Research On Neutral-point Balance And Stability Control Technology Of Three-level Grid-connected Inverter Under Non-ideal Power Grid

Grid-connected converter is an important part of renewable energy power generation system,which plays a key role in the stable and high performance operation of the system.However,since the renewable energy power generation system is usually at the end of the power grid,the power grid often presents the characteristics of weak power grid,which puts forward higher requirements for the control of grid-connected inverter.In this paper,the current control of grid-connected converters is coupled with the phase-locked loop and the feed-forward control loop of the grid voltage through the common coupling point voltage,and the three-level Neutral-Point potential interacts with the grid to affect the stable operation of the system.Based on the above analysis,this paper presents a simplified based on zero sequence component injected into three-level neutral voltage balance control,has the advantage of fast dynamic response is easy to project implementation,puts forward the improved phase-locked loop control strategy and fundamental component of positive sequence d shaft power grid voltage feed-forward combination of harmonic control strategy of the controller,to improve the adaptability of control system for grid impedance changes and system stability.The main research work and innovation of this paper are as follows:1)For the three level topology neutral voltage will interact with the ideal power grid problems,dc side capacitor voltage model is established in this paper,detailed analysis of the power grid harmonic and negative sequence component and influence on neutral voltage modulation and power factor,and analyzed three times the base when the neutral voltage offset and frequency fluctuations when the grid current waveform characteristics.In this paper,a simplified based on zero sequence component injected three-level neutral voltage balance control,to simplify the three-level vector diagram into a two level 6 vectors,the building up of middle point in each of the two level vector diagram accurate mathematical relationship between current and zero sequence component,analyzes its features found only needs to have a carrier and six equation can realize three-level carrier modulation and neutral voltage balance control.2)In consideration of the weak current offline dynamic response of the phase-locked loop will affect the performance of current control problem,this paper established considering the effects of phase lock loop three-phase grid inverter control system small signal model with LCL,combined with impedance analysis method,the dynamic responses of the phase-locked loop respectively on the modulation of grid voltage and current feedback value q axis component of introducing the interference terms,this will lead to the low output impedance of the q axis spectrum phase margin rapidly decreases,and affect the stability of grid current control in offline,tend to lead to system instability state,to improve the phase-locked loop to track the performance of the power grid voltage phase information,In this paper,an improved PLL control strategy based on symmetric component method is proposed,which USES an adaptive all-pass filter to extract positive sequence components and then uses an adaptive second-order generalized integrator to suppress harmonic interference.3)In view of the weak current off-line power grid voltage feed-forward control would undermine the stability of the control system,this article first briefly deduces the design method of feedforward control,and points out that the grid voltage feed-forward of current loop is introduced into the positive feedback channel and presents the characteristics of all-pass,then through to the output impedance of the open-loop transfer function analysis and system analysis,weak current offline feedforward control can lead to low output impedance phase margin spectrum has fallen dramatically.Based on the above analysis,the feedforward of d-axis fundamental wave component of grid voltage positive sequence is proposed,and the control strategy of harmonic controller is added to the grid-connected current loop controller to improve the current control performance and the adaptability of weak current network.Finally set up T-type three-level three-phase LCL type grid inverter simulation and experimental platform,the neutral voltage balancing algorithm,improved phase-locked loop control and fundamental component of positive sequence d shaft power grid voltage feed-forward combination of harmonic controller control strategy part has carried on the simulation and experimental verification,the results show the correctness of theoretical analysis and the feasibility of the proposed control method.