Instantaneous phase-angle estimation algorithm under unbalanced voltage-sag conditions

If a negative sequence is generated by voltage sag and/or unbalance, it appears as an oscillating error in a synchronous reference frame (SRF). In power conditioning equipment, the exact value of a positive sequence is needed to achieve the unity power factor and constant output voltage, whereas the exact value of a negative sequence is needed for unbalance compensation. To measure the positive sequence separately from the negative sequence, a low pass or notch filter having a narrow bandwidth is normally used. However, such a filter causes a lot of phase delay, thus the response time of the system tends to be lengthened. A method of estimating the positive- and the negative-sequence voltages separately is presented, without a significant delay, by utilising the weighted least-squares estimation (WLSE) method with the covariance resetting technique. A frequency update law is also proposed to accommodate the frequency varying environments. The authors demonstrate through simulation and experiment the superior performance of the proposed scheme in measuring the positive- and the negative-sequence voltages at the time of abrupt transition. This method can be applied to uninterruptable power supplies (UPS), pulsewidth modulation (PWM) AC/DC converters, active filters, AC voltage compensators etc