Multi-frequency Dq Impedance Measurement Method And Electrical Stability Analysis Of Vehicle-grid System

Dq impedance plays a significant role in impedance-based small signal stability analysis.A series of electrical stability problems will be caused by various mismatching parameters of the single-phase vehicle-grid cascade system in electrified railways.However,due to confidentiality,it is not easy to obtain all the specific parameters of the single-phase vehicle-grid system,thus making it challenging to analyze the impedance characteristics of the system through detailed modeling.Moreover,the single-phase vehicle-grid system’s traditional dq impedance measurement method has many constraints with low efficiency.A multi-frequency dq impedance measurement method was proposed for the single-phase vehicle-grid system to solve the above problems.The influence of each parameter in the vehicle-grid cascade system on the electrical stability trend was analyzed using this method combined with the maximum peak criteria.Firstly,the single-phase vehicle-grid system is described,and the dq impedance models of three equivalent traction networks and single-phase converter on the vehicle side are given.Before the algorithm is put forward,three multi-frequency disturbances are defined,the pseudo-random binary sequence and ternary sequence are modified as PWM driving signals.The disturbances of specific frequency characteristics are introduced into the system to be tested through the switching behavior of the disturbance circuit,to ensure that the measurement is carried out according to the principle of the black-box model,and the system normally runs in the measurement process without the phenomenon of voltage and current un-synchronization.Secondly,a multi-frequency dq impedance measurement algorithm is proposed for the single-phase vehicle-grid system objects.The algorithm can accurately calculate the dq impedance of each measurement frequency point in a short measurement period.Both simulation and experimental results are provided to validate and confirm the effectiveness of the proposed algorithm.The most significant advantage of this algorithm is that it changes the way that the dq impedance measurement of the single-phase system needs to inject disturbance at a specific frequency in a cycle,which dramatically reduces the measurement process time and improves the measurement efficiency.The final measurement results can also be used for stability analysis and state prediction of the system.Finally,based on the proposed multi-frequency dq impedance measurement method,using the measured value and combined with the MPC criterion,the stability analysis is performed for the grid-side equivalent parameters and control loop parameters of the vehicle-side.The MPC circle radius was modified by the critical value from simulation and the HIL platform.The stability region of each parameter in the complex plane was redetermined,reducing the conservatism of the traditional Generalized Nyquist criterion based on theoretical value,solving the problem of stability misjudgment,and providing a new reference for the parameter design of the vehicle-grid cascade system.