The Research On Detection And Control Method Of Dynamic Voltage Restorer

With the modern power quality problems growing, more and more users have higher requirements for the voltage quality of power system, the compensation devices has been developed rapidly to suppress a variety of voltage disturbances in power system. Among the modern power quality problems, the voltage sag is one of the most prominent and serious dynamic voltage quality problems. Dynamic Voltage Restorer (DVR) as the most economical and effective custom power electronic devices has become a research hotspot in the power quality control technology. The main purpose of this papar is to research the detection and control technologies in DVR. At first, we study the design approach of the single phase amplitude-phase locked loop(1MPLL), which is mainly used in the single-phase DVR or the three-phase independent compensation DVR that is applied to three-phase four-wire power distribution network; Then, A design approach of the three phase amplitude-phase locked loop(3MPLL) is proposed to design the three-phase three-wire DVR; Furthermore, After identifying the voltage sag and generating the reference compensation with the above amplitude-phase locked loop, the control strategy about the DVR are explored; Finally, we estimate and analysize the symmetrical components which are widely used in power quality monitoring and evaluation system.An optimized design approach about1MPLL is presented in this paper based on the ideas of objective function optimization; it is used to analyze the steepest descent direction of the objective function with the gradient descent algorithm, which ensures a fast and global convergence of phase-locked loop system. With reasonable objective function and appropriate parameters, amplitude-phase locked loop can fast and accurately track on the features (such as amplitude, phase and frequency and so on) of the fundamental frequency signal under interferences, which has the better abilities in frequency adaptability and anti-harmonic interference. Single phase locked loop presented in this papar has the advantages of simple structure, clear physical meaning and convenient parameter design, and provides synchronization information of distribution network voltage as well as the amplitude of voltage sag. Therefore, it is suitable for DVR system particularly. The DVR decide whether to initiate compensation with the detected amplitude information, when the voltage sag is appeared, the compensation algorithm is applied to generate real-time reference compensation output signal based on phase information from phase-locked loop. Compared with the traditional DVRs which detect amplitude and phase separately and inevitably increase the amount of computation, can not be used in real time, the proposed algorithm is more conducive to the actual implementation.A design method of a three-phase locked loop based on the conception of adaptive model reference is proposed, we designs positive and negative frequency adaptative sequence voltage component state observer and frequency adaptive rate by Lyapunov Stability Theory. This method provides the global asymptotic stability to the phase-locked loop and can detect the amplitude and phase of the positive sequence component; it is not affected by the negative sequence component theoretically, and avoids the disadvantages of SFR-PLL, in which the global stability is not discussed. The system structure is simple and conducive to set the system parameters, the algorithm takes into account the impact of frequency fluctuations on the phase-locked loop accuracy. Therefore, the system has a strong robustness for frequency fluctuations.The controller design method based on model predictive control and robust Hn control theory is presented to offset the insufficient robustness of the current DVR control methods. Predictive control methods based on non-parametric (impulse response or step response) models are complex with large amount of computation and more tuning parameters, it is difficult to be applied in the real-time control systems. In this paper, the DVR is altered by an inner current controller, and a predictive controller is designed with a simplified prediction model on the augmented control object. The model predictive control method reduces the impact of model errors on the performance of scroll optimization, feedback correction and other measures. The controller determines the current control action based on the future output state of the system; therefore, it reduces overshoot, improves the response speed of the system and meets the fast and accurate voltage sag compensation purposes of DVR. Since robust Hx control considers the uncertainty of controlled object parameter in advance, the control system has a strong robust performance. The uncertainty of the controlled object model is casued by LC filter parameter perturbations in DVR main circuit, we select the appropriate boundary function by considering the robust stability and robust tracking performance and designs the controller based on Hx control theory. The controller makes the DVR system has strong robustness against the parameter perturbationsA real-time symmetrical component estimation method of three-phase system based on minimum mean square learning rule neural network is presented; the symmetrical components can be used to identify voltage sags and other power quality monitoring applications. A symmetrical component model of three-phase voltage is established, the minimum mean square learning rule neural network is revised according to symmetrical component estimation of three-phase system, the algorithm estimate the symmetrical components by the neural network in real-time, which has less calculation, faster dynamic response and higher accuracy, can be used in power system in real time.