Study On Robust Control Strategy Of Single-phase Singles-stage Photovoltaic Inverter Operating In Off-grid And Grid-connected Mode

Nowadays,photovoltaic power generation,which has drawn much attention from nations all over the world,has become an emerging industry because of its advantages such as cleanness,convenience,efficiency and security.To a great extend,the characteristics and advantages of photovoltaic power generation depend on the high-performance control technologies of its inverters.When the photovoltaic power generation system is operating in the off-grid mode,it is an urgent technical problem to realize the effective synchronization of the output voltage of the inverter as well as the high robust voltage tracking effect due to the lack of utility grid voltage support.When photovoltaic power generation system is operating in grid-connected mode,direct current control and direct power control are usually implemented.Compared with direct current control,direct power control(DPC)does not need to convert the power variables into the corresponding current variables,but directly controls the instantaneous output power of the inverter,which has faster response and simpler structure.Although DPC is widely implemented in three-phase systems,the study on DPC for single-phase system is rarely reported.Therefore,the voltage control strategy and direct power control strategy of singlephase single-stage photovoltaic inverter system in off-grid/grid-connected mode are designed in this dissertation.The specific works in this dissertation can be summarized as follows:(1)The mathematical models of single-phase single-stage photovoltaic inverter system in off-grid/grid-connected mode are established,and the calculation of instantaneous single-phase grid-side power in grid-connected mode is deduced.(2)A new robust voltage control strategy using time-varying gain extended state observer(VGESO)-based adaptive Super-Twisting algorithm(ASTA)for voltage source inverte system with load disturbances as well as parametric perturbations.Firstly,a VGESO is designed to reconstruct the unmeasured auxiliary state variables and the lumped uncertainties including load changes and parameter perturbations.Then on the basis of the VGESO,a sliding mode controller employing ASTA is synthesized so that the output voltage of inverter system is capable of tracking its reference value rapidly and accurately,and the capability of resisting the load disturbances as well as parametric perturbations is strengthened.(3)A novel DPC strategy based on the combination of H-infinity control based on reduced order extended observer(RESO)and Super-Twisting algorithm is proposed for single-phase single-stage photovoltaic inverter system in grid-connected mode.Firstly,a RESO-based Hinfinity voltage controller is designed for the outer voltage square loop to keep the DC-Bus voltage stable under the affect of photovoltaic changable output power,which is attributed to the real-time estimation and feedforward compensation of the RESO.Secondly,a sliding mode controller employing super twisting algorithm is designed for the inner power loop,which is capable of reducing the total harmonic distortion(THD)rate of the grid-side current,realizing the decoupling control of the instantaneous active power and reactive power,and enhancing the robustness of the system to parametric perturbation.