Voltage sag detection technique for a dynamic voltage restorer

Dynamic voltage restorers (DVRs) are used to protect sensitive loads from the effects of voltage sags on the distribution feeder. This paper presents and verifies a novel voltage sag detection technique for the control system of a DVR. In some cases it may be necessary for the DVR control system to not only detect the start and end of a voltage sag but also to determine the sag depth and any associated phase shift. The DVR, which is placed in series with a sensitive load, must be able to respond quickly to a voltage sag if end users of sensitive equipment are to experience no voltage fluctuations. A problem arises when fast evaluation of these parameters is required, as standard information tracking or detection methods such as the Fourier transform or phase-locked-loop (PLL) are too slow. As a result of this the voltage sag detection method this paper proposes a new state-space matrix method, which is able to compute the phase shift and voltage reduction of the supply voltage much quicker than the Fourier transform or a PLL. The paper also illustrates that the state space matrix method returns results that can be directly interpreted whereas other methods such as the wavelet transform, which return results that can be difficult to interpret.