Improved control strategies for three-phase grid-connected photovoltaic systems under grid-fault conditions

Abstract

During grid fault conditions, a distributed generation should remain connected for a pre-determined amount of time, and also provide reactive power to support the grid voltage. This is called low-voltage ride through (LVRT). LVRT control method for wind power generation systems under unbalanced and harmonic conditions is a well-developed research topic. However, too little attention has been paid to the LVRT control method for three-phase grid-connected photovoltaic (PV) systems under grid fault conditions. This thesis proposes improved control methods for a three-phase three-leg and a three-phase four-leg PV power converter under grid fault conditions. For a three-phase three-leg PV system, an improved positive-negative-sequence control scheme and an instantaneous active-reactive power control strategy are suggested. These schemes are used to cancel the double grid frequency oscillations in the active power and reactive power of a three-phase grid-connected PV during unbalanced grid condition. These methods are also effective to reduce the oscillations of Direct Current (DC)-link voltage that can be detrimental for DC-link capacitor. In order to track the desired unbalanced or harmonic reference current, enhanced proportional resonant (PR) current controllers with harmonic compensator have been designed using Bode frequency analysis. This study also suggests enhanced control method for a three-phase four-leg grid-connected PV system under unbalanced fault conditions using the combination of proportional integral (PI) and enhanced PR controllers using symmetrical components. Enhanced synchronization method for a three-phase four-leg grid-connected PV power converter operating in a three-phase four-wire system under unbalanced grid fault conditions using the magnitude and the phase angle of the positive, negative and zero sequence components is also presented. The proposed control strategy for the three-phase three-wire PV has the ability to cancel the double grid frequency oscillations in the active power, reactive power and also up to 55.5% reduction in the amplitude of the voltage oscillations under unbalanced grid fault conditions. The enhanced scheme for three-phase four-leg PV power converter operating in a three-phase four-wire system under unbalanced grid fault conditions has also the ability to cancel the oscillation of both the active and the reactive powers simultaneously.