Research On Delay-dependent H_∞Control Methods For Unified Power Quality Conditioner In A Microgrid

As the transition from traditional grids to smart grids, microgrids can meet users’demand for a variety of power quality. But distributed generations and nonlinear loads in amicrogrid may probably cause some power quality problems such as the voltagefluctuation, harmonics and negative-sequence currents. In order to enhance the powerquality of a microgrid, the unified power quality conditioner (UPQC) gradually becomes ahot topic for research. This device can be used to compensate many power qualityproblems and an ideal option for improving the power quality of a microgrid. However,considering the detection methods, the limits of software and hardware, modeling errors aswell as parameter drift phenomena, there will certainly be delays and parameteruncertainties in the UPQC of a microgrid, which can make the whole system unstable andcan not work properly. This paper focuses on the stabilizing control methods for theUPQC in a microgrid under the influence of delays and parameter uncertainties.First of all, the power quality problems of a microgrid, the basic structure of theUPQC in a microgrid and the equivalent circuit of the UPQC are introduced briefly. Thestate-space model of the UPQC is established by using the Kirchhoff’s laws. Based on this,the cause and effect of delays in the UPQC are analyzed. The model of UPQC delaysystem is built and a control method for the UPQC under the influence of delays isinvestigated by using the Lyapunov-Krasovskii stability theory, the theory of delaysystems as well as the H∞control theory. Furthermore, the cause and effect of theparameter uncertainties in the UPQC are analyzed and the model of UPQC uncertain delaysystem is established. Considering the conservatism problem of the stability condition,another Lyapunov-Krasovskii functional is constructed to deduce a control method for theUPQC under the influence of delays and parameter uncertainties. Finally, the effectivenessof the proposed control methods is demonstrated by simulations. The mutual relationship between the H∞performance and the upper bound of the delay is analyzed.