Small signal stability analysis of a battery connected standalone photovoltaic system
This paper presents the small signal stability (SSS) analysis of a standalone photovoltaic (PV) system with battery as backup source for power management between the source and the load. The stability of this system needs to be analysed for its proper and smooth operation. Lead-acid battery is commonly used in high power PV applications due to its low cost and availability in large size. Three independent control loops are proposed to control the standalone PV system; MPPT control loop to extract maximum power from PV module under varying solar irradiation, battery control loop for bidirectional power flow between battery, dc-link through buck-boost converter to maintain constant dc input voltage, and inverter control loop for maintaining tight voltage regulation and achieving fast dynamic response under sudden load fluctuations. State-space modeling of the standalone system is discussed for the SSS analysis. The stability of the above control loops are verified by using Bode diagram. Finally the proposed method is applied to 2 kW, 110 V, 50 Hz two-stage single-phase standalone PV system. The simulation and the experimental results are presented to validate the proposed control strategy and its stability.