Suppression of Multiple Fluctuations in DC-bus Voltage of the Grid-connected Photovoltaic Energy Storage System Using Extended State Optimal Control

Introduction: The wide application of photovoltaic (PV) generation has practical significance for reducing fossil energy dependence and environmental pollution. To smooth the fluctuation of PV’s output power and eliminate the coupled double-line frequency ripple in two-stage grid-connected PV systems, it is indispensable to find an effective solution. This study aims to propose a control scheme based on supercapacitor energy storage and extended state optimal tracking control method to solve the problem of disturbance and oscillation fluctuations in the DC-bus voltage in the two-stage distributed grid-connected PV systems. Methods: Firstly, the dynamic model of the SC subsystem is established. Secondly, a mathematical model including all the unstable states of reference input, step, and double line frequency disturbance signals is built. Then, the extended state space model is constructed based on these two models. Furthermore, the optimal state feedback controller is designed by using the optimal tracking strategy. Results: In the case of the DC-bus voltage control test, when the PV output power fluctuates, the SC makes a rapid response and compensates for the power deficiency. After such a dynamic adjustment process, the DC-bus voltage stabilizes at the reference value of 400 V, which achieves the accurate regulation of the DC-bus voltage. The maximum absolute error is 1 V, the maximum relative error is 0.25%, the maximum overshoot is 0.75%, and the settling time is less than 10 ms. In the case of the gird-connected operation test, the dynamic responses of the DC-bus voltage and its error also demonstrate the excellent performance of accurate tracking and suppression of multiple fluctuations. Accordingly, the MPPT control functions well. For instance, taking 1.5 s, the PV output power drops suddenly, and the active power demand increases at the same time, which makes the power difference reach the maximum. Then, the SC promptly increases its output current to provide power support. At this time point, the maximum overshoot of the voltage tracking is 4.5%, and the maximum absolute error is less than 1 V. Discussion: This study provides a novel idea and approach to address the problem of suppressing the multiple fluctuations coupled to the DC-bus voltage in the two-stage grid-connected PV system via the single-phase DC/AC inverter. However, the results are limited to simulations. Hence, based on the work of this article, further studies should be conducted in applying the method in real operation systems and adapting it to deal with the multiple fluctuations suppression of the grid-- connected PV systems under asymmetrical grid conditions. Conclusion: The proposed control scheme can not only effectively restrain the fluctuations in the PV output power and load demand but also suppress the double line frequency ripple so that the DC-bus voltage can always be accurately regulated to its reference value, which also guarantees the reliable realization of the PV’s MPPT control. Additionally, the dynamic and static performances of the system, such as overshoot, rapidity, and tracking error, are satisfactory