Harmonic Oscillations in a Grid-connected PV Generation Farm Caused by an Increased Number of Parallel-connected PV Generating Units and Damping Controllers

This paper examines the harmonic oscillations in a grid-connected PV generation farm (PVGF) caused by the parallel connection of an increased number of PV generation units (PVGUs). An equivalent model of the grid-connected PVGF is derived, which clearly explains why there are internal and external oscillation modes in the grid-connected PVGF. An indicator of impedance multiplication (IIM) is proposed to quantitatively estimate the impact of the increased number of PVGUs in parallel connection. The analysis in this paper reveals the mechanism about why the damping of external oscillation modes may decrease when more PVGUs are in parallel connection under the condition that the IIM is positive. An example grid-connected PVGF is presented in this paper to demonstrate and evaluate the derived analysis and conclusions. A method for designing the damping controllers to ensure a negative IIM is proposed. With the damping controllers being installed, the risk of growing harmonic oscillations caused by the increased number of the PVGUs in parallel connection can be effectively eliminated.