The line-side converter, which can represent an ac–dc–ac drive electric vehicle when analyzing the low-frequency oscillation in electric railways, is modeled in dq frame with the proposed single-phase dq -decomposition method. With this model, an underdamped mechanism of the vehicle-grid interaction is confirmed. Besides, a low-frequency stability prediction approach is presented, based on the dominant-pole analysis and verified by the simulation and the testing result. The proposed modeling method and prediction approach benefit in three aspects: It is more convenient to rebuild the model with a change of either the line-side converter controller or the system condition, much quicker to predict the low-frequency instability and more explicit to guide the modification of the controller software to damp the low-frequency oscillations.

Analysis of Low-Frequency Oscillation in Electric Railways Based on Small-Signal Modeling of Vehicle-Grid System in dq Frame

The voltage low frequency oscillation (LFO), between traction network and electric locomotives or electric multiple units in high speed railway, is a typical phenomenon happening in cascade systems with power electronic converters accessed, which is regarded as the stability problem of a vehicle-grid (V2G) system in general. To address the problem, the mathematical model of the V2G system is deduced in detail first, and a generalized dq -decoupled small-signal model of a single-phase pulse width modulation rectifier is established in this process. Then, a novel stability criterion based on forbidden region for dq-decoupled multi-input multi-output cascade systems is proposed to analyze the stability of the V2G system. The proposed criterion has less conservatism than singular-value criteria and norm criteria, and can be applied for systems with either high or low power factor. The comparison with other criteria and simulation results verifies that the proposed forbidden region-based criterion performs well in the analysis of the critical condition of LFO. Moreover, the impact factors of LFO, both grid parameters and vehicle parameters, are researched based on the proposed stability analysis criterion and the simulation verification, which provides design guidance for the V2G system to restrain LFO.

Vehicle-Grid System Modeling and Stability Analysis With Forbidden Region-Based Criterion

Due to the low-frequency oscillation (LFO) between traction network and electric multiple units (EMUs) of high-speed railway, many traction blockades occur in different EMU depots. Aiming at this problem, the mathematical model and small-signal analysis model of EMUs-traction network cascade system (ETNCS) are built in this paper. The system stability mainly affected by the electrical coupling between source and load module is analyzed and discussed in detail. Considering the autotransformer leakage reactance, traction network impedance, and distributed capacitance, the output source module resistance of ETNCS is calculated based on the generalized symmetrical component method. Using the generalized Nyquist criterion and Bode diagram, the ETNCS's stability is analyzed and the quantitative indicators of system impedance matching are given. Then, the simulation model of full ETNCS is built in this paper, which can reproduce the LFO phenomenon. In addition, the theoretical analysis results and the feasibility of inhibition for LFO are verified based on MATLAB/simulink simulation. Finally, the LFO signal modal analysis is implemented by using Prony and estimating signal parameters via rotational invariance techniques (ESPRIT) methods. The conclusions show that the proposed stability and modal analysis methods could be used not only to analyze the LFO more clearly, but also to provide the technical reference for the stable operation of EMUs.

Stability Research of High-Speed Railway EMUs and Traction Network Cascade System Considering Impedance Matching

The traction blockade in the depots of multiple electric multiple units (EMUs) is caused by the low-frequency oscillation (LFO) of high-speed railway traction network overvoltage. To suppress LFO, a strategy for optimizing the load characteristics of EMUs rectifier using the auto-disturbance rejection control (ADRC) is put forward. First, a first-order mathematical model is established to describe the characteristics of EMUs rectifier. Second, the nonlinear state observer and feedback control law are designed for state observation and disturbance estimation. By constructing a new nonlinear state-error feedback function, the ADRC is improved. This function’s convergence effect is better than the linear state-error feedback function, and it can solve the problem of the jitter problem of control force to some extent. Third, the outer-loop controller for pulsewidth modulation (PWM) rectifier voltage is designed. The improved ADRC can effectively deal with the contradiction between overshoot and rapidity. Its performance is better than the traditional proportional integral (PI) controller, which is applied in all of the EMUs at present. Finally, the full model of EMUs-traction network electrical coupling system (ETNECS) is constructed. Compared with multivariable controller, the simulation results show that the improved ADRC can ensure the ETNECS stability and suppress the LFO more efficiently.

Suppression of Low-Frequency Oscillation in Traction Network of High-Speed Railway Based on Auto-Disturbance Rejection Control

In recent years, low-frequency oscillation (LFO) events sometimes occur between the traction network and the electric motive units (EMUs) in vehicle-grid systems of a high-speed railway when multiple EMUs access the traction network. Some measures have been taken from the vehicle side to suppress LFO by changing the control parameters or control strategies of the converters. This paper presents a proposed scheme of suppressing LFO by applying a static synchronous compensator (STATCOM) to the traction network. The stability of the vehicle-grid system is first analyzed using the norm criterion. Next, for unstable vehicle-grid systems, the approach of utilizing a modular multilevel converter STATCOM (MMC-STATCOM) connected to the grid is proposed to suppress the LFO. Further, when the MMC-STATCOM is connected to the grid, the parameter sensitivity analysis is implemented in detail; the analysis results indicate that the MMC-STATCOM-based approach operates well in suppressing LFO, regardless of the change of the system parameters. The simulation results validate the stability analysis and the efficient suppression of LFO for the vehicle-grid system.

Vehicle-Grid System Stability Analysis Based on Norm Criterion and Suppression of Low-Frequency Oscillation With MMC-STATCOM

In AC rail networks, the new locomotives using four-quadrant rectifiers are responsible for low frequency disturbances on overhead line voltage. In recent years, around the world, interactions between locomotives and power supply have been observed. In some sectors of the 25kV-50Hz French rail network, serious disturbances have affected the rail system operation while flicker was observed on the upstream electrical network. To prevent such dysfunctions, an analysis method is proposed. Any locomotive or multiple-unit train is characterized as an admittance matrix by using a Voltage Source Converter. After that, a stability study can be performed on each critical area with a view to determine the acceptable number of locomotives equipped with active front-end rectifiers.

Measurement of locomotive input admittance to analyse low frequency instability on AC rail networks

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Measurement of locomotive input admittance to analyse low frequency instability on AC rail networks