Study On Control Technique Of Dynamic Voltage Restorer

With the high-precision load becomes more and more widespread, the requirement of power quality has been improved, even if short-time voltage sag will cause impact on the load. Dynamic voltage restorer (DVR), as a power electronic device to suppress voltage sag, has become a heat area of investigation.The action of DVR, which is offering compensation voltage to maintain the stability of load voltage, depends on quickly and accurately dectecting the voltage sag. When voltage sag occurs, the process of amplitude change and phase change will affect the performance of detection method. Thus, first of all, this paper analyzes voltage sag characteristics of seven different types, aims to provide a reference basis for the design of dectection method. An excellent voltage sag detection method needs to extract the fundamental component in the distorted grid voltage quickly and accurately, and also be suitable for detecting three-phase and single-phase voltage sag. The voltage sag detection method, based on discrete fourier transfer (DFT) algorithm, has high capability of filter but long detection time. However, the voltage sag detection method, based on short-time DFT algorithm, has short detection time but low capability of filter. Therefore, this paper proposed a voltage sag detection method based on improved short-time DFT algorithm. By means of disturbance filter, which is combined with several notch filters and a low-pass filter, the filter characteristic of short-time DFT algorithm has been improved, and the detection time is not delayed a lot. The proposed algorithm can gurantee shorter detection time and higher dection accuracy.The high cost of DC power supply will limit the capability of energy storage device, thus, to extend compensation time becomes very important. To avoid over-modulation, the amplitude of compensation voltage will limit the energy which can be released, and the output active power will determine the rate of discharge. Therefore, by means of adjusting compensation strategy, the compensation time can be changed. The essence of minimum-energy compensation strategy is to supply maximum compensation time. As a zero active power compensation, the compensation time is maximum, but as a minimum active power compensation, the compensation time is not always maximum, because the amplitude of compensation voltage is high, which will decrease the released energy. Therefore, this paper proposed an improved minimum-energy compensation strategy, the relationship between compensation time and phase of load voltage is established by the formula of capacitor discharge, and then, the optimum phase of load voltage is solved. The principle and characteristic of single-phase and three-phase compensation strategy has been analyzed. To avoid sudden change of the phase of load voltage, this paper proposed an optimal phase transition method for the improved minimum energy compensation strategy.DVR, as a device of voltage sag compensation, demands fast transient response, and the disturbance of grid voltage is a main impact on control performance. This paper described the DVR control strategy based on discrete state space, established the closed-loop control system model which is combined with proportional-integral (PI) and dicrete full state feedback, and solved the decoupling component of load voltage and grid voltage/load current. The discrete state feedback control is used to damp the LC resonance peak, which will enhance control bandwidth and stablilty. The feed-forward decoupling control is used to suppress the disturbance of grid voltage and load current, which will enhance transient performance. Although the control parameter design method based on direct pole placement is relatively simple, but it can not give a intuitive relationship between parameter design and control performance, especially the PI zero and its damping pole. Therefore, this paper proposed a conrol parameter design method based on virtual resistance. Firstly, by means of full state feedback control, an adjustable virtual resistence can be series in filter inductor. According to the requirements of stable state erreor, phase margin and Routh criterion in w domain, the variation range of PI parameter and virtual resistance can be solved, and then the appropriate control parameters can be selected. This parameter design method can reduce the number of parameters and simplify the design process.