STUDY OF CONTROL SCHEMES USED IN PARALLEL OPERATION OF THREE PHASE INVERTER

Abstract

Renewable Energy Sources (RES) are considered as the replacement of conventional energy sources. These RES can use wind energy, solar light, bio waste and can also be in the form of small hydro power units. These RES has very poor power quality and contains voltage fluctuations and variable frequency. These factors make RES a stability risk for the main utility grid. As a solution, currently inverters with different design techniques are being used as an interface between RES and main utility grid. The current study proposed a new technique "droop control" for inverter design. The conventional droop control technique which was already used in inverter design, has difficulty in synchronizing parallel connected inverters with different droop gains and line impedances. This control scheme does not use any predefined droop values for inverters and all inverters are responsible for the estimation of their own droop values with respect to their output power. Therefore, inverters are not bound to use same and static droop values which are considered as a vital communication link. The studied design technique has given very good results in a simulation run. When the Simulink model was run in parallel connected inverter with different line impedances, it was a success as both inverters started operating with same droop values as they were sharing an equal load. - Parallel connected inverters, the circulating current problem is still a major concern. The term circulating current describes the uneven current sharing between the units. This circulating current leads to, current distortion, unbalanced operation, which possibly damages the converters, and a reduction in overall system performance. Therefore, current sharing control methods become necessary to limit the circulating current in a parallel connected converter system. The performance of the discussed methods is verified in both constant and varying load conditions using MATLAB simulation.