Synchronous Harmonics Research Paper

CHAPTER 3

POWER QUALITY AND ITS PROBLEMS

3.1 Harmonics

Power systems are designed to operate at frequencies of 50 or 60 Hz. However, certain types of loads produces currents and voltages with frequencies that are integer multiples of the 50 or 60 Hz fundamental frequency. These frequencies components are a form of electrical pollution known as harmonic distortion. There are two types of harmonics that can be encountered in a power system. Synchronous harmonics. Asynchronous harmonics.

Synchronous harmonics are sinusoids with frequencies which are multiples of the fundamental frequency. The multiplication factor is often referred to as the harmonic number. The synchronous harmonics can be subdivided into two categories. Sub-harmonics:

Transformer exciting current, arc furnaces, rectifiers and many other loads will produce harmonics in the utility lines. Most utilities limit the allowable harmonic current levels to the values shown in IEEE 519.

3.1.1 Total Harmonic Distortion (THD)

The total harmonic distortion of a signal is a measurement of the harmonic distortion present in current or voltage. It is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Harmonic distortion is caused by the introduction of waveforms at frequencies in multiplies of the fundamental.

THD(%)=√(∑_(i=2)^α▒x_i^2 )/|x_1 | (2.2)

The THD is a very useful quantity for many applications. It is the most commonly used harmonic index. However, it has the limitation that, it is not a good indicator of voltage stress within a capacitor because that is related to the peak value of voltage waveform.

3.1.2 Distortion Factor

The distortion factor Fd is defined as the ratio between the fundamental and the signal in RMS values. It is given

They may be trapped by power factor correction capacitors and overload them or cause resonant over-voltages. They can distort the feeder voltage enough to cause problems in computers, telephone lines, motors, and power supplies, and may even cause transformer failures from eddy current losses. The harmonic currents may be trapped by installing series LC filters resonant at the offending frequencies. These filters should be designed to offer low impedance at the resonant frequency compared to the source impedance at that frequency. But, again, there is a hidden “gotcha.” If a filter is installed that has a series resonance at the 7th harmonic, it will also have a parallel resonance with the utility at a lower frequency when the source inductance is added to the filter inductance. If this parallel resonance should lie on or near the 5th harmonic, there is the possibility of the resonant over-currents described earlier. The installation of series resonant traps will always introduce parallel resonances at frequencies below the trap frequencies. Good practice dictates that multiple resonant traps be installed first at the lowest harmonic frequency