Stability Analysis And Output Voltage Quality Improvement For VSIs With LC Filters

Voltage-controlled voltage source inverters with LC-filters are widely used in distributed power generation systems,uninterruptible power supplies,and high-voltage direct current transmission systems.The second-order resonance of the LC filter and the delay of digital control profoundly affect the stability of the inverter closed-loop control system.In addition,a large number of non-linear loads are connected to the LC-type inverter,causing the output voltage to be distorted.In this thesis,the single-phase full-bridge LC-type inverter based on proportional quasi-resonant voltage controller is studied,and the stability and output voltage distortion problems are studied.The stability of LC-type inverter closed-loop control systems with different control structures with LC filter resonant frequency in the range of 0 to Nyquist frequency are discussedin this thesis.The stable regions and stable conditions are also given.When the resonant frequency of the LC filter is greater than 1/3 of the sampling frequency,the proportional coefficient in the voltage controller is adjusted making the open loop gain at 1/3 of the sampling frequency less than 1,so that the LC type inverter can stably output a sinusoidal AC voltage.When the LC filter resonance frequency is less than 1/3 of the sampling frequency,it is necessary to use the current inner loop to dampen the resonance peak to stabilize the closed loop system.It can be seen that considering the digital control delay,the current inner loop is not equivalent to pure resistance,but the frequency associated impedance in series with the passive components in the LC filter.The impedance also changes the resonant frequency of the LC filter while damping the resonant peak.When the impedance at the actual resonant frequency of the LC filter consists of a pure resistance,the double closed-loop control system is possibly stable.In this thesis,based on the the resonant frequency of the LC filter,the stable region is devided into several parts,and according to the Nyquist stability criterion,stable conditions are given.The capacitor voltage linear differential feedback can be in place of the capacitor current proportional feedback.This Thesis analyses the stability of the internal loop capacitor voltage differential feedback based on the common digital differentiator and voltage outer loop control system.The phase lag characteristic of the backward Eular differentiator narrows the effective active damping region.For this reason,the lead phase compensator is used to compensate the phase lag of the backward Eular differential differentiator,and an improved digital differentiator similar to the ideal differential characteristic is obtained.The root locus diagram illustrates the frequency characteristics of the capacitor-voltage differential inner-loop capacitor voltage outer loop double closed-loop control structure based on the improved digital differentiator and the frequency characteristics of the capacitor current inner loop capacitor voltage outer loop double closed-loop control structure are almost the same.The stability characteristics of them are also almost the same.Aiming at the problem of output voltage distortion of LC-type inverters with nonlinear loads,the output impedance model of LC-type inverter with double closed loop control is established,and the mechanism of output voltage distortion is revealed.According to the mechanism,a research for improving the output voltage quality of LC type inverter is done.In this thesis,the frequency characteristics of the resonant controller are acknowledged in another angle,and making full use of its characteristics that can compensate the arbitrary phase and amplitude at a given frequency.The multi-resonant feedforward control strategy is designed,and the feedforward strategy parameters design method is also given.The impedance gain at the frequency is reshaped to be close to zero,which reduces the influence of the load current on the output voltage and improves the output voltage quality.Experiments were carried out on PSIM simulation software and hardware prototypes.The experimental results verified the correctness of the conclusions given in this thesis.The stability analysis conclusions and the strategy for improving output voltage quality are also applicable to the three-phase LC-type inverters.At the same time,the method of stability analysis also has reference significance for the LC type inverter load-bearing robustness analysis.