Analysis And Improvement Of Transient Voltage Caused By The High Power AC Electric Arc Furnace

With the development of the country's economy and industry,the capacity of electric arc furnaces(EAF)has become larger and larger,and the impact of AC EAF on the power quality of the power grid has become more and more serious.When the arc is short-circuited open,not only steady-state power quality problems will occur,but transient power quality problem such as voltage sag will also occur.The research on electric power quality caused by EAF has been carried out for many years,but the problem of electric power quality is still prominent.How to establish an accurate AC EAF model suitable for transient voltage analysis and how to improve power quality of EAF is still an urgent issue to be solved.This paper focuses on the transient power quality problem of AC EAF,and studies how to accurately model and improve the power quality problem of EAF.First of all,according to the different burning states of the arc during the operation of the AC electric arc furnace,the proportional function,the hyperbolic function and the exponential function are combined to obtain the expressions of the arc voltage and the arc current under the different burning states of the arc,Then,white noise is added to simulate the random change of the arc,and the Chua's chaotic circuit is used to modulate the arc voltage to obtain the final EAF model.This model is not only suitable for studying the steady-state power quality of EAF,but also suitable for studying the transient power quality of EAF.The built AC EAF is connected to the power supply system for simulation analysis,and compared with the actual operating electrical characteristics to verify the validity and correctness of the built model.Secondly,Secondly,the power quality problem caused by the AC EAF is simulated,and the results show that the voltage fluctuation value,harmonic content,and three-phase unbalance caused by the EAF exceed the allowable value.Then conduct theoretical analysis and simulation research on the power quality of the EAF when three-phase short-circuit,twophase short-circuit,and one-phase open occur.The results show that: when a three-phase short circuit occurs,within 10 ms,the reactive power impulse is 6 times that of normal operation,the impulse current is 2.66 times of the rated current,and the effective value of the common connection point(PCC)voltage drops by 40.37%,resulting in a voltage sag.When a-phase and b-phase are short-circuited,and c-phase is disconnected,the current waveforms of a-phase and c-phase at PCC are almost the same,and the magnitude and phase are close to the same.The magnitude of the inrush current of b-phase is about twice that of a-phase and c-phase.Corresponding to the current change trend,the voltage of a-phase and c-phase dropped but the amplitude is small,the effective value of a-phase voltage dropped by 6.66% compared with normal operation,the effective value of c-phase voltage dropped by 3.03% compared with normal operation,the voltage of b-phase has a significant drop,the effective value of the voltage dropped by 29.81% compared to normal operation,and the three-phase unbalance is serious.Finally,a unified power quality conditioner(UPQC)model is built.Compared with other power quality adjustment devices,UPQC can simultaneously improve a variety of power quality problems.When the grid voltage fails,it can maintain the terminal voltage of load at the rated value and the waveform is a sine wave.When the load is a non-linear load such as an EAF,it can absorb the harmonics generated by the load and perform reactive power compensation to maintain the stability of the grid voltage.UPQC adopts dq0 detection method to detect current and voltage,and adopts direct control strategy.The UPQC model is connected to the EAF power supply system model for simulation.The results show that UPQC can improve the voltage fluctuation,harmonics and three-phase unbalance of the EAF.