HARMONIC MITIGATION FOR POWER QUALITY IMPROVEMNT BY DESINING PHASE-SHIFTING TRANSFORMERS FOR NON-LINEAR LOADS

Abstract

Electricity supply should invariably show a perfectly sinusoidal voltage signal at every customer location. Due
to number of reasons, utilities often find it hard to preserve such desirable conditions. The deviation of the voltage and
current waveforms from sinusoidal is described in terms of the waveform distortion, often expressed as harmonic
distortion which degrades the power quality. Harmonic mitigation techniques are available for harmonic problems in
three phase power systems. Users of adjustable speed drives (ASD) and other three phase (rectified) non -linear loads
have many choices available when it comes to harmonic mitigation. In the consideration of various alternatives, much
depends on the user’s objectives as well as the severity of harmonics contributed by internal loads. As the even
harmonics are generally cancelled out due to symmetry of waveform of quantity bu t odd harmonics are present in the
system, where lower order harmonics have worse effect. Number of methods is available for mitigation of harmonics
which are explained here. Main concentration of the paper is on mitigation of harmonics by designing different
PST(phase shifting transformer).PST extensively used for improving power quality to reduce harmonics in AC mains. As
rectifier widely used in industry and it is also the non linear load. So PST is also used to reduce THD for any non linear
loads of the system.
The Phase Shifting Transformer (PST) is essential gradient that provides desired phase shift in supply voltage by
different connections like star, delta, polygon and zigzag. The level of harmonic currents may be reduced by using phaseshifting techniques and low impedance plays a crucial role in reducing voltage distortion. These topologies can be found
in aircraft power systems, motor drives, Dc Arc furnace and other applications that require low total harmonic distortion
of the input line current.
The quality and reliability of the electrical system can be considerably improved through the use of PST