Research On The Rapid Detection Methods Of Voltage Sag

In recent years,with the development of power grids and the continuous increase of new types of electrical loads,the importance of power quality has increased significantly.Improving power quality has become the focus of research.Among many power quality problems,voltage sag is considered as a very serious threat,which can cause damage to sensitive equipment and bring about significant economic losses.In actual work,optimizing the power system structure can reduce the probability of a voltage sag.Another way to suppresses voltage sag is to install management equipment.Accurately and effectively detect the various characteristics of voltage sag is the critical prerequisite of the effective control of voltage sags.The main work of this paper is as follows:Firstly,this paper briefly introduces the concept of power quality,elaborates on the definition of voltage sag and the reasons for its occurrence.It summarizes several commonly measures for voltage sag control,and analyzes the research status of voltage sag detection.Some voltage sag detection algorithms are listed and compared,including the methods based on peak voltage,RMS,single-phase voltage conversion average,dq transform,wavelet transform,etc.Meanwhile,parts of algorithms are simulated,and it summarizes the advantages and disadvantages of different detection methods.Secondly,this paper proposes a single-phase voltage sag detection algorithm based on the ensemble empirical mode decomposition(EEMD)and the two-points method.When voltage waveform contains harmonic distortion and noise,the calculation results of two-point method will have large fluctuations,and have significant errors.To solve this problem,the EEMD method is used to extract the fundamental component of the voltage and filter out harmonics and noise.Then,the amplitude and phase angle of the fundamental voltage is calculated through two-point method.At the same time,a method for constructing the data sequence which required for online calculation is also given,so that the proposed method can meet the needs of online detection.The simulation results show that the EEMD method can effectively filter the harmonic interference component,and dealing with the extracted fundamental component by the two-points method can quickly and accurately detect the voltage sag.Thirdly,an online detection method of voltage sag based on improved HHT is proposed.The traditional HHT method has the problem of end effect.In order to avoid the effects of end-point effects,autoregressive integrated moving average model has been used to extend the voltage series,and the prediction error is reduced through the cosine window function.The extended sequence is analyzed by HHT algorithm.Then,the Hilbert spectrum of voltage data and the characteristic information of voltage sag can be obtained.The simulation results show that the end effect is suppressed by the proposed method.The start and end moments of the voltage sag can be quickly and accurately located.The errors of calculation results of voltage amplitude and frequency are within the allowable range.Finally,a voltage sag governance scheme for dual power supply network is designed.When a voltage sag occurs,the scheme begins to determine the location of the sag disturbance source,and it can also determine its type.When a single-phase short circuit occurs on the user side,the fault location can also be located.By issuing corresponding instructions,it can effectively ensure the normal operation of important loads.