Robust fractional-order super-twisting sliding mode control to accurately regulate lithium-battery/super-capacitor hybrid energy storage system

One of the main challenges in front of the inventors and researchers to compensate the power quality problems is the existence of an efficient and reliable electric energy source for dynamic voltage restorer (DVR). Recent advances in the technologies of lithium-battery (LB) and super-capacitor (SC) have made them possible to be used as hybrid energy storage system (HESS) for DC-power supply of DVR. But then, a robust and efficient controller must be essentially designed to compensate the intricate correlation between system components, unknown nonlinearities, parameter uncertainties related to the HESS. Hence, fractional-order super-twisting sliding mode control (FOSTSMC) is proposed for LB/SC-HESS to smoothly and accurately smoothly track the current reference and control the DC-link voltage of the asymmetric half-cascaded multilevel inverter of DVR during different compensation conditions. As the primary controller, FOSTSMC has compensated the unknown nonlinear parts and parameter uncertainties related to LB/SC-HESS through the real-time disturbance estimation using sliding mode state and disturbance observer (SMSDO). Both the battery current and DC-link voltage due to the physical measurability are chosen to design the control scheme based on the proposed FOSTSMC. The proposed controller has been compared with the proportional integral derivative and the sliding mode control under different probable scenarios to evaluate and validate its observability and controllability. To better present the effects of FOSTSMC, AHCMLI, and HESS-based DVR, they have been quantified as follows: Three real-time stability benchmarks that is, overshoot, undershoot and settling time are respectively attained for FOSTSMC: 0.6, 0.4, and 0.03; for sliding mode control: 1.1, 0.7 and 0.019; for proportional integral derivative: 0.9, 2, and 0.117. That is to say, FOSTSMC can smoothly and accurately track the current reference and control the DC-link voltage during different compensation conditions as compared with other controllers. Asymmetric half-cascaded multilevel inverter can provide 33 steps with consideration of 12 unidirectional switches, whereas binary, trinary, and other multilevel structures can provide less than 27 steps with consideration of 12 switches. The quasi-AC voltage synthesizer-based DVR can completely and accurately compensate the voltage sag and swell with THD of 2.12 and 2.35, respectively. The THD of mal-compensation voltage sag and swell using the conventional DVR are respectively 11.57 and 13.86.

Automated summary

In order to address power quality issues in the DVR, an efficient and reliable power source and controller are required to compensate for uncertainties and non-linearities in HESS, enabling smooth tracking of current references and control of DC-link voltages.