DC link voltage control of power conditioning unit during LVRT

Abstract

The increasing demand of photovoltaic (PV) solar systems and use of power conditioning unit (PCU) in these systems lead to new requirements for the operation and management of the distribution grid, especially during low voltage ride through (LVRT) operation period. LVRT requires PV system to remain as grid connected and it will help to maintain grid voltage and frequency stable during the unexpected condition. This paper proposes a dc-link voltage control of power conditioning unit during LVRT operation. To observe the changes in grid voltage during the LVRT, the high modulation ratio (HMR) is maintained by controlling dc link voltage to attenuate the high frequency harmonics injected into grid. The low voltage operation affects the power quality at point of common coupling (PCC), as a result of the reduced grid current quality induced by the oscillated dc-link voltage. The dc-link voltage can be decreased to a particular value to increase HMR when needed, without weakening the LVRT capability, in order to attenuate the higher order harmonics of output waveform. The control method suggested here will attenuate the double-line frequency dc-link voltage ripple during the asymmetrical grid faults. Due to this, the dc-link voltage will attain the safe operational range value by transferring power ripple to the front-end dc input source. The results are verified in MATLAB/SIMULINK.