Research On Detection Of Voltage Sags And Its Countermeasures

As one of the most serious problems of power quality, voltage sag gets close attention of domestic and foreign researchers, because of its hazards, wide impact and frequent occurrence. Voltage sag for several frequency cycles will lead to the operation interruption of the sensitive equipment and result in huge economic losses. The detection of voltage sag characteristic parameters and its suppression measures are the basic work to improve transient power quality problems and reduce the frequency of voltage sag. In this paper, voltage sag detection method and suppression measures are researched. Main contents are as follows:( 1) This paper introduces the concept, causes, harms and suppression methods of voltage sag. On this basis, it analyzes and compares several kinds of voltage sag detection algorithms, including the methods based on RMS, peak voltage, the fundamental component, dq transform, Hilbert-Huang transform, short time Fourier transform, wavelet transform and S-Transform. Meanwhile, parts of the methods are simulated, and the advantages and disadvantages of various algorithms and their applicable scope are summed up.( 2) In this paper, a new voltage sag detection method based on local mean decomposition is proposed. A new method of waveform extension which names horizontal and vertical proportion extension method is proposed to improve the end effect of LMD. Firstly, the lifting wavelet is applied to denoise original signals and horizontal and vertical proportion extension is performed on the denoised signals. Then the extended signals are decomposed by LMD. Finally, the feature information of voltage sag is extracted from instantaneous amplitude and instantaneous frequency obtained by LMD. The simulation waveforms and actual waveform of voltage sag are detected and analyzed by using this method, and the simulation results illustrate that the proposed method has good noise immunity and its end effect is not obvious, which can detect the amplitude, starting and ending time and frequency of voltage sag effectively.( 3) The suppression measures of voltage sag are simulated and analysised in this article which use an actual power of domestic oilfield as an example. The model of the power grid is build on PSASP. By calculating the trend of the grid and voltage sag under a variety of short-circuit case, and combing with its own situation, characteristics of the grid voltage sag were obtained. On this basis, the article analyzes the effect of the limiting reactors and STATCOM to suppress sensitive target bus voltage sag. The results show that limiting reactors can effectively suppress sensitive targets bus dips degree under fault conditions of non-circuit, but they will cause some loss of power during normal operation; STATCOM can provide compensation to the grid quickly and efficiently, and the greater the installed capacity, the better the compensation. In addition, practical measures to suppress the field grid voltage sag are put forwarded by combing the actual situation of the grid and the results of previous simulation analysis.