Investigations on Voltage Regulation and Current Sharing in a DC Microgrid

Abstract

The increase in energy demand and depletion of fossil fuels have led to the development of distributed generation, which has given birth to the concept of the local microgrid. Microgrid integrates renewable energy sources into the grid. However, its working is highly dependent on power electronic converters, leading to stability problems and power quality issues. Initially, an AC microgrid was introduced, which converts all dc sources from renewable energy generation into ac sources. But it faces problems such as difficulties in synchronization to supply frequency, reactive power control, and skin effect. To overcome these problems, a dc microgrid came into existence, subsequently. It works on droop control which minimizes voltage regulation and enforces proportional current sharing. The major problem in droop control is the choice of droop constant. A low droop constant leads to low voltage regulation and unequal current sharing, but a high droop constant leads to increased voltage regulation and proportionate current sharing. Hence, there has to be a trade-off between voltage regulation and current sharing on the choice of a droop constant. To address these issues, a feature of droop control is also introduced to change the slope of output voltage and current on load variation. This thesis focuses on a complete DC microgrid structure to minimize voltage regulation across each load and proportional current sharing among generation sources. It works on adaptive droop control in both islanded and grid-connected modes. The proposed secondary distributed droop controller is discussed to fulfill the set objectives after performing its stability analysis. An inequality curve is drawn for each case to show the deviation of the distribution of currents from the equality line. The surplus power of the DC bus is stored in the battery through a dual active bridge converter operated in the triple-phase shift control to keep a broader range of power transfer with a limited current. The transfer of power is done based on the phase angle between the high-frequency transformer's primary and secondary terminal voltages. The dual active bridge converter follows two types of controllers to perform charging and discharging operations. The conventional controller is implemented during charging mode with an SoC estimator to avoid overcharging. On the other hand, the dual active bridge converter follows the distributed droop controller during discharging operation to provide equal current sharing among the battery as well as other sources, simultaneously minimizing load voltage regulation. Finally, the grid converter is operated in the direct power PWM control, which controls the instantaneous active and reactive power transfer. This method uses the space vector modulation technique, which deploys a suitable voltage vector to track the reference real power and reactive power values. Thus, the grid current is drawn such that it has low harmonic contents and nearly unity power factor. III Hence, the proposed system is shown to be capable of feeding both AC and DC loads overcoming various constraints. Moreover, enhanced current sharing accuracy and low voltage regulation are obtained with energy restoration capacity and supply them back during the night time. Keywords: Current Sharing; direct power control; distributed droop controller; dual active bridge converter; grid integration; inequality curve; pulse-width modulated converter; voltage Regulation.