LPV Control of Current Source Inverter synchronized with the grid

Abstract

Power inverters are widely used in grid connected applications, specially with the increasing use of renewable energies, to improve power quality and increase efficiency. However some parameter associated to the grid and/or the inverters, such as the grid frequency, needs to be known or properly estimated. This estimation introduces an uncertainty in the system and suffer transients due to the estimator. Linear Parameter Varying (LPV) control can be used to account for those uncertainties. This paper presents a feedback control based on a LPV control law to improve the power quality of a grid connected Current Source Inverter (CSI), significantly reducing the total harmonic distortion (THD) of the grid generator current. A linear model of the interconnection is presented, where the frequency of the grid is assumed variable being estimated together with its phase through a Phase Locked Loop (PLL). The LPV control law is implemented through state feedback achieving both, harmonic suppression and reference tracking, using the estimation of the frequency of the PLL. Implicitly, this allows for the estimation of the magnitude and phase of the harmonic distortion canceling it up to the CSI current limit. An analytic proof of the filtering guarantees of the method is presented along with simulation results that show the practical viability of this technique. It is shown that this control approach is capable of an appealing adaptation to changes in the frequency of the power grid with a low computational burden being able to also cope with disturbances in the estimation of the frequency due to the PLL.