Research On Dynamic Voltage Restorer Linked To DC-Modulated AC/AC Converters

The main function of DVR is to mitigate voltage sags on transmission lines so that the load is not affected by any variation of voltages. Nonetheless, there are some aspects that must be taken into account; the DVR is normally linked to the transmission line using a transformer. The main functions of the injection transformer include voltage boost and electrical isolation. The design of the transformer has to take in consideration the voltage drop, power losses, phase angle and saturation during the energisation transients. Hence, there is a great incentive to investigate whether a DVR can be designed without a transformer such that the reliability and the effectiveness of this new design can be enhanced and less costly. A new idea of using DC-DC modulation AC/AC technique instead of a transformer to link the Dynamic Voltage Restorer(DVR) to the transmission line is proposed in this thesis. Dynamic voltage restorer is used as series compensation to mitigate voltage disturbances. It injects a series voltage on the transmission line to compensate voltages distortions. This thesis presents a transformerless DVR that can mitigate long duration voltage sags and swells with lower losses and a flexible AC/AC conversion ratio. The use of DC-modulated AC/AC converters is a new technique that allows the DVR to be connected directly to the distribution network without using a costly and bulky injection transformer with no magnetization and conversion issues. Buck, Boost and Buck-boost converters are used to replace the transformer. With these converters the voltage ratio directly depends on the duty cycle of each converter. Compare to the transformer it has a more flexible voltage ratio. Simulation results are presented to demonstrate the efficiency against voltage disturbances of proposed DC-modulation technique used to replace the transformer in DVR application.The analysis in this project will be divided into two steps. Firstly, the author calculates the mathematical model of the DVR and the DC/DC converters. Secondly, the author will use the software PSIM to simulate the system operations.