What is Harmonics, Creation, Elimination in Power system

What is Harmonic in Electrical:

Harmonic in electrical is nothing but an integer multiplication of fundamental frequency. Harmonic are an unwanted distorted waveform which frequency is higher than the fundamental frequency. Since all of our electrical equipments are rated in either 50 Hz or 60 Hz depending upon the country, the high frequency signal cause decreasing the performance of the system, increasing the temperature, increasing the neutral voltage etc.. Generally harmonic are produced by nonlinear electronics loads such as electronics ballest, VFDs, Battery charger, all electronics device etc.

See: Why India Use 50Hz Power frequency and US use 60 Hz Frequency

How electrical harmonics Occurs in Power system:

Let consider above example. Take single VFD. As you know VFD consists of AC to DC converter and DC to AC converter and other electronics components. The main reason for harmonic occurrence is, we are using high speed switching while converting dc waveform into ac waveform. See the output of the six step VFD inverter, how signal has distorted. However, we are giving pure sine wave but we are getting distorted sine wave. The remaining part of the distorted wave is called harmonics. Also note that the frequency of the remaining harmonic waveform. It is higher than the fundamental input frequency.

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How Electrical Harmonics Occurs in Power system

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Fundamental harmonic in Electrical:

Normally we are getting the power from the generator whose frequency is called fundamental frequency. The fundamental frequency will be 50 Hz or 60 Hz. All the electrical and electronics appliances are designed to operate in this frequency. It is also called as first harmonic.

Second harmonic in electrical:

The waveform whose frequency is 100 Hz (2 * 50 Hz). Therefore, the second harmonic wave has twice the frequency of fundamental harmonic. See the wave form. There when fundamental harmonic reaches zero, it reaches high value vice versa. That’s why second harmonic creates reverse direction which means the negative sequence current flows in the electrical circuit. This current opposes the rotating magnetic field in the induction motor. It causes to producing less mechanical torque in electrical motor. It is also called as negative sequence harmonic.

Third harmonic in electrical:

The third harmonic whose frequency is triple time of the fundamental harmonic. The frequency of the third harmonic will be 150Hz (3* 50 Hz). This is very dangerous harmonic. You see the waveform, when the fundamental harmonic current reaches zero, the third harmonic current also reaches to zero like that the fundamental harmonic reaches to high value it reaches high value at negative side. That’s why it creates zero sequence current in the power system. Since it causes increasing the neutral voltage in the system. Increasing neutral voltage, the relay operates the circuit breaker. This is the effect of third harmonic currents. It is also called as “triplens” harmonic.

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Fourth Harmonic:

The fourth harmonic frequency would be 200 Hz multiple of 4 in fundamental frequency. You can see the waveform, there the fundamental harmonic reaches to high value, the harmonic current also reaches to high and vice versa for negative side also. That’s why it increases the flow of current in the conductor and increases the electrical equipment temperature. It is also called as positive harmonic.
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5th Electrical Harmonic:

The 5th harmonic frequency is 250 Hz, typically 5*50Hz. See the wave for when the fundamental come to zero it goes to high and vice versa. So that, 5th harmonic we will get the reverse phase order. And it will rotate in opposite direction. Thus 5th harmonic will be found in braking region. 5th harmonic opposite rotation of the induction motor. Same like second Harmonic.

Conclusion about Electrical harmonic:

Harmonic are unwanted distorted current or voltage waveform which causes increasing conductor temperature, cause to produce less torque and increasing the neutral voltage. Unlike the positive ( 4th, 7th, 10th) and negative sequence harmonic (2nd, 5th, 8th) currents that cancel each other out, third order or triplen harmonics (3rd, 6th, 9th, …) do not cancel out. To do stop third harmonics current generally high inductive reactor are used in power system. However, the flow of unwanted harmonic current (whatever type of harmonics) in the power system reduces the system efficiency.

Learn More: What is Crawling of Induction Motor & How to Avoid it?