An instantaneous phase angle detection algorithm under unbalanced line voltage condition

If a negative sequence is generated by a voltage sag and/or unbalance, it appears as an oscillating error in a synchronous reference frame (SRF). In power conditioning equipment, an exact value of positive sequence is needed for achieving the desired goal of the system, e.g., unity power factor and constant output voltage, whereas the exact value of negative sequence is needed for compensation. To measure the positive sequence separately from the negative sequence, one uses normally a low pass filter having a narrow bandwidth. However, such a filter causes a lot of phase delay or measurement delay, thus the response time of the system tends to be lengthened. The authors propose a method of estimating the positive and the negative sequence voltages separately without a significant delay by utilizing the weighted least-squares estimation (WLSE) method having the covariance resetting technique. They 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 UPSs, PWM AC/DC converters, active filters, series voltage compensators, etc.