Impedance Emulating Control Of Grid-connected Converters

Grid-connected converters are the critical equipment for the interaction of renewable energy and power loads with the grid.With the process of power electronics in power systems,the control performance of grid-connected converters plays an increasingly important role in the operation of power systems.The improper design of the controller not only makes it difficult to obtain the desired power quality,but also causes the grid-connected system to oscillate and destabilize,posing a safety hazard to the power system.In this paper,the controller design and stability analysis of grid-connected converter are studied.A gridconnected controller design method based on impedance emulating is proposed,which realizes the passive grid-connected synchronization of the converter without phase locked loop and improves the power quality and weak grid adaptability of the system.The main research contents are shown as:(1)A synchronization controller design method based on impedance emulating is proposed for the grid-connected synchronous control of converters,which reduces the difficulty of controller design and provides support for the physical and engineering aspects of controller design.Taking the single-phase rectifier as an example,the resistance emulating,harmonic impedance emulating,and inductance emulating control strategies are introduced with the help of proven circuit theory.Based on the self-synchronization characteristics of the passive circuit,the converter is shaped into a controlled impedance source to realize the passive autonomous grid synchronization of the converter without phase locked loop.By emulating the high resistance characteristic of the resonant circuit and the inductor phase compensation characteristic,the converter current quality as well as the power factor is achieved,and the system power quality is improved.In addition,considering that it is difficult to obtain the accurate equivalent input inductance value in practice,an inductor online identification method based on extremum seeking is adopted to realize the system unit power factor operation.(2)The emulating impedance control is extended to three-phase systems,and a control framework based on the concept of emulating common-mode/differential-mode resistance is proposed for the control of rectifiers.The proposed method achieves the three control objectives of constant active power,symmetrical three-phase current and constant reactive power under unbalanced conditions.The instantaneous power expression of the rectifier under unbalanced conditions is derived,and the equivalent input circuit characteristics of the emulating required in the non-standard stationary coordinate system are analyzed.The concepts of common mode resistance and differential mode resistance are proposed,and it is pointed out that the common mode resistance and differential mode resistance can be used to regulate the average and ripple power of the rectifier,respectively.The proposed method does not require grid voltage sensors and has good parameter robustness and asymmetric line impedance adaptation capability.(3)To solve the instability problem of traditional resistance emulating control under inverter and light load conditions,two improvement methods,negative inductance emulating stabilization controller and generalized resistance emulating controller are proposed to achieve system stability and improve the operating range of the converter.Based on the idea of reversing the sign of the inductor to eliminate the unstable poles of the system,the stabilization control strategy based on negative inductance emulating is proposed to realize the safe operation of the converter in the inverter mode.The conventional resistive element is generalized by introducing additional degrees of freedom and a generalized resistance emulating control strategy is proposed.Through root trajectory analysis,the new degree of freedom effectively improves the system stability and solves the problem of restricted operation in inverter and light load conditions caused by the traditional resistance emulating control.In addition,the construction ideas of negative inductance and generalized resistance break through the limitations of actual physical circuits and improve the way of controller design based on virtual dynamics.(4)To evaluate the effect of resistance emulating synchronous control on system stability,a small-signal model of the grid-connected converter is established,and it is pointed out that the resistance emulating control can reduce the frequency coupling propagation path and better weak grid adaptability than the PLL-based grid-following control.Based on the time-periodic system theory,the small-signal model of singlephase AC-DC converter is constructed.Based on the harmonic signal flow diagram method,the frequency coupling propagation path of the grid-connected system is analyzed,and it is found that the resistance emulating control can effectively avoid the frequency coupling dynamics introduced by PLL.The dq-axis conductance model of the three-phase AC-DC converter is built and the effects of grid impedance and voltage loop bandwidth on the system stability are analyzed by the generalized Nyquist stability criterion.The results show that the grid-connected system under the resistance emulating control has a wider stability range compared to the PLL-based grid-following control.