Since the beginning of railway electrification, power quality has been a main problem in railway networks because of their special characteristics. Many ways of power quality improvement have been investigated and applied to ac and dc traction systems through railway electrification history. This paper presents a perspective on power quality issues through railway electrification development and investigates the necessity of power quality and system requirements for appropriate power quality. Compensation strategies are classified and compared. This aims to provide a comprehensive perspective of the power quality issue in railway power/distribution networks for researchers and engineers working on railway electrification. Less than 100 research articles are referenced for researchers to obtain good background information.

Power Quality Issues in Railway Electrification: A Comprehensive Perspective

In recent years, evidence has suggested that the global energy system is on the verge of a drastic revolution. The evolutionary development in power electronic technologies, the emergence of high-performance energy storage devices, and the ever-increasing penetration of renewable energy sources (RESs) are commonly recognized as the major driving forces of the revolution. The explosion in consumer electronics is also powering this change. In this context, dc power distribution technologies have made a comeback and keep gaining a commendable increase in research interest and industrial applications. In addition, the concept of flexible and smart distribution has also been proposed, which tends to exploit distributed generation and pack together the distributed RESs and local electrical loads as an independent and self-sustainable entity, namely a microgrid. At present, research in the area of dc microgrids has investigated and developed a series of advanced methods in control, management, and objective-oriented optimization that would establish the technical interface enabling future applications in multiple industrial areas, such as smart buildings, electric vehicles, aerospace/aircraft power systems, and maritime power systems.

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Next-Generation Shipboard DC Power System: Introduction Smart Grid and dc Microgrid Technologies into Maritime Electrical Netowrks

The power converters based on a modular structure have gained importance for electric transportation applications, including electric vehicles (EVs), EV charging stations, railway traction, and electric ships. Modular multilevel converters (MMCs) are recognized as one of the promising topologies to improve the energy conversion efficiency and fault-tolerant ability of power conversion systems. This paper aims at a review of latest research contributions regarding the evolution of circuit topologies, operational challenges, control schemes, and fault-tolerant strategies of the MMC. Finally, the current state of the art, opportunities and the future perspective of MMC in electric transportation applications are addressed.

Modular Multilevel Converters for Transportation Electrification: Challenges and Opportunities

Model predictive control (MPC) has a number of desirable attributes which are difficult to achieve with classical converter control techniques. Unfortunately, the nature of power electronics imposes restriction to the method, as a result of the limited number of available converter states. This, combined with the spread spectrum nature of harmonics inherent with the strategy, complicates further design. This paper presents a method for removing this characteristic without compromising the desirable functionality of predictive control. The method, named modulated MPC, is applied to a two-level three-phase converter and compared with a number of similar approaches. Experimental results are used to support theoretical analysis and simulation studies.

/pdf/modulated-model-predictive-control-for-a-three-phase-active-2deyp01mjf.pdf

Modulated Model Predictive Control for a Three-Phase Active Rectifier

This paper presents a predictive control scheme that is suitable for the torque and flux control of multilevel inverter-fed induction machines. The control strategy combines the use of a proportional-integral controller to obtain good steady-state behavior and a predictive controller to achieve fast dynamic torque response. In this way, torque and stator flux references can be reached within one sample period. With the use of multilevel space phasor modulation, low torque and flux ripple are possible with fixed sample rate. Experimental and simulation results are presented in order to demonstrate the effectiveness of the proposed strategy

Predictive Torque Control for Inverter-Fed Induction Machines

Low-frequency stability is a major concern in 16.7Hz railway grids. Using state-of-the-art railway control concepts low-frequency oscillations in the range of 2–5 Hz are experienced, well documented but not finally solved in several publications. In this paper an advanced multivariable control concept is demonstrated. Besides achieving very good dynamic behavior, the approach improves the introduced stability problem. The proper operation of the control is verified using measurement results. Furthermore the paper includes a time constant analysis of railway grids and elementary assumptions for stability investigations. A state-of-the-art control is compared to the advanced multivariable control. The comparison leads to the essential difference partly causing the low frequency stability problem. Finally a time domain simulation verifies the stability problem caused by the state-of-the-art control and shows the improved performance of the multivariable control.

Improvement of low-frequency railway power system stability using an advanced multivariable control concept

Indirect Stator-Quantities Control (ISC) combines the principle of stator-flux-orientation proven successful in Direct Self Control (DSC) with Pulse-Width Modulation (PWM). High torque dynamics and robust behaviour against input voltage disturbance are achieved in the whole operation range including field-weakening. After careful correction of inverter-voltage errors and dc portions in the stator fluxes and currents not necessary for the actual operation point the difference of the stator-current space vectors in the machine and in the controlled model is employed for speed and stator resistance estimation, allowing to dispense with speed sensors. A special flux management allows infinitely slow change between driving and braking.

Speed-sensorless stator-flux-oriented control of induction motor drives in traction

Recent AC railway traction vehicles contain two main power electronic components in the power train, the four-quadrant converter and the three-phase machine-side inverter. They are joined by a common DC-link capacitor and - often - a resonant circuit tuned to twice the supply frequency. Converter and inverter are mainly controlled separately. The overall performance of the traction vehicle can be improved considerably by an integrated control of converter and inverter, explicitly taking into account the dynamics of the DC-link filter. This requires, however, enhanced control and simulation effort. The simulation has to deal with a hybrid system, described on the one hand by continuous differential equations and on the other hand by converter switching functions. The continuous part of the system is described by state-space equations solved by the Bulirsch-Stoer implementation of the Richardson extrapolation, the switchings are modeled using Petri Nets. This combination allows high-speed simulations of the complete system at suitable precision. For verification the simulation results are compared to measurements made at a 50-kW laboratory test set-up of the complete power train. At this early stage of the project, simple control schemes are used for first tests of simulation and test set-up. Measured and simulated quantities are in very good agreement, allowing to proceed to more sophisticated control schemes.

Advanced simulation concept for the power train of an AC locomotive and its verification

The control of four-quadrant converters (4QC) of traction vehicles fed by an ac railway grid has to meet very restrictive limitations with regard to harmonics and stability. The challenge is increased by continuously changing the position-dependent grid impedance and voltage harmonics already contained in the grid voltage. Testing a 4QC control in a lab, therefore, requires a railway grid representation that both emulates variable grid impedance and grid-voltage harmonics. The paper presents an innovative solution to this problem that employs a 4Q inverter with a suitably designed new control scheme. As the rated power is not high, compared to the traction vehicle fed, the control has to be effective, taking each switching event of the inverter into account. For the design of the control, a suitable simulation is needed, which is also presented in this paper. A comparison of simulation and measurement results is given, proving excellent results.

Dead-Beat Control Algorithm for Single-Phase 50-kW AC Railway Grid Representation

Today the development process for power electronics includes modeling, simulation, control design, implementation in hardware and finally the test of the hardware at a suitable test bench. In case of drive systems, changing to a different type of machine or to a different rating of the machine requires only relatively few modifications of the control and its parameters. Nevertheless, at least the prototype has to be tested under conditions close to reality. In many cases even the test of each converter and its control is required before shipping to the customer.

Machine emulator: Power-electronics based test equipment for testing high-power drive converters

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