Abstract：When the switching frequency of the four-quadrant rectifiers in different multiple units deviates，the

current will experience low-frequency oscillation，which is not conducive to the operation of the traction power

grid. Therefore，a matrix small-signal modeling method for SPWM comparators was proposed to analyze this issue，which achieves a more accurate description of the switching frequency characteristics of the single-phase PWM rectifier. Then，the matrix small-signal modeling of parallel four-quadrant rectifiers was further established，and then the model was used to analyze the causes and rules of differential frequency oscillation during mixed running of different multiple units. Finally，the correctness of the established model and analysis conclusions were verified through experiments.

      Key words：four-quadrant rectifier；traction power grid；matrix small-signal model；differential frequency

oscillation