The sag detection strategy for seamless transfer APF-UPS system

In this paper, when voltage sag of the normal grid voltage occurs, detecting the peak voltage for fast mode transfer method is proposed. The conventional detector of sag voltage used in synchronous d-q frame by Digital Phase-Locked-Loop(DPLL). However using this method, the delay time is longer. Despite the sag occurrence of grid voltage, it cannot reflect a sudden change of the grid voltage in the moment of voltage sag, which causes a resulted peak value to be significantly distorted and momentary interruption. Sag detection method is proposed by reducing the delay time caused by these sag. Its effectiveness and capability in instantly detecting are verified via PSIM simulations and experimental results.