In a linear circuit the current follows the voltage.The current increase or decrease proportionally with increase/decrease of voltage. The VI characteristics of the devices is linear is called linear devices. The example of the linear device is a resistor. The linear devices follow the Ohm’s Law. If the voltage applied across resistor is sinusoidal the current flowing through it is also sinusoidal.
In a nonlinear devices, the current flowing through the device does not follow the Ohm’s law. The current flowing through the non linear devices is not follow the voltage waveform. If the applied voltage is sinusoidal, the current flowing through the non linear device is nonsinusoidal.
At the time of charging of the transformer the magnetizing current drawn by the transformer does not follow the sinusoidal voltage and current is non sinusoidal.
According to Fourier series, the non sinusoidal current can be resolved into the fundamental current and the integral multiples of the fundamental frequency. The integral multiples frequency of the fundamental frequency is called Harmonics.
What is fundamental frequency?
Fundamental frequency of the sinusoidal waveform is the first harmonic order frequency and it is equal to the supply frequency. The fundamental frequency is the lowest frequency f on which complex waveform is built. In india and UK the fundamental frequency is 50 Hz and in USA the fundamental frequency of alternating voltage is 60 Hz.
A sinusoidal waveform is an alternating voltage or current which varies according to sin function.
i = I_{m} sinwt
v = V_{m} Sinwt
w = 2Π f
Where,
v and i is instantaneous value of the waveform
V_{m}, I_{m} is the maximum value of the waveform
f is the lowest frequency or fundamental frequency
What is Harmonics?
Ideally voltage and current waveform is sinusoidal. However, because of the increased popularity of electronics power devices and other nonlinear loads, the waveform quite often become distorted. The distorted waveform can be resolved into fundamental and integral multiples of fundamental frequency according to Fourier series. Thus, the distorted waveform will contain harmonics.
What is Harmonic Frequency?
Harmonic order can be assigned according to frequency of voltage or current waveform.
Harmonic order

Frequency(Hz)

2

100

3

150

4

200

5

250

6

300

7

350

8

400

9

450

If current or voltage waveform is symmetrical about xaxis the even number of harmonics would be zero.
Further, the harmonics order can be represented as positive and negative sequence harmonics. The harmonic frequency which has same phase sequence that of fundamental frequency has are called positive sequence harmonics. The harmonic frequency which has opposite phase sequence that of fundamental frequency has are called negative sequence harmonics.
Harmonic Number

1

2

3

4

5

6

7

8

9

10

11

12

Phase Sequence

+

_

0

+

_

0

+

_

0

+

_

0

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