achieve energy savings and to improve the energy efficiency in electrified rail transport. For this, it is essential to add the topology of the electrification system to the data considered in the design of the train trajectories. [ABSTRACT FROM AUTHOR]

Simulation tools are crucial to efficiently design the infrastructures and operations of DC electrical railway systems, including potential innovative technologies such as reversible traction power substations and energy storage systems. For this purpose, it is essential to accurately estimate the evolution of the voltage and power flows within the DC traction network, with fast computation time. This study, therefore, proposes a new simulation approach for fast and accurate voltage estimation and power flow analysis of DC railway systems. It is based on the use of non‐linear switched models for traction power substations and trains. The modified nodal analysis is extended to consider such models, including the voltage drop control of the different subsystems, avoiding the necessity to use complex numerical iterative solvers. This new approach is validated and compared to an existing dynamical model and a conventional static model. The comparisons prove the relevance of the new approach, which provides validated and accurate results (<2% error compared to the validated dynamical model) with fast computation time (speed up of 500 compared to the dynamical model). It can, therefore, be used to study, design, size, and optimise DC traction systems with new technologies aimed at saving braking energy. [ABSTRACT FROM AUTHOR]

Single-phase 25kV AC traction power supply system (TPSS) is widely used for electric power feeders in high-speed rail. Modern locomotives draw a large amount of unbalanced, inductive power from two phases of the grid through the traction transformer and lead to severe problems of voltage unbalance and reactive power existence. These problems are mitigated in the promising co-phase TPSS with the equipment of Railway Power Conditioner (RPC) or Hybrid Power Quality Conditioner (HPQC), in which HPQC can operate with a lower voltage and cost. However, HPQC installation involves the adoption of a bulky and expensive coupling transformer. In this paper, a co-phase TPSS employing asymmetric back-to-back modular multilevel converter (MMC) is proposed and it is named asymmetric three-leg hybrid HPQC (ATHPQC). Through the simulation verifications in this research work, the proposed design can effectively remove the additional and costly step-down coupling transformer, which is required by symmetric HPQC. For comprehensiveness of the ATHPQC design, internal circulating current modeling and analysis are investigated. Control system is implemented, including external compensating current control, internal balancing control and PWM module. ATHPQC is a promising alternative TPSS for next-generation high-speed rail. [ABSTRACT FROM AUTHOR]

To improve the comprehensive performance of the metro traction power supply system (MTPSS), a new multipulse integrated metro traction power supply system (MI-MTPSS) is proposed in this article. The configuration and characteristics of MI-MTPSS are introduced, and based on the practical bilateral-ring power supply structure, a series of MTPSS-combinations with different standby equipment have been constructed. Meanwhile, a multiattribute and multidimensional based comprehensive evaluation method is proposed and adopted for evaluating the MTPSS-combinations comprehensively and quantitatively. According to the practical data, the practicability and reliability of the system are verified, and the MI-MTPSS is expected to be used in an industrial application in the future. [ABSTRACT FROM AUTHOR]

This study does research on resonance and harmonic transmission problems in a new continuous cable traction power supply system (CCTPSS), CCTPSS can realise long-distance co-phase power supply and effectively reduce phase separation and is applicable for extended power supply in areas with the weak power grid. Firstly, the system structure is introduced, a cable traction network is divided into three kinds of subnets for its complicated topology and huge system scale. The equivalent circuit model and mathematical model of each subnet are established with distribution parameters, especially the cable capacitance effect be considered. Then, according to the two-port network analysis theory, the system equivalent circuit is constructed by considering the electrical connection relationship, system harmonic impedance expression is calculated. Resonance influence factors and harmonic transmission mechanisms are analysed. Finally, theoretical analysis validity is verified in simulation; harmonic transmission characteristics CCTPSS including traction cable and overhead catenary, are studied. The new system shows better resonance performance compared with the existing AT traction power supply system. [ABSTRACT FROM AUTHOR]

To solve the negative sequence (NS) problem and enhance the regenerative braking energy (RBE) utilisation in an electrified railway, a novel energy storage traction power supply system (ESTPSS) is proposed in this study. In the new system, a power flow controller is adopted to compensate for the NS, and a super‐capacitor energy storage system is applied to absorb and release the RBE. In addition, through the cooperation of each part, the proposed power supply system can provide continuous power without neutral sections. The NS compensation principle of the new system is illustrated. Depending on the operational state of the traction load, the control model is presented for different working patterns, including a regenerative braking pattern, a peak clipping pattern and a valley filling pattern. Moreover, a hierarchical control strategy including upper energy management and low‐layer converter control is proposed, which effectively coordinate switches between the different working patterns. Finally, the validity of the control method and the excellent performance of the system are demonstrated by a case study. The results indicate that compared to a conventional TPSS, the ESTPSS increases the RBE utilisation of the system by 83%. [ABSTRACT FROM AUTHOR]

The present study describes and analyses a set of quasi‐static railway power systems models and simulations considering on‐board and off‐board energy storage systems but also reversible and non‐reversible substations and regenerative braking trains. The advantages and drawbacks of each technology are discussed. Then, several case studies considering different technological combinations are introduced and simulated, while the attained results are compared and analysed. In this regard, to cover all the casuistic, two simple explanatory cases are first exposed and then eight realistic scenarios are examined, which are simulated for a long time interval and also explained considering aggregated results. [ABSTRACT FROM AUTHOR]

Accurate solution of DC ungrounded rail power system load flow requires detailed modelling of the negative path, resulting in large numbers of circuit nodes and branches. The solution of the associate nodal voltage equations not only requires a heavy computational effort, but could also encounter ill numerical conditions. This two‐part study presents a novel methodology utilising the longitudinal configuration of rail traction power networks to derive a ladder‐circuit model and to devise an efficient solution algorithm. This method belongs to the so‐called sequential methods yet goes a step further to divide a DC traction power system into substations and ladder circuits between them. The modelling of individual substations makes it easier to incorporate detailed characteristics of receptive substations and various train load models, while the generic ladder‐circuit topology paves the way for simple and efficient numerical solutions. The proposed load flow solution requires manipulation of small‐dimensional matrices and equations, making it suitable for either manual or computer calculations of the DC portion of the sequential method for the load flow analysis of the entire traction system. [ABSTRACT FROM AUTHOR]

Electrified railway traction power supply system is directly powered by 110 kV or 220 kV (some 330 kV) high-voltage power grid. The uncertainty, non-linearity, and asymmetry of traction loads like the electric locomotives and electric multiple units (EMUs) exert the negative sequence and harmonics impact on the high-voltage transmission network. In order to evaluate the impact of the uncertainty and asymmetry of traction loads on the high-voltage transmission grid, a probabilistic three-phase power flow model of transmission network considering the probability model of traction loads is proposed. First, the power probability model of traction loads is established according to the running characteristics of traction loads. Second, the Monte Carlo simulation method is applied for the probabilistic three-phase power flow model calculation considering the traction loads. Finally, the simulation calculation is carried out on the balanced IEEE-14 bus three-phase test system. The results show that the asymmetric traction loads cause the unbalance of the grid nearby to deteriorate. The further away from the traction substation, the smaller impact of the traction loads.

High-speed railway (HSR) traction transformer, as a connection between traction power supply system and the power grid, is critical in electrified railway design, which requires a drastic change of traction load and frequent external short circuits. In the actual operation process, the traction transformer can produce minor faults that are difficult to detect such as the deformation of the winding, which may result in a serious problem. This study analyses the topology structures and winding fault characteristics of six HSR traction transformers consisting of single phase, Vv, YNd11, Scott, YNvd and impedance matching wiring. Both circuit models of normal operation and fault operation of HSR traction transformer are established. Results show that real-time diagnosis of HSR traction transformer can be accomplished through measuring the voltage signal of the input and output points during operation and establishing real-time monitoring statistics of small faults standardisation（I^2 and squared prediction error）based on fast independent component analysis algorithm, as well as the signal statistics distribution, thereby improving the safety and reliability of HSR operation.

New energy storage vehicles which use supercapacitors (SCs) as the unique source of traction power are developing rapidly recently. To satisfy the operating voltage requirement of energy storage vehicle, hundreds of SCs are needed to be connected in series. In this case, there is a voltage imbalance between the energy storage cells during rapid charging, resulting in reduced energy efficiency and service life of the energy storage device. A layered equalisation scheme is proposed to solve the problem of voltage imbalance. A single-input multiple-output synchronous rectification equalising circuit is proposed to ensure voltage consistency of series supercapacitor monomer in bottom module. The proposed circuit resolves the existing slow balancing speed and low-voltage balancing precision problems in the current supercapacitor equalising circuits. Meanwhile, an equalisation strategy based on module SOC is adopted to solve voltage imbalance problem of the upper supercapacitor module. The results of MATLAB/Saber co-simulation and experiment demonstrate that the layered equalisation system achieves the target of fast equilibrium and high precision balance.

Leakage currents from DC traction power supply system of metro will reduce the lifetime of the buried pipeline. This study illustrates a real contribution in the analysis of dynamic stray current and the evaluation of the corrosion status of the buried pipeline. The metro locomotives dynamic operation process was established through traction calculation, and then analysed the dynamic influence of stray current distribution under transient finite element model. Finally, a comparison of corrosion influence of stray current is also presented. According to the simulation results, the distribution of stray current is affected by many factors, the relative position of the cathodic protection pipeline and unprotected pipeline has significant influence on stray current.

The windings of a traction transformer are always be threatened by the fluctuation of the load. Frequency response analysis (FRA) is regarded as the most effective method to diagnose the winding deformation. Appropriate FRA modelling method may help to predict typical winding faults for large transformers. The existing model usually simplify dozens of winding disks into several disks, it is applicable when analysing small displacement. However, when the disks between windings are seriously misplaced, the conventional model may not be suitable. This study presents an improved lumped parameter circuit model considering intersection capacitances and mutual inductance between windings. The feasibility of the proposed model is verified based on a 10 kV test transformer. Finally, the model is applied for a 220 kV traction transformer, the variation of parameters and features are analysed for axial displacement of different windings. The results show that the axial displacement may lead to increase of resonance frequency and decrease of amplitude. The low‐frequency band (1–30 kHz) may be regarded as visible features for diagnosing axial displacement of H winding and the middle frequency band (50–200 kHz) may be a characteristic band for diagnosing axial displacement of T winding. [ABSTRACT FROM AUTHOR]