Study On Effect Of Voltage Sag On Sensitive Equipment And Evaluation Method Of Severity

With the development of power grid and the significant increase of distributed generation,the electric power system have become more and more complex and diverse.Voltage sag events,caused by short-circuit fault,overload,switch operation,large motor starting and the access of distributed generation,more frequently occur.On the other hand,with the augmentation of the sensitivity of electrical equipment to power quality,especially for adopting computer,power electronics and automation technology to control precise process,new energy generating set and electric vehicle charging infrastructure,voltage sag events will cause immeasurable loss to equipment production and operation.Frequent voltage sag events have become one of the main power quality problems,which have great influence on reliable power supply and normal operation of electrical equipment.This paper studies the effect of voltage sag on sensitive equipment and sets up their tolerance curve.As for evaluation system of voltage sag proposed by IEEE P1564,This paper Points out that it doesn't consider the agglomeration effects of multiple voltage sag.On the basis of this,the evaluation method for multiple voltage sag severity is studied.This article main content is as follows:First,for sensitive equipment,Including ac contactor,electronic appliances,lighting lamps,this paper analyzes the influence of voltage sag on them in theory,and set up their tolerance curve through experimental study.For ac contactor,it's equivalent magnetic circuit model is established,and the instantaneous expression of electromagnetic suction is derived;According to the equivalent circuit of switch power supply in electronic appliances,this paper analyses the changing characteristics of DC side voltage when the AC voltage sag occurs.For lighting lamps,such as high intensity discharge,LED lamp,fluorescent lamp,combined with the equivalent circuit and their material properties,the influence of voltage sag on them is analyzed.Their tolerance curves are set up through experimental study.Moreover,this paper studies the characteristics of effect of different starting angle and phase angle jump on them.Second,Based on steady state equivalent circuit of induction generator,this paper gets the electromagnetic torque-slip characteristic curve of induction generator,analyses the influence of voltage sag on induction generator and defines the critical duration and critical value under voltage sag.Taking that the slip of induction generator during voltage sag can't exceed the critical slip as critical condition,the critical duration of induction generator is deduced.And taking that the maximum electromagnetic torque of induction generator during voltage must outweigh the mechanical torque as critical condition,the critical voltage amplitude of induction generator is derived.The correctness and effectiveness of the proposed method is validated by the calculation of 3MW induction generator connected to the power distribution network.Last,this paper analyses the agglomeration effects of multiple voltage sag on the time scale.For irregular multiple voltage sag,converting its voltage timing curve into voltage duration curve,and defining its average over-limit duration time difference and over-limit probability.Comparing the pros and cons of many kinds of conventional severity index,this paper builds a more elaborate improved voltage sag severity index by superposition of the sensitivity of multiple duration intervals which voltage sag goes through.Combining the improved voltage sag severity index and voltage duration curve,this paper establishes a evaluation method for multiple voltage sag severity.According the analysis on test data of a 110 kV substation and IEEE 1159.2,the correctness of the proposed scheme is validated.