Closed-loop state variable control of dynamic voltage restorers with fast compensation characteristics

Voltage sags are the most prevalent type of power quality disturbances, and the dynamic voltage restorer (DVR) can provide load voltage support with a power rating only a fraction of that of the load. However, most reported control strategies are using open-loop feed forward control, because of the fast voltage compensation requirement, in order to reduce the impact of voltage sags to sensitive loads. It is shown that the open-loop control produces poorly damped response due to the presence of the switching harmonic LC filter. Closed-loop control permits active damping of the LC filter, and a closer tracking of the reference load voltage. Multi-loop feedback PI control has attracted considerable attention, but it has difficulties meeting the fast dynamic response requirement because of the dynamic separation of two cascaded loops. This paper presents a closed-loop state variable control strategy to handle the time varying control references in voltage sag compensation applications. It removes the inner current control loop that is included in the multi-loop control structure, instead, the derivative of the output current is used to increase the dynamic response of the control system. Furthermore, a software based current observer is built to obtain the output current and its derivative, and to minimize the noise effects. The theoretical considerations are verified through simulation and experimental tests.