A novel control strategy for an ultra-capacitor based dynamic voltage restorer with controllable dc-link voltage

Issues related to power quality have become more critical in recent times due to the increased penetration of renewables into the power grid. Voltage sags and swells are amongst the most important issues associated with a power grid, and extensive research has been directed towards the mitigation of these issues. In the DVR control strategies discussed so far in literature, complex vector control techniques are employed in the inverter for the generation and injection of compensating voltages. Also, the absence of a buck-boost converter in some of the existing DVR systems necessitates energy storage of high voltage rating. The DVR system discussed in this paper has a DC/AC inverter, a bidirectional buck-boost converter and an ultra-capacitor as the energy storage device. This paper proposes a new control strategy, wherein the output voltage (DC-link voltage) of the bidirectional buck-boost converter is dynamically varied based on the magnitude of sag/swell. The SPWM inverter operating at constant modulating index then inverts this DC voltage. Ultra-capacitor, due to the many advantages it offers over the conventional battery, has been used as the active source for the DVR. Simulations have been carried out for three phase balanced voltage sags and swells to validate the control strategy. Experimental results for a reduced scale hardware prototype are underway, some key results of which are included in this paper.