Introduction: This paper is the result of the research “Design and control of single phase voltage compensator based on AC Chopper” developed in the AD-AF Academy of Viet Nam in 2019. Problem: This article develops power electronic devices based on AC chopper and provides solution for the voltage sag which is long-lasting and deep amplitude of fading at rural power grid. Objective: Improve the quality of the electricity system in rural Vietnam. Methodology: With the improvement of power system, power quality has become the major concern of user and grid companies. The Voltage sag has turned into the research priority because of the most frequently happening and great economic loss. Based on the rounded analysis of traditional voltage compensation devices, this article develops power electronic devices based on AC chopper and provides solution for the voltage sag which is long-lasting and deep amplitude of fading at rural power grid. Results: On the one hand, it reduces the cost and space of the device. On the other hand, it solves the short compensation time of UPS and DVR. Conclusion: The trial operation proves that this single-phase low-voltage compensation device has obvious compensation effect for most resistive loads and some inductive loads, less harmonic content, long continuous compensation time and stable performance. Originality: This compensation device integrates the advantages of UPS and DVR, and also solves their shortcomings and deficiencies. Limitations: The authors need to spend more time to the development and design three-phase low-voltage compensation device.

Introduction: This paper is the result of the research “Optimal LQG controller to open rudder supplying water to turbine of small and medium hydro power plants” developed in the Electric Power University in Vietnam in 2019. Problem: To maintain the frequency of the emitted voltage of the generator at the nominal value of 50 Hz, the authors present a solution to apply the optimal control theory to create a command to control the rudder angle to adjust the water flow into the turbine. Objective: The article presented about the order value formation algorithm, to stabilize the frequency of transmission voltage at 50 Hz standard value. Methodology: In this paper, the laws to control the rudder supplying water to turbine of small and medium hydro power plants are synthesized by optimal control theory. To establish optimal control rules, the paper proposes using the Kalman filter to estimate the state of the object. By that, the frequency of generated voltage will be stabilized under changing load. The efficacy of the steady and dynamic performance of the control strategy was verified using Matlab/Simulink software. Results: The proposed system can compensate for power fluctuations and is effective in terms of power regulation. Conclusion: The algorithm presented in the paper is the basis for setting up the software when when designing and manufacturing the turbine - generator combination. Applying this algorithm, the process of adjusting the transmitted power according to the required load will be performed with quality. Originality: This paper’s contribution lies in its employment of an effective optimal LQG control for varying operating conditions. Limitations: The authors need to spend more time to the development and and test this algorithm and and implementing it in practice.