HamidReza Mahdian - Arak Science and Research branch Islamic Azad University Arak, Iran
MohammadReza Hashemi - Arak Science and Research branch Islamic Azad University Arak, Iran
Ali Asghar Ghadimi - Arak University, Faculty of Electrical Engineering

Conventional techniques of phasor estimation which based on constant frequency with dynamic changes in the frequency of a power system are severely affected. Inaddition, the frequency changes from its nominal value had led to change in line and utility reactances, that causes a change inrelay operation. Consequently, frequency in protection, monitoring, control of power quality and performance of system is highly sensitive and is a key parameter. Currenttechniques that estimate the frequency are sampled voltage and current. If we consider pure sinusoidal voltage system, bymeasuring the time between zero crossings of the signal, the frequency can be easily calculated. Furthermore, the wide spread use of power electronics devices in the power generation, transmission, distribution and utilization of electrical energy, has led to change the actual shape of thevoltage wave form. Consequently, in practice given signal from power system is associated with noise and distortion, so we need to be able to use more advanced techniques that frequency can be calculated with reasonable speed and accuracy. With acceptance of this difficulty. the fact that thebehaviour of the power system is a flexible system that cannot be predicted accurately, is even more visible. So we need a way to calculate frequency in minimum possible time with maximum accuracy. So we can eliminate distortion caused by the frequency. Furthermore, for connecting with AC system and AC power injection, power electronic converters require accurate knowledge of voltage phase, so that the injectedvoltage must be in phase with grid voltage.

Phase-Locked Loop (PLL), voltage-sourced converters (VSC), synchronization, current control, unbalanced voltage