Study On The Stability Of Digital Controlled Grid-Connected Inverter And Paralleled Inverters

Grid-connected inverter is the interface between renewable power source and the grid. Digital control is widely used in grid-connected inverter as thanks to its consistency and anti-interference capability, therefore keeping digital controlled grid-connected stable is of vital importantce renewable generation system. With the dramastic development of renewable energy generation system, inverters are parallelled to make up a high power system. The coupling between inverters make stability analysis of paralleled inverter more complex. Concentrating on stability analysis of digital-controlled grid-connected inverter and paralleled inverters, stability analyses model of three-phase inverter is first established, making the stability analysis of three-phase inverter in accordance with single-phase ones. Influence of delay on stability of single-loop and dual-loop controled single inverter are then systematically carried out, and stability improvement methods are proposed. Finally, system stability of paralleled inverter are analyzed, giving theoretical foundation for system stability improvement of paralleld inverters.Inverters contain maily three-phase inverter used in mid-or high-power applications and single-phase inverter used in distributed system. To obtain zero steady state error, three-phase inverter are commonly controlled in dq frame. When controlled in dq frame, due to the coupling between d-axis and q-axis, system stability is more complex and not totally same as that in singe-phase applications. Some researches ignors the coupling factors in dq axis, and obtains satisfactory conclusion, but the relationship between have been carefully analyzed. This paper established the stationary frame equivalent model of PI controller in dq synchronous frame. The similarity and difference between PI/PR controller in stationary frame and the PI controller in synchronous frame are analyzed. Based on the accurate equivalent model, the precondition to directly ignore the coupling factor and use PI model to analyze system stability is evaluated. The experimental result proved the superiority over PI model. The accurate equivalent model deals with the stability problem of three-phase system, making the stability analysis universal to single-or three-phase inverter.Delay introduced by digital control changes phase-frequency characteristic(PH-F) and thus affects system stability. In privious work, the delay time is regarded to be decided by the interr upt mode and PWM load mode of DSP, and is equal to or half of carrier period. Fixing the delay time results in an incomplete analysis on influence of delay. Taking single-current control in LCL-type grid-connected inverter as an example, the influence of delay on system stability is systematicly analyzed. Based on the fact that the phase lag in any frequency is in proportional with delay time, the critical delay time to keep system stable is analyzed. The delay time control methods are further proposed. After controlling the delay time, system stability is improved, system that was supposed to be unstable truns to be stable. The analysis suits to other single-loop controlled systems.As capacitor voltage feedforward is the superposition of grid voltage feedforward and gird-side inductor voltage feedback, the inverter-side current control with capacitor-voltage control is used as an example to study system stability considering flexiable delay of dual-loop controlled inverter. The inner loop can be regarded to be an active damping to the control plant, the resonance peak of LCL filter is suppressed after introding feedforward. However, if the delay in feedfowrard path is not carefully designed, there may be another resonance peak, compromising or even conteract the damping effect or the inner-loop. Influence of delay on damping effect is analyzed, the critical delay time of conteract the damping effect is calculated. A feedforward strategy with reduced delay is proposed, making the sysmte adaptive to weak grids. The simulation and experiment results verfied the analysis. The analysis suits to other dual-loop controlled system.The power quality, or even the system stability are reported to deteriorate when more inverters are connected in parallel. Although analysis of particular system give explanation, the conclusion is limited in particular system and hard to spread to other systems. Considering the influence of different control strategies and connection architecture, a universal analysis model of parallel-connected inverter is established, and then a universal stability criterion is further proposed. The voltage source is moded as a Thevenin branch and the current sources as Nordun branches, the stability criterion is deduced by evaluating the stability of PCC voltage. The stability criterion is based on only one transfer function, which greatly simplifies the analysis. The stability criterion derive into a serial of conclusions in special situations, and these conclusions agrees with existing analysis, revealing its universality. System stability considering delay and different control strategies are analyzed through the criterion and vierfied through simulations, verifing effectiveness of criterion and give clues on system stability improving techniques.