Study On Negative Damping Characteristics Analysis And Suppression Method Of Phase Locked Loop For Grid-Connected Inverter

With the development of distributed generation systems,the proportion of power electronic devices in power systems has gradually increased.In this situation,grid-connected inverters are the most representative power electronics.The grid-connected inverter is used as the interface device between the distributed generation system and the grid.It determines the quality of the grid-side power.The phase-locked loop(PLL)in the grid-connected inverter plays an important role in maintaining the stability of the system.The PLL controls the output power of the inverter by detecting the voltage phase on the grid side.Therefore,the PLL is of great significance to the safety and stability of the system.Based on the PLL,this paper has probed into its impedance characteristics and improved methods.The impedance model of the grid-connected system is established,and then the circuit model is simplified according to Norton's theorem.Based on the Nyquist stability criterion,this paper analyzes the stability conditions of the single inverter and the multi-inverter system.On this basis,it is pointed out that the addition of PLL causes oscillation in the low frequency band of the system.Then,the damping characteristics of the PLL are analyzed from the closed-loop output admittance angle.The phase-frequency curve is located near 180°.Therefore,the negative damping characteristic of the additional admittance of the PLL is one of the main reasons for the instability of the system.The correctness of the theoretical analysis is verified by simulation and experiment.As the negative damping characteristic of the PLL makes the system oscillate,this paper aims to correct the phase-frequency characteristics of the additional impedance of the PLL by improving the structure of the PLL.From the perspective of control theory,a cascaded PLL structure is proposed to increase the order of the PLL system and reshape the additional impedance of the PLL.The small admittance method is used to derive the additional admittance of the cascaded PLL.On this basis,changing the control parameters of the cascaded PLL can further improve the system performance.On the other hand,a method based on the filter matrix to change the output impedance characteristics of the PLL is designed.The method is a loop control algorithm.A filter transmission matrix quadrature signal generator is designed based on the combination of the phase shift operators and the first order filters.By embedding it in PD,it is possible to limit the bandwidth of the PLL,reduce the gain of the low-frequency PLL,and perform series correction on the system.The validity of the design method is verified by simulation and experiment.Considering the mechanism of additional negative damping,this paper uses the external compensation to achieve the suppression of the negative damping of the PLL.The proposed grid voltage proportional differential feedforward strategy is equivalent to adding an output impedance in parallel with the inverter output impedance.By adjusting the impedance of the inverter itself,the system instability problem caused by the additional impedance of the PLL is compensated.Because the grid voltage differential feedforward control strategy is less robust,a virtual damping feedforward compensation strategy based on closed-loop characteristics of PLL is proposed.This method can offset the negative damping of the PLL and improve the system stability margin.The validity of the design method is verified by simulation and experiment.