Nonlinear sideband effects in small-signal input dq admittance of six-pulse diode rectifiers

This paper focuses on the modeling of small-signal input dq admittance of six-pulse diode rectifiers, providing comparison between well-known averaged value models (AVMs), the switching simulation model and hardware measurements. The analytical AVM (AAVM) is a widely used solution for modeling a six-pulse diode rectifier, but it is shown that it has to be modified in order to model the input dq admittance more accurately. Analytical expressions for all four admittances present in the dq matrix are derived and compared to switching model small-signal dq admittances. Furthermore, a parametric AVM (PAVM) is included into analysis, “slightly modified” and analytical expressions for all four dq admittances are derived in this case also. However, it is concluded that both AVMs predicts two admittances, Y_dd(s) and Y_qd(s), very precisely even beyond the switching frequency. On the other hand, the prediction of other two admittances, Y_dq(s) and Y_qq(s), is accurate only up to one fourth and one half of the switching frequency, respectively. The main sources of differences are found to be sideband resonant points that are mainly present in the response to q-channel injection. The main reason is that q-channel injection modulates commutation angle and yields significant sideband admittances around multiples of the switching frequency, which is typical behavior for nonlinear systems. Furthermore, a hardware set-up is built, measured and modeled, showing that the switching simulation model captures nonlinear sideband effects accurately. In the end, a six-pulse diode rectifier feeding a constant power load is analyzed with the modified AAVM and through the detailed simulations of switching model, proving effectiveness of the proposed modifications. The nonlinear sidebands are inevitable phenomenon in a six-pulse diode rectifier as sidebands are found in all the presented test-cases.