NOVEL ACTIVE POWER FILTERS FOR HARMONICS SUPPRESSION

[attachment=17623]

INTRODUCTION
CONCEPT OF HARMONICS
Harmonics can be defined as entities having a frequency in integral multiples of the fundamental frequency. This means, if we have a fundamental frequency of ‘f’ Hz, the entities 2f, 3f, 4f, 5f etc. are called as harmonics. During the past decades many power electronic devices such as 6-pulse converters, variable speed motor drives, and other power electronic equipment for energy flow control have been installed in power distribution systems. Such devices have gained a great deal of popularity due to the significant savings in energy costs that they provide. Unfortunately, power electronic devices are a source of harmonic distortion in power distribution systems since they draw a supply current that is non-sinusoidal.



Harmonic Distortion :
This term can also be referred to as harmonic pollution. Harmonic distortion of voltages and currents in power systems means that these quantities are non sinusoidal. These are caused by the presence of non-linear loads in the system that produce distorted current. In most cases, these currents are periodic; therefore, using Fourier analysis these distorted voltages and currents can be described in terms of harmonics.



CLASSIFICATION OF HARMONICS

Harmonics are classified as follows -
1. According to integral multiple (greater than 1) of fundamental frequency as even harmonics and odd harmonics. Even harmonics are entities having a frequency of even multiples of the fundamental frequency, while the odd harmonics have frequency of odd multiples of the fundamental frequency i.e. if we have a fundamental frequency of 50 Hz, the even harmonics would have frequency of 50x2=100Hz, 50x4=200Hz, 50x6=300Hz and so on. The odd harmonics have a frequency of 50x3=150Hz, 50x5=250Hz, 50x7=350Hz and so on.
2. Inter harmonics are those which have a frequency of non integer multiples of fundamental frequency. These appear as discrete frequencies or as a wide band spectrum i.e. assuming a fundamental frequency of 50Hz, inter harmonics will have a frequency of 50x1.5=75Hz, 50x2.4=120Hz, 50x5.5=275Hz.