Research On The Key Techniques Of Parallel Operation Of Inverter System

The inverter parallel operation is the trend of inverter power system from traditional centralized to distributed power supply system.The inverter parallel technology is the technical basis for realizing high reliability,high redundancy and high scalability AC power system,and it is also one of the key technologies for realizing new energy generation such as wind and solar power.To summarize the existing domestic and foreign parallel technology,this thesis deeply researches the key technical problems of inverter parallel system in different working situations.The specific research content is as follows:The current sharing degree and the voltage total harmonic distortion（THD）are important indexes of Uninterrupted Power System（UPS）.This thesis illustrates the limitations of traditional PR-based controllers in suppressing voltage harmonics,and proposes an output impedance reshaping mechanism by increasing the adaptive virtual capacitive reactance so that the voltage at the harmonic frequency can be reduced to zero.Based on the instantaneous average current sharing method（IACS）and combined with the experience in UPS research,the current-sharing controller based on virtual impedance technology is proposed to increase the circulation impedance so that the current sharing degree can be improved.Experimental results show that the system obtains effective current sharing performance with the proposed method,while the voltage harmonics are suppressed at a low level.The traditional droop control will deviate the output power under the mismatched line impedance in single AC bus parallel system.To improve the power distribution accuracy,based on the ―P/ω,Q/E‖droop control theory,this thesis analyzes the mechanism of the error of the power distribution caused by the difference of line impedance,and an enhanced voltage compensation controller is proposed.The line voltage difference is compensated to the fundamental voltage based on adaptive algorithm so that the reactive power distribution effect is improved;while the harmonic power distribution is improved by adding adaptive virtual impedance at harmonic frequencies.This thesis analyzes the stability and dynamic performance with the small-signal model,and the results verify the effectiveness of the algorithm under nonlinear load.Grid-connected inverters have two different operating modes: islanded and grid-connected mode.To achieve a smooth transition between the two modes,this thesis introduces the outer loop on the basis of the traditional double closed-loop control to form a cascaded structure and proposes a seamless transition control strategy.The outer loop controller is designed to increase the damping and inertia from grid-connected to islanded mode so that voltage,current and frequency fluctuations can be suppressed.In addition,this thesis optimizes the Second-Order General Integrator（SOGI）structure to solve the real-time problem of filtering out DC bias,and nonlinear operator is introduced to increase the speed of phase-locking.Finally,the effectiveness is verified through the simulation and experiment.Aiming at the amplitude,frequency deviation and power imbalance caused by droop control in the distributed system,based on the finite-time control theory,this thesis proposes a secondary control protocol under the undirected topology to solve the problems of voltage tracking and the power synchronization.In the secondary frequency recovery,a single control protocol is constructed based on the sign function to achieve both frequency recovery and accurate active power distribution;in the secondary voltage recovery,the voltage compensation control strategy proposed in Chapter 3 is extended to the distributed system,and then a secondary voltage-reactive power control protocol is proposed to solve the problem of voltage recovery and reactive power distribution caused by the mismatched line impedance.Based on the Lyapunov stability criterion,the rigorous theoretical proof of the convergence for the finite-time controller is carried out.Finally,the simulation results are shown to verify the advantages of the power distribution,convergence speed and large-scale system adaptability.