Three-phase photovoltaic inverters during unbalanced voltage sags: Comparison of control strategies and thermal stress analysis

Unbalanced voltage sags are common and lead to instabilities in grid-connected photovoltaic systems. Such unbalanced faults generate issues in the control of negative-sequence reference voltage. Consequently, active power oscillations appear in the system, which has a negative impact to the DC bus voltage control. In order to solve this problem, four different power control strategies for unbalanced voltage conditions are analyzed in this paper: Instantaneous Active-Reactive Control (IARC), Positive- and Negative-Sequence Control (PNSC), Average Active-Reactive Control (AARC), and Balanced Positive-Sequence Control (BPSC). Focusing in dynamic response of the system and in the thermal stress of the switches, this paper provides a clear comparison between the aforementioned technique when applied in three phase photovoltaic systems. In terms of DC bus voltage control, IARC and PNSC present no oscillations and better response. However, IARC, BSPC and AARC are the ones with better thermal response.