| With the development of modern science and technology and the automation of production rising, kinds of sophisticated electrical equipments are widely used for electricity and power consumers demand higher quality power supply.The most common power quality problem is voltage sag. Dynamic voltage restorer (DVR) is one of the most effective tools to solve voltage sag. As a new power quality control device, DVR can move in a few milliseconds. DVR has an unparalleled advantage in the elimination of voltage fluctuation and flicker, and solving the phase voltage asymmetry and short-term power interruption. As with good dynamic performance and high cost, DVR has become the first choice when power users solve the problems of dynamic power quality, and especially the most economical and effective electrical device for solving the voltage sag problem.In this paper, working principle of any parts of DVR main circuit topology is demonstrated comprehensively. Compensation strategy, detection algorithms and control methods for the DVR are studied. The cascaded inverter is choosed as the main circuit topology and super capacitor as the energy storage device, which do not need extra energy storage device and series transformer, and help cut costs, reduce area and improve system reliability. Choose improved single-phase dq detection algorithm as the voltage sag detection algorithm which change single-phase instantaneous voltage from theαβstationary coordinate system transformation to the dq rotating coordinate system and can greatly reduce the computation.SPWM is choosed as the control technology of inverter which enables two-way transmission of energy; In addition, SPWM can use to overcome harmonic pollution generated by uncontrolled rectifier on the network side.Three basic compensation strategies are described in detail and they are pre-sag compensation, in-phase compensation and minimum-power compensation. They are compared by phasor diagram analysis. The relationship between injected active and load power factor are derived after the analysis of minimum-power compensation for DVR. Based on this, the improved minimum-power compensation strategy is proposed. The topological structure of DVR connected to the system is improved in this way that a thyristor-switched inductor is added across the load which is be carried out when DVR is compensating sags. Minimum power consumption is achieved with deliberate reduction of the power factor by the thyristor-switched inductor. Finally, minimum power compensation is simulated in Matlab7.0 and the feasibility of the design scheme is verified by simulation results which show that the proposed method reduces the power injection, enlarge the voltage sag range of pure reactive compensation and being able to mitigate the severer sag with longer duration.
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