A Fault Tolerant Control Strategy for Three-Level T-Type Inverter in LVRT/HVRT Operation

Three-level T-type inverter (3LT(2)I) has been widely applied in photovoltaic (PV) system. When the grid fault occurs, the low-voltage ride-through (LVRT) and high-voltage ride-through (HVRT) capabilities of 3LT(2)I are the key to increase the system reliability. However, the switch fault is easy to occur in LVRT/HVRT operation due to the overcurrent or overvoltage. And the existing fault tolerant control (FTC) strategies are not applicable to the grid fault conditions. Therefore, this paper proposes an FTC strategy for 3LT(2)I in LVRT/ HVRT operation. First, the active/reactive currents are modified and the negative sequence component of grid voltage is injected into the reference modulation waveforms to realize the fault ride-through. Second, the impossible vectors caused by the switch fault are abandoned to realize FTC. Then, the available vectors are selected according to the three-phase current polarities and the duty cycles are calculated to realize the neutral point (NP) voltage oscillation suppression. In addition, a compensation is introduced to eliminate the NP voltage deviation. Finally, simulations and experiments verify the validity of the proposed strategy.