Abstract
Parallel-connected uninterruptible power supply (UPS) systems have been used to maintain power supply to the critical load in order to increase power capacity and system reliability. This paper presents a robust and precise voltage control strategy for parallel-connected UPS systems. Each parallel-connected UPS system consists of a three-phase inverter with an output inductor-capacitor (LC) filter directly connected to an AC common bus in order to feed the critical load. Fractional-order sliding mode control (FOSMC) is proposed to maintain the quality of the output voltage despite linear, unbalanced and/or nonlinear load condition. The Riemann-Liouville (RL) fractional derivative is employed in designing the sliding surface. The voltage control strategy effectively eliminates the parametric uncertainties, external disturbances, and reduce the total harmonic distortion (THD) of the output voltage. Furthermore, it also maintains very good voltage regulation such as dynamic response and steady-state error under the nonlinear or unbalanced load conditions. The stability of the proposed controller is proven by applying Lyapunov stability theory. Droop control approach and virtual output impedance (VOI) loop are investigated to guarantee the accurate active and reactive power-sharing for parallel-connected UPS system. Finally, the implementation of the control scheme is carried out by using MATLAB/Simulink real-time environment.
Highlights
The increasing demands for sustainable, clean, and environmentally friendly energy call for distributed renewable energy resources to frequently be used to support electricity demand
MATLAB/Simulink simulating performance proposed illustrated through the real-time real-time single and multipleparallel-connected parallel-connected uninterruptible power supply (UPS)
The main objective of this paper was to design a nonlinear voltage control strategy for single and and multi-parallel connected system to maintain the quality of the output voltage despite multi-parallel connected UPS system to maintain the quality of the output voltage despite linear, multi-parallel connected system to maintain the quality of the output voltage despite linear, linear, unbalanced, andchange step load change current conditions
Summary
The increasing demands for sustainable, clean, and environmentally friendly energy call for distributed renewable energy resources to frequently be used to support electricity demand. The critical safety-related loads require extremely reliable and high-quality power to function efficiently. Uninterruptible Power Supplies (UPSs) play a key role to provide a continuous power supply and protection against interruptions. When a power outage takes place, a UPS must deliver an emergency backup power to a load and prevent damages. An ideal UPS provides unity power factor, low Total Harmonic Distortion (THD) sinusoidal, high efficiency, low cost, high reliability, electric isolation, and low transients response time during the transition from grid mode to backup mode and vice versa. The most important task of UPS is to maintain the pure sinusoidal output voltage with low THD under linear and distorted load conditions. According to Energies 2018, 11, 3475; doi:10.3390/en11123475 www.mdpi.com/journal/energies sinusoidal output voltage with low THD under linear and distorted load conditions.
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