Control Strategy Of Low Switching Frequency Full Power Converter In Permanent Magnet Synchronous Motor

The direct-driven generator set takes the permanent magnet synchronous motor(PMSM)as the core.It has the advantages of high reliability and high power density.Therefore,it is the most promising driving way for high-power generator sets.As the core component of the unit,the operation quality of the full power converter is very important.However,with the improvement of the power level,the switching frequency of the converter is set to low to reduce the switching loss.In the digital control system,the influence of digital delay will increase due to the low switching frequency,which leads to the performance degradation of the full-power converter based on the traditional control strategy.To improve the stability and dynamic performance of the full power converter when PMSM works at low switching frequency,it is necessary to study the key issues of control strategy.Firstly,the source of the delay in the digital control system of full power converter is analyzed.Furthermore,the grid-side converter and the machine-side converter are modeled.On the basis of the mathematical model,the traditional control strategies of the grid-side converter based on quasi-proportional-resonance(QPR)control and the machine-side converter based on zero direct-axis current control are studied,and the dynamic performance is declined due to the digital delay when PMSM works at low switching frequency.Secondly,the passive damping method without considering the effect of digital delay at low switching frequencies is discussed.According to the Nyquist stability criterion,it reveals the mechanism by which the current loop cannot operate stably,and points out that the change of LCL filter and QPR control parameters can not improve the stability of the system.To solve the problem,the lead compensation link with low pass characteristic is proposed,and it can realize the compensation of the phase lag near the power grid.Simulation and experimental results show that the grid-side converter with compensation can operate stably,and it can improve the dynamic performance of the current loop.Thirdly,due to the existence of digital delay,the traditional PI controller limitations of the decoupling capability and dynamic performance are studied.The angle lag is eliminated by introducing phase leading angle into Park inverse transform,and the effect of digital delay is eliminated by the predictive feedforward.Therefore,the proposed PI controller can realize the decoupling of the torque and flux.To further improve the dynamic performance of PI controller,the PI controller is designed based on the motor discrete domain model.Through the configuration of feedforward coefficient,the proposed PI controller realizes deadbeat response.Simulation and experimental results show that the controller can achieve full decoupling of torque and flux at low switching frequency,which effectively improves the dynamic performance of the machine-side converter.Finally,for the condition that multiple parameters are simultaneously identified,a multiple-parameters identification method is proposed to solve the problem of errors in the identification results due to the under-ranking of the stator voltage equation.The inductance identification is realized by the d-axis steady-state voltage equation.To solve the problem of deficiency-rank,the zero-order hold discrete voltage equation is further transformed to eliminated the influence of the rotor flux.With the model reference adaptive algorithm,the stator resistance identification is realized.Finally,with the q-axis steady-state voltage equation,it can realize rotor flux identification,and as a result the multiple-parameters identification is realized.By analyzing the influence of discretization error on the identification result,the nominal current compensation is introduced to improve the identification accuracy.Finally,the accuracy of the proposed method is verified by simulation and experimental results.