Research On Robust Stability And Robust Performance Of Grid-Connected Inverters In Weak Grids

Under the background of vigorously developing new energy and comprehensively promoting the large-scale development and application of wind power and solar photovoltaic power generation,more and more renewable energy is connected to the medium and low voltage distribution grids through grid-connected inverters.But the medium and low voltage distribution grids have the characteristics of large unit line impedance,large resistive component in impedance,frequent load turning on and turning off,and frequent voltage fluctuations.They are typical weak grids.The grid-connected inverter control includes grid-side current control,AC voltage control,virtual synchronous machine control,etc.The line impedance in a weak grid has a significant impact on these controls.In a weak power grid,the uncertainty of the line impedance affects the stability of the grid-side current control,causing the inverter to have a robust stability problem.In additon,the uncertainty of the line impedance also affects the dynamic performance of the AC voltage control,causing the inverter to have a robust performance issue.The resistive component in line impedance is relatively large,which leads to obvious coupling between the active power control and reactive power control in virtual synchronous machine.Besides,the uncertainty of the line impedance affects the power decoupling performance,making the power decoupling have a robust performance problem.Therefore,in a weak grid,the stability analysis and control strategy design of grid-connected inverters are more complicated than in a strong grid.Aiming at the above problems,this paper studies the grid-connected inverter connected to a weak grid.Firstly,the robust stability problem in grid-side current control is studied.In order to enhance the robustness of the current loop,a loop correction method based on a reference model is proposed.Aiming at the stability problem,the current loop is firstly modeled.The dynamic process of a phase-locked loop under a weak grid is also analyzed,and the mathematical model of the phase-locked loop is derived.If the bandwidth of the PLL is low,the PLL will not affect the stability of the current loop.Then the robust stability of an inverter in a weak grid is analyzed.When the line inductance increases,the stability of the inverter will become worse.The improved grid voltage feed-forward is analyzed.This method is equivalent to adding a lag-lead compensator in series with the current controller.This method can improve the stability of the current loop,but still has the problem of weak robustness.In order to make the inverter more robust,a loop correction method based on a reference model is proposed.This method replaces the middle and high frequency components in the actual system response with the medium and high frequency components in the reference model response,so that the inverter has both good dynamic performance and robust stability in a weak grid.Then,the problem of parameter optimization design and robust performance in AC voltage control is studied.In order to improve the dynamic performance of the voltage loop,a modulation signal feedback control strategy is proposed.The optimal design of the nominal system is the basis of the robust performance analysis.Aiming at the problem of parameter optimization design in AC voltage control,a parameter optimization design method of current controller is first proposed.The calculation result shows that designing the time constantτof the current loop as 3T_s can suppress the resonance peak in the closed-loop transfer function.Besides,it can also maintain a good dynamic performance of the current loop.Then,the influence of voltage controller parameters and damping resistance on system performance is analyzed by slice graphs.The parameters of voltage controller are optimized by three-dimensional graphs.Aiming at the robust performance problem existing in AC voltage control,the robust performance of an inverter in a weak grid is analyzed.When the line impedance is greater than the nominal value,the overshoot of the unit step response will become larger.In order to make the inverter have better robust performance in a weak grid,a modulation signal feedback control strategy is proposed,which can make the inverter still have better dynamic performance when the line impedance is greater than the nominal value.Finally,the stability and power coupling of virtual synchronous machines are studied.In order to enhance the robustness of power decoupling,a power decoupling method based on feedforward decoupling and extended state observer is proposed.Aiming at the stability of virtual synchronous generator,the power transfer functions of the virtual synchronous machine are calculated using some matrixs.The reason of synchronous frequency resonance in the power transfer function is analyzed by singular value decomposition.The analysis results show that the resonance peak corresponds to DC gain in the abc coordinate system.In order to suppress the resonance in active power and reactive power,it is proposed to apply a 50Hz band-pass filter to the sampled AC current signals to filter out the DC bias in AC current signals.Aiming at the power coupling problem of virtual synchronous machine,the power coupling degree of virtual synchronous machine under different line impedances is analyzed by calculating the relative gain of the power transfer function matrix.In order to suppress the coupling between power loops,a feed-forward decoupling method based on virtual power angle and virtual voltage is proposed.This method can make the nominal system obtain good power decoupling performance,but when the line impedance deviates from the nominal value,the power decoupling performance will be worse.In order to improve the robustness of power decoupling,an extended state observer is proposed to observe the coupling between active power and reactive power in real time,and the power coupling is suppressed by disturbance compensations of power angle and voltage.The proposed method can still achieve dynamic decoupling of active power and reactive power when the line impedance deviates from the nominal value.