Online keeping control of a grid-tied PV inverter under abnormal grid voltage and frequency based on a boundedness indicator

Abstract

In this article, two targets of a grid-tied photovoltaic (PV) inverter are investigated: 1) The grid-tied PV inverter always remains online even under abnormal grid voltage or frequency; 2) And it does not deliver power to ac grid when ac grid is abnormal. To achieve such targets, an online keeping control method for the grid-tied PV inverter is proposed under either abnormal grid voltage or frequency. The control system design is based on a boundedness indicator of ac grid, where the boundedness indicator is the combination of a voltage indicator and a frequency indicator through two bounded filters. The boundedness indicator is further introduced into the maximum power point tracking (MPPT), the PV voltage setting, and the PV voltage regulation. Through the proposed design, the grid-tied PV inverter can have normal operation with maximum power acquisition, when both grid voltage and frequency are within predefined ranges. And it stops to deliver real power to ac grid when either grid voltage or frequency is out of the predefined range. The grid-tied PV inverter can continuously connect to ac grid to have good fault ride-through capability, when solar resource is available. The proposed design is validated through both experimental and simulation investigations of a dual-stage grid-tied PV inverter with smooth transients between normal operation and grid fault operation.