A very fast unblocking scheme for distance protection to detect symmetrical faults during power swings

Power swing blocking function in distance relays is necessary to distinguish between a power swing and a fault. However the distance relay should be fast and reliably unblocked if any fault occurs during a power swing. Although unblocking the relay under asymmetrical fault conditions is straightforward based on detecting the zero- or negative-sequence component of current but symmetrical fault detection during a power swing presents a challenge since there is no unbalancing. This paper presents a very fast detection method used to detect symmetrical faults occurring during a power swing. Based on a 50 Hz component getting on three-phase active power after symmetrical fault inception and using Fast Fourier Transform (FFT), the proposed detection method can reliably and quickly detect symmetrical faults occurring during power swing in one power cycle, i.e. 0.02 second. This detection method is easy to set and immune to the fault inception time and fault location. Power swing and fault conditions are simulated by using software PSCAD/EMTDC
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. FFT is performed by using On-Line Frequency Scanner block included in the software.