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JEET, Vol. 11, No. 1, January 2016 
Fast Detection Algorithm for Voltage Sags and Swells Based on Delta Square Operation for a SinglePhase Inverter System
WooCheol Lee 
Area 
B  Electric Machinery and Power Electronics 
Abstract 
In this paper, a new sag and peak voltage detector is proposed for a singlephase inverter
using delta square operation. The conventional sag detector is from a singlephase digital phaselocked
loop (DPLL) that is based on dq transformations using an allpass filter (APF). The dq
transformation is typically used in the threephase coordinate system. The APF generates a virtual qaxis
voltage component with a 90¡Æ phase delay, but this virtual phase cannot reflect a sudden change in
the grid voltage at the instant the voltage sag occurs. As a result, the peak value is drastically distorted,
and it settles down slowly. A modified APF generates the virtual qaxis voltage component from the
difference between the current and the previous values of the daxis voltage component in the
stationary reference frame. However, the modified APF cannot detect the voltage sag and peak value
when the sag occurs around the zero crossing points such as 0¡Æ and 180¡Æ, because the difference
voltage is not sufficient to detect the voltage sag. The proposed algorithm detects the sag voltage
through all regions including the zero crossing voltage. Moreover, the exact voltage drop can be
acquired by calculating the qaxis component that is proportional to the daxis component. To verify
the feasibility of the proposed system, the conventional and proposed methods are compared using
simulations and experimental results. 
Keyword 
Sag and Peak Detector,AllPass Filter (APF),Digital PhaseLocked Loop (DPLL)

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