Zero crossing determination by linear interpolation of sampled sinusoidal signals

Measuring and monitoring quality parameters of AC power systems requires several calculations, such the actual frequency and relative phase angles between each one of the three phases. These calculations must result in accurate values to comply with the function requirements; for synchronism function, frequency inaccuracy must be less than 0.01 Hz, and phase angle inaccuracy must be less than one degree. To perform these calculations, the zero crossing instants must be determined for each monitored signal. Conventional methods use a hardware circuit to perform these determinations resulting a high-cost system, because each monitored signal must have its own circuit. Averaging is not a good alternative due to the long time required besides there is no improvement in phase angle accuracy. The purpose of this article is to introduce a low-cost one cycle method to perform these calculations by software in a DSP using samples of the monitored signals.