Modeling And Secondary Side Measurement Method Of Generalized Impedance Of Grid-connected Converters Under Arbitrary Power Factor

In the modern power grid,as the proportion of wind power,photovoltaic and other new energy power generation with converters as the interface continues to increase,the AC grid connected by new energy gradually weakens,showing a weak power grid(this article refers specifically to the low short-circuit ratio power grid)characteristic.In the weak power grid scenario,the interaction between converters in a high percentage of new energy power systems is prominent,and the system is facing serious problems of small interference and stability.The generalized impedance method derives the system impedance matrix based on the polar coordinate system,and through the Schur method,the system impedance model is reduced from 2x2 to SISO model.Compared with the traditional impedance method,this method not only obtains the system SISO model,but also can use the Nyquist criterion for quantitative stability analysis,and then proposes the concept of the original-dual complex circuit,pointing out that the generalized impedance can be regarded as the original-dual complex circuit Special case,the impedance of the grid-connected converter is generally accurately measured by the active measurement method.At present,there is a generalized impedance measurement method for injecting the disturbance signal from the secondary side of the converter.The advantage of this method is that the secondary side injection costs less,and the operation is more convenient.However,the above studies are all directed to the working condition of unit power factor,and the modeling and measurement when non-unit power factor is yet to be studied.The main research contents and results of this article are as follows:1)For the grid-connected converters with low unit frequency and middle and high frequency bands under non-unit power factor,the different dynamic sub-cases are considered,and the generalized impedance measurement methods for injecting disturbances in the current inner loop are given respectively.First,through mathematical derivation,the port characteristic equation of the converter after the secondary side injection disturbance is given and transformed into the polar coordinate form of the synchronous coordinate system;secondly,the port characteristic of the polar coordinate system is transformed into the original-dual complex space,Constructing the originaldual complex circuit of the grid-connected converter under disturbance,the principle of disturbance injection test is explained in a physical sense;secondly,the calculation method of generalized impedance measurement is given by mathematical derivation;finally,Matlab / Simulink is used The simulation in detail verifies the effectiveness of the measurement algorithm.2)For the grid-connected converter controlled by the Udc-Q outer loop in the low and middle frequency bands under non-unit power factor,the measurement method of the polar coordinate system impedance matrix injecting disturbance into the current inner loop is given.First,through mathematical derivation,the impedance model of the gridconnected converter in the polar coordinate system is established,and the port characteristic equation of the converter after the secondary side injection disturbance is derived;secondly,the converter impedance and operating conditions are used The relationship between the two is given,and the impedance measurement method for variable conditions is given.First,the disturbance transfer matrix is measured under noload conditions,and then the measured disturbance transfer matrix is used to measure the generalized impedance of the converter;finally,the simulation using Matlab / Simulink The effectiveness of the measurement algorithm is verified in detail.