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

An active power compensator (APC) based on single-phase back-to-back power converter is proposed in this paper to solve problems of power quality of electric railway power supply system. This system adopts a single-phase feeding connection, which is called cophase power supply scheme. In this scheme, APC connects the balance transformer between feeding phase for power supply and another phase for compensation. It has some characteristics, such as active power balancing, reactive power compensating, and harmonics filtering. In order to achieve these characteristics, the control scheme requires seven combination models. In this paper, a multifunctional control algorithm is proposed to realize every conceivable model. A cophase system with APC based on field programmable gate array (FPGA) and YNvd balance transformer is also designed and evaluated. The experimental results obtained from this prototype illustrate that the compensating ability is extremely high in steady-state and dynamic responses, and the power quality of a substation with distorted loads can be improved integrally.

Single-Phase Back-To-Back Converter for Active Power Balancing, Reactive Power Compensation, and Harmonic Filtering in Traction Power System

A multi-purpose balanced (MPB) transformer is proposed in this paper for electric railway applications. The MPB transformer is developed from impedance-matching (IM) transformers widely used in China's railway systems. The proposed MPB transformer has three main functions: (1) it provides a symmetrical two-phase voltage of 27.5 kV for railway overhead lines to power electric trains; (2) it supplies a three-phase balanced voltage of 10.5 kV for railway substations; and (3) it can accommodate a three-phase harmonic filter with improved power factor and reduced manufacturing cost. In this paper, the operating principles of the MPB transformer are elaborated, the design of the harmonic filters is discussed, and the results of computer simulations are provided. A 16-MVA MPB transformer was designed and commissioned, and real-time field measurements are provided.

A Multi-Purpose Balanced Transformer for Railway Traction Applications

This paper presents a general filtering and unbalance compensation scheme for electric traction systems using a direct power control-based algorithm. For a balanced three-phase three-wire system, the proposed method is able to control the power flow exchange between the grid and the load so that the instantaneous complex power is maintained constant. As a consequence, any nonlinear unbalanced load is seen by the three-phase supply as a balanced linear load. The proposed filter is evaluated on power substations with open delta (V-V) and Scott transformer feeders, and for two-level and dual-converter in the power stage. The scheme has been simulated and experimentally validated. The results from experimental and simulation tests show the controller advantages and the applicability of the proposed method in railway systems.

http://prof.usb.ve/jaller/PPI_papers/TPEL2013.pdf

Harmonic and Unbalance Compensation Based on Direct Power Control for Electric Railway Systems

In this paper, the behavior of various critical parameters of transformer oil is interpreted with a view to enhancing its life time. Systems abnormalities such as excess loading, switching, and ambient conditions are responsible for the accelerated aging and sudden failure of transformer. The level of insulation plays a significant role in the life expectancy of the transformer. The diagnostics of the critical parameters for the defined ranges of aging of transformer oil attempts to determine the level of insulation and serve to ensure its efficient operation. The approach includes formulation of suggestions for determining the insulation level of the power transformer. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Investigations of transformer oil characteristics

In a transformer, the harmonic model is essential for the filter design and analysis of harmonic power flow. Among all of the harmonic analyses of nonlinear devices, the core nonlinearity caused by the transformer hysteresis has received the most interest. Although many harmonic models have been proposed to interpret the nonlinearity of the transformer core, most of them require complex calculations. In this study, we derive the mathematical formulas of the exciting current for a single-phase transformer by the describing function methods. The mathematical expression allows us to obtain the Fourier series for the exciting current; and, then, it provides the various order harmonic magnitudes and phase angles of the exciting current. This work also presents a mathematical model characterized by simple calculation and with high precision for the harmonics of exciting current. The transformer is under no load condition. Actual measuring verifies the effectiveness of the proposed model.

A harmonic model for the nonlinearities of single-phase transformer with describing functions

The harmonic current of the Le Blanc transformer of the Taiwan railway system is measured to determine how it reduces the power supply capacity to the trolley (nonlinear load). Improvements to the harmonic current in point-of-common-coupling (PCC) after adding the double-tuned filter to the load side, are also described.

Harmonic Study of the le Blanc Transformer for the Taiwan Railway Electrification System

The western Taiwan railway transportation system consists mainly of a mountain route and ocean route. The Taiwan Railway Administration (TRA) has conducted a series of experiments on the ocean route in recent years to identify the possible causes of unknown events which cause the trolley contact wires to melt down frequently. The conducted tests include the short circuit fault test within the power supply zone of the Ho Long substation (Zhu Nan to Tong Xiao) that had the highest probability for the melt down events. Those test results based on the actual measured maximum short circuit current provide a valuable reference for TRA when comparing against the said events. The Le Blanc transformer is the main transformer of the Taiwan railway electrification system. The Le Blanc transformer mainly transforms the Taiwan Power Company (TPC) generated three phase alternating power supply system (69 kV, 60 Hz) into a two single-phase alternating power distribution system (M phase and T phase) (26 kV, 60 Hz) needed for the trolley traction. As a unique winding connection transformer, the conventional software for fault analysis will not be able to simulate its internal current and phase difference between each phase currents. Therefore, besides extracts of the short circuit test results, this work presents a EMTP model based on a Taiwan Railway substation equivalent circuit model with the Le Blanc transformer. The proposed circuit model can simulate the same short circuit test to verify the actual fault current and accuracy of the equivalent circuit model. Moreover, the maximum short circuit current is further evaluated with reference to the proposed equivalent circuit. Preliminary inspection of trolley contact wire reveals the possible causes of melt down events based on the simulation results.

Bing-Nan Chen

Papers

A harmonic model for the nonlinearities of single-phase transformer with describing functions

Harmonic Study of the le Blanc Transformer for the Taiwan Railway Electrification System

A Short-Circuit Current Study for the Power Supply System of Taiwan Railway