The voltages used for synchronization are filtered by band-pass filters with an adjustable tuning frequency. The transfer function of one band-pass filter is:
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The three-phase voltages on the primary side of the transformer are thus filtered by an adaptive band-pass filter. The six secondary (Y and Δ side) line voltages (phase to phase) calculated from the primary voltages are used to determine the zero-crossing times. For each voltage, there are two zero crossings in one cycle, one with a positive slope and another with a negative slope for a total of 12 zero crossings in each cycle.
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