Isolated Energy Feedback Electronic DC Load And Optimal Control In Weak Grid

With the increasing demand of power and battery,the rapid,accurate and stable simulation of multiple load characteristics has become the developing direction of electronic load.More and more power electronic devices causes weak grid and its grid connection optimization has become a research hotspot.This paper analyzed the technical progress of the energy-fed DC electronic load,studied the load simulation and the energy-fed circuit,the load simulation strategy with high precision and quick response,the energy-fed grid connection strategy with high precision and anti disturbance,the filtering scheme and feed-forward strategy for the weak grid.All in all,an isolated energy-fed DC electronic load prototype had been developed for experiments in this paper.The main contents of the paper were as follows:(1)The power circuit of isolated energy-fed DC electronic load was studied.ZVS phase-shifting full bridge was selected for load simulation to analyze the working mode so as to verify its driving and soft switching characteristic.H4 inverting full bridge was selected for grid connection to analyze the harmonic characteristics of monopolar frequency doubling modulation so as to verify its output harmonic with high-frequency advantage.(2)A load simulation strategy with low static error,fast response and low overshoot based on self-organizing fuzzy PI controller was studied.Aiming at the static error and low robustness of PI,fuzzy control and input domain proportional quantization factor was introduced to adjust the PI coefficients adaptively.The simulation results showed that,compared with the conventional fuzzy control,the constant current 7A load simulation was completed in 0.8ms,the simulation time was reduced by 73.3%,the overshoot was reduced by 69.9%,and the dynamic switching time was less than 0.67 ms.(3)Based on “PI+QPR” double closed-loop controller,an energy-fed grid connection strategy was studied.In order to ensure energy flow and the resistance of grid frequency fluctuation,the power balance double closed-loop and its inner loop QPR controller's parameters were designed.3k W system simulation result showed that the double closed-loop QPR controller reduced the current harmonic to 3.44% at 50 Hz and 3.47% at 49.5Hz,which had good resistance to the grid frequency fluctuation.(4)Filters and feedforward optimization for weak grid with inductive reactance and harmonics were studied.The LCL filter with superior high frequency performance was selected and designed,and passive damping resonance suppression of capacitor parallel resistance was adopted.This paper studied the passive damping change of the filter under the weak grid,designed the proportional compensation grid voltage feedforward for the harmonic from weak grid,and full-load system simulation result showed that weak grid with 9.26% harmonic outputted 4.00% current harmonic,meeting the grid connection standard.(5)An isolated energy-fed DC electronic load prototype was developed for the power circuit,load simulation,energy-fed grid connection and weak grid connection experiments.The experiment results showed that the power circuit parameters were reasonable;the simulation error of various load characteristics were less than 4%,and the constant current load switched only in 1.7ms;the ideal grid-connected current harmonic was 2.641% at full load,and the power factor was 0.998;the weak grid-connected current harmonic was 2.988% at full load,and the power factor was as high as 0.998,so the system had good resistance to weak grid,meeting the grid connection standard.