25-08-2017, 09:32 PM
Harmonic pollution
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INTRODUCTION
The objective of the electric utility is to deliver sinusoidal voltage at fairly constant magnitude throughout their system. This objective is complicated by the fact that there are loads on the system that produce harmonic currents. These currents result in distorted voltages and currents that can adversely impact the system performance in different ways.As the number of harmonic producing loads has increased over the years, it has become increasingly necessary to address their influence when making any additions or changes to an Installation.
In general harmonics are periodic disturbance in voltage and current,any non-sinusoidal periodic wave form can be contributed as combination of sinewave of certain frequencies,amplitude phase angle.In simple terms harmonics are multiple of the normal main power frequency.For example,3rd order has got frequency 150hz and 5th order has frequency 250hz.In a wave shape only the fundamental componenet of voltage and current contribute to power transmission.This non active power due to harmonics is referred as distortion power.
The harmonic is introduced in the system at both ends i.e. at the generating end and at the receiving end(loads).A pure sinusoidal voltage is a conceptual quantity produced by an ideal ac generator built with distributed stator and the field windings that operate in uniform magnetic field.Since neither the winding distribution nor the magnetic field are uniform in a working ac machine,voltage waveform distortions are created and the voltage time relationship deviates from pure sine function.As the deviation is in the form of ac function which implies that the voltage distortion contains harmonics.
In contrast some loads cause the current to vary disproportionately with the voltage during each half cycle.These loads are classified as nonlinear loads and the current and the voltage waveforms are nonsinusoidal,containing distortions,where by the 50hz wave forms has numerous additional waveforms superimposed upon it,creating multiple frequencies within the normal 50 hz sine wave.The multiple frequencies are the harmonics of the fundamental frequency.The current distortions produce voltage distortions.This is of no concern where there is a low impedence path from the power source,which has sufficient capacity so that loads placed upon it will not affect the voltage.
To fully appreciate the impact of this phenomena, there are two important concepts to bear in mind with regard to power system harmonics. The first is the nature of harmonic-current producing loads (non-linear loads) and the second is the way in which harmonic currents flow and how the resulting harmonic voltages develop.
Linear and non-linear loads
A linear element in a power system is a component in which the current is proportional to the voltage. In general, this means that the current wave shape will be the same as the voltage (See Figure 1). Typical examples of linear loads include motors, heaters and incandescent lamps.
These voltages sum and when added to the nominal voltage produce voltage distortion. The magnitude of the voltage distortion depends on the source impedance and the harmonic voltages produced.
Classification of harmonics
Each harmonics has a name,frequency and sequence.The sequence refers to phasor rotation with repect to the fundamental frequency i.e. in an induction motor a positive sequence harmonic would generate a magnetic field that rotated in the same direction as the fundamental.A negative sequence harmonic would rotate in the reverse direction.The first nine harmonic with their effects are listed below.