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Railway traction power system (TPS) impedance affects the stability and operation performance of connected power electronics devices (i.e., electric trains) with the closed-loop control system. In order to identify the detailed TPS impedance, the wideband-response-based impedance measurement technique has become attractive for its fast execution time and accuracy, especially the chirp-based technique for its bilateral controllability of injected wideband disturbance. However, for the reason that the disturbance device is directly powered by the measured system, the fundamental component separates some harmonic clusters in the frequency domain and leads to unsatisfactory uniformity of the injected wideband disturbance. In this article, a segmented-chirp-based technique is proposed to eliminate this phenomenon caused by the fundamental component, where the chirp signal is segmented and carried on the fundamental only when the amplitude is relatively smooth on top (or bottom), which facilitates a uniform wideband distribution to keep a good measurement accuracy. Meanwhile, the presented modular disturbance circuit with a flexible topology can better adapt to the various industrial engineering conditions. Finally, the detailed measurement results based on the simulation and downscaled experimental system validate the effectiveness of the proposed measurement scheme.

Wideband impedances of the single-phase four-quadrant converter (4QC)-based high-speed train are of great significance for small-signal stability analysis in the electric railway system. However, the conventional impedance measurement strategy relies on sweeping the desired frequencies repeatedly that extremely slows down the impedance measurement speed, which, usually, seems incapable for the actual dynamic industrial systems. Addressing this shortcoming, this article proposes a novel impedance measurement approach based on the wideband excitation to greatly enhance its rapidity. Further, to eliminate the frequency overlapping effects under the wideband excitation, the proposed alternate injection strategy realized by the multitone signal can separate the effective frequencies from the coupling frequencies in the $\alpha \beta $ frame. Afterward, the dq -frame data covering all the desired frequencies can be obtained by combining the conventional method and the proposed $\alpha \beta $ -value exchange method ( $\alpha \beta $ -VEM), which can be applied for the impedance calculation. Finally, the results of both simulation and real-time hardware-in-the-loop (HIL) tests can well verify the effectiveness of the proposed approach.

The accurate information of the wide-bandwidth impedance versus the frequency is urgently needed for evaluating the system resonances, instabilities, and operations of the railway traction power system (TPS), and to avoid/control the harmonic resonance and oscillation issues. As the system topology and detailed parameters of the TPS are not fully known even timely varying, we have to obtain the detailed wide-bandwidth impedance information through exciting the harmonic disturbance into the system, and then, calculating the response information. Therefore, a controlled wide-bandwidth impedance measurement approach is presented in this paper, in which, a butterfly-type disturbance circuit and chirp pulsewidth modulation signal model are incorporated to generate the desired controlled-bandwidth harmonics with a high aggregation as well as the average amplitude. Impedance measurement results of the proposed approach have been validated through both simulation and experiment. Considering the measured errors, the proposed method is efficient in testing the wide-bandwidth impedance of the single-phase railway traction system.

In the ac grid-converter system, the mismatching between the load-impedance and source-impedance have caused many instability issues, such as low-frequency oscillation, harmonic instability and sub-synchronous oscillation. Therefore, the individual impedance specification of every subsystem is very important to avoid impedance mismatching. In this paper, a typical impedance model and of the single-phase ac grid-converter is built in modified sequence-domain. And then, based on the forbidden-region-based criterion (FRBC), a novel impedance specification of the ac grid-converter system is proposed, which provides a sufficient condition for system stability and guide the individual impedance design of the source and load. Compared with the existing impedance specification of the ac grid-converter system, the proposed impedance specification is simpler and has less conservatism. Simulation results verify the effectiveness and the correctness of proposed impedance specification for the ac grid-converter system.

Electric railway systems occur instability and oscillation frequently for the mismatched impedances between the traction power system (TPS) and the voltage-source-converter (VSC)-based electric train. However, the parameters of both TPS and electric train (hereinafter TPS-train) are not fully known so that utilizing the mathematical deduction to quantify the impedances is difficult. This paper gives a method to measure the impedances at controlled frequency band. A chirp-PWM signal is introduced and utilized to drive the insulated gate bipolar transistor (IGBT) modules of disturbance circuit, which can produce a desired broad spectral excitation at controlled frequency band. The impedances can be then calculated with the corresponding response voltages and currents after connecting the disturbance to the tested system. Then, the stability and oscillations can be identified using the measured TPS and electric train impedances. The proposed method is easy to implement with its simple hardware structure and software control, and also it gives a high measurement accuracy. Simulations verify the effectiveness of this method.