Shunt active power filters are custom power devices used to eliminate current harmonics and improve the power factor in systems with non-linear loads. Several different methods can be used to control active power filters, whereby the reference current that is detected from the load current and processed by the active power filter controller forms a very crucial role in determining the active power filter performance. This paper presents improved technique of generating the compensated supply current using four different methods namely, the Instantaneous Reactive Power Theory (PQ Theory), the Modified Instantaneous Reactive Power Theory (Extension PQ Theory), the Synchronous Reference Frame Theory (SRF Theory), and the Modified Synchronous Reference Frame (Extension SRF Theory). The system was tested and modeled in MATLAB/ Simulink simulation package with a shunt active power filter to compensate the harmonics current injected by the loads. The simulation studies assumes non-ideal condition such as Non-symmetrical Voltage Supply and Distorted Voltage Supply. The results obtained showed that the Total Harmonic Distortion of the compensated supply current was improved.