| In recent years,with the rapid development of the economy and industry,there has been an increasing demand for energy.However,traditional fossil energy reserves are limited and non-renewable,and have a negative impact on the environment and climate.To address these challenges and accelerate economic decarbonization,China has proposed the ’double carbon’goal,which involves gradually constructing a new power system with new energy resources as the main source of power.This power system will exhibit the ’double high’ characteristics of a high proportion of new energy resources and high proportion of power electronic equipment.The improvement of productivity and industrial progress will increase the demand for impact loads,and the number of impact loads connected to the ’double high’ power grid will continue to increase.However,the ’double high’ characteristics of the power system,as well as the nonlinearity and impact of load,will present serious challenges to power quality.Based on this background,this paper studies the analysis and improvement of power quality of nonlinear and impact loads under the background of ’ double high ’ power grid.To begin with,this study aims to establish a ’double high’ power grid model and an impact load model.A model of the ’double high’ power grid based on a photovoltaic grid-connected power generation system is established,analyzing the working principle and output characteristics of photovoltaic cells.Maximum power point tracking is carried out using the conductance increment method with an additional threshold,and grid-connected power generation is achieved through the voltage and current double-loop control strategy.Simulation verification is conducted to ensure that the model is capable of being connected to the grid.An ac arc furnace has been chosen as the impact load,and a nonlinear time-varying resistance model has been established.To simulate the irregular changes in arc length,a dynamic arc length modulation mechanism has been introduced.As a result,the final dynamic model of the arc furnace has been obtained.Verification through simulation is conducted to ensure that the established model conforms to actual working conditions.Secondly,this study analyzes the power quality problems caused by electric arc furnaces during normal operation and three-phase short circuits.During normal operation,the threephase unbalance degree,harmonic content and voltage fluctuation value of 35 k V gridconnected point exceed the allowable value.When a three-phase short circuit occurs,the system experiences significant voltage sag within 0.02 s,resulting in severe reactive power impact.The effective value of the phase voltage of the 35 k V grid-connected point is reduced by 27.92%,and the reactive power is increased by 109.78 %.The effective value of the secondary side phase voltage of the arc furnace transformer is reduced by 28.86%,and the reactive power is impacted from several MVar to 30 MVar.The results of the analysis demonstrate that the impact load incorporated into the ’double high’ grid model can be used for studying steady-state and transient power quality problems.Finally,this study uses a dynamic voltage restorer(DVR)to control voltage sags.Compared with the steady-state power quality problems that have always existed in the system,transient power quality problems are random and accidental,causing greater harm,with voltage sags having the most serious consequences.The DVR can respond quickly in a short time and effectively compensate for voltage sags.The model of DVR is built,and its operational principles and components are analyzed in this study,and a suitable structure is selected for this research.The compensation strategy adopts in-phase compensation,and dq0 detection method is utilized to detect voltage sags.Simulation results show that when the DVR is put into operation,the ’double high’ grid voltage sag caused by the three-phase short circuit of the electric arc furnace is effectively compensated,with remarkable treatment effects. |
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