A new hybrid simulation integrating transient-state and steady-state models for the analysis of reversible DC traction power systems

Highlights

• A reversible traction power system based on four-quadrant converters is introduced.

• A new hybrid simulation method is proposed.

• The simulation integrates train motion, steady-state and transient-state models.

• It is able to evaluate both the system-level and equipment-level performance.

Abstract

This paper presents a new hybrid simulation method for reversible traction power supply systems (RTPSSs) supplied by four-quadrant converters (4QC). RTPSSs overcome several drawbacks of conventional DC electrification systems, such as wasting of braking energy, output voltage fluctuation, and low power factor at light load conditions. The paper introduces the system topology of a RTPSS and illustrates its advantages over standard DC systems. A hybrid simulation method is proposed to study the performance of the RTPSS taking into account the dynamics of multiple trains. This hybrid simulation integrates train motion, steady-state and transient-state models, and is able to evaluate both the system-level steady-state performance (voltage, current, power and energy) and the equipment-level transient-state performance (harmonics, ripple, power factor and stability). Structure and signals flow of this hybrid simulation system have been introduced, and the methodology of modeling has been presented. The validity and applicability of the proposed hybrid simulation method have been proved by a case study of a 10 km railway line with 6 substations. To date, no other work has accomplished the steady-state and transient-state study using a single integrated simulation.

Introduction

In a Traction Power Supply System (TPSS), several traction substations are powered by one or two main substations through three-phase medium voltage distribution networks (MVDN). The traction substations include transformers, diode rectifiers and switchgears. The merits of traditional TPSS are simplicity, robustness and low-cost. However, some obvious drawbacks exist:

• waste of regenerative braking energy. Especially when a small number of trains travel on the line, the regenerative braking energy cannot be reused within the DC network. It is usually dissipated by on-board braking resistors, causing temperature rise in tunnels and at the stations.

• fluctuation of DC voltage. Because of the unregulated characteristic of diode rectifiers, the output voltage has large fluctuations with variable loads.

• low power factor problem at AC grid interface. Studies indicate that it is usually caused by the equivalent capacitance of large amount of grid cables [1], [2].

Some solutions have been proposed to overcome the above-mentioned drawbacks. In [3], [4], [5], thyristor-based inverters are introduced to feed the regenerative braking energy back to the MVDN. With the development of power semiconductors and converter control technology, IGBT inverters have been increasingly employed in TPSSs [6], [7], [8], [9], [10], [11], [12], which allows not only energy recovery, but also active power filtering [13], [14], [15], [16]. Traction substations with IGBT inverters are called inverting substations or reversible substations. In order to reduce the voltage fluctuation of the DC network and improve the energy efficiency, the conventional diode rectifier is replaced by the controlled rectifiers in [8], [9], [17]. The installation of storage devices (such as super-capacitors or flywheels) [18], [19], [20], [21], [22], [23], [24], [25] at substations or tracksides could be a good way to absorb the surplus regenerated energy and regulate DC voltage. However, compared with inverters, storage devices require more installation space, higher cost, and more safety constraints [17]. To improve power factor under light load condition, the traditional schemes based on Static Var Compensator (SVC) and Static Var Generator (SVG) are usually adopted in the TPSS [1], [2], [26], [27], [28]. The total capacity of SVGs in a typical line is normally several MVars.

The above solutions lead to an increase of equipment cost, system complications and increase of maintenance cost. Considering the excellent characteristics and fast development of Four-Quadrant Converters (4QC) [29], [30], a reversible traction power supply system (RTPSS) equipped with pure high-power 4QC is recommended in this paper.

A simulation approach is indispensable to study and assess the performance of the RTPSS. According to the time duration and objectives of the simulations, three categories of models can be considered for the study of RTPSS: multi-train motion model, steady-state model (average static model) and transient-state model (instantaneous dynamic model) [31], [32]. Multi-train motion simulation is used to model train movement and traction power requirements [33]. Trains require different traction power while they travel along the power supply network. Kinetic train motion, track alignment, and operation timetable are usually considered in formulating the multi-train operation simulations [34]. Based on the steady-state mathematical model and iterative calculations, the steady-state simulation can be used to analyze power flows [20], [35], [36], evaluate energy consumption [31], [32], [33] and design the power supply system [4], [37]. Steady-state simulations do not depend on specific software environment, and no control loops are required in the model. Moreover, the time step of steady-state simulations is relatively long, typically T_s = 1 s [38], [39], [40]. Steady-state simulations provide a global overview of the system during a whole day with short computational time [20]. Transient-state simulations study the control and dynamic performance during a shorter time step, typically 20 ms or less). Transient-state simulations, which are based on the electrical circuit model and closed-loop control, can be used instead to verify control algorithms, evaluate power quality, and analyze system stability. Transient-state simulations usually rely on the specific simulation software or tools, such as Simulink, Psim, and Pspice. Transient-state simulations are mainly used in equipment-level simulations [41], [42] or occasionally in short-circuit simulations [43].

In most previous studies, train motion simulation, steady-state simulation and transient-state simulation are carried out separately. The study of multi-train motion has not considered the capacity of TPSS. The study of steady-state power flow has not considered the response ability of the control loops and cannot provide information on instantaneous waveforms, harmonic content, ripple and so on. The study of the transient-state simulation usually focuses on limited devices during short time, which cannot be used to study the system performance. Therefore, the previous studies lead to inefficiency on the system study and performance evaluation. Firstly, it is time-consuming to build these three simulation models on different software environment and platforms. Secondly, it has not realized data interaction and sharing between different simulations. In this paper, a hybrid simulation method integrating steady-state and transient-state models is proposed to achieve a combination of train motion, steady-state and transient-state simulation. It can be used as an efficient tool for the study of the power supply system based on controllable converters and accomplish the verification and assessment of the system performance with consideration of multi-train operations.

This paper is structured as follows; the Section 2 introduces the system schematic and advantages of the RTPSS, as well as the topology of the reversible traction substation. In Section 3, the structure of the hybrid simulation system is proposed, and the modeling of train motion, steady-state and transient-state are illustrated. In Section 4, a case study is carried out to verify the function and validity of the proposed hybrid simulation method. The conclusions are presented in Section 5.