| With the input of a large number of power electronic devices,the electrical coupling problem of electric multiple units(EMUs)-traction network cascade system(ETNCS)is becoming more and more serious.Among them,the low-frequency oscillation(LFO)and traction blockade caused by multiple EMUs connected to the traction network at the same time seriously affect the organization of train operation.To solve this problem,taking ETNCS as the research object,impedance ratio analysis as the research method,and improving the stability of ETNCS as the research goal,a LFO suppression method based on on board energy storage and virtual synchronous machine control technology is proposed.According to the research idea of "stability analysis of ETNCS→mechanism analysis of LFO→the recurrence of LFO→active damping suppression measures based on on board energy storage→damping compensation based on virtual synchronous machine control",the LFO problem and suppression measures are deeply studied:Firstly,the impedance model of ETNCS is established based on impedance analysis method.Taking CRH3 rectifier as the research object,based on the existing research,the DC side secondary filter and equivalent load are considered,and the accurate impedance modeling method of traction rectifier is proposed.Based on the impedance ratio criterion,the critical stability condition of the ETNCS is analyzed by Bode diagram,and the LFO phenomenon is reproduced by simulation.In order to study the mechanism of LFO,taking CRH5 rectifier as the research object,the paper deduces the impedance model under dq system,and reveals that the voltage loop PI controller introduces negative damping into the ETNCS in the low frequency range,thus it is clear that the insufficient damping is the important reason for the LFO,and provides theoretical basis and research ideas for the suppression measures.Secondly,based on the mechanism of LFO,an active damping suppression method based on on board energy storage is proposed.The energy storage medium is connected with the DC side of the rectifier through the DC/DC converter,and the voltage fluctuation is suppressed by outputting the energy opposite to the DC side oscillation power.Due to the lack of inertia in traditional PI control strategy of DC/DC converter,the improvement of DC voltage stability is limited.A control strategy based on virtual synchronous DC motor(VDCM)is proposed.The key parameters such as virtual moment of inertia and damping coefficient are designed by combining small signal and root locus analysis.Finally,the impedance ratio stability analysis of the rectifier based on on board energy storage verifies that the proposed method not only has superior dynamic performance,but also enhances the system stability.Thirdly,based on the damping compensation of ETNCS,the control strategy of virtual synchronous machine for traction rectifier is proposed.By introducing virtual moment of inertia and damping coefficient,the characteristics of synchronous machine are simulated to provide support for traction network voltage and frequency.A stability modeling analysis of virtual synchronous machine control technology based on harmonic state space(HSS)model is proposed,which is linearized near the steady-state working point,and the equivalent impedance of traction rectifier in frequency domain is derived and drawn.Based on the stability analysis of impedance ratio,it is verified that the proposed control strategy not only compensates the damping of the system,but also enhances the anti-interference characteristics of the system in the low frequency band,thus effectively suppressing the LFO.Finally,by building Matlab/Simulink of ETNCS and d SPACE low-power experimental platform,the proposed control strategy and suppression measures are simulated and experimentally verified,and the effectiveness of the proposed suppression measures is verified. |
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