Tuesday, 20 September 2016

Know about Harmonics and its Effects

Harmonics are the deviation or the distortion of the waveform of its Sinusoidal waveform due to the addition of waveforms of different frequencies.
As per Ohms Law V=I/R,Voltage is proportional to the Current means Current also have a sine waveform except current(which has power factor) does not align perfectly to voltage waveform.
                                         current waveform,voltage waveform,sinewave

The Power supply operates at a standard of 50Hz that means 50Cycles per sec,the voltage changes from positive and negative polarity and back again 50 times per sec.The below is the sinewave of the supply in that the voltage(240V) changes between +240 to -240V thus called alternating voltage.

Figure 2 shows the superimposition of the 3rd order harmonic to the base sine wave due to that the resultant waveform is Distorted waveform(Base+3rd Harmonic).The entire waveform does not follow the Sinusoidal pattern causes unbalance in the load.


                                     sine waveform,supply waveform

Waveform with superimposed harmonics,3rd harmonics,2nd harmonics
The below Fig shows the distorted waveform which is the resultant of the addition of the 3rd harmonic with the fundamental pure sinewave.
Fundamental to Distorted harmonics waveform


The below fig shows the Sine Wave from the Source without harmonics and the fig shows only one cycle having Vmax is the Peak value,Frequency f in Hz and Time period T.

                                    Sinewave of the power supply


The Equation of a Sinewave is written as:

                                     V=VmaxSine(wt)

Note: The harmonics depend on Phase angle,Frequency and also the amplitude.
If the fundamental frequency is given as; E = Vmax(2πƒt),then the complex waveforms will be,

                         

Complex WaveForms due to Harmonics

                       Complex waveform due to the addition of harmonics                   
Harmonic Sequencing

From the below table, Fundamental frequency is either 50Hz or 60Hz. And the 2nd order harmonic frequency will be 100Hz or 120Hz ,the 3rd order harmonic frequency is 150Hz or 180Hz and so on...,

The harmonics are voltages or currents that operate at a frequency that is an integer multiple of the Fundamental Frequency.

Name
Fund.
2nd
3rd
4th
5th
6th
7th
8th
9th
Frequency, Hz
50Hz/60Hz
100/120
150/180
200/240
250/300
300/360
350/420
400/480
450/540
Sequence
+
0
+
0
+
0

Note that in some countries the fundamental frequency will be either 50Hz(UK) or 60Hz(US).

Sequence
Rotation
Harmonic Effect
+
Forward
Excessive Heating Effect
Reverse
Motor Torque Problems
0
None
Adds Voltages and/or Currents in Neutral Wire causing Heating

Common Problems Caused by Harmonics

Overloading Neutral Conductors
The 3-phase system consists of three phase conductors and a neutral conductor. These three phase conductors carry the same amount of current and they used to cancel each other in the neutral conductor makes the load balanced But due to the usage of motors,fans,rectifiers,SMPS causes neutral with increased or twice the current flow of phase current.Hence with balanced load with the currents cancel each other in the neutral conductor, the size of the conductor is minimum, but due to the nonlinear loads, the current draw is higher hence the size of the neutral conductor is bigger than the counterpart.

Overheating Transformers & Losses
The Transformers feeding power to the unbalanced loads like rectifiers,motors, the eddy current loss in the windings is proportional to the square of harmonic current and its frequency thus the losses associated with the transformer with the non-linear load will be twice of the linear load.This condition leads to the overheating of the Transformer which degrades the insulation causes complete failure.

Nuisance Tripping of Circuit Breakers

The RCCB used to check the current between phase to neutral if the neutral current is not same with the phase current then RCCB disconnects power to the Load and also RCCB does not sum the higher frequency components thus sometimes it used to trip.