The Analysis And Control Of Voltage Sag In Distribution System With Micro-grids

As the growth of the demand and the consumption of oil, coal, natural gas and other traditional energy, we need to find renewable energy to sustain the growth of the future power.The unnecessary emissions caused by the combustion of fossil fuels are threatening our environment, clean energy sources like solar and wind energy should be gradually developed.Micro-grid can synthetically utilize a variety of energy sources, so it can satisfy the diversity of consumers’ demand. As an effective technical means of the distributed power grid, it has gotten wide attention. At the same time, the micro-grid is usually connected to the grid through a large number of non-linear electronic devices, which change the structure of the original system and bring many non-negligible power quality problems. The voltage sag has become one of the most outstanding problems. The voltage sag not only cause equipment damage, and may even cause huge economic losses. Therefore, researching voltage sag of the power grid system with micro-grid has great significance in improving the power quality.Firstly, this paper briefly introduced the basic concept of distributed power generation technology and micro-grid. Based on the basic structure and operation mode of micro grid, the mathematical model of wind power and photovoltaic distributed power generation were established. Then voltage sags in distribution power system with the wind and photovoltaic hybrid micro-grid were discussed. In the process of the voltage sag, the change of voltage amplitude and the power in the fault grid were further analyzed. In order to detect and compensate the voltage sag, the dynamic voltage restorer(DVR) was proposed.Secondly, aimed at the detection of voltage sag in distribution system with micro-grid, this paper proposed a fundamental voltage amplitude detection algorithm based on the single-phase rotating coordinate system. Using the formulas of trigonometric function, the dc component of the voltage sag amplitude were deduced under the d-q rotating coordinates axial. And the calculating formula of detecting voltage amplitude was deduced when frequency changed. From aspects of harmonic distortion, frequency change and phase angle jump, the performance of the proposed algorithm were verified. The simulation results showed that the proposed algorithm can avoid detection delay, and had good response speed and amplitude detection precision.Finally, to compensate voltage drop of the power grid system, an enhanced comprehensivecompensation strategy of DVR was proposed. Based on the basic control strategies of DVR, the DVR power flow and the maximum compensation time was analyzed. The strategy made transition from pre-sag compensation to the minimum energy compensation. When the DVR modulation index or dc voltage approached limits caused by deep sag, an iterative cycle control was adopted to avoid overshoot. The simulation results showed that the method can accurately compensated voltage amplitude and phase angle in real-time and reduced the energy loss,significantly increased the DVR compensation time.