Estimated Droop Control for Parallel Connected Voltage Source Inverters : Stability Enhancement

Abstract: Renewable Energy Sources (RES) are considered as the replacement of conventionalenergy sources. These RES can use wind energy, solar light, bio waste and can alsobe in the form of small hydro power units. These RES has very poor power qualityand contains voltage uctuations and variable frequency. These factors make RES astability risk for the main utility grid. As a solution, currently inverters with dierentdesign techniques are being used as an interface between RES and main utility grid. Thecurrent study proposed a new technique "estimated droop control" for inverter design.The conventional droop control technique which was already used in inverter design, hasdiculty in synchronizing parallel connected inverters with dierent droop gains and lineimpedances. The proposed "estimated droop control" does not use any predened droopvalues for inverters and all inverters are responsible for the estimation of their own droopvalues with respect to their output power. Therefore, inverters are not bound to usesame and static droop values which are considered as a vital communication link. Theproposed design methodology has made inverters independent from this only virtual linkof communication due to which the reliability of a system has increased. The proposeddesign technique has given very good results in a simulation run. When the Simulinkmodel was run in parallel connected inverter with dierent line impedances, it was asuccess as both inverters started operating with same droop values as they were sharingan equal load. The most important test was with dierent line impedances because inconventional droop control it is dicult for inverters to keep their synchronism withdierent line impedances and an unequal load sharing make inverters to deviate fromtheir nominal values and to generate dierent tracking signals for each set. This problemhas been successfully solved with estimated droop control as in this method each inverterset its droop gains according to its output power, which helps an inverter to operate atnominal values with dierent droop gains and line impedance.