Influence of off-grid/grid-connected operation on stability of large-scale photovoltaic system

Abstract

Off-grid or grid-connected operation of large-scale photovoltaic system may cause stability problems. When the system performs off-grid operation, energy storage, as balance node, regulates power and frequency, then also influences stability. Under grid-connected operation, the external power system balances power and frequency regulation, and energy storage only equilibrium wave power. Although a detailed model for a single photovoltaic generation system has been established, the connection of numerous photovoltaic units is difficult to model and simulate, and it can easily cause dimension disaster. In order to analyze stability effectively of off-grid/grid-connected operation of large-scale photovoltaic system, we first establish the equivalent model of a photovoltaic farm through the method of dynamic aggregation of wind turbines, and then obtain indicators affecting the stability considering the reactance matrix of the transmission line. When the real part of the oscillation mode sensitivity index is greater than zero, oscillation mode approaches the imaginary axis, indicating that the PV system stability deteriorates. Conversely, the PV system is regionally stable. Finally, the effects of the number of photovoltaic units and transmission line length on stability are theoretically analyzed, and the theoretical derivation is validated through simulations. More PV units and a longer transmission line increase the likelihood of instability through the sensitivity of the oscillation mode.