Understanding Total Harmonic Distortion: What It Is and Why It Matters

• 2024-11-20

Introduction of Total Harmonic Distortion

• Total harmonic distortion (THD) is the measure of the deviation of voltage or current waveform considering ideal sinusoidal shape.

• True Power Factor= Distortion Factor x Displacement Factor

• Electrical equipment manufacturers follow the recommended harmonic standards to limit the harmonic emission from their products and ensure the protection of the equipment from the supply voltage or current harmonics.

BASICS of Harmonic Distortion:

The real-time deviation of supply voltage from the above-mentioned shape, amplitude, and frequency standards is what we have to observe.
These distortions occur in a situation We call 'Power Quality' issues.

As the density of non-linear devices on a per capita basis drastically increases world-wide, Power Quality is becoming a matter of importance. The proliferation of non-linear loads introduces sinusoidal voltage components with small frequencies into the power system.

These voltage components-called voltage harmonics- superimpose with the fundamental frequency voltage and hence in turn make the utility supply voltage non-sinusoidal.

When one looks at linear distortion and non-linear distortion, harmonic current and harmonic
frequency can lead to signal distortion.

The non-sinusoidal-voltage supply leads to heating losses in machines, malfunctions sensitive equipment, and interferes with communication.

Total harmonic distortion (THD) is the measure of the deviation of voltage or current waveform from ideal sinusoidal shape.

With Fourier analysis THD quantifies the unwanted harmonics present in the voltage or current waveforms.

THD analysis is important to understand before we try to maintain power system stability.

 
UNDERSTANDING THD

The input voltage specification printed in any equipment is the RMS voltage of the fundamental frequency (f) component, V1.

The harmonics effect disrupts & transforms the ideal voltages or currents to an undesirable shape, amplitude, and frequency.

The harmonics can be further disintegrated into several sinusoidal components at frequencies which are integer multiples of the fundamental frequency, f.

If v(t) constitutes a sinusoidal voltage component with a frequency equal to n times f, it forms the nth harmonics vn(t). The nth harmonic vn(t)supplies its share to the amplitude of the non-sinusoidal voltage v(t). N could be any value 2,3, or 4, and so on, but even harmonics rarely exist.
 

THD can be mathematically shown in percentages with the below equation, where Vn is the RMS voltage of nth harmonic, and V1is the RMS voltage of the fundamental frequency.

Effects of Harmonics in Power Supply:

The presence of harmonics can be harmful to power system components such as electrical machines, electronic devices, measuring instruments, switchgear, transmission cables, and lighting systems.

A harmonic component can affect the fundamental signal, output signal, and input signal depending on your THD impact factor. Voltage harmonic distortion, crossover distortion, and intermodulation distortion can all affect the harmonic content of your fundamental signal.

Motors, generators, and transformers are major part of electrical system. During saturation the ferromagnets in these machines act as the source of the harmonics. We are familiar with the hysteresis and eddy current losses in machines, which are frequency dependent.

The presence of higher-order harmonics in the input supply worsens the magnetic losses in motors and transformers, hence the temperature.

Coming to the rotor side of motors, the distortions in rotor currents cause torque pulsations or reduced torque. The current distortion leads to increased copper losses and heating in the electrical
machines, hence insulation suffers.

Generators are source of 3rd harmonics into the power system. The current and voltage harmonics spread from the generator to other loads present in the electrical system.

CONCLUSION:

THD analysis is done by finding the percentage of harmonics superimposed in the waveforms.

When dealing with quality of power systems, it is required to keep an eye on both voltage and current harmonics. Multispan’s PQM-16+ and other models from the PQM series make it easy to monitor power quality. They accurately measure THD and individual harmonics for voltage and current up to the 32nd order, helping you meet IEEE 519 standards and ensure reliable system performance

Both of these distortions have drawbacks: less efficiency, an increase in current, insulation problems, incorrect relay tripping’s, and switchgear failures.

Non-linear loads are the main problem makers, and there is a need for Proper design of these non- linear devices to maintain THD standards.