Passivity-based Stabilization Of Grid-connected Inverter

With the massive consumption of fossil energies,the development of human civilization is severely challenged by energy crisis and environmental pollution.To tackle the challenges,renewable energy such as wind and solar energies have been developed and utilized on a large scale.Currently,grid-connection is the main way to utilize renewable energy,where the grid-connected inverter plays a significant role.To guarantee its stable operation in the complex grid,the concept of passivity gains much attention.In essence,the purpose of passivity-based design is to avoid the resistive part of the output impedance of the inverter to be negative in the entire frequency band.Therefore,this thesis devotes to a passivity-based design scheme for the grid-connected inverter,which juggles both universality and ease of implementation.Firstly,the mathematical models of both the inverter-side and grid-side current controlled LCL-type grid-connected inverters are established.These models tell that the stability of the inverter can be devided into internal stability and external stability.Based on the impedance-based stability criterion,the property of a passive system is also elaborated,and the passivity requirements are obtained for external stability.Secondly,based on the passivity-based requirements,the optimal capacitor-current active damping gains of both inverter-side and grid-side current feedback controls are derived,respectively,which helps to shape the output impedance of the inverter passive.However,it is worth noting that the output impedance is critically passive at one-sixth of the sampling frequency（ω_s/6）,where the passivity is subjected to the inevitable fluctuation of the main-circuit parameters in practical applications.Thirdly,an output impedance shaping scheme is proposed.It is universal and easy for implementation.By adding a proper compensator to cascade with the current regulator,a virtual impedance is equivalent to be in series with the output impedance of the inverter.It helps to shape the output impedance,and improve the passivity aroundω_s/6.Accordingly,the passivity becomes robust against the main-circuit parameter fluctuation.Some valuable findings are concluded.For the inverter-side current feedback control,the phase-lead compensator is suited.For the grid-side current feedback control,which compensator is suitable depends on the relation ofω_s/6 and the resonant frequency(ω_L1C)of the inverter-side inductor and the filter capacitor.Whenω_L1C>ω_s/6,the phase-lead compensator is suited;whenω_L1C<ω_s/6,the phase-lag compensator is suited;whenω_L1C=ω_s/6,any type of compensator will destroy the passivity of the system.Therefore,the case ofω_L1C=ω_s/6 should be avoided while designing the LCL filter.Empirically,a forbidden range ofω_L1C considering the fluctuation ranges of both the inverter-side inductor and the filter capacitor is recommended,such thatω_L1C will never be close toω_s/6.Further,the influence of the resonant part of the PR regulator on the passivity is revealed.It finds that it tends to increase the burden of the phase-lead compensator but decrease that of the phase-lag compensator.Finally,a 6 k W prototype of a single-phase LCL-type grid-connected inverter was built and the experimental results verified the correctness of the theoretical analysis.