Electrical harmonics are electric voltages or currents that have frequencies that are multiples of the fundamental frequency of the power system. Electrical harmonic effects listed as per below

Drawbacks of Harmonic

Overheating of electrical equipment: Harmonics can cause excessive heating of electrical equipment, such as transformers, motors, and generators. This can lead to premature equipment failure and downtime.

Voltage distortion: Harmonics can cause distortion in the voltage waveform, which can lead to voltage fluctuations, flickering lights, and other voltage-related problems.

Power quality issues: Harmonics can cause power quality issues such as voltage sag, voltage swell, and voltage interruption. These power quality issues can result in equipment malfunction and downtime.

Interference with communication systems: Harmonics can interfere with communication systems, such as radio and TV signals, leading to poor reception and transmission quality.

Increased energy costs: Harmonics can lead to increased energy consumption, resulting in higher energy bills.

To mitigate the negative effects of harmonics, electrical engineers can use techniques such as harmonic filters, active power filters, and static var compensators. These techniques can help reduce harmonic distortion, improve power quality, and increase the efficiency of electrical systems.

Current Harmonics are generated due to non-linear loads, i.e. the loads which draw non-sinusoidal currents from a sinusoidal voltage source. Current harmonics lead to voltage harmonics due to finite source impedance.

Harmonics are multiples of fundamental frequency, such as 2nd, 3rd, 4th, 5th etc. This is called order of harmonics. DC value in AC is called as sub-harmonic at zero frequency. Normally in a three-phase system even harmonics are not generated due to waveform symmetry. Hence we will consider only DC and odd harmonics below.

Effect of DC in AC or sub-harmonic at zero frequency:

The DC in AC is generated due to unequal firing of diodes or thyristors connected to positive and negative of DC link in converters. DC component in AC leads to:
a) Saturation of transformers, thus increasing magnetic losses.
b) When an AC voltage containing DC component is applied to AC motor, it leads to more starting current.
c) DC component leads to braking torque, which needs to be overcome while motor is running. Hence it increases motor losses in running conditions.

Effect of Triplen order Harmonics:

The harmonics which are multiples of third harmonic such as 3rd, 9th, 15th, 21st (as only odd harmonics are considered) are known as triplen harmonics. They generate zero sequence currents.
In a three phase four wire system the neutral cable carries only the unbalance part of three phase currents, as the balanced part adds to zero. But when the frequency of currents is three or multiple of three times the fundamental frequency, such currents add in neutral. Thus neutral may get
overloaded and can burn eventually due to overheating.

Effect of 5th, 11th, 17th, 23rd etc order Harmonics:

These are also (6N - 1) harmonics. Apart from the other effects such as proximity effect, skin effect etc. common to all harmonics, these harmonics will generate negative sequence torque, which is against the direction of motion. Hence if the motor is running in clockwise direction, these harmonics will try to rotate the motor in anti-clockwise direction, at much faster rate depending upon the order of
harmonics. Thus they will create additional losses. They will also result in pulsating torques especially at lower speeds.

Effect of 7th, 13th, 19th, 25th etc order Harmonics:

These are also (6N +1) harmonics. Apart from the other effects such as proximity effect, skin effect etc. common to all harmonics, these harmonics will generate positive sequence torque, which is in the direction of motion. Hence if the motor is running in clockwise direction, these harmonics will try to rotate the motor in clockwise direction, but at much faster rate proportional to the order of harmonics. Thus they will create additional losses. They will also result in pulsating torques especially at lower speeds.

Lower Order harmonics: Though less harmful than higher order harmonics – are not preferred as they are more difficult to filter than higher order harmonics.

Higher Order harmonics: are more harmful than lower order ones due to pronounced eddy current losses, hysteresis losses, copper losses (due to skin effect). They also have more proximity effects. They lead to more RFI and EMI related problems.

Difference between TDD and THD with reference to harmonic standard?

TDD means Total Demand Distortion
THD means Total Harmonic Distortion
Demand current is the fundamental component of the maximum current that a plant draws from the source.

Percentage TDD means (Harmonic Current / Demand Current) X 100.
Demand current is less than the maximum continuous current capacity or the rated current capacity of the plant.

Suppose the Demand Current of the plant is 1000 Amps. A certain non-linear load, for example a Variable Frequency drive, is drawing 500 Amps and at this current level it is having THD of 30%.
Assume that this is the only non-linear load that the plant is having. Hence in this case the harmonic current in the plant will be 500 x 30 / 100 = 150 Amps.
TDD in this case will be ( 150/1000) X 100 = 15%. Whether the plant complies with IEEE standard will be decided by TDD and not by THD.

Sources of Harmonics

Harmonics are generated by various electrical equipments connected to the electrical grid.
• Thyristor controlled devices like Variable Speed Drives, Soft Starters, Rectifiers
• Elevators, Inverters, UPS, Battery Chargers
• Arc Furnaces
• Electric welding machines
• In some cases transformers, rotating machines

Presence of harmonics is not good for both utility operators as well as individual customers. Various adverse effects of Harmonics are as follows:
• Undesirable resonance effect seen between inductive and capacitive components
• Interference in communication lines and devices
• Due to introduction of harmonics it may generate  irregular voltage and current values in the grid and may unnecessarily increase overall demand in utility grid.
• Increased losses due to distorted waveform. Equipments  may get overheated. E.g. transformer overheating, increased losses due to harmonics
• Some of the precision electrical equipments like relays may fail to perform accurately as they are calibrated to work with fundamental waveform at rated values. Harmonics cause false conditions and may lead to equipment malfunction
This in turn puts strain on the network

How to Control Harmonics

Presence of harmonics is not good for both utility operators as well as individual customers. Various adverse effects of Harmonics are as follows:

1. Use of Harmonic Filters: In cases where it is not possible to limit the non linear load, use of Harmonic filters is recommended.
2. Limiting non linear load devices (e.g. inverters, UPS) to below 30% of total load on a transformer.
A Harmonic filter helps to trap the harmonic frequency thereby freeing up the fundamental frequency. In this, the filters are tuned to specific harmonic such as 3rd, 5th, 7th , 11th.

A Harmonic filter provides low impedance to tuned harmonic frequency and bypasses it to earth and thus eliminates harmonics from power supply.

Conclusion

It is extremely important to control harmonics due to their undesirable effects. Utilities and consumers both are fast realizing the importance of controlling harmonics in the network. Many utilities are making it mandatory for consumers with high non linear load to install harmonic filters.