Per-Phase Active Power Distribution Strategy for Three-Phase Grid-Tied Inverters Under Unbalanced Conditions Without DC Sources

A novel per-phase active power distribution (PAPD) strategy for three-phase grid-tied inverters (TPGIs) without the need for a dc source is proposed in this article. Under the grid fault or the unbalanced load situation, the TPGI plays an important role to adjust the power flow to improve the quality and stability of the power system. However, the dc source is always needed for active power delivery, so the TPGI should be installed near the dc source, which lacks flexibility. With the novel approach of the proposed PAPD strategy, the TPGI can supply reactive power according to a low-voltage ride-through (LVRT) code to support the unbalanced grid voltage sag and adjust the active power flow among three phases to balance the loads, without the need for a dc source. To meet the complicated operation conditions, the proposed PAPD strategy is composed of two parts: the active power limitations (APLs) and the current balancing control (CBC) method. The APL determines the active power flow limitation to avoid the overcurrent problem. The CBC method can equalize phase currents and balance the current stress among power switches, so the reliability of the TPGI can be improved. A 5-kVA prototype TPGI has been built and tested under different fault conditions. Hardware experiments obtained from different operation cases validate the performance of the proposed PAPD strategy.