Research On Voltage SAG Monitoring Device Based On DSP

Voltage sag is the most typical, and the most serious dynamic power qualityproblem. It’s the main reason leading to malfunction or damage of sensitive electricalequipment. Therefore, monitoring the voltage sag characteristic features is an importantbasis for power quality assessment and governance.Firstly, the paper pays great attention to the three reasons of voltage saggeneration, which includes short circuit fault, the transformer operation and inductionmotor starting. It also offers the analysis on the characteristics of voltage sag caused bydifferent situations through MATLAB simulation. On this basis, several voltage sagdetection methods are analyzed separately in detail. On the basis of discussing andcontrasting the detection of voltage sag characteristic features by RMS method, thefundamental component method, wavelet transform and dq transformation, the papergives an assessment of advantages and disadvantages of these algorithms.The traditional algorithms take over-reliance on the detection of sag depth whendetecting the beginning and ending moment of sag. They take too much time to capturethe occurring of sag by measure the sag depth. Therefore, the paper proposes a detectionmethod of sag disturbance based on the image edge detection operator. The method canidentify the starting and ending moments of sag accurately by using of the geometriccharacteristics of sag waveform. With the advantage of detection speed, the method canlocate the point of time just in two sampling time after the occurring of sag. Besides, thepaper also researches on the inhibition of the background gradient of the image edgedetector output.In order to shorten the delay of the detection of sag depth and phase transitions, thepaper makes an improvement of the existing single-phase dq transformation algorithm.Thus, the delay angle can be reduced to arbitrarily small when the virtual three-phasesystem is constructed. By this way, the detection of sag depth and phase transitionsbecomes almost “instantaneously”. In addition, the paper gives an analysis on the"transaction" phenomenon of the improved single-phase dq transformation algorithmwhich may occur near the perturbation. It also offers the solution based on the open-closing morphological operations.On the basis of research on algorithms of voltage sag detection, the paper describesa designation of sag detection equipment based on DSP TMS320F28335. The systemcan function well on sag detection, basic power parameters measurement, faultrecording, human-system interaction and ethernet communications. Moreover, in orderto receive the monitoring information of each sag detection equipment through ethernetcommunications, the paper offers a design of the upper master control software based on LabVIEW. At last, it shows the experimental results of this system. The results showthat it can meet the demand of accuracy and real-time performance.