Grid Synchronization Techniques and Analysis in Distributed Power Generation Systems under Unbalanced Conditions | Original Article
The grid connected Distributed Power Generation Systems (DPGS) experiences more sensitive behaviour against small variations occurs in grid parameters. The variations or unbalance in grid parameters are mainly due to the presence of voltage sags, frequency variations, and distorting harmonics in the grid. The adverse effect of unbalanced grid conditions may create the failure in synchronized operation between DPGS and its connected grid.In this work to provide continuous synchronization for DPGS during both balanced and unbalanced condition of grid, two novel grid synchronization techniques based on Phase Locked Loop (PLL) studied are (i) Modified Double Decoupled Synchronized Reference Frame PLL (ii) Modified Double Second Order generalize integrator PLL. The main aim of the presented modules is to provide sequence components of grid voltage. These sequence components helps in determining magnitude and phase angle of unbalanced grid. The presented PLL configurations are worked based on synchronous reference frame algorithms i.e. the Park’s conversion method is applied to convert 3-phase ABC voltage vector to rotating d-q reference frame components. The generated positive sequence d-component is used to determine magnitude of utility voltage and the positive sequence q-component is used to get phase angle of grid voltage using Phase Locked Loop (PLL) circuit. The PLL circuit also maintains the frequency and phase angle of DPGS same as the determined new phase angle value of grid voltage. The equivalent simulation circuit of each module is developed using MATLAB Simulink. Thesimulation of each module is carried out by creating different unbalanced conditions of grid. From the simulation results it can be observe that the generated positive sequence d- component provides the magnitude of grid voltage accurately during both balanced and unbalanced conditions. Similarly the determined positive sequence q-component is utilized by Phase Locked Loop circuit for providing phase angle plot of grid voltage accurately. The time required for each module in determining phase angle and magnitude of grid voltage after creating faultis within 20-25ms i.e. within two cycles of faulted voltage. The presented techniques are also tested to observe their response in the presence of harmonic distortions. In all grid unbalanced conditions both techniques provide satisfactory results with free of harmonics.