Stability Analysis And Control Strategy Research Of Grid-connected Inverter Considering Grid Background Harmonics

The development of distributed power generation systems based on renewable energy such as wind and solar energy can effectively alleviate the energy crisis.Grid-connected inverter has become an important interface for power conversion between new energy generation and the grid due to its flexible operation mode and good controllability.However,with the gradual increase in the penetration rate of new energy in the power grid,the characteristics of the power grid become more and more like a weak grid.On the one hand,large-scale changes in the impedance of the grid may cause system stability problems,on the other hand,the background harmonics of the grid will seriously affect the waveform quality of the grid-connected current.An improved feedforward control strategy is proposed here for the inverter directly connected to the grid and the inverter connected to the grid through a transformer,which improves the stability of the system while ensuring the ability to suppress the background harmonics of the grid.The control strategies of the three-phase LCL grid-connected inverter in the dq synchronous rotating coordinate system and the αβ two-phase static coordinate system are compared and the mathematical model of the grid-connected inverter in the αβ two-phase static coordinate system is established firstly.With the goal of eliminating grid background harmonics,the full feedforward transfer function of grid voltage is derived.On this basis,the influence of control delay on the stability of the inverter system adopting the full feedforward control strategy of grid voltage is analyzed.It is found that the control delay causes the inverter output impedance to produce a negative phase shift,which seriously deteriorates the system stability,and makes the inverter output impedance unable to reach infinity,thereby reducing the ability to suppress the background harmonics of the grid.For the inverter system directly connected to the grid,an improved feedforward control strategy based on multi-resonant links is proposed.By adding multi-resonant links in the feedforward channel,the suppressed background harmonics are extracted and fed forward.The parameters of the resonant links are designed to completely eliminate the phase delay at the harmonic frequency.The Bode diagram shows that the phase delay of the inverter output impedance is effectively compensated and the ability to suppress harmonics is guaranteed.The simulation on the PSCAD platform also verifies the effectiveness of the proposed method.For the inverter connected to the grid through a transformer,the problem of the feedforward point change caused by the leakage inductance of the transformer as a part of the filter is firstly analyzed.Through the equivalent transformation of the control block diagram,it is found that the capacitive current proportional feedforward active damping is cancelled,and the feedforward function can be equivalent to the virtual impedance connected in parallel at both ends of the capacitor.The resonance peak of the LCL is damped by the virtual impedance.Aiming at the problem that the real part of the virtual impedance is negative in a specific frequency range,which may cause system instability,a method of virtual impedance reshaping is proposed.A low-pass filter is designed in the feedforward system to ensure that the real part of the virtual impedance is positive.Finally,the model was built on the PSCAD simulation platform for verification.