Correlation of the distorted signal with sine and cosine functions is one of the best ways of calculating power-frequency phasors. However, if the data window becomes shorter than one cycle, DC transient components introduce large errors. The presented algorithm removes this weakness. It splits the signal into two parts mutually shifted in phase, and correlates both of them with the sinusoidal function only. Comparing the characteristics obtained in the time domain and in the frequency domain, it becomes evident that the new method is superior to the standard one.