Various non-linear loads such as adjustable speed drives (ASD), rectifiers, ovens, computer power supplies, etc. they draw non-sinusoidal currents containing harmonics from the supply which in turn cause voltage harmonics. Current harmonic causes increased losses in the power system, excessive heating in rotating machinery, interference with nearby communication and control circuits, etc. It has become vitally important to maintain the sinusoidal nature of voltage and currents in the power system. Various international agencies such as IEEE and IEC have issued standards that place limits on various current and voltage harmonics. The limits for various current and voltage harmonics specified by IEEE-519 for various frequencies are given in Table 2.1 and Table 2.2. Table 2.1 IEEE 519 Voltage Limits Bus Voltage Minimum Individual Harmonic Components (%) Maximum THD (%) 69 kV and below 3 5115 kV to 161 kV 1.5 2.5 Above 161 kV 1 1.5 The objectives and functions of the filters of active power have been expanded by reactive power compensation, voltage regulation, etc. to harmonic isolation between utilities and consumers, and to the damping of harmonics throughout the distribution as harmonics propagate through the system. Active power filters are installed at individual customers or in substations and/or on distribution feeders. Depending on the compensation objectives, various types of active power filter topologies have evolved, an adequate description of which is given in [3-4].Table 2.2 Current Limits IEEE 519SCR=Isc/Il h