This paper proposes a smooth transition control strategy for voltage source inverters between standalone (SA) and grid connected (GC) modes of operation. In both GC and SA modes of operations, the inverters use the droop control method to regulate the real power flow without the need of external communications between them. One of the distributed energy sources working in the microgrid system is designated as a dispatch unit to facilitate interconnection with utility grid and achieve smooth transition between the modes of operations. In the transition modes the dispatch unit takes extra responsibility and ensures the continuous power delivery to the load. For smooth transition from grid connected to stand alone, the dispatch unit compensates for the grid current instantly after the transition and then gradually it makes the other sources share that current. The dispatch unit also adjust its output power to make the transition from standalone to grid connect as that power affect the frequency through the droop control. The performance of the proposed control technique has been verified with simulation and experimental studies.
This paper proposes a closed loop control method to reduce the voltage harmonics of the weak utility grid through distributed energy systems (DES). It is desired to have uniform voltage and frequency around grid. The grid voltage gets distorted due to the nonlinear behavior of the distributed loads and sources. It is possible to correct the distorted utility grid locally at the point of connection with the DES. The proposed method adds to the responsibility of the DES in estimating the grid voltage harmonics and injecting additional current to rectify the grid voltage pollution. Recursive Least Squares Estimation (RLSE) technique has been implemented to estimate the grid voltage harmonics, which works harmoniously with closed loop harmonic voltage controller. DES with the proposed control technique reduces the voltage harmonic in addition to providing the sustainable energy to the utility grid. The results are verified with the simulation and experimental tests on a 5 kW utility interactive energy conversion system.
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