Comprehensive Evaluation Of Voltage Sag In Distribution Network

In recent years,the continuous improvement of national economy and technology promoted the integrated chip manufacturing,intelligent production line,the rapid development of computer network technology and other industries.These new loads are very sensitive of power quality in power distribution network,especially the problem of voltage sag.Frequent sag events has brought huge economic losses to power users.In addition,the regional voltage sag level is also one of the main standards for power supply enterprises to investigate the power quality in the distribution network.Therefore,the comprehensive evaluation of voltage sag in distribution network is helpful to clarify the severity of voltage sag and can provide a basis for the prevention and management of voltage sag events.This paper firstly analyzes the subjective and objective weighting methods of voltage sag,and points out their advantages and disadvantages.The severity of the temporary drop increases significantly when the temporary drop value decreases more or the duration is longer.The punishment-incentive effect in the local state variable weight method gives the variable weight to the index change to reflect its influence on the evaluation results.For the voltage sag assessment on the load side,this paper is based on the tolerance curve of sensitive equipment,and uses the failure rate of sensitive equipment to reflect the severity of voltage sag,which is closer to the impact of the voltage sag event on the equipment.Finally,the accuracy and rationality of the proposed method are proved by the example analysis.Secondly,the comprehensive assessment of voltage sag of distribution network should reflect the information of the system side and the load side.Therefore,a voltage sag assessment based on two sides using complex correlation coefficient method is proposed,which combines the sag information of the system side and the load side to grasp the severity of the sag event as a whole event and considers the redundancy of the information.The superiority of this method is demonstrated by comparing it with other combination evaluation methods.In addition,the concept of concentric relaxation sag domain is centered on the fault point of the system side to study the effect of sag events on sensitive equipment.Based on the two-side assessment method of voltage sag,the thermal map of voltage sag is formed by calculating the different degrees of severity of voltage sag atdifferent positions in the sag region.The range of influence of voltage sag events and the severity of sag in each level can be defined in the voltage sag heat map,and the responsibility for the economic loss caused by the voltage sag can be divided.At the same time,the horizontal comparison of the thermal maps of different descending events can provide a more intuitive understanding of the descending situation,and the classification of descending events of different severity can facilitate the subsequent descending classification management.The method is proved to be reasonable and effective by the simulation in IEEE33 node system.Finally,a set of voltage sag monitoring system is designed and the selection of monitoring points is completed to provide hardware foundation and data support for the voltage sag assessment on the load side.The front terminal in the monitoring system takes STM32 chip as the core,and transmits the monitoring information to the background server through RS485.The host computer developed based on the E language platform realizes the visual management of the transient monitoring data.In a word,the accurate assessment of voltage sag level of distribution network with scientific and reasonable methods can provide a basis for the prevention and management of voltage sag events,and is conducive to the healthy development of voltage quality in power grid.